Symposium SF07—Frontiers of Intermetallics Science for Structural and Functional Materials Design

Intermetallic titanium aluminide alloys based on the ordered γ-TiAl phase are innovative materials for lightweight high-temperature applications. In addition to their low density of roughly 4 g/cm³, their high specific Young’s modulus and strength even at elevated temperatures, and their good oxidation and burn resistance, especially their excellent creep properties make these alloys a material of choice for challenging structural applications. Following intensive research and development activities, γ-TiAl based alloys have recently entered service in the automotive and aircraft engine industries, e.g. as low-pressure turbine blades in environment-friendly jet engines, as engine valves in sports and racing cars, or as turbocharger turbine wheels. In the course of the past decades, the development of these complex multi-phase alloys has benefited greatly from the application of (in situ) synchrotron X-ray techniques. Diffraction and scattering techniques, in particular, have offered access to the atomic structure of the material and provided insights into a variety of microstructural parameters. Advanced experimental setups, which are steadily refined, have even allowed the exploration of elaborate manufacturing processes and yielded insights that have so far been inaccessible by means of conventional methods. Here, a practical introduction and overview of recent progress in this field of research are provided. Current prospects at modern synchrotron radiation sources will be illustrated by means of selected recent case studies pertaining to different stages in the development of modern γ-TiAl based alloys (i.e., fundamental research, manufacturing, and fine-tuning of properties for application). In this context, available setups for in situ high-energy X-ray diffraction and small-angle X-ray scattering experiments will be discussed in terms of their advantages as well as their limitations. This talk reaches out not only to the TiAl scientific community, though. By addressing the key question as to how to actually simulate real-life conditions such as applied loads and high temperatures in a laboratory environment, researchers with a general background in materials science may also find in it a collection of ideas that are indeed applicable to many different materials systems.

The aim of this work is to study the deformation mechanisms in the ordered β_o phase containing some ω_o precipitates of a TNM-TiAl alloy (Ti_51.05Al_43.9Nb₄Mo_0.95B_0.1 in at.%) and to look for the origin of the low ductility at room temperature of this alloy.
The alloy produced by Spark Plasma Sintering exhibits a near lamellar microstructure made of lamellar γ/α₂ colonies surrounded by γ and β_o grains. Tensile tests were performed at room temperature to determine the mechanical properties and the ductility of the alloy. The rupture surface of the deformed material were investigated by scanning electron microscopy.
Two types of TEM (Transmission Electron Microscopy) experiments were used to explore the behaviour of these dislocations and to study the deformation mechanisms. The first one lies in investigating the deformation microstructure in thin foils extracted from bulk samples previously deformed in tension at room temperature, while the second type consists in performing in situ tensile tests inside a TEM that allows the direct observation of the dislocation dynamics under stress and in real time.
At room temperature, the material exhibits a limited ductility with cleavage surfaces along the lamellae interfaces of the colonies. The β_o grains contain nano-precipitates of the ω_o phase and deform plastically by both <111> superdislocations and <001> dislocations. The <111> superdislocations are dissociated into two superpartial dislocations separated by an antiphase boundary. They glide in {011} planes and have the tendency to be localized into pile-ups, an effect demonstrated as the result of the ω_o precipitation that harden the β_o grains and favour deformation concentration. The <001> dislocations are found to be split into two identical superpartial dislocations separated by a stacking fault. They are elongated along their screw orientation and also anchored at these small nano-precipitates of ω_o phase. In situ straining experiments performed at room temperature show that they move by jumps between these elongated configurations.
These results are discussed to explain the low ductility of this alloy. The stress concentration due to the pile-up formation on grain boundaries is assumed to induce fracture along lamellae interfaces in neighbouring lamellar colonies.

Titanium aluminium alloys are constantly evolving as tremendous efforts have been set to improve their mechanical properties at elevated temperatures. Thus, new alloys were developed and tailored via heat treatments to fulfil the demanding requirements of aviation and automotive applications. However, these optimizations notoriously have led to decreased room temperature ductility and reduced fracture toughness. Within this study, two advanced TNM alloys have been investigated by in-situ micromechanical notched cantilever tests and sophisticated transmission electron microscopy techniques. Using these methods, the mechanical properties of distinct interfaces regarding the fracture toughness, J-integral and fracture stress could be evaluated at any point during the experiment. Furthermore, the crack propagation could be tracked in-situ and sudden fracture events were linked to the failure of distinct microstructural components. The presented method allowed to identify strengthening mechanisms such as e.g. particles along common α₂/γ interfaces or bridging. To further deepen the understanding of the fracture mechanisms along this interface type, strain maps of the fully lamellar microstructure were conducted edge-on for both alloys. To do so, 4D scanning transmission electron microscopy including scanning nanobeam electron diffraction was performed. From these experiments, strain accumulations caused by misfit dislocations or silicide particles were identified and linked to fracture events. Furthermore, these results were discussed in the light of existing literature.
Based on these results, further alloy design and tailored microstructures will be adapted to enable a higher room temperature ductility and fracture toughness of these alloys.

Research on developing new TiAl alloys for high temperature applications is introduced. At KIMS, we have developed new TiAl alloys which have excellent room temperature and high temperature properties. Especially, the new alloy showed excellent oxidation resistance in the temperature range from 900 to 1000^oC by forming stable Al₂O₃ oxidation layer. Process developement of casting, forging as well as 3d printing on the newly developed KIMS alloys was introduced. Especially, small size turbine wheel and blade were manufactured be centrifugal casting process. The results from the testing and validation of TiAl blade were shown that KIMS alloy can be used as a turbine blade above 900^oC. In addition, we proposed some underlying mechanism of high temperature strength of KIMS alloy from TEM and SEM observations. Finally, the operation test of micro gas turbine is now under examination to confirm the possibility of the application of the new alloy in the gas turbune engine.

Ti5553 (Ti-5Al-5Mo-5V-3Cr wt.%) is a titanium alloy with promising applications in safety-critical structures due to its high strength-to-weight ratio and near-β microstructure which opens pathways for tailoring mechanical performance based on annealing conditions. In order to utilize its advantageous properties, efficient manufacturing must be possible. Laser powder bed fusion (L-PBF) involves the layer-by-layer fabrication of complex geometries such as lattices which help reduce component weight. Moreover, L-PBF expedites part deployment since components can be built on-demand without multi-step assembly. Thus, the successful realization of L-PBF Ti5553 relies on a fundamental understanding of the alloy’s additively manufactured microstructure and mechanical behavior. In support of production efforts at Lawrence Livermore National Lab and Kansas City National Security Campus, the present work investigates the hierarchical microstructures and mechanical properties of L-PBF Ti5553 bulk and lattice parts. The characterization techniques include room-temperature tensile tests, nanoindentation, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The presence of second-phase (ω) nanoprecipitates in the lattice material distinguishes its microstructure from that of the bulk and moreover, indicates the challenges in predicting bulk mechanical response based on lattice properties. The present results provide a starting framework for the successful printing of Ti5553 bulk and lattice parts by comprehensively examining the processing-structure-property-paradigm of the deposited material system.

Additive manufacturing (AM) enables the production of complex, net-shape geometries. Additionally, in AM of intermetallic compounds and the related superalloys, which has received less attention, the microstructure and crystallographic texture of the product can be arbitrarily controlled by selecting appropriate process parameters, thereby enabling unprecedented superior properties. This paper discusses recent progress pertaining to texture evolution mechanisms and control methods, with an emphasis on laser-beam and/or electron beam powder bed fusion (PBF). One of the unique characteristics of PBF is that the crystallographic texture can be varied as a function of position within the product by controlling the scan strategy. The transient behavior of the texture and the factor used to control it via the scan strategy are discussed. In addition, the texture evolution behavior of highly symmetrical Ni-base superalloys as well as relatively low symmetrical TiAl intermetallic compound and disilicide is discussed. The importance of the crystallographic, multiplicity, of the preferential crystal growth direction is described to understand the evolution behavior of the texture in such heat-resistance materials.

This study was supported by Grants-in-Aid for Scientific Research (JP18H05254) from the Japan Society for the Promotion of Science (JSPS). This study was also partly supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), Materials integration, for revolutionary design system of structural materials, from the Japan Science and Technology Agency (JST).

Additive manufacturing (AM) provides inherent advantages for the processing of intermetallics. Most often intermetallics are high performance materials more expensive than competing conventional alloys. Thus, the near net shape processing capability of AM causing no material loss is advantageous. In addition, the properties of most intermetallics are very sensitive to variations of the chemical composition making the good chemical homogeneity of the powder based AM methods beneficial. Accordingly, TiAl low-pressure turbine blades for the current GE9X engine are produced by Electron Beam Melting. However, AM processes pose also many problems for intermetallics. Many intermetallic materials including TiAl are brittle compared to conventional alloys and the high heating and cooling rates experienced during AM can cause cracking and damage. The complex phase diagram of many intermetallics exacerbates this problem. TiAl for example shows a number of phase transformations and two-phase regions or in advanced alloys also three-phase and even more-phase regions between melting point and room temperature. If those phase transformations are associated with volume change or if too large volume misfits between phases exist internal stresses will occur. At lower temperatures, TiAl parts are too brittle to withstand those stresses and will fail during AM. This is one reason to use Electron Beam Melting instead of Laser Beam Melting for AM of TiAl despite the latter being less costly. Heating of the powder bed and the already processed section of the part is easier in Electron Beam Melting. With TiAl being less brittle at higher temperatures, this enables crack free processing.
In this contribution, the results of the characterization by high-energy X-ray diffraction of TiAl alloys during AM are presented. Directly after solidification, the phase constitution of TiAl is far away from the situation in the finished part. It resembles very closely the phase constitution directly at the melting point. The assumption of the final microstructure and phase constitution takes place in the layers below the surface. These experience a cyclic heat treatment due to the melting and solidification of the on top powder layer. The maximum temperature of this thermal cycle decreases with increasing distance from the melting and solidifying top layer. From the development of lattice constants and phase fractions, it is possible to reconstruct the internal processes in the material during this complex heat treatment scheme. The phase diagram is no good guide to understand the situation because the phase constitution and microstructures are away from equilibrium. Only by direct measurement one can acquire information about the microstructure formation in the TiAl part necessary to optimize the process or adapt the material to the process.

An integrated inverse design approach to accelerate the development of novel titanium aluminides for LPT blades in jet engines, specifically focusing on powder processing, such as MIM (Metal Injection Molding) and AM (Additive Manufacturing), has been undertaken, in collaboration with industries and universities, under the project of “Materials Integration for Revolutionary Design System of Structural Materials” in Cross-ministerial Strategic Innovation Promotion Program (SIP) in Japan. We have successfully developed a novel TiAl alloy with mechanical properties superior to the currently existing alloys and fabricated into 200 mm LPT blade by both MIM and AM processes. Our integrated computational materials design system is composed of three modules: “Mechanical-property Prediction Modules (MPM)”, “Microstructure Design Module (MDM)”and “Process Design Module (PDM)”. For example, the system works as follows: specific values of the properties required by industries, say, tensile strength and fracture toughness, are at first input into the MPM, and the MPM calculates and outputs the volume fraction of the most important microstructure constituent to meet the required properties, based on our mechanical property database (DB). Then, the volume fraction value is input into the MDM, and the MDM optimizes alloy composition and heat treatment condition to have the microstructure, based on our multi-component phase diagram DB. The proposed alloy with the composition is then cast into bar ingots and powdered by EIGA by industries. All of the powders are used for MIM (<45μm) and for AM-EBM (>45 μm) without any waste and fabricated into LPT through each PDM. Tokyo Tech is in charge of the integrated system of MPM and MDM, so that this talk focuses on some points to integrate the modules to the system. For MPM, the key is the digitalization of β/γ cellular microstructure because the bcc β-Ti phase introduced by the cellular transformation (α₂+γ -> β+γ) significantly affects the mechanical properties. For MDM, the most critical point is the reliable knowledge on the phase diagram with oxygen. We found that oxygen tremendously affects the phase equilibria among β-Ti, α-Ti (α₂-Ti₃Al) and γ-TiAl phases and eventually changes the transformation pathways to control the microstructures. Without this information, thus it is impossible to build up the system since a large amount of oxygen is picked up in MIM process in comparison with AM process. The details of MPM and MDM including the calculation methods will be independently presented by our co-authors in this symposium.
A part of this study has been carried under the research of SIP II in JST (Japan Science and Technology Agency).

In order to reduce the cost and time required to develop the TiAl low-pressure turbine blade for jet engines, we have built an inverse problem approach using two modules: one is the mechanical properties prediction module (MPM) to predict the microstructures that satisfy the required properties, and the other is the microstructure design module (MDM) to specify the composition range and heat treatment conditions for tailoring the microstructures. In this presentation, the MDM is highlighted. In this module, the two-step heat treatment of the lamellar formation and subsequent aging is assumed. The temperature and composition dependence of the volume fraction of each phase, as well as the supersaturation for the discontinuous precipitation of α₂-Ti₃Al + γ-TiAl → β-Ti + γ, are calculated in the framework of CALPHAD method. Furthermore, the effects of oxygen on the phase equilibria and kinetics are considered aiming at its application to powder processes such as metal injection molding. Therefore, the key is to understand the effects of oxygen experimentally and to construct a thermodynamic database that accurately reproduces them. Two series of quaternary Ti-Al-M-O alloys (M: transient metal elements) with various oxygen levels were studied at the temperature range between 1573 K and 1273 K using the combination of wavelength dispersive spectroscopy and soft X-ray emission spectroscopy. Then, thermodynamic parameters for two quaternary systems as well as the constituent sub-systems were assessed. Selected important interaction parameters and their composition dependence were optimized based on the ab-initio calculation using KKR-CPA method. The newly assessed database reproduces phase boundaries in multi-component systems with an accuracy of 1% for metal elements and 0.2% for oxygen concentrations. In the presentation, we will demonstrate an alloy design based on this module.

In order to accelerate the development of TiAl LPT blade in jet engines and reduce the extensive experimental efforts, some computational approaches to predict the required mechanical properties, in addition to the module to specialize the alloy composition to be able to control microstructure, are needed. We built up the sophisticated materials integration system for the inverse problem consisting of two modules of mechanical-properties prediction (MPM) and microstructure design (MDM). In the presentation, then, focus is places on the MPM how the ultimate tensile strength (UTS) can be predicted. Note that the output of this module is a concrete value of the microstructural constituent governing the property. Therefore, the quantitative analysis by digitalizing the microstructural constituents in needed as one of the databases (DB) of the module. Another important DB to predict the UTS is the work hardening rate of the microstructural constituents. There are two important microstructural constituents in the alloys: one is the lamellar microstructure consisting of α₂-Ti₃Aland γ-TiAl phases and the other the cellular microstructure consisting of β-Ti and γ phases, since we revealed the introduction of the cellular microstructure is effective in increasing the mechanical properties. Then, volume fraction of this β/γ cellular microstructure (V_c) was quantitatively analyzed from back-scattered electron images took from a scanning electron microscope by an image analysis software. The work hardening rate of each microstructural constituents were evaluated by two ways, one is calculating by an analysis of forward problem that dividing the differential between experimentally measured UTS and 0.2% proof strength (σ₀) by elongation (ε), the other is directly measurement by digital image correlation technique with in-situ tensile test. The UTS of alloys were able to be predicted by simple linear compound law using the UTS of α₂/γ lamellar microstructure and β/γ cellular microstructure, and V_c. And these UTS of each microstructural constituent could also be predicted using their σ₀, ε and work hardening rate. The calculated UTS could reproduce the experimentally measured UTS well, it has a margin of error of only ±5%. By using this method, the volume fraction of the microstructure constituent to meet the required properties that contribute to an input value into the MDM can be output. This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Material Integration” for revolutionary design system of structural materials.

TiAl alloys are sensitive to loss of ductility by exposure in air at temperatures around 700°C and for durations as low as one hour. Though this effect is detrimental for the development of the TiAl alloys as materials for hot parts in automotive and aeronautical engines, it remains poorly understood. Thus, we address here the questions of the penetration kinetics of oxygen in a Ti-48Al-2Cr-2Nb alloy of near γ microstructure, and of the impact of this penetration on the tensile properties of this material at room temperature. For this purpose, tensile specimens have been exposed in a 20 % O₂ + 80 % Ar atmosphere, for temperatures ranging from 400°C to 700°C, and for exposure durations of one hour. The tensile tests show that, in addition to the known effect of loss in ductility, the yield stress increases significantly. Therefore, the origin of this effect has been looked for through investigations of the microscopic deformation mechanisms by transmission electron microscopy. In particular, the dislocation morphologies have been determined in samples of different oxygen concentrations. To quantitatively determine the oxygen penetration kinetics, the diffusion coefficient of oxygen in the γ phase has been determined theoretically by DFT calculations. In parallel, experimental values have been obtained for the first time, using ¹⁸O isotope and SIMS concentration profile analyzes, in the following conditions: temperature from 500°C to 700°C, exposure durations from 1 h to 48 h. Surprisingly, the experimental diffusion kinetics were significantly lower than those theoretically predicted. This discrepancy is discussed in terms of potential effects of the Nb and Cr substitutional solutes on the interstitial diffusion mechanisms of oxygen. The main result of these studies is that oxygen dissolution and diffusion in the γ phase is probably not at the origin of the increase in yield stress.

The outstanding material-specific properties of intermetallic γ-TiAl based alloys make them excellent candidates for high-temperature structural applications in the aerospace and automotive industry. In order to seize new application areas, an increased service temperature and improved mechanical properties are necessary. Considering that modern engineering γ-TiAl based alloys consist mainly of γ-TiAl phase with some amounts of α₂-Ti₃Al phase and β_o-TiAl phase, depending on the amount of Al and additional alloying elements, strengthening of the γ phase is a key point to further advance this class of materials. A promising alloying element meeting the requirements to be an effective solid solution strengthener for the γ phase is Zr. While its beneficial effect on the mechanical properties is already known in the scientific community, a detailed study of its influence on the phase transformations in the Al range of engineering γ-TiAl based alloys is still missing. Consequently, the present work investigates six model alloys based on the Ti-(42-46)Al-(2-4)Zr (at.%) system regarding the influence of Zr on the microstructure and phase transitions. Microstructural characterization reveals a stabilization of the γ phase due to Zr, leading to an increase of the γ fraction at expense of α₂ in the microstructure, which consists predominately of lamellar α₂/γ colonies and a minor amount of globular γ phase after hot-isostatic pressing. The material’s hardness is increased due to both solid solution strengthening and lamellar refinement of the α₂/γ colonies by Zr. The phase transformation behaviour and the related impact of Zr were investigated by differential scanning calorimetry (DSC) as well as high-energy X-ray diffraction (HEXRD) utilizing synchrotron radiation. Especially, two different types of in-situ HEXRD heating experiments were conducted in order to investigate, on the one hand, the solid-state transformations below the material’s solidus temperature, and, on the other hand, by employing a newly developed experimental setup, the liquid-solid transformations at higher temperatures. Both DSC and HEXRD experiments clearly show that the Zr additions increase the temperature stability range of the γ phase, which ultimately transforms into α-Ti(Al) phase upon heating. However, phase transformations above the γ phase dissolution temperature, especially, involving the liquid phase, are drastically shifted towards lower temperatures when alloying with Zr, thus effectively narrowing the α single-phase field region. Complementarily, heat treatments above the solidus temperature reveal microstructural features associated with these phase transformations, e.g. formation of liquid phase between the α and β grains in the three-phase α+β+L phase field region. Subsequent water-quenching results in the transformation of the liquid phase into γ phase and of the β phase into α₂ laths, respectively, while in the alloy variants rich in Al, a portion of the high-temperature α phase transforms into massive γ phase. Furthermore, the combination of X-ray diffraction and ab-initio calculations based on density functional theory (DFT) grants valuable insights into the effect of Zr and Al on the lattice parameters of the γ phase. More precisely, Zr causes a strong reduction of the tetragonality of this phase due to an asymmetric influence on the lattice parameters, i.e. a significant elongation of the shorter crystal axis, while the longer axis remains mostly unaffected.

Polycrystalline Ni-based superalloys are vital materials for disks in the hot section of aerospace and land-based turbine engines due to their exceptional microstructural stability and strength at high temperatures. In the drive to increase operating temperatures and hold times in these engines, hence increasing engine efficiency and reduction of carbon emissions, creep properties of these alloys becomes increasingly important. At these higher temperatures, new deformation modes become active. Several alloy compositions and microstructure variants of commercial disk alloys are being explored, including γ’ strengthened alloys as well as compositions promoting γ’ and γ’’ co-precipitation. Microtwinning and stacking fault shearing are important operative mechanisms in the critical 600-800°C temperature range. Advanced electron-microscopy-based techniques have been used to gain new insights into these mechanisms and the rate-limiting processes during high temperature deformation. Atomic-scale chemical and structural changes associated with stacking fault and microtwin interfaces within γ’ precipitates have been identified and indicate that local phase transformations (LPT) occur commonly during creep of superalloys. Furthermore, the important deformation modes can be modulated by LPT formation, enabling a new path for improving high temperature properties. In another phenomenon related to LPT, some superalloys exhibit preferential precipitation along annealing twin boundaries (ATBs). It is surprising that ATBs act as heterogeneous nucleation sites since they are low energy boundaries. However, in superalloys that exhibit this behavior, LTP appears to occur at these ATBs and aide in nucleation of γ’ (and γ’’) precipitates. The implications of this effect with respect to localization of slip is being characterized using in situ deformation experiments in the SEM. This work is part of a GOALI-DMREF program funded by the National Science Foundation in which collaborators are exploring the thermodynamic driving force for LPT formation using DFT modeling, and phase field dislocation dynamics modeling is being used to explore the interaction of dislocations with γ’ microstructures under the cooperative shearing and local compositional changes associated with the LPT mechanism.

Heusler type NiMnGa is a prototype ferromagnetic shape memory alloy (FSMA) which can be driven not only by temperature change but also by magnetic field. In conventional shape memory alloys (SMAs), shape change occurs by martensite variants reorientation (MVR), and the shape recovery occurs during heating through reverse martensitic transformation (MT) into parent (austenite) phase. Thus, the motion frequency is very limited by the heating/cooling speed. On the other hand, FSMA can be driven isothermally by magnetic field, since the actuation is caused by the reversible twin boundary motion (twinning and detwinning) of martensite variants under equivalent magnetostress. Then, FSMA is considered as a new actuator material with fast response and large stroke. The maximum actuation strain of NiMnGa is around 5.8% in 5M martensitic state and 9.6% in 7M martensitic state. A drawback of NiMnGa is that the large magnetoactuation can appear in the single crystal state only. This is because in polycrystalline state the constraint force from the neighbor grains is larger than the magnetostress and thus suppresses MVR by magnetic field. In order to overcome this problem, we have innovated polymer-matrix-based composite materials containing single-crystalline NiMnGa grain particles. By using Bi microalloying we have developed a technique which allows easy disintegration of polycrystalline NiMnGa into single grains which are magnetostrain active. At present, the value of magnetostrain of the silicone composites containing 20vol% 5M- and 7M- martensitic NiMnGa particles (Chiu et al., Scripta Mater., 2022) or 30vol.% 5M-martensitic NiMnGa particles (Sratong-on et al., Sci. Rep., 2019) is 4.0% under 0.7 T. In this presentation, we summarize recent progress of the NiMnGa FSMA / silicone polymer composites. This research is partially supported JSPS by Kakenhi 20K20544, 22H00256 and 22K14491, the Research Center for Biomedical Engineering, and World Research Hub Program of International Research Frontiers Initiative, Tokyo Institute of Technology.

Shape memory alloys, such as Ti-Ni-based (Ti-Ni, Ti-Ni-Cu), Cu-based (Cu-Al-Ni, Cu-Zn-Al), and Fe-based (Fe-Ni-Co-Al-Ta-B, Fe-Mn-Al-Ni) alloys, display shape memory effect and pseudoelasticity due to thermoelastic martensitic transformation. They have offered many technology advantages for various applications such as eyeglass frames, coupling devices, medical devices, and seismic dampers. In recent years, some Co-based shape memory alloys such as Co-Ni-Ga have been reported and they have attracted much attention because of their ability to be used at high temperatures above 373 K. In this study, we present the results of novel Co-Al-Si shape memory alloys showing the thermoelastic martensitic transformation from a B2-structured parent phase. The crystal structures of parent and martensite phases were determined by transmission electron microscopy (TEM), and the transformation temperatures were investigated by measuring the temperature dependence of electric resistivity. Furthermore, it was found that some alloys show excellent pseudoelasticity at temperatures from 298 K to 453 K. Thus, the new Co-based shape memory alloys show great promise for applications requiring high-temperature pseudoelastic properties.

Superelastic crystalline solids are able to recover their original shape even after the large amount of deformation is applied. The shape recovery of most superelastic crystalline solids usually occurs through a reversible phase transition between martensitic and austenitic phase. Although this shear process provides up to ~10% of elastic strain, there are several issues that limit their elastic performance. Usually, this shear process requires high activation temperature to reverse the phase transition. So, it is difficult to exhibit superelasticity at cryogenic temperature. Also, the shear process often drives dislocations to move and causes large accumulation of dislocations, which eventually prevent the reversal of phase transition. Therefore, it is desirable to seek out a new class of superelastic material that can overcome these issues through a completely different superelasticity mechanism. In this presentation, we report our first discovery of the giant superlelasticity in a novel intermetallic compound SrNi₂P₂ via a unique phase transition process, lattice collapse-expansion and the influence of cryogenic temperature.
SrNi₂P₂ single crystals were grown using solution-growth technique, and in-situ uniaxial compression and tension tests on [001]-oriented SrNi₂P₂ micropillars, which were fabricated using focused-ion-beam milling, were conducted at room temperature, 200K, and 100K. Room temperature mechanical testing revealed the giant superelasiticity with a remarkably huge elastic strain limit near 20% if both compression and tension are considered. Up to our knowledge, this giant elastic strain corresponds to one of the highest elastic strain limits ever reported for crystalline solids. This excellent superelastic performance is achieved through multiple-stage lattice collapse and expansion under compression and tension by forming and breaking P-P bonds in two co-existing different crystal structures in SrNi₂P₂. Based on Density Function Theory (DFT) calculation and High-Resolution Transmission Electron Microscope (HRTEM), P-P bonds are formed or broken partially depending on the stress state, leading to the multi-stage lattice collapse and expansion. Fraction of P-P bonds seems to be controlled primarily by entropy under a given stress, so the total number of P-P bonds is determined by thermodynamics under a give stress and temperature. Unlike shear process of martensite-austenite transition, no dislocation is involved in the superelasticity process of SrNi₂P₂. Simple making and breaking bond process seems to guarantee ultrahigh fatigue life with presumably nearly infinite number of cycles. Our state-of-the-art in-situ cryogenic nanomechanical testing revealed that forming and breaking P-P bonds is strongly sensitive to cryogenic temperature. At lower temperature, phase transition becomes easier under compression but more difficult under tension. This difference can be explained by thermal contraction. Due to the thermal contraction at lower temperature, which reduces P-P distance, making P-P bonds becomes easier, but breaking P-P bonds becomes more difficult. One noticeable result is that even under cryogenic environment (~100K), SrNi₂P₂ still exhibit superelasticity, implying that SrNi₂P₂ is capable of shape recovery at very low temperature unlike conventional superelastic crystalline solids such as Ni-Ti intermetallic compounds. Our experimental and computational results provide a fundamental insight into understanding the superelasticity mechanisms of SrNi₂P_2. Furthermore, our work suggests its strong potential as a cryogenic superelastic material that could be useful to develop an impact resistance material for aerospace engineering under cryogenic environment.

Amalgamation is the alloying of two metals, one of which is in liquid form. While broadly applied in metallurgy or dentistry, amalgamation process is yet majorly disregarded for nanotechnology. Recently, we developed the nanoscale amalgamation reaction for the synthesis of intermetallic and alloyed nanoparticles. [1] Starting from monometallic seeds, we carry out a thermal decomposition of metal-amides to dispatch low-melting metals to the surface of nanoparticles and thus trigger a time-efficient and homogeneous alloying to bimetallic compositions. Our new synthesis is convenient and reproducible universal method for high-quality intermetallic nanoparticles, providing uniform compositions and phase purity already after a few minutes of reaction (whereas short reaction time is the key for excellent size distributions). Finally, nanoscale amalgamation gives access to unique bimetallic nanoparticles of very dissimilar metals (for example easily reducible Au, Pd or Pt with much more active Ga or Zn), which is impossible via traditional co-precipitation methods. Combining metals at the nanoscale is a powerful strategy for many applications: large surface area for efficient catalysis, size-dependent properties for ultralow-power phase-change memory and high-Q factor plasmonic applications, small diameter for effective nanomedicine or ultrafine-grain thermoelectrics and additive manufacturing are just to name a few. Our synthesis supplies high-quality intermetallic nanoparticles for this wide range of emerging applications.

[1] J. Clarysse, A. Moser, O. Yarema, V. Wood, and M. Yarema, Science Advances, 2021, 7, eabg1934.

The familiar metals and alloys in our daily lives are mostly crystalline materials, and they elastically deform because of temporary stretching or contracting of bonds between atoms. The ideal elastic strain for a crystalline metal is the strain at which the lattice itself disintegrates and hence set a firm upper bound of the elastic strain of the material. Theoretically, this strain value can be in the order of 10% for most crystalline metals with absolutely no defects. However, a macroscopic block of conventional crystalline metals practically suffers a very limited elastic deformation of <0.5% with a linear stress–strain relationship obeying Hooke’s law. In this presentation, we present our recent results on the experimental observation of a large tensile elastic deformation with an elastic strain of >4.3% in a Cu–Al–Mn single-crystalline alloy with an L2₁-ordered structure at its bulk scale at room temperature. The large macroscopic elastic strain that originates from the reversible lattice strain of a single phase is demonstrated by in-situ microstructure and neutron diffraction observations. Furthermore, the tensile elastic reversible deformation, which is nonhysteretic and quasilinear, is associated with a pronounced elastic softening phenomenon. The increase in the stress gives rise to a reduced Young’s modulus, unlike the traditional Hooke’s law behavior. This non-Hookean huge tensile elastic deformation behavior is discussed in terms of the strong lattice anharmonicity in the present bulk Heusler-type Cu–Al–Mn crystalline alloy. The experimental discovery of a non-Hookean huge elastic deformation offers the potential for the development of bulk crystalline metals as high-performance mechanical spring materials for use as metallic seals and connectors, or for new applications via “elastic strain engineering.”

Shape recovery in shape memory alloys (SMAs) occurs by reverse martensitic transformation from martensite to austenite phases. SMAs are used in wide applications such as medical devices, actuators, and household electric applications etc. If SMAs can be used at high temperatures, their application will expand, for example, in aerospace field. TiNi intermetallic compounds is the most used SMAs. The crystal structures of the martensite and austenite phases in TiNi are B2 and B19’ in Strukturbericht expression, respectively. However, the martensitic transformation temperature (MTT) of TiNi is lower than 100 °C. To use SMAs at high temperatures, MTT should be increased. Various attempts have been made to increase the MTT of SMAs, such as addition of alloying elements to TiNi or selection of other intermetallic compounds with high MTT. We have focused on TiPt and TiPd as high-temperature SMAs (HT-SMAs) due to their high MTT above 500 °C. Martensitic transformation, microstructure, and shape recovery of TiPt or TiPd alloys have been investigated. The addition of alloying elements was a certain effect on strength improvement of austenite, but not sufficient. It also had the disadvantage of lowering the MTT in many cases. Recently, multi-component alloys, in particularly medium- or high-entropy alloys, have been attracted as new structural materials because their severe lattice distortion and sluggish diffusion are expected to improve the high-temperature strength of SMAs. In this presentation, phase transformation, microstructure and shape recovery of multi-component SMAs together with TiPt or TiPd base SMAs will be introduced.

Actinide-based materials possess unique physics due to the presence of 5f electrons. Owing to their importance as critical nuclear energy materials, their study has mainly focused on these applications, leaving plenty of fundamental physics aspects open for investigation. Examples include the three magnetic phases of Th, which are directly correlated to its impurity density, and the charge density wave anomaly present in α-U at low temperatures. The effective examination of unique quantum phenomena in these materials requires high purity monocrystalline samples. However, the formation of large area single-crystalline actinide compounds is particularly underexplored relative to other material systems because of limited source availability and safety regulations. High quality actinide containing samples will help provide more accurate experimental values for physical properties used by atomistic computer modelling approaches. In turn, this will enable more accurate models of compounds with strongly correlated electron systems. However, current thin-film synthesis techniques for actinide-based materials make use of alloys or functionalized compounds which leave behind impurities and affect crystal quality. Molecular beam epitaxy (MBE) presents an attractive avenue for the study of actinide heterostructures because of the high degree of control over purity, dimensionality, strain, and interfaces between monolithically grown materials. MBE also uses high purity elemental sources and is known for producing highly crystalline material. Thus, by using this technique for synthesis of these complex materials, we can reduce the presence of elemental impurities and defects and simplify the characterization and interpretation of the physical properties. Idaho National Laboratory has recently installed an MBE chamber equipped with electron beam sources, several high temperature cells, and a nitrogen plasma source to facilitate the deposition of low vapor-pressure elements. This work looks to establish the foundational knowledge for the synthesis and characterization of actinide-nitrides with complex oxidation states; wherein, initial studies will employ transition metals as early-stage surrogates for actinide-based nitride materials. We aim to synthesize single crystal, epitaxial (UN) and thorium nitride (Th₃N₄), both of which are promising nuclear fuel candidates for upcoming reactor architectures. The properties of these crystal phases has been an active research field and the synthesis and characterization efforts will be greatly benefited from MBE grown samples. Experimental validation of computational models will result in faster nuclear fuel validation and implementation of nuclear reactors for clean energy solutions.

Nitride materials have a diverse set of applications, but the system is significantly underexplored compared to oxides. Several recent papers have revealed that there are numerous stable nitride compounds which have yet to be created. Vacuum deposition is an ideal method for the discovery and study of novel materials as it allows for stabilization of materials outside of thermodynamic limits through engineering of the growth kinetics. It also allows for delicate stoichiometry control, enabling careful tuning of composition-dependent physical properties, and materials with high crystallinity and purity. The preparation of monocrystalline samples from ultra-pure atomic sources will allow for examination of the physical properties of these materials without the added complications of grain boundaries or impurities, and to elucidate any directional dependence of these properties which may be present in low-symmetry crystals. This work aims to synthesize single crystalline thin films in this relatively uncharted material space using molecular beam epitaxy (MBE). Growth windows, the vacuum deposition equivalent to commonly employed thermodynamically limited phase diagrams, will be established for a variety of materials. These outline the parameters required to achieve specific phases and include substrate temperature, flux ratios, growth rate, and strain. The focus for the work here is initially given to the exploration of transition metal nitrides containing Cr, Zr, Mn, Nb, Ti, and Ni. The factors which control the formation of different crystalline phases and stoichiometries will be investigated for binary and ternary compounds. While various new ternary phases involving these elements have been predicted to be stable, the physical properties have not been predicted. However, the properties of the binary constituents and the select few known ternary compounds are of interest to various facets of research including: magnetics, semiconductors, superconductors, plasmonics, thermoelectrics, high hardness, stability in extreme environments, and anti-corrosive coatings, these novel ternary compounds will likely have similar properties. Additionally, developing the growth windows for compounds containing elements with complex oxidation states will provide a platform for additional studies involving heavier elements with 5d, 4f, and eventually 5f-electrons. Utilizing these elements in carefully engineered crystal structures is particularly interesting because of their high sensitivity to temperature, strain, increased spin-orbit interactions, as well as magnetic and electric fields. MBE is known for the precise deposition of thin films with sharp interfaces. This will allow for the synthesis of multilayer heterostructures of different materials with compatible crystal symmetry, enabling the combination of multiple properties, study of interface effects, and comparison to random alloys with the same overall composition. Select elements can also be combined with the well-studied Group-III Nitride material system, laying the foundation for future integration of these novel materials with existing semiconductor optoelectronics.

The hydrogen economy strongly depends on two critical reactions for the production of hydrogen and the generation of green energy, namely the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR). Pt is the best catalyst in both cases (HER and ORR) and allows high reaction rates to be reached in an acidic environment with very small overpotentials. Nevertheless, Pt cost and scarcity limit the widespread application of this strategy to use hydrogen as a clean energy source. Thus, there is a significant interest in discovering cheaper and more abundant catalysts to replace Pt.

In this investigation, a high-throughput simulation strategy based on first principles simulations has been implemented to discover intermetallic compounds with high catalytic activity for the HER and ORR through the application of elastic strains. The methodology was initially validated for 13 late transition metals. To this end, the adsorption energy of the intermediate species (H*, O*, and OH*) in the catalytic process was determined by density functional theory calculations on surface slabs with different orientations (fcc(111), hcp(0001), and bcc(101)) subjected to different strain states (uniaxial, biaxial, shear, and a combination of them) up to the mechanical stability limits, as indicated by phonon calculations. This information was used to calculate the catalytic activity of each transition metal for both HER and ORR, and the theoretical results were in agreement with the experimental data in the literature [1,2].

Afterward, this methodology was used to explore the catalytic activity for the HER and the ORR of intermetallic compounds with stoichiometries AB and A3B and fcc, hcp, and bcc lattices. To this end, the potential intermetallic candidates were screened from the Materials Project database, and those that were toxic, radioactive, or unstable were discarded, leading to a sample of approximately 300 compounds. Those with oxygen adsorption energies lower than -2.5 eV were also discarded due to the formation of oxides under working conditions and the adsorption energies of the intermediate species (H*, O*, and OH*) were calculated for the most stable surfaces of each material using density functional theory calculations. This information was used to determine the catalytic activity and further calculations for the intermetallic compounds with the best performance introducing the effect of elastic strains. The results show that the scaling relations (that limit the catalytic performance of transition metals) are not maintained in the case of intermetallic compounds due to the variation in the most favorable adsorption site for the different adsorbates. Moreover, elastic strains can be used to improve the catalytic performance of intermetallic compounds by changing the free energy barrier associated with the rate-limiting reaction or even the rate-limiting reaction. The present investigation provides a general framework to assess the effect of elastic strains on the catalytic activity of intermetallic compounds and the design of new catalysts with improved performance and/or selectivity of the intermediate species.
References
[1] C. Martínez-Alonso, J. M. Guevara-Vela, J. LLorca, Phys. Chem. Chem. Phys. 2022, 24, 4832-4842.
[2] C. Martínez-Alonso, J. M. Guevara-Vela, J. LLorca, Phys. Chem. Chem. Phys. 2021, 23, 21295-21306.

The use of intermetallic Pt–Co nanocrystals (NCs) for the electrocatalytic oxygen reduction reaction is quickly gaining interest thanks to the higher electrochemical stability of the intermetallic L1₀ phase compared to a random alloy A1 phase. However, the thermal treatment that enables the intermetallic phase transformation also causes considerable NC aggregation, resulting in a significant loss of electrochemically active surface area. Herein, we report the use of microwave radiation to induce the intermetallic phase transformation in Cu-doped Pt–Co NCs. We demonstrate that microwave radiation reduces NC aggregation while allowing for a complete phase transformation in only 30 s. These microwave-treated NCs demonstrate higher mass activity for the oxygen reduction reaction while maintaining electrochemical stability similar to the thermally annealed samples.

High performance electric motors for electric vehicles, efficient wind generators and a variety of commercial and industrial motors all require high-performance permanent magnets (PM) which can maintain their energy product at temperatures up to 200°C. For the Nd2Fe14B based PMs, this has been achieved primarily by replacing the Nd with heavy rare-earth elements (Tb, Dy and Ho). Of the rare-earth elements (REE), these are truly rare, with the cost of Dy typically about 10x that of Pr and Nd. The heavy REEs increase the temperature range by enhancing anisotropy field and lowering its temperature dependence. They also lower the remanence of the alloy relative to the light REE. While applying the Dy only to the surfaces of the magnetic reduces the overall amount needed, it does increase the processing steps needed for the final product. An alternative approach to enhancing the temperature range is to reduce the grain size (dia.< 2 µm), so that grain sizes are closer to those of the magnetic domain size, hence improving the resistance to demagnetization. However, reducing the grain size in sintered magnets is fraught with several challenges. Not only does the grain size need to be reduced, but the distribution of grain sizes also needs to be top-size limited since a small number of large grains can have a disproportionately large effect on decreasing coercivity. Finer grained samples are more susceptible to oxidation, harder to fully magnetically align and, most importantly, need to be sintered to full density without excessive grain growth. We will present results from various methods for reducing grain size, passivating the grains, and using sintering aids to minimize grain growth.

Soft magnets play a vital role in the efficient energy conversion in a variety of important industries including wide-bandgap semiconductors, electric vehicles, aeronautics and aerospace, particularly at high temperatures. Improving the efficiency of modern power electronics and electrical machines via advanced soft magnets has the potential to significantly contribute to global energy savings, thereby leading to a reduction of the associated carbon footprint. Here, we present microstructural characterization and property measurements on two novel FeCoMnAl alloys, one single-phase B2 and one nanostructured B2 + b.c.c., which have good soft magnetic properties up to ~873 K. Both alloys exhibits a high saturation magnetizations, high Curie temperatures, low coercivities, and high electrical resistivities. TEM-based ALCHEMI analysis showed that in the B2 phase the Al atoms preferentially occupy one sublattice site whereas the other elements tend to partition between both sublattice sites. DFT calculations predict magnetic properties consistent with the experimental results and indicate that Fe, Co, and Mn elements predominately contribute to the ferromagnetism.

New materials exhibiting an attractive combination of magnetic, mechanical and electrical properties can play a vital role in improving the performance of next generation rotating electrical machines. However, developing such a material using conventional approaches is expensive, time consuming and misses googols of possible compositions and processing parameters. On the other hand, the accelerated discovery approach relies on high throughput synthesis, characterization, property evaluation, predictive machine learning (ML) and modelling to rapidly screen a huge number of compositions. The assessment of the structure as well as the magnetic, mechanical, and electrical properties of compositionally graded Co-Fe-Ni alloy samples processed by laser additive manufacturing (AM) was rapidly carried out. The AM process facilitated the rapid synthesis of a compositionally graded material library and the subsequent assessment of several properties. A large change in these properties was observed as a function of composition. Saturation magnetization varied from 60 to 230 emu/g, coercivity from 0.5 to 66.4 Oe, resistivity from 9.4 to 72.6 μΩ cm, microhardness from 90 to 400 Hv, ultimate tensile strength from 465 to 1167 MPa etc. Novel compositions, e.g., Co₃₀Fe₆₀Ni₁₀, Co₁₀Fe₈₀Ni₁₀, etc., with an optimum magnetic-mechanical-electrical property set have been identified by our approach. This work is supported by the AME Programmatic Fund by the Agency for Science, Technology and Research, Singapore under Grant No. A1898b0043.

Hydrogen will play a major role in the establishment of a renewable-energy-based society and realization of carbon neutrality. However, one of the major challenges is the storage and transportation of hydrogen: the H₂ gas has large volume and its liquefaction is necessary. The current available technology for hydrogen liquefaction is conventional gas compression systems based on the Joule-Thompson effect, which is costly and inefficient at low temperatures. Cryogenic magnetic refrigeration (CMR) is a prospective environmentally friendly technology for temperature below 120 K, and it can theoretically exhibit a higher efficiency as compared to those of conventional gas compression systems. In the CMR, the gaseous hydrogen is pre-cooled down to ~77 K by using liquid nitrogen, and, at the second stage, the magnetic liquefier provides a further decrease in temperature down to 20 K (H₂ liquefaction temperature). However, the lack of magnetic refrigerant materials with high magnetic entropy change in a wide temperature range of 77-20 K required for the hydrogen liquefaction is a bottle-neck for practical applications of MR cooling systems
In this talk, we will first demonstrate how data science can be used to develop magnetocaloric materials with giant and reversible magnetocaloric effect at the cryogenic temperatures. We conducted machine learning on a dataset extracted from literature on Fe₂P based alloys to predict optimum alloy composition which results in a decrease of transition temperature below 100 K. Combination of machine learning and experimental validations resulted in realization of rare-earth free (Mn,Fe,Co)₂(P,Si) based compounds which exhibit a large magnetocaloric performance in isothermal magnetic entropy change (ΔS_m) of 7.5–11.5 J/kgK at the temperatures below 100 K [1]. This study demonstrates that data-driven development of magnetocaloric materials can efficiently boost the optimization of their properties, thus aiding the practical applicability of magnetic refrigeration technology. In the second part of the talk, we will show a series of materials with a giant magnetocaloric effect (MCE) in magnetic entropy change (-ΔS_m > 0.2 Jcm^-3K^-1) in the Er(Ho)Co₂-based compounds, suitable for operation in the full temperature range required for hydrogen liquefaction (20-77 K) [2]. We found that the giant MCE becomes reversible, enabling sustainable use of the MR materials, by eliminating the magneto-structural phase transition revealed by detailed cryogenic and in-situ microstructure characterizations. We will discuss how this discovery can lead to the application of Er(Ho)Co₂-based alloys for hydrogen liquefaction using MR cooling technology for the future green fuel society.
[1] J. Lai et al. “Machine learning assisted development of Fe₂P-type magnetocaloric compounds for cryogenic applications” Acta Mater. 232 (2022) 117942.
[2] Xin Tang, et al. “Magnetic refrigeration material operating at a full temperature range required for hydrogen liquefaction” Nature Comm. 13 (2022) 1.

Permanent magnets are utilized in many applications such as the main power motor in electric vehicles. It is known that the coercivity, the resistance to changes in magnetization, of magnets can be significantly improved by suppressing the formation of magnetization reversal nuclei and preventing the growth of the reversal nuclei[1]. Micromagnetic simulations predicted that the local change in magnetic properties such as magnetocrystalline anisotropy is crucial to prevent the growth of the reversal nuclei[2]. Thus, the coercivity may be improved by optimizing microstructures consisting of the phases with different magnetic properties. The existence of various phases in microstructures has been reported in high coercive Nd-Fe-B permanent magnets[3], but the key microstructural features to improve the coercivity are still not well understood. It is thus essential to identify one-by-one relationship between microstructural features and their magnetic properties. One of the methods to directly measure the local magnetic properties is to measure the magnetic domain wall (DW) width. DW width varies with the change in the ratio of exchange stiffness and magnetocrystalline anisotropy where the DW is located. The larger DW width indicates larger exchange stiffness or smaller magnetocrystalline anisotropy. However, direct and precise measurement of DW width requires high spatial resolution, since DW is a nanometer-sized magnetic structure.
Differential phase contrast scanning transmission electron microscopy (DPC STEM)[4] is an imaging technique of local electromagnetic fields inside specimens. Combined with recently developed magnetic-field-free atomic resolution STEM [5], DPC STEM now enables visualization of nanoscale magnetic structures in magnetic free environment. Furthermore, one can obtain structural information in the same field of view through other STEM techniques such as high-angle annular dark field images and electron energy loss spectroscopy (EELS).
In this study, we directly measured DW width in La-substituted Nd-Fe-B magnets by DPC STEM. In this magnet, Nd is non-uniformly substituted by La, and the magnetocrystalline anisotropy is expected to locally fluctuate with Nd concentration. To confirm the correlation of DW width and Nd concentration, we measured DW width by DPC and analyzed Nd composition by EELS at the same area. The results show the DW width increases with the decrease in Nd concentration. Details will be reported in the presentation.
1) O. Gutfleisch, J. Phys. D: Appl. Phys. 33 (2000).
2) G. Hrkac, et al. Appl. Phys. Lett. 23 (2010).
3) T. G. Woodcock, et al. Acta Mater. 77 (2014).
4) N. Shibata et al., Acc. Chem. Res. 50 (2017).
5) N. Shibata et al., Nat. Commun. 10 (2019).

Fabrication of intermetallic nanostructures over wafer-scale distances is critical for applications such as catalysis, energy storage, and microelectronics. Current top-down and bottom-up fabrication approaches, such as CVD or colloidal synthesis, are limited to simple material compositions, have poor morphology and size variance, and lack control of crystallinity. Recently, thermomechanical nanomolding, whereby a polycrystalline feedstock is pressed through a mold with 1D pores at an elevated temperature (~0.5T_m), has been shown by Jan Schroers’ group at Yale University to form periodic arrays of single crystal, defect free nanowires. Since composition is determined by the feedstock material, this technique can be easily used to form nanowires of a wide range of intermetallic compounds.
Building upon this work, we investigated thermomechanical nanomolding for fabrication of 2D nanostructures. Using 2D Si trenches with a SiO₂ liner as a mold, we successfully nanomold both single element and intermetallic compounds over ~1 cm distances. SEM, EBSD, and S/TEM imaging confirm that 2D linear trenches can be formed with no apparent gaps; however, the final structures are not always single crystal, in contrast to 1D nanomolding. While 1D nanowire growth is driven by interfacial diffusion, the lower surface to volume ratio and post-mold structures indicate that other growth mechanisms, such as deformation twinning and grain reorientation, play an important role during nanomolding.

The performance of advanced soft ferromagnets relies on the formation of an optimized nanocrystalline structure through controlled devitrification from amorphous precursors. This structure is conventionally achieved via small additions of early/late transition metal elements (Nb, Cu, etc.) that allow for tuning of devitrification kinetics but also degrade saturation magnetization. Here, building from an in-depth study of as-quenched amorphous nanostructures produced by different rapid solidification conditions, we identify that specifics of small quenched-in crystallites strongly determine devitrification kinetics in FeSiB system free of transition metal additions.
Samples of composition Fe₇₉Si₁₁B₁₀ were fabricated into two forms: 1) melt-spun ribbons of ~25 μm in thickness and 2) water-quenched microwires of ~150 μm in diameter. While identical chemical composition and overall X-ray amorphous structure of both as-quenched forms are confirmed, nanoscale differences are revealed by high-resolution transmission electron microscopy. In specific, the water-quenched microwires feature small (~30 nm in diameter) Fe(Si) crystallites dispersed throughout their volume, in contrast to the melt-spun ribbons that exhibit amorphous structure with Fe(Si) nanocrystalline islands (~2 nm in thickness, ~50 nm in diameter) confined to the surfaces. Analysis of calorimetric data reveals that the microwires devitrify via nucleation from pre-existing nuclei, resulting in reduced activation energy, followed by diffusion-controlled grain growth. On the other hand, the ribbons exhibit continuous nucleation followed by interface-controlled growth. It is contemplated that the different devitrification behavior exhibited by the two sample types is attributed to their unique nanostructural state, which is subtly affected by the rapid solidification conditions. These results provide insights into the control of devitrification to achieve high-moment, high-performance nanocrystalline soft ferromagnets without alloying additions.
Acknowledgments: IEEE Magnetics Society Educational Seed Funding, Northeastern University, and ICMM/CSIC, Spain

Topological intermetallic nanomaterials have novel symmetry-protected electronic states that are advantageous for applications such as catalysis¹, electronics², and quantum computing³. Fabrication of intermetallic nanomaterials is hindered by the lack of high throughput synthesis methods that allow for tight control of phase, morphology, and stoichiometry. Here, we present the use of thermomechanical nanomolding (TMNM) to fabricate nanowires of topological intermetallic materials. Developed by Jan Schroers’ group at Yale University, TMNM is a scalable fabrication process where a bulk feedstock is pressed through a mold with nanosized pores at elevated temperatures and pressures. Starting with a polycrystalline feedstock, TMNM forms defect-free, single crystalline nanowires with consistent growth orientation and composition.⁴ TMNM is materials agnostic and has been successfully used to create nanowires of several different topological materials.⁵ Furthermore, high pressures and temperatures used in the molding process can be exploited to study phase stability with the possibility of producing metastable phases through nanoscale confinement effects.

We used TMNM to fabricate nanowires of several different topological intermetallic materials. Due to the presence of symmetry-protected surface states, these topological materials have enhanced electronic properties that make them attractive candidates for next-generation interconnect materials. High-resolution S/TEM analysis reveals formation of high-quality, single-crystalline nanowires with consistent composition. Surprisingly, we observe that high levels of vacancies can be tolerated in these single-crystalline molded nanowires without forming any extended defects, such as nanoscale voids or phase separation. The molded nanowires show promising electrical properties where their resistivity values do not increase with decreasing dimensions of the nanowires, in contrast to the increasing resistivity of Cu interconnects at the nanoscale. Thus, we present TMNM as a high throughput fabrication method to form nanowires of topological intermetallic materials with controlled structure and electrical properties with promising applications in energy-efficient computing technologies.

References
1. D. Wang, Q. Peng, Y. Li, Nano Res. 3, 574-580 (2010).
2. H.J. Han, D. Hynek, Z. Wu, L. Wang, P. Liu, J. V. Pondick, S. Yazdani, J.M. Woods, M. Yarali, Y. Xie, H. Wang, and J.J. Cha, APL Mater. 8, 011103 (2020).
3. S.M. Frolov, M.J. Manfra, and J.D. Sau, Nat. Phys. 2020 167 16, 718 (2020).
4. N. Liu, Y. Xie, G. Liu, S. Sohn, A. Raj, G. Han, B. Wu, J.J. Cha, Z. Liu, and J. Schroers, Phys. Rev. Lett. 124, 036102 (2020).
5. Z. Liu, N. Liu, and J. Schroers, Prog. Mater. Sci. 125, 100891 (2022).

Topological semimetals (TSMs) possess topologically protected surface states near the Fermi level with high carrier densities and high mobilities, holding distinct potential for low-dissipation on-chip interconnects that may outperform current copper interconnects for continued dimensional scaling of CMOS technologies. To explore the feasibility of nanoscale interconnects using TSMs, synthesis of TSMs with controlled dimensions, structural characterization, and transport properties at the nanoscale are essential. Here, we report synthesis of various TSM nanowires (MoP₂, CoSi, FeGe) with controlled dimensions and crystallinity by chemical vapor deposition and nanomolding. The demonstration of nanostructured TSM nanowires provides opportunities for careful investigations of design rules for TSMs-based nanoscale interconnects.

The creep strength of the first-generation MoSiBTiC alloy (Mo-5Si-10B-10Ti-10C, at%) is superior to that of SiC_f/SiC composites, and the specific strength of this alloy is comparable to that of ceramics matrix composite at 1250°C. On the other hand, its high strength properties make hot forging and processing into complex shapes difficult. Therefore, the processing of the MoSiBTiC alloy by powder metallurgy has been investigated. In this study, the powder and sintered compact of the first-generation MoSiBTiC alloy were prepared by electrode induction gas atomization (EIGA) and spark-plasma-sintering (SPSed compact).
The spherical powder of the first-generation MoSiBTiC alloy was produced by the EIGA method. The microstructure of the powder was much more refined than that of the alloy produced by the arc-melting method (Cast sample). Interestingly, some solidification steps in the Cast sample disappeared in the powder. These differences are attributed to the rapid cooling effect of the EIGA method. Dense and sound powder compacts were successfully obtained by the SPS process. The composition of the SPS compacts was close to the nominal composition of the first-generation MoSiBTiC alloy. Therefore, it was found that there were no significant compositional changes during the powder process. The microstructure of the SPSed compact was considerably (almost one order) finer than that of the Cast sample, with random crystallographic orientation distribution and less anisotropic morphology along the solidification direction observed in the Cast sample.

Mo-Si-B alloys with the addition of ZrC, so-called MoSiBZrC alloys, combine moderate room-temperature fracture toughness and superior high-temperature strength. This is because of the synergistic effect of toughening and high-temperature strengthening by the ZrC phase. However, MoSiBZrC alloys, as well as MoSiBTiC, suffer from catastrophic oxidation at and above 700 °C, caused by the rapid volatilization of MoO₃. For this reason, many efforts have been made to improve oxidation performance. For instance, our previous studies revealed that Cr addition can remarkably improve the oxidation resistance of the MoSiBTiC alloy. Also, it was reported that Nb addition to Ti alloys decelerates the diffusion of oxygen ions in the oxide scale and contributes to the formation of compact oxide layers. Therefore, in the present study, the effect of Cr and Nb additions on the oxide scale formation of MoSiBZrC alloy was analyzed.
Alloy ingots with selected compositions were prepared by conventional arc-melting in an Ar atmosphere. The oxidation tests were carried out at 800 °C. Compared with the base MoSiBZrC alloy, the alloys with Cr and/or Nb additions exhibited considerably reduced weight losses during oxidation. The Cr addition increased the oxidation resistance of the MoSiBZrC alloy because the high amount of Cr dissolved in the Mo solid solution phase and formed as protective Cr₂(MoO₄)₃. The Nb addition decreased the thickness of the oxide layer. In the alloys with Cr and Nb addition, the CrNbO₄ layer formed beneath the Cr₂(MoO₄)₃, which further decelerated the diffusion of O^2- ions. Moreover, in the Cr-added alloys, a substantial amount of cracks were observed in the substrate near the oxide-substrate interface, but the crack density decreased by the Nb addition. It is considered that the Nb addition might effectively work to suppress the spallation of the oxide scale.

The L1₂-ordered structure is one of the most wide-known structures in the field of high-temperature structural materials owing to its superior high-temperature strength. This is believed to be attributed to the anomalous increase of yield stress with increasing temperature (referred to as yield stress anomaly, YSA), which is caused by a thermally activated (111)-(010) cross slip of Anti-phase boundary (APB) coupled b=1/2[101] superpartial dislocations. However, not all L1₂-compounds are necessary to exhibit the YSA phenomenon. The L1₂-Fe₃Ge was considered one of those for a long time. This was related to the fact that slip on the (010) plane is exclusively activated in L1₂-Fe₃Ge with their polycrystal specimens that either the (111) slip or the YSA was believed not to occur. Besides, unfortunately, the preparation of single-crystal specimens in bulk size is extremely difficult. For that reason, almost nothing is known at present about the deformation behavior of the L1₂-Fe₃Ge in detail.
In the present study, we investigate the compressive deformation behavior of L1₂-Fe₃Ge in their micropillar single-crystal specimens at room temperature. Emphasis is placed on the deformation microstructures of the L1₂-Fe₃Ge through detailed transmission electron microscopy (TEM) investigation, paying special attention to whether or not the YSA occurs in this L1₂-compound.
For the compression orientation near [001], the (111) slip in L1₂-Fe₃Ge is successfully observed. This is the first time to experimentally observe the (111) slip in L1₂-Fe₃Ge. The critical resolved shear stress (CRSS) for (111) slip is estimated as ~240MPa, which is resulted to be approximately 8 times higher than that for (010) slip (~35MPa). The extremely huge gap in the CRSS between the (111) slip and the (010) slip is considered responsible for the absence of the (111) slip in polycrystal deformations. Typical dislocation structures corresponding to the Kear-Wilsdorf lock are observed after deformation on (111) slip, suggesting a possibility of the occurrence of YSA in L1₂-Fe₃Ge, even at room temperature. This exciting discovery brings L1₂-Fe₃Ge an opportunity to serve as the γ’ strengthening phase in γ/γ’ two-phase superalloys and hence opens a new category in the field of high-temperature structural materials, the Fe-based superalloy strengthened by L1₂-Fe₃Ge variants.

Laser powder bed fusion (L-PBF) process is one of the metal additive manufacturing techniques capable of producing complex geometrical forms. The AlSi10Mg alloy with a nominal composition of Al-10Si-0.3Mg (mass%) is one of the representative Al alloys used for the L-PBF process. The L-PBF manufactured AlSi10Mg alloy exhibits a fine cellular microstructure consisting of the α-Al matrix with supersaturated Si solid solution and surrounding Al/Si eutectic network. The characteristic microstructure is considered to contribute to a superior strength of the L-PBF processed Al-Si alloys compared to the conventional cast alloys. In general, Al-Si-Mg alloys are artificially aged to achieve high strength by promoting the fine precipitation of Si and the β-Mg₂Si phase (including the metastable phase) in the α-Al matrix. It has been reported that nanoscale precipitates of the Mg₂Si phase introduced by aging treatments contributed to the further hardening of the L-PBF manufactured AlSi10Mg alloy. The insight can open an opportunity for further strengthening of L-PBF manufactured Al-Si-Mg alloys by controlling a combination of the Mg₂Si phase with the Si phase. In this study, we designed an alloy composition of Al-10Si-4.5Mg (mass%) based on the calculated phase diagram of the Al-Si-Mg ternary system. Thermodynamic calculations assessed relatively high fractions (approximately 8% in equilibrium) of the Mg₂Si phase in the designed alloy composition. We made an attempt at the L-PBF processing using an Al-10Si-4.5Mg alloy powder. The microstructural characteristics were clarified for the L-PBF processed samples and then the age-hardening behavior associated with the precipitation of the Mg₂Si phase will be investigated.
The gas atomized Al-10Si-4.5Mg alloy powder with a mean particle size of approximately 20 mm was applied for the L-PBF manufacturing using a ProX DMP 200 machine (3D Systems). The applied process parameters were as follows: scan speed (v) = 0.6~1.8 m/s, laser power (P) = 51~140 W, laser spot size ~100 μm, powder layer thickness = 30 μm, and hatch distance = 100 μm. The relative density was measured by the Archimedes method. The Al-10Si-4.5Mg alloy samples with an approximate dimension of 15 mm × 15 mm × 10 mm were manufactured under the conditions of v = 1.0 ~ 1.8 m/s and P = 77 ~ 128 W. The samples manufactured under v = 1.0 m/s and P = 128 W exhibited a highest relative density of 92 %. Microscopic cracks perpendicular to the building direction were often observed in all of the manufactured samples. Such crack propagation would be a dominant contributor to the reduced relative densities of the Al-10Si-4.5Mg alloy samples. The manufactured samples exhibited melt-pool structures composed of many elongated α-Al phases (primary solidified) surrounded by numerous granular Mg₂Si phases and fine Si phases (formed in eutectic reactions). The microstructural morphology was in good agreement with the calculated Scheil solidification sequence of liquid (L)+α → L+α+Mg₂Si → L+α+Mg₂Si+Si in the alloy composition. On the other hand, a relatively coarse equiaxed cellular microstructure decollated with granular Mg₂Si phase was observed at the melt-pool boundaries. The relatively coarse and brittle Mg₂Si phase localized at melt-pool boundaries may contribute to the crack propagation perpendicular to the building direction. The hardness of the as-manufactured sample was over 180 HV, which was much higher than that of the AlSi10Mg alloy (~130HV). This superior hardness would be attributed to the granular Mg₂Si phase. Si and Mg solute contents inside the α-Al matrix were approximately 2.2 and 1.0 mass%, respectively. The high solute contents indicate the possibility of further precipitation strengthening by the post heat treatments. The age-hardening behavior and its associated precipitation of the Mg₂Si phase (together with Si phase) will be presented.

Aluminum (Al) alloys are widely used in transportation equipment and aerospace because of their lightweight and high specific strength. Al-Fe alloys have been investigated as candidate materials for high-temperature applications, but the coarsened θ-Al₁₃Fe₄ phase (mC102) has a detrimental effect on the mechanical performance of the cast Al-Fe alloys. In rapidly solidified alloys, the refined Al₆Fe metastable phase (oC28) would increase strength without compromising ductility. The refined Al₆Fe metastable phase would transform into the coarsened θ stable phase at elevated temperatures, resulting in the reduction of high-temperature strength in thermal exposure. The stabilization of the Al₆Fe phase by the addition of a third element (Mn or Cu) would be effective to improve the microstructural stability at elevated temperatures. In the present study, Mn and Cu were selected as the added third elements in terms of the crystal structure of Al₆Fe phase. The added Mn is expected to replace the Fe sublattice of the Al₆Fe phase (oC28) and form a stable Al₆(Fe, Mn) phase in the Al-Fe-Mn ternary system. In the Al-Fe-Cu ternary system, α phase is in equilibrium with the Al₂₃CuFe₄ phase with the same orthorhombic crystal structure (oC28), suggesting that the Al₆Fe phase would be stabilized by Cu addition. Herein, for understanding the effects of Mn and/or Cu additions on the formation of Al₆M (M: Fe, Mn, Cu) phase in the solidification microstructures of Al-Fe alloys, we fabricated Al-Fe-Mn, Al-Fe-Cu ternary alloys, and Al-Fe-Cu-Mn quaternary alloys solidified at different cooling rates. The solidification microstructures and high-temperature strength were systematically investigated.
Various Al-Fe alloys with compositions of Al-1%Fe, Al-1%Fe-1%Mn, Al-1%Fe-1%Cu, and Al-1%Fe-1%Cu-1%Mn (mol%) were prepared by solidification at different cooling rates. The raw materials were melted in an induction furnace in the temperature range of 800-900°C for 1800 s in an Ar atmosphere. The molten materials were prepared at two different cooling rates. Some were cooled by switching off the power source to the furnace (cooling rate: 0.3 °C×s^-1). The others were cast into an iron mold to form rod-shaped ingots at a higher cooling rate (145 °C×s^-1).
In the furnace-cooled Al-1%Fe alloy, the coarsened θ-Al₁₃Fe₄ stable phase was formed in the α (fcc) matrix. The cast Al-1%Fe alloy exhibited several elongated α phases surrounded by refined α/Al₆Fe eutectic structure. In the Al-1%Fe-1%Mn alloy, the Al₆(Fe, Mn) phase appeared regardless of the cooling rate. The refined α/Al₆(Fe, Mn) two-phase eutectic microstructure occupied almost the entire area in the cast sample. In the furnace-cooled Al-1%Fe-1%Cu alloy, the Al₂₃CuFe₄ phase was formed in the final solidification zone. The cast Al-1%Fe-1%Cu alloy showed the same microstructural morphology as the cast Al-1%Fe binary alloy, and the Cu tended to distribute into the Al₆Fe phase. The fine eutectic microstructure was observed in the cast Al-1%Fe-1%Cu-1%Mn alloy as well as in the cast Al-1%Fe-1%Mn alloy. The added Cu and Mn elements were enriched in the intermetallic phase, which may contribute to the stabilization of the Al₆M phase.
The 0.2% proof stress of the cast Al-1%Fe alloy was about 90 MPa at room temperature and decreased with increasing temperature. The 0.2% proof stress at 300°C was about 50 MPa. With the addition of 1 mol% Mn and Cu, the 0.2% proof stress at room temperature increased to 120~130 MPa. The combined addition of Mn and Cu increased the 0.2% proof stress to about 150 MPa, and the reduction in strength level by increasing test temperature appeared less pronounced for the Al-1%Fe-1%Mn-1%Cu quaternary alloys. The moderate reduction in strength could be due to the fine eutectic structure containing the Al₆M phase stabilized by Mn and Cu elements.

Recently, novel alloy design methods using high-concentrated solid solutions, such as high-entropy alloys, are attracting attention. By introducing the strengthening phase to a high-concentrated solid solution with excellent mechanical properties, it is expected to produce high-performance materials. Because the number of combinations of multiple elements is too large, a guideline for selecting constituent elements for searching for new high-concentrated solid solution-based multi-phase alloys is needed. Our goal is to develop a rapid screening scheme for multi-component material development.
The Pettifor maps are useful maps for binary compounds A_xB_y with various x/y ratios in an easy-to-understand notion in terms of the Mendeleev number [1]. Given that it can describe a wide range of binary compounds, the order of the Mendeleev number is related to factors governing the binary interaction energy, such as electron concentration. Therefore, this idea is expected to be effective in describing the stability of not only binary compounds but also binary solid solutions. By creating the map for solid solutions and combining it with the conventional Pettifor map for binary compounds, both combinations that form a stable solid solution and the compound that may equilibrate each other can be found in a short time survey without checking a huge number of phase diagrams one by one. For a typical example, in the case of a ternary alloy with Co and Al additions to pure Ni, these maps will tell us Co forms an fcc continuous solid solution with Ni, together with the candidates of Al-containing compounds such as Ni₃Al-L1₂, NiAl-B2 and CoAl-B2.
For the relationship between the binary system and multi-component system, our previous works revealed that many multi-component compounds are derived by substituting a specific site of a binary compound for other elements, e.g., (Ni, Co)₃(Al, Ta, W) derived from L1₂-Ni₃Al [2,3]. This knowledge leads us to expect that, information such as the crystal structure of binary solid solutions is helpful to evaluate the stability of multi-component solid solutions. The main aims of the present work are to investigate the stability of multi-component solid solutions thermodynamically, create the map for binary primary solid solutions and confirm our idea is effective in selecting the constituent elements in multi-component systems.
Under such considerations, the map proposed in this study includes detailed information about each binary solid solution such as its crystal structure, its maximum solubility and the presence of its competing phases within the 1,411 binary phase diagrams in a single map. This map clearly shows which solutes dissolve largely in which solvents, allowing an immediate evaluation of the potential for multi-component high-concentrated solid solution formation. Then, by comparing this map with the Pettifor maps, the compound to be selected as a stable phase in a multi-component system can be estimated. For example, considering the Ti addition to CoCrFeMnNi alloy, the stabilization of the solid solution by combining Co, Cr, Fe, Mn and Ni can be confirmed in the map of this study. Also, in the conventional Pettifor maps, as the candidate compounds composed of the constituent element with Ti, there are only the Laves phases (TiCo₂, TiCr₂, TiFe₂ and TiMn₂) in the AB₂ map and the B2 compounds (CoTi, FeTi and MnTi) in the AB map. In previous experiments [3], it was confirmed that the high-concentrated fcc phase equilibrates with AB₂ Laves compound in Ti-added CoCrFeMnNi alloy. Thus, our scheme can easily estimate the combination of a stable high-concentrated solid solution and the candidate compounds. This study will enable the rapid development of high-performance multi-component materials.
[1] D. G. Pettifor, J. Phys. C, Solid. State., 19, 285 (1986)
[2] S. Yamanaka, K. Ikeda and S. Miura, MRS advances, 4(25-26), 1497 (2019)
[3] S. Yamanaka, K. Ikeda and S. Miura, J. Mater. Res., 36, 2056 (2021)

Pearlitic steel has been used in a wide variety of practical applications, such as rails, bridge cables and high-strength wires because of their ultra-high strength, which is closely related to their lamellar structure composed of alternating ferrite (α) and cementite (θ-Fe3C) layers. The strength of pearlitic steel wires has known to be improved further by cold drawing, which results in the refinement and alignment of the lamellar structure. However, the detailed mechanism of the microstructure evolution during cold drawing process has not been fully clarified yet. This is mainly because of the lack of information on plastic deformation behavior of cementite. Recently, we have systematically investigated room temperature deformation behavior of cementite single crystals as a function of the loading axis orientation by utilizing micropillar compression method and have successfully identified five different slip systems operative at room temperature and their critical resolved shear stress (CRSS). In the present study, we have applied the micropillar compression method to investigate plastic deformation behavior of individual pearlite lamellar grains, whose loading axis orientations are (nearly) parallel to the lamellar boundary planes with the Bagaryatsky orientation relationship ((112)_α//(010)_θ, [-110]_α//[001]_θ). Operative slip systems in each lamella and their transfer behavior through lamellar interfaces were investigated experimentally through the trace analysis of slip traces appeared on side surfaces of micropillar specimens. When the loading axis was nearly parallel to [-110]_α and [001]_θ, both α-Fe and θ-Fe3C lamellar were confirmed to plastically deform by the activation of their primary slip systems with their slip traces propagating continuously through many alpha-theta interfaces. In contrast, when the loading axis was about 15 degrees away from [-110]_α and [001]_θ, non-primary slip systems were activated in both phases, while the continuation of slip traces was maintained. Possible factors that control the operative slip systems in both phases were discussed based on the information on the operative deformation modes in θ-Fe3C obtained by our previous micropillar experiments, geometrical relationship between slip systems in alpha and theta phases, and strain compatibility at alpha-theta interfaces.

The shape change of the ferromagnetic shape memory alloy (FSMA) is manipulatable via applying an external magnetic field [1]. The NiMnGa-based alloys are considered promising FSMAs for applications as magnetic field-driven devices; however, their high brittleness inhibits their practicability. Therefore, in this work, a grain separator, Bi element, was intentionally introduced into the polycrystalline NiMnGa alloys to separate the grains and make NiMnGa alloys even brittle. The introduced concentration of the Bi element was variating from 0.00 at.% to 0.05 at.% to optimize the doping amount and the properties of the bulk NiMnGa specimens. The Bi-doped polycrystalline NiMnGa alloys were then mechanically disintegrated into single-crystalline particles by a well-controlled crushing system. The NiMnGa single crystal particles were utilized for the fabrications of the composites with the volume fraction (vol.%) of the single-crystal particles at 20% by an integration of the single-crystal particles with a silicone polymer matrix. The single crystal NiMnGa particles/silicone polymer composite materials were successfully fabricated for realizing the applications of the high-speed actuator.
The Bi doping studies were performed using concentrations of 0.00, 0.01, 0.03, and 0.05 at.%. Elemental mapping (i.e., Ni, Mn, Ga, and Bi) of each alloy was conducted to reveal the distribution details of each element. It is found that with the addition of the Bi separator to the NiMnGa polycrystalline bulk materials, the grains were successfully separated. In addition, it was shown that the higher concentration of the Bi doping, the more grain boundaries could be observed. This effect was already saturated at 0.03 at.%. The average grain size, analyzed by using ImageJ software, was around 210 μm for NiMnGa polycrystalline bulk alloys doped by 0.03 and 0.05 at.% Bi. A co-existence of the 5M- and the 7M-martensite phases was found in the single crystal NiMnGa particles at 27 ^oC, whereas at 39 ^oC only a single 5M-martensite phase, and at 51 ^oC only the parent austenite phase (A), were observed. Hence, the following sequence of phase transformations was observed 7M/5M→5M→A [2].
The micro-strains of particles in composites with volume fraction of particles of 20 vol.% (co-existence of 5M- and 7M-martensite phases), Composite A (CA), and 30 at. % (5M-martensite phase), Composite B (CB), were measured by a micro-CT and analyzed by a polar decomposition method [3]. The composites were compressed from 0% to 50% of the total deformation strain. It was found that the particle composed of 7M-martensite performed the largest micro-strain among all the tested particles. On the other hand, the 5M-martensite particles in CA and CB composites exhibit similar micro-strain. It is worth noting that the loading-unloading hysteresis of CA was reduced greatly owing to the released spatially interference among the particles.
References
[1] H.E. Karaca, I. Karaman, B. Basaran, Y.I. Chumlyakov, H.J. Maier, Acta Mater. 54(1) (2006) 233-245.
[2] C. Seguí, V.A. Chernenko, J. Pons, E. Cesari, V. Khovailo, T. Takagi, Acta Mater. 53(1) (2005) 111-120.
[3] P. Sratong-on, V.A. Chernenko, J. Feuchtwanger, H. Hosoda, Sci. Rep. 9 (2019) 3443.
[4] W.-T. Chiu, P. Sratong-on, M. Tahara, V.A. Chernenko, H. Hosoda, Scr. Mater. 207 (2022) 114265.

With the demand for high efficiency in green technology fields such as energy conversion systems, wind turbines, and electric vehicles, Nd₂Fe₁₄B sintered magnets have to store high energy density and be stable for devices in various operating temperatures. For high coercivity in accordance with this requirement, several attempts are being investigated to develop heavy rare-earth element (HRE)-lean or free Nd-Fe-B magnets. A promising approach to achieve coercivity enhancement while avoiding scarcity of resources and the expense of remanence due to antiferromagnetic coupling between HRE and Fe is to control the microstructure, such as grain size. The size-dependent coercivity tends to maximize when the magnetic dimension reaches a critical diameter (about 260nm for Nd₂Fe₁₄B) where only a single domain is supported. Furthermore, the fine grains suppress domain reversal at high temperatures, improving the thermal stability of the coercivity and its suitability to the motor operating environment.
As an attempt at grain refinement, conventional metallurgy techniques mainly controlled the milling parameters to reduce the size of feedstock particles but had limitations in size reduction. Accordingly, bottom-up synthesis accompanied by a reduction-diffusion (R-D) process, which overcomes the limitations of top-down methods and has advantages such as low energy consumption, low raw material cost, and uniform morphology, has emerged as an attractive way to control the particle size. Motivated by the advantages of chemical-based synthesis, the size control of Nd₂Fe₁₄B particles has been conducted in a few studies through concentration of the solution and R-D conditions as variables, but magnetic properties predominantly depended on crystallinity and composition rather than size. In addition, there is an inherent limitation that it is difficult to even sinter due to low productivity, imperfection, and oxidation of nanoparticles, as a result, research to confirm the size effect on magnetic properties of Nd₂Fe₁₄B by a chemical-based method has not yet been reported. Therefore, it remains a major assignment to develop the magnetic properties of a sintered magnet by forming submicron grains using chemically synthesized particles of an appropriate size.
In this study, we adopted an effective approach using submicron Fe powder to synthesize Nd₂Fe₁₄B by applying the size dependence of final magnetic particles on transition metal precursors. Chemical-based methods including ultrasonic spray pyrolysis (USP) and hydrogen reduction process for the synthesis of submicron seed powder were proposed and performed. And then in order to fabricate a fine-grained Nd-Fe-B sintered magnet with high magnetic properties, the R-D process and sintering conditions were optimized using the synthesized submicron iron. Finally, the interaction between oxygen content and physical and magnetic properties according to the size of Nd₂Fe₁₄B was systematically investigated and the effect of size on magnetic properties was elucidated by controlling the particle size of Nd₂Fe₁₄B using Fe seed particles with a size varying from nano to microscale.

Next-generation advanced permanent magnets require high magnetocrystalline anisotropy that is donated by chemical ordering. While it remains highly challenging to control chemical order in the “holy grail” FeNi permanent magnet system (aka tetrataenite), the analogous proxy system of MnAl provides a good platform for investigating and understanding the chemical ordering in ferromagnets. To this end, the disorder (hcp ε-phase) – order (L1₀ τ-phase) phase transformation in Mn₅₅Al₄₅ compound is investigated here. Structural, magnetic, and calorimetric data confirmed the attainment of L1₀ ordered τ-MnAl upon heating two different quenched forms of metastable, disordered ε-MnAl: melt-spun ribbon solidified from a molten precursor and water-quenched one from the already solidified ribbon. While both quenched Mn₅₅Al₄₅ samples exhibit nominally phase-pure (~ 99 wt.%) ε-MnAl phase, the ordering transformation of the melt-spun ribbon occurs ~40 degrees lower than that of the water-quenched one. Kinetic analysis reveals that the ordered τ-phase in the melt-spun ribbon develops from pre-existing nuclei, in contrast to that in the water-quenched alloy that exhibits a constant nucleation rate with fewer quenched-in nuclei. It is contemplated that specifics of as-quenched microstructure that originate from quenching conditions tailor the chemical order in the MnAl system. The results of this work provide insights into controlling phase stabilities and chemical ordering kinetics in other relevant ferromagnetic systems of applied interest as permanent magnets.
Acknowledgments: This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE SC0022168 and performed at Northeastern University.

Permanent magnets enable clean energy technologies such as wind turbine generators and hybrid vehicle regenerative motors. The expanding importance of these materials married with unpredictable supply restrictions underscores the importance of developing high-performance magnet substitutes of lower critical element content. To this end, the rare-earth-lean (Sm,Zr)(Fe,Co,Ti)₁₂ (“1-12”) compound has theoretical high-temperature performance that can surpass that of the NdFeB supermagnets, if only sufficient magnetocrystalline anisotropy energy could be converted into high coercivity to resist demagnetization. Advancing this objective, novel synthesis approaches have recently produced 1-12 powder containing single, defect-free crystallites of extremely high coercivity. In this work we correlate magnetic data with structural data to hone in on the characteristics of these high-coercivity crystallites, with the goal understand the impact of synthesis conditions on the intrinsic and technical magnetic properties of this compound.

Magnetic and particle size data were collected from 1-12 ferromagnetic powders made by a tailored synthesis process that combined mechanical activation with high-temperature calciothermic reduction. The data were analyzed to correlate processing conditions, including thermal treatment temperature and acetic acid concentration (utilized to remove unreacted calcium metal) from the powders. While all powder crystallites possess nominally the same composition, magnetic hysteresis loops indicate the presence of multiple hard magnetic phases, with a maximum of 15% of the powder exhibiting a record 2.2 T coercivity at room temperature. The underlying structural characteristics of the powder crystallites, including the evolution of particle transformations to form the desired 1-12 compound, are investigated to elucidate the origins of magnetic performance and provide guidance for its amplification.

This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE SC0022168.

Metal-Hydrogen alloys have been studied due to their widespread applications; in particular, the palladium-hydrogen system manifests superconductivity [1]. Palladium, alloyed with noble metals (Ag, Au, Cu) also shows superconductivity when hydrogenated with a ratio of 80% hydrogen; in this case, the superconducting transition temperature is maximum, T_C=15.6 K [2] for a concentration of Pd₇₀Ag₃₀. In this work we computationally generated and studied an amorphous structure of the intermetallic palladium-silver alloy Pd_100-xAg_x, x=30 at% and compared it with the crystalline counterpart. The amorphization process follows the undermelt-quench approach [3] on a 216-atom supercell of the PdAg alloy using ab initio molecular dynamics. Hydrogen was interstitially inserted for a ratio H/(Pd+Ag) = 0.8 in the supercells. The obtained structures were characterized with the Pair Distribution Function (PDF). In order to compare crystalline and amorphous hydrogenation, the topology and the electronic structures were obtained. In this work they will be contrasted, analyzed and discussed.

[1] Skoskiewicz, T. (2022). Superconductivity in the Palladium-Hydrogen and Palladium- Nickel-Hydrogen Systems. In Görlich (Ed.), Volume 11, Number 2 June 16 (pp. 825-828). Berlin, Boston: De Gruyter. https://doi.org/10.1515.
[2] Stritzker, B. High superconducting transition temperatures in the palladium-noble metal-hydrogen system. Z. Physik 268, 261–264 (1974). https://doi.org/10.1007/BF01669889.
[3] A. A. Valladares. in Glass Research Progress, edited by: Jonas C. Wolf and Luka Lange, Nova Science Publishers, Inc., 2008. Pp. 61-123.

The study of superconducting alloys has shown that the intermetallic compounds are an attractive family of materials. Those that contain bismuth may exhibit superconductivity [1, 2, 3]. Within these alloys we find the intermetallic crystalline phases α-PdBi₂ and β-PdBi₂ with T_c =1.73 K and 4.25 K respectively [4]. In this work we report the study of the topology and electronic structure of both the amorphous and crystalline phases of the above-mentioned intermetallic compounds. The amorphization was accomplished by using the process called the undermelt-quench approach [5]. This process is based on ab initio molecular dynamics and consists in heating the supercells to temperatures just below the liquidus temperature and then cooling it down to close to absolute zero. This process was applied to the α and β phases of the material, using supercells with a total of 216 atoms for the binary system. In order to compare the crystalline and the amorphous systems, the topology and electronic structure were obtained and these results will be presented and discussed.

[1] N. N. Zhuravlev and G. S. Zhdanov, Zh. Eksp. i Teoret. Fiz. 25, 485 (1953).
[2] N. E. Alekseevskii, N. N. Zhuravlev, and I. I. I.ifanov, Zh. Eksp. i Teoret. Fiz. 27, 125 (1954).
[3] N. N. Zhuravlev, Soviet Phys.—JETP 5, 1064 (1957).
[4] B. T. Matthias, T. H. Geballe, and V. B. Compton: Rev. Mod. Phys. 35 (1963) 1.
[5] A. A. Valladares. in Glass Research Progress, edited by: Jonas C. Wolf and Luka Lange, Nova Science Publishers, Inc., 2008. Pp. 61-123

The β-type Ti-based alloys have recently attracted much attention due to their exceptional properties such as low Young’s modulus, shape memory effect and gum-metal-like behavior. Among them, β-type Ti-based shape memory alloys such as Ti-Nb-Ta-Zr and Ti-Mo-Al, which undergo thermoelastic martensitic transformation from a β (body-centered cubic) parent phase to an α’’ (orthorhombic) martensite phase, show great promise for applications in medical devices due to their superelasticity and biocompatibility. However, the transformation strain by superelasticity is usually small to be less than 3% in these conventional β-type Ti-based shape memory alloys. In this talk, we report a novel B2-structured Ti-Al-Cr shape memory alloy with a greater superelastic strain. The crystal structure and lattice parameters of both parent phase and martensite phase were determined using the X-ray and neutron diffraction methods. The mechanical properties of the alloy were evaluated by uniaxial tensile or compressive tests on single-crystal samples at room temperature. It was found that the alloy exhibits excellent superelastic behavior with a recoverable strain over 7%. The experimentally observed transformation strain is consistent with the calculated one using the lattice parameters on the base of lattice deformation theory. This newly-developed Ti-Al-Cr shape memory alloy is a promising candidate for medical applications. This alloy is also characterized by its lower density than that of the conventional β-type Ti-based alloys containing high-density Nb or Mo, having potential as a lightweight shape memory alloy.

In situ high-temperature X-ray diffraction (HT-XRD) is an useful technique that can provide very valuable information on in situ crystallographic properties such as lattice parameter and phase transformation at a certain temperature. Through In situ HT-XRD, we have studied the crystallographic characterization of zirconium-based samples containing minor alloying elements of tin, niobium, and iron at a temperature range of 30–870 °C with intervals of 40 °C. The a- and c-axes lattice parameters of the hexagonal Zr crystal structure were calculated from the in situ HT-XRD spectra using the Pawley refinement. We confirmed that the a-axis lattice parameters of the hexagonal Zr crystal structure for the zirconium-based samples with tin element are decreased compared to those for a pure zirconium sample, in contrast to the zirconium-based samples with other minor alloying elements (i.e., niobium and iron). This effect is the greatest when 3 wt% tin was added in zirconium. On the other hand, the c-axis lattice constants for all the zirconium-based samples are not declined by the minor alloying elements as compared to the pure zirconium.

Solid state precipitation and growth of different phases inside solid solutions is an important part of the design and manufacturing of metallic alloys. Intermetallic precipitates are one of the most effective methods to strengthen metals because such precipitates often resist dislocation motion.

One optimal microstructure for improving the strength of metals is a high density of small precipitates. It has recently been demonstrated that in Mg alloys this microstructure can result from the defects generated by the mechanical deformation [1]. Vacancies and voids are commonly found among these defects.

The agglomeration of vacancies has been in the center of our attention because these structures may play a crucial role in the nucleation of intermetallic precipitates that remains poorly characterized. It is known that voids can change the composition in their vicinity by attracting or repelling solute [2]. This could promote or suppress the nucleation of precipitates.

Furthermore, the formation of voids from agglomeration of vacancies is itself a nucleation process that is not fully understood. Current models assume that voids nucleate from a uniform concentration of vacancies [3]. However, in direct molecular dynamics simulations we have observed that agglomerations of vacancies could remain locally stable before creating a well-defined internal surface. This could suggest that the nucleation of a void with a defined internal surface could involve a two-step process.

Nucleation is known to be a “rare event”, which means extracting kinetic information from computational simulations is challenging. By means of Replica Exchange Transition Interface Sampling (RETIS) we can quantify aspects of the void nucleation process through the sampling of independent phase-space paths. The resulting data is useful for elucidating the kinetics of void nucleation. Using this data, we aim to validate or improve existing void nucleation models.


[1] S.E. Prameela, P. Yi, Y. Hollenweger, B. Liu, J. Chen, L. Kecskes, D. Kochmann, M.L. Falk, T.P. Weihs, “Strengthening magnesium alloys with nanoscale precipitates and solute clusters,” Mechanics of Materials, 2022, in press.

[2] X. Wang, C. Hatzoglou, B. Sneed, Z. Fan, W. Guo, K. Jin, D. Chen, H. Bei, Y. Wang, W. J. Weber, Y. Zhang, B. Gault, K. L. More, F. Vurpillot and J. D. Poplawsky. “Interpreting nanovoids in atom probe tomography data for accurate local compositional measurements”, Nat. Communications (2020) 11:1022

[3] K. F. Kelton, A. L. Greer. “Nucleation in Condensed Matter: Applications in Materials and Biology” (Elsevier, Netherlands, 2016)

Observing the physical properties of each microstructure is the key to understanding the various properties of metals. For instance, corrosion, such as oxidation and reduction, can be influenced by microstructures. Recently, as corrosion begins at the nanometer or micrometer scale, local corrosion studies related to microstructures using microscopic techniques are being conducted. However, to understand the corrosion mechanism, it is not sufficient to observe morphological change by ex-situ experiments. Furthermore, understanding the overall corrosion mechanism requires observing microstructures in real time and even predicting surface changes. In this presentation, various physical properties such as height, surface potential, and capacitance gradient according to microstructure in advanced high-strength steel, which is applied automotive, were investigated by atomic force microscopy (AFM). Furthermore, we performed the prediction of morphological change through a combination of in-situ electrochemical AFM (EC-AFM) images and deep learning techniques for understanding the corrosion mechanism. We show that the combination of deep learning to in-situ EC-AFM images can show not only identification of microstructures, but also insight into the corrosion mechanism. Consequently, this approach can be easily applied to other intermetallic-based alloys.

The microstructure template of a disordered matrix reinforced by ordered-intermetallic precipitates offers a potent design strategy for high temperature materials, enabling strength alongside damage tolerance, which has been central to the success of fcc Ni-superalloys. Such a strategy is equally applicable to bcc-based systems, which offer advantages of increased melting point, lower cost, and/or lower density. However, whilst bcc-superalloys based upon a refractory metal, Ti, Cr or Fe matrix, strengthened by ordered-bcc intermetallic precipitates are possible, further research and development is need for them to become a commercial reality.

In this talk, opportunities for bcc-superalloys systems will be discussed, from binaries to ternaries and more complex alloys, including Refractory High Entropy Superalloys. It will then discuss our latest developments in tungsten-based bcc-superalloys, beta-Ti superalloys and chromium superalloys. Prospectives will be given for the onward development of the bcc-superalloy concept for application in nuclear fusion, Gen-IV fission, gas turbines and concentrated solar power.

Max phases have attracted attentions because of its high strength, high melting point and low density [1]. The present authors conducted a broad reserach for establishing the BCC-V based materials with Max phase V₂AlC as a dispresoid for strengthening. It was found that BCC-V matrix alleviates the delamination tendency of MAX phase during plastic deformation even at relatively lower temperature, however, the microstoructure stability of V-Al-C ternary alloys is not enough. In this study effect of various quaternary elements on the microstructure stabilization is investigated.
Based on the V-Al-C ternary phase diagram [2], we prepared alloy ingots with and without quaternary additives such as Cr, Ti, Mo and W. An Ar-arc melting machine was used to melt the sample several times, and a part of ingot sealed in a evacuated silica tube was subjected to a heat-treatment at 1473 K for 1 week. Microstructure observation using FE-SEM and WDS analysis for determining the composition of each phase is conducted, and the EBSD analysis was also conducted to understand the crystallographic orientation relationship between BCC-V and MAX phase.
It was confirmed that the alloys with and without the heat-treatment have two-phase microstructure, however, plate-like V₂AlC-MAX phase in as-cast alloys with Ti spheroidized during the heat-treatment. On the other hand, the MAX phase in alloys with Cr, Mo or W keep its plate-like structure even after the heat-treatment. It was confirmed that V has large solubility of Cr, Mo and W, while Ti tends to be in MAX phase because Ti forms MAX phase (Ti₂AlC). In our previous study it was found that the BCC-V and MAC phase has a certain crystallographic orientation relationship. The lattice mismatch between BCC-V and MAX in ternary alloys are evaluated to be about 9%, and the value increases in alloys with Ti because Ti increases the lattice constant of MAX by replacng smaller V atom. On the other hand either Mo and W increase the lattice constant of BCC-V solid solution, resulting in decreasing the lattice mismatch between BCC-V phase and MAX phase. Smaller inter-phase boundary energy between BCC-V and MAX phase caused by smaller lattice mismatch might be a reason why Mo and W stabilize the microstructure of alloys. In the presentation a part of the mechanical testing will be presented.
Acknowledgement
This work was supported by a grant from JSPS KAKENHI for Scientific Research on Innovative Areas “MFS Materials Science (Grant Numbers JP18H05482)".
References
[1] M. W. Barsoum and T. El-Raghy: Am. Sci., 2001, 89, 334-343.
[2] B.Hallstedt, CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry, 2013, 41, 156-159.

A new model, known as the diffuse multi-layer fault model (DMLF), has provided an accurate high-throughput method for evaluating the energetics of planar faults within alloys of complex composition. Specifically, the DMLF, which approximates a planar fault through a series of proximate structures that capture the local bonding environment within the vicinity of the fault, has been used recently to assess the energetics of planar faults within FCC alloys [1] as well as the {111}a/2(110) antiphase boundary observed within FCC-derived L1₂ ordered intermetallics. [2] Within this work, we report on the successful extension of this model to the antiphase boundaries present on {110} planes within the BCC-derived B2 and L2₁ ordered intermetallics. Through coupling the DMLF to a large database of symmetrically distinct atomic configurations on a host BCC lattice, proximate structures have been identified for the {110}a/2(111) antiphase boundary within the B2 and L2₁ intermetallic and the {110}a/4(111) antiphase boundary observed in the L2₁ intermetallic. Density functional theory calculations are employed to validate the accuracy of the formation energies of these newly identified proximate structures relative to those calculated via the more conventional, and computationally costly, supercell method. The use of the B2 and L2₁ proximate structures to predict the thermodynamic stability of planar faults in BCC-derived ordered intermetallics with a complex composition will also be discussed.

[1] K. V. Vamsi, M. A. Charpagne, and T. M. Pollock, "High-throughput approach for estimation of intrinsic barriers in FCC structures for alloy design," Scr. Mater., 204, 114126.
[2] K. V. Vamsi and T. M. Pollock, "A new proximate structure for the APB (111) in L12 compounds," Scr. Mater., 182, 38-42.

Refractory compositionally complex alloys (RCCA) are promising candidates for high-temperature structural applications [1]. Many of the reported alloys consist of A2 or B2 phases with additional intermetallic phases often located at grain boundaries. However, to achieve good mechanical performance at elevated temperatures as well as sufficient ductility at room temperature, the proper formation of a strengthening phase is crucial. Recently, Senkov and co-workers [2] reported a microstructure with coherent cuboidal precipitates and promising high-temperature strength in the Al-Mo-Nb-Ta-Ti-Zr system. However, the alloy lacks ductility at room temperature, which could be
attributed to an ordered B2 matrix phase with disordered A2 precipitates.
Following along above conceptual path we report here on the current status of our investigations within the Ta-Mo-Ti-Cr-Al system which exhibits a promising combination of strength and oxidation resistance at elevated temperatures [3]. The objective is to attain a suitable multi-phase microstructure of A2 matrix and B2 precipitates. Thermodynamic calculations were employed with an in-house database to predict specific transformation sequences of ordering and diffusion-controlled phase separation within this system. As already observed in the Ta-free subsystem MoTiCrAl, Al has a significant effect on the ordering transition of A2 towards B2 [4]. However, in the case of the MoTiCrAl system, no multi-phase phase field has been predicted and confirmed after the homogenization. This changes by the addition of Ta, as a multi-phase field is predicted [5]. To minimize the formation of inherently brittle Laves phase, the Cr concentration was adjusted accordingly [6]. The microstructure of two compositions (high and low in Al, respectively) were investigated experimentally by scanning and transmission electron microscopy, while the phase transitions were determined by means of differential scanning calorimetry. The microstructure of the alloy with high Al concentration exhibited a B2 matrix with A2 precipitates; in contrast, an A2 matrix with B2 precipitates was determined in the Al-lean alloy. The necessary reaction sequences are discussed in detail and provide information about the phase diagram structure necessary to reproducibly obtain the desired two-phase microstructures. Microstructural peculiarities, such as segregation at planar defects as well as their implications on the mechanical properties will be discussed.

References:
[1] D. B. Miracle, M.-H. Tsai, O. N. Senkov, V. Soni and R. Banerjee, Refractory high entropy superalloys (RSAs), Scripta Materialia, vol. 187, pp. 445-452, 2020
[2] O. N. Senkov, C. Woodward and D. B. Miracle, Microstructure and Properties of Aluminum-Containing Refractory High-Entropy Alloys, JOM, vol. 66, pp. 2030-2042, 2014
[3] B. Gorr, F. Müller, S. Schellert, H.-J. Christ, H. Chen, A. Kauffmann and M. Heilmaier, A new strategy to intrinsically protect refractory metal based alloys at ultra high temperatures, Corrosion Science, vol. 166, p. 108475, 2020
[4] S. Laube, H. Chen, A. Kauffmann, F. Müller, B. Gorr, J. Müller, B. Butz, H.-J. Christ and M. Heilmaier, Controlling crystallographic ordering in Mo–Cr–Ti–Al high entropy alloys to enhance ductility, Journal of Alloys and Compounds, vol. 823, p. 153805, 2020
[5] S. Laube, S. Schellert, A. Tirunilai, D. Schliephake, B. Gorr, H.-J. Christ, A. Kauffmann and M. Heilmaier, Microstructure tailoring of Al-containing compositionally complex alloys by controlling the sequence of precipitation and ordering, Acta Materialia, vol. 218, p. 117217, 2021
[6] F. Müller, B. Gorr, H.-J. Christ, H. Chen, A. Kauffmann, S. Laube und M. Heilmaier, Formation of complex intermetallic phases in novel refractory high-entropy alloys NbMoCrTiAl and TaMoCrTiAl: Thermodynamic assessment and experimental validation, Journal of Alloys and Compounds, vol. 842, p. 155726, 2020

The modern material class of equiatomic multi-component alloys, especially high-entropy alloys (HEAs) gained tremendous attention in the scientific community over recent years. This can be attributed to two main reasons: Firstly, the new concept of combining several elements (at least 5 principal elements with concentrations between 5 and 35 at. % [1]) in contrast to conventional alloys, mostly containing only two or three elements in addition to the main alloy constituent. This results in a broad variety of possible combinations thus leading to completely novel materials. Secondly, recently developed refractory metal based high-entropy alloys (RHEAs) have shown properties that are superior to the ones of current state-of-the-art alloys, which are based on several unique thermodynamic effects [2–5]. However, besides the outstanding mechanical properties, abrasion resistance and thermal resistance, a vast variety of chemical elements used in RHEAs also belong to the category of biocompatible elements, hence leading to potentially new biomedical materials. In consideration of this background and due to the excellent biocompatibility of the constituents [6], an equiatomic composition of Ta, Nb and Ti as multi-component base alloy was chosen for investigations.
The alloy examined was produced using an arc melting furnace under Ar atmosphere, metallographically prepared and investigated respectively. Scanning electron microscopy (SEM) analysis revealed the presence of a dendritic microstructure, with an enrichment of high-melting elements in the dendrites, as well as Ti in the interdendritic regions (verified by means of EDS mappings). Microstructure analysis by means of X-ray diffraction (XRD) showed, that there are two types of body-centered cubic (bcc) crystal structures (Im-3m I: a = 3.287 Å; Im-3m: a = 3.291 Å) present in the as-cast state. To get a better understanding of the microstructure evolution, heat-treatment experiments regarding different temperatures and times were performed. Moreover, the biocompatibility of the novel alloy Ta-Nb-Ti was evaluated by means of cell (f.e. osteoblasts) attachment, as well as monocyte inflammatory response analysis and compared to samples of elemental Ta, Nb, alloy Co-28Cr-6Mo and alloy Ti-6Al-4V. First results indicate competitive osteoblast attachment, as well as comparable expressions of fibrosis markers in comparison to the conventionally used biomedical materials. In addition, the Ta-Nb-Ti alloy showed reduced inflammatory capacity, indicating a high potential for use as prospective biomedical material.

References
1. Yeh, J.W.; Chen, S.K.; Lin, S.J.; Gan, J.Y.; Chin, T.S.; Shun, T.T.; Tsau, C.H.; Chang, S.Y. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes. Adv. Eng. Mater. 2004, 6, 299-303+274.
2. Regenberg, M.; Hasemann, G.; Wilke, M.; Halle, T.; Krüger, M. Microstructure Evolution and Mechanical Properties of Refractory Mo-Nb-V-W-Ti High-Entropy Alloys. Metals (Basel). 2020, 10, 1530.
3. George, E.P.; Curtin, W.A.; Tasan, C.C. High entropy alloys: A focused review of mechanical properties and deformation mechanisms. Acta Mater. 2020, 188, 435–474.
4. Xiao, Y.; Zou, Y.; Sologubenko, A.S.; Spolenak, R.; Wheeler, J.M. Size-dependent strengthening in multi-principal element, face-centered cubic alloys. Mater. Des. 2020, 193, 108786.
5. Shittu, J.; Pole, M.; Cockerill, I.; Sadeghilaridjani, M.; Reddy, L.V.K.; Manivasagam, G.; Singh, H.; Grewal, H.S.; Arora, H.S.; Mukherjee, S. Biocompatible High Entropy Alloys with Excellent Degradation Resistance in a Simulated Physiological Environment. ACS Appl. Bio Mater. 2020, 3, 8890–8900.
6. Andersen, P.J. 1.1 Metals for Use in Medicine. In Comprehensive Biomaterials II; Elsevier, 2017; Vol. 1, pp. 1–18 ISBN 9780081006924.

The intermetallic B2 phase NiAl possesses promising properties as a high-temperature material with a very high melting point, low density, good thermal conductivity, and excellent oxidation resistance. However, the high-temperature strength and the room temperature fracture toughness show poor performances which disqualify the usage of single-phase NiAl for structural applications as a high-temperature material. However, the addition of 28 at. % Cr and 6 at. % Mo to NiAl leads to an eutectic alloy, which forms a two-phase rod-like or lamellar eutectic microstructure during directional solidification. These in-situ composites show superior mechanical properties over single-phase NiAl. New additive manufacturing methods, such as selective electron beam melting (SEBM) provides very high cooling rates which produces nanostructured composites with very small lamellar spacings between the B2-NiAl and the bcc Cr solid solution. The room temperature fracture toughness of NiAl-(Cr,Mo) composites benefits from a smaller lamellar spacing and the creep strength can be superior to the creep strength of TiAl-alloys.
In this study, dense and crack-free specimens of eutectic NiAl-(Cr,Mo) in-situ composites have been processed via selective electron beam melting using an Arcam A2 machine. In dependence on the processing parameters, a lamellar network-like microstructure can be obtained. For alloys with a lower Mo content, also rod-like structures introduced by discontinuous precipitation can be found in the additively manufactured composites. These discontinuous precipitations are triggered by an in-situ heat treatment in the building chamber or an ex-situ heat treatment. The different types of microstructures are investigated by their mechanical properties, microstructure, and chemical composition. APT investigations are used to further analyze and understand the formation of the nanostructured phases. The high-temperature properties are analyzed by compression and creep experiments, whereby the post-creep defect structure is further investigated by TEM to understand the deformation mechanisms. Furthermore, the room temperature fracture toughness of the different morphologies is studied by testing FIB-milled micro bending cantilevers. The best creep results and high-temperature flow stress are shown by the network-like composites, while the lamellar composites show superior fracture toughness. Consequently, the correlation between microstructure and mechanical properties requires a trade-off between room temperature- and high-temperature properties.

Additively manufactured high entropy alloys are in the early stage. More studies are required on how their compositions, processing routes, and microstructural evolutions intertwine to influence their (mechanical) properties for various fields of applications. In this study, the equiatomic AlCoCrFeNi high entropy alloy was fabricated via two additive manufacturing routes: binder jetting (3DBJ) and direct energy deposition (DED). The response of the samples to isothermal heat treatment was investigated and compared with the as-printed samples. The microstructural evolution was studied using backscattered electrons images from scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) was used to probe the constituent’s composition/distribution. The different phases present in the microstructure were investigated using X-ray diffractometer (XRD) and electron-backscattered diffraction (EBSD) techniques. Moreover, the mechanical properties data captured via nanoindentation were evaluated and mapped to show a representation of how they vary through the bulk alloy. Interplays of these essential parameters are responsible for the various mechanical properties exhibited by the alloy. The isothermal heat treatments significantly modified the microstructures of the alloy. However, the 3DBJ and DED samples respond differently, as diffusion mechanisms like grain boundary precipitation, precipitate-free zones (PFZ), and widmanstatten structure were more observable in the 3DBJ samples. The 3DBJ samples exhibit higher values of mechanical properties than the DED samples. The higher mechanical properties in 3DBJ samples are also linked to intermetallics such as sigma phase and ordered BCC (B2). Consequently, the effects of these phenomena are summarized by the difference between the properties of the as-printed samples (2 - 13 GPa hardness, 130 - 240 GPa reduced modulus) and those of the heat-treated samples (4 – 8 GPa hardness, 160 – 200 GPa reduced modulus). The ongoing exploits provide significant insight into how to exploit the immense opportunities AlCoCrFeNi high entropy alloy has for fields of applications such as the tools, automobile, structures, and aerospace industries.

A two phase alloy of Cr, bcc, solid solution matrix strengthened by a Cr₃Si, A15 ordered, intermetallic phase was investigated with the goal of achieving an alloy with properties "Beyond Nickel-Based Superalloys". In comparison to the commonly used Ni-base superalloys, these Cr-Si alloys may enable higher working temperatures and lower densities slightly above 7 g cm^-3. In addition, Cr is the only refractory metal which intrinsically builds an oxide scale suitable for protecting the alloy from oxidation. Si improves the oxide scale formation even further as well as a high temperature strength by precipitation of an intermetallic phase Cr₃Si. We observed that the addition of further alloying elements such as Mo, Ge, and Pt leads to an improvement in creep properties (Mo), oxidation resistance (Ge), and nitridation resistance (Pt) by maintaining the two-phase microstructure. Creep tests were conducted at temperatures at 980°C and higher and reveal promising results. The microstructures, scales, and reaction products were investigated by XRD, SEM, EPMA and Image Analysis.

There nearly is no other industry placing such high demands on materials as aircraft construction. Due to the mature design of the aircraft engines, further competition will take place with innovative materials and manufacturing processes. In the vicinity of the combustor, where engine parts are burden with temperatures higher than 500 °C, Ni-based superalloys have been state-of-the-art for many years. However, Ni-based superalloys come along with a high density (~ 8.5 g/cm³). Weight reduction is an important issue to achieve the target of increased thermodynamic efficiency including less fuel consumption and a reduced amount of exhaust emissions. To this end, research activities focus on new light-weight materials with good high-temperature properties to increase the thrust-to-weight ratio in aircraft engines by replacing Ni-based superalloys. Vanadium points out as an interesting candidate, since it offers the lowest density (ρ = 6.11 g/cm³) in comparison to other high-melting metals. Moreover, alloyed with Si and B, a multi-phase microstructure consisting of a vanadium solid solution (V_ss) phase next to the intermetallic phases V₃Si and V₅SiB₂ [1,2] forms, which offers a low ductile-brittle-transition as well as enhanced high temperature strength and creep resistance next to an improved oxidation resistance [1]. However, conventional ingot metallurgical processing of V-Si-B material and subsequent machining is energy- and time-consuming due to the different behavior of the ductile solid solution phase and the brittle silicides. This work presents the first study on additive manufacturing of a high melting point near-eutectic V-based alloy via additive manufacturing (AM) process. The feasibility of printing pre-alloyed V-Si-B powder material was demonstrated. V-Si-B powder was manufactured via gas atomization (GA) process out of solid raw materials meeting the requirements for AM regarding flowability and particle size. After developing suitable process parameters for the generation of crack-free samples, the microstructure of the materials was analyzed in detail using SEM/EDS and EBSD analyses. An overview about the ambient and high temperature material properties of a V-9Si-5B alloy is given. In terms of high-temperature mechanical properties, the brittle-to-ductile transformation temperature (BDTT) and the creep rate at a potential application temperature were determined as well as first investigations on the oxidation resistance.

[1] M. Krüger, Scripta Mater. 2019, 121, 75–78.
[2] G. Hasemann, M. Krüger, M. Palm, F. Stein, Mater Sci Forum. 2018, 941, 827-832.

The V-Si-B system has gained scientific interest as a new low-density, refractory metal-based structural intermetallic alloy system. The alloy design is strongly influenced and driven by the developments in the field of Mo-Si-B alloys and shares some interesting structural and microstructural features. Very recently, the formations of ternary eutectic V_SS-V₃Si-V₅SiB₂ microstructure has been reported [1] which contains the same isomorphous phases as the ternary eutectic in the well-studied Mo-Si-B system: a refractory metal-based solid-solution phase (Mo_SS or V_SS) and the two intermetallic phases with either an A15 (Mo₃Si and V₃Si) or a D8_l (Mo₅SiB₂ and V₅SiB₂) structure. In contrast to the Mo-based system, where the intermetallic Mo₃Si represents the major phase within the ternary eutectic microstructure, the V_SS-V₃Si-V₅SiB₂ eutectic features the ductile solid-solution phase as the major component. This fact makes the V-Si-B ternary eutectic very interesting in terms of its mechanical properties.
The present work is focused on the compressive stress-strain behavior of different near-eutectic V-Si-B alloy. Compression tests were performed using an electro-mechanical universal testing machine and a constant crosshead speed corresponding to an initial (engineering) strain rate of 10^-3 s^-1. The yield stresses were determined by the 0.2% offset method. The temperature dependence of its yield stress between room temperature and 1000 °C was investigated in the as-cast and annealed state (1400 °C for 100 hrs) of the fully eutectic alloy V-9Si-6.5B and were compared to literature values of different V-based alloys [2–4].

References
[1] W.G. Yang, G. Hasemann, M. Yazlak, B. Gorr, R. Schwaiger, M. Krüger, J. Alloys Compd. 902 (2022) 163722.
[2] H. Bei, E.P. George, E.A. Kenik, G.M. Pharr, Z. Met. 95 (2004) 505–512.
[3] S. Xie, E.P. George, MRS Proc. 980 (2007) 0980-II08-05.
[4] M. Krüger, Scr. Mater. 121 (2016) 75–78.

Recently, considerable effort has been devoted to the development of ultra-high temperature structural materials as alternatives to Ni-based superalloys to improve the energy efficiency of gas turbine systems. Among the several potential candidates, Mo-Si-B alloys have received much attention due to their high melting point and high temperature strength, but also have some remaining challenges to improve ductility, lower density and enhance environmental resistance. In the Mo-Si-B system microstructures with a Mo solid solution (Mo_ss) Mo₃Si and Mo₅SiB₂ (T₂) phases have been the focus of attention. However, the Si solubility in the Mo_ss phase diminishes the ductility and toughness. In order to address this issue, a new series of Mo-Si-B alloys in the Mo_ss+T₂ +Mo₂B three phase region has been designed to examine the effect of the lower Si solubility limit in the Mo_ss phase on the microstructure, hardness and oxidation behavior. The results showed that the Mo-Si-B alloys in the Mo_ss+T₂ +Mo₂B three phase region have higher fracture toughness and the same level oxidation resistance as alloys in the Mo_ss+T₂ + Mo₃Si three phase region. Selected additions of Al and Ti enable a density reduction to below 9 gm/cm³. In order to examine the Al addition effect on the Mo_ss+T₂ +Mo₂B three phase region the microstructures and oxidation of Mo-Si-B alloys at different Al addition level was examined at 800, 1100, 1200, and 1300°C for both isothermal oxidation and cyclic oxidation exposure along with an analysis of the oxidation kinetics and oxide layer morphologies.
While refractory metal intermetallic alloys (RMIA) offer an attractive option to extend operational capability beyond what is currently available with Ni-base superalloys the processing of the RMIAs to manufacture dimensionally controlled shapes in a commercially cost-effective way is challenging. For RMIAs the directional solidification approach used for Ni base superalloys has serious difficulties. For example, there are very limited choices for suitable mold materials that are nonreactive with the alloy melt. The solidification path for large ingots in RMIAs involving intermetallic silicide and boride phases usually yields product phases that are not part of the equilibrium phase microstructure. Due to the high-temperature stability and sluggish diffusion in refractory metals, homogenization requires very high temperatures (>1500^oC) and long annealing times (>1 day) that are not commercially cost effective. A conventional powder metallurgy processing approach for RMIAs involves several steps including the hot pressing of RMIA powders, high-temperature sintering, and then usually a hot isostatic press (HIP) treatment to achieve full density. Even with these multiple processing steps, it is difficult to produce complex shapes with accurate geometric control. To circumvent these challenges, an additive manufacturing (AM) route offers a new, rapid and effective strategy. With AM methods such as directed energy deposition (DED) and liquid powder bed fusion (LPBF), alloying is accomplished by the melting and reactive synthesis of component powder mixtures to full density so that the size scale of any nonequilibrium solidification products is limited that facilitates post-alloying homogenization. This approach has been demonstrated to yield the fabrication of homogeneous alloys with well-defined shapes. Besides the successful use of AM, the guidance from computational thermodynamics to identify compositions that meet density, strength and ductility goals and the use of dimensionless numbers to guide the AM process has been essential to achieve fully dense component with the desired microstructures.

The refractory multi-principal elements alloys (RMPEAs) are promising structural materials to help increase power efficiency in high-temperature oxidation environments, while the intrinsic poor oxidation resistance of the refractory elements limits their application. The oxidation behavior of a refractory high entropy alloy WMoTaNbV is investigated at 1300°C. By using an innovative two-step coating strategy with Mo-precoat followed by Si-B pack cementation, a multilayer Mo-Si-B coating with strong oxidation resistance and compatibility as well as a diffusion barrier is developed. The coating can protect the substrate at 1300°C for more than 750 h of thermal cycles. An RMPEA boride phase develops at the alloy/coating interface after longtime service and acts as an additional diffusion barrier to maintain the coating integrity. The oxidation test results demonstrated that this two-step coating system provides robust oxidation protection at high temperatures and can be widely applied in RMPEAs.

Refractory metal based alloys exhibit intrinsically high creep resistance at anticipated application temperatures beyond 1100 °C due to high solidus temperatures. However, apart from ductility issues at low temperatures, oxidation resistance at elevated temperatures is often observed a fundamental problem for the further development of this group of alloys.

Among others, Mo-based alloys have attracted particular research interest [1]. However, the disordered, body centered cubic solid solution, rich in Mo and required to obtain reasonable toughness and low brittle-to-ductile transition temperatures, usually forms volatile MoO₃ already at temperatures below 1000 °C. The progress of oxidation occurs rather fast and typically leads to a complete disintegration of the part within short periods of time. Thus, this so-called pesting behavior is a catastrophic type of oxidation.

In the present study, we provide recent results on the development of pesting-resistant high Mo containing alloys specifically addressing this fundamental problem when oxidation resistance is considered [2-6]. The alloys with this surprising and not expected property are Ti-rich, two-phase or three-phase alloys from the Mo-Si-Ti system. All of the identified alloys feature: (i) Fine-scale microstructures of the solid solution and one or two silicides originating from eutectic reactions, solid state transformations or combination thereof. (ii) Surprisingly adherent and passivating TiO₂/SiO₂ duplex and/or TiO₂ top layers after oxidation in laboratory air that limit the oxidation progress and the contribution of evaporation at temperatures of 800 up to 1200 °C. (iii) Low densities in conjunction with reasonable creep resistance. A holistic overview is provided about the nature of the TiO₂ which is typically not considered a passivating oxide and the microstructural and chemical requirements of pesting resistance.

Apart from Mo-Si-Ti, further alloys were developed also featuring fine-scaled microstructures of Mo-rich solid solution and silicide phases. Indeed, another alloy from the Mo-Si-Cr system exhibiting pesting resistance was identified but significantly differentiating in the mechanism to suppress MoO₃ formation at 800 °C [7]. The common and differentiating features of the two developments are highlighted to provide further insights into research opportunities.

[1] J.H. Perepezko, M. Krüger, M. Heilmaier, Mo-Silicide Alloys for High-Temperature Structural Applications, Materials Performance and Characterization, 10, 22-145, 2021.
[2] D. Schliephake, A. Kauffmann, X. Cong, C. Gombola, M. Azim, B. Gorr, H.-J. Christ, M. Heilmaier, Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti alloys, Intermetallics, 104, 133-142, 2019.
[3] S. Obert, A. Kauffmann, M. Heilmaier, Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys, Acta Materialia, 184, 132-142, 2020.
[4] S. Obert, A. Kauffmann, S. Seils, S. Schellert, M. Weber, B. Gorr, H.-J. Christ, M. Heilmaier, On the chemical and microstructural requirements for the pesting-resistance of Mo-Si-Ti alloys, Journal of Materials Research and Technology, 9, 8556-8567, 2020.
[5] M. Weber, B. Gorr, H.-J. Christ, S. Obert, A. Kauffmann, M. Heilmaier, Effect of Water Vapor on the Oxidation Behavior of the Eutectic High-Temperature Alloy Mo-20Si-52.8Ti, Advanced Engineering Materials, 22, 2000219, 2020.
[6] S. Obert, A. Kauffmann, S. Seils, T. Boll, S. Kauffmann-Weiss, H. Chen, R. Anton, M. Heilmaier, Microstructural and Chemical Constitution of the Oxide Scale formed on a Pesting-Resistant Mo-Si-Ti Alloy, Corrosion Science, 178, 109081, 2021.
[7] F. Hinrichs, A. Kauffmann, A.S. Tirunilai, D. Schliephake, B. Beichert, G. Winkens, K. Beck, A. Ulrich, M. Galetz, Z. Long, H. Thota, Y. Eggeler, A. Pundt, M. Heilmaier, A novel nitridation- and pesting-resistant Cr-Si-Mo alloy, Corrosion Science, submitted, 2022.

The first generation MoSiBTiC alloy with the nominal composition of 65Mo–5Si–10B–10Ti–10C in at% exhibits excellent high temperature creep resistance and specific strength for application at the temperature up to at least 1300°C. But the oxidation behavior should also be paid attention to since it is a big issue for such Mo-based alloys and could be well affected by the microstructure at the material surface. In this study, gas atomized MoSiBTiC powder was used to produce the compact by spark plasma sintering (SPS) method, and a relative density of almost 100% was achieved. Compared to the conventional arc-melted MoSiBTiC alloy, also prepared in this work, SPS compact showed greatly refined microstructure and a small compositional change during the process. By the microstructure refinement, the oxidation performance of the SPS compact slightly deteriorated at 800°C but improved at 1100 °C. Moreover, the effect of microstructure refinement on the oxidation behavior at a higher temperature such as 1500 °C is not remarkable. This distinction at the three temperatures is considered to be attributed to different reasons. Surface modification using the pack cementation method was also applied to the SPS compact to further improve the high-temperature oxidation resistance. With the development of an approximately 100 µm thick silicide layer on the top surface, the sample exhibited remarkably enhanced oxidation resistance at the temperature up to at least 1300 °C.

The concept of Materials Integration is to computationally link among processing, structure, property, and performance for accelerating materials research and development [1,2] and is comparable to that of Integrated Computational Materials Engineering. To embody the Materials Integration, we have developed a system named MInt (Materials Integration by Network Technology) [3]. In MInt, one can implement any types of prediction models based on computational simulations, theoretical or empirical formulas, and machine learning algorithms as a module. Furthermore, one can connect several modules to design a workflow that comprehensively predicts from processing through microstructure to property and/or performance. MInt executes a workflow by delivering the output data from a module to the input port of the next module and stores all the computational results in order. MInt is a platform to accumulate the materials scientists’ knowledge digitalized as modules and workflows, which are stored not in isolation but in the form of graph network. At this moment, the modules and workflows implemented in MInt are mainly for conventional structural alloys such as steels, aluminum, and Ni-based superalloys, but one can expand its application fields by adding new modules and workflows.

Data-driven approach plays an important role in Materials Integration. Firstly, it is useful to make the prediction modules more accurate; for example, data assimilation technique can adjust model parameters included in the computational simulation or the theoretical or empirical formula to improve predictability. Secondary, it helps to build a comprehensive workflow even if all the mechanisms are not fully understood. For a missing link, machine learning provides a non-linear regression model based on experimental data without physical background. On top of that, certain data-driven methods, such as sparse modeling and feature importance analysis, contribute to finding the physical picture. Lastly, it provides a realistic means of solving the inverse problem, i.e., finding the optimal material and processing that meets the desired performance. Since MInt provides an application programing interface to execute the workflow and extract the computed results, one can apply any types of optimization algorithms including Bayesian optimization, Monte-Carlo tress search and so on. In this talk, I would like to share some examples of where the data-driven approach has worked.

[1] Demura M, Koseki T. Mater Japan. 2019;58(9):489-493. doi:10.2320/materia.58.489 [2] Demura M. Mater Trans. 2021;62(11):1669-1672. doi:10.2320/matertrans.MT-M2021135 [3] Minamoto S, Kadohira T, Ito K, Watanabe M. Mater Trans. 2020;61(11):2067-2071. doi:10.2320/matertrans.MT-MA2020002

The performance of superalloy forgings hinges on the careful design of the thermomechanical history to promote distributions of oft-dependent microstructural features. Establishing predictive process-structure-property (PSP) models to tailor manufacturing routes requires immense cost due to the time-consuming tasks of quantifying statistically significant observations of different predictors and performance metrics across multiple lengthscales [1]. Power analysis indicates that for a simple multivariate linear model of one performance metric, P, dependent on six input process predictors (k = 6) (i.e., P = f(k₁, k₂, …k₆)) with 80% predictive power would require over 120 observations; to predict n performance metrics (P₁, P₂, ...P_n), a statistical study protocol would require 120n observations (ANOVA would require 175n). Measuring a statistical number of observations of predictors and metrics is challenging at both ends of the lengthscale spectra. Nanoscale precipitates, like the distribution of the ��′ and ��′′ precipitates in Inconel 706 have required transmission electron microscopy which does not readily lend itself to transition from qualitative analysis to statistical measures [3]. While conventional mechanical testing, on the other hand, requires relatively large volumes of material for a single observation. In both cases, these predictors and metrics are observed destructively resulting in development efforts that take many years to establish the database necessary for PSP models. Since PSP models benefit from an iterative design paradigm, this motivates the development of high-throughput measurements, both experimental approaches [4, 5] and physics-based predictions [6] if the dramatic design paradigm shift predicted by the Materials Genome Initiative.
In this paper, high-throughput measures of the precipitation distributions in Inconel 706 from experimental observations and physics-based simulations are used to reduce the necessary “physical” observation space necessary to predict PSP models. Heritage data from five years of development work conducted by Howmet Aerospace Forgings were used to demonstrate the method. DEFORM simulations of thermal history were used to supply time-temperature boundary conditions to CALPHAD simulations in ThermoCalc/PRISMA of the resulting ��′ and ��′′ precipitate distributions at discrete locations within thermomechanically processed Inconel 706. Computer vision algorithms were developed to observe features in scanning electron micrographs. These features were used to tailor the CALPHAD interfacial energy and predict features from other processing conditions. In this manner, the number of physical observations of ��′ the ��′′ distributions is reduced by 25x (100’s to 4); and it relies on more readily available experimental and computational approaches allowing for industrial use with limited available technical resources. When combined within the heritage data frame, gradient boost machine learning PSP models were trained following a 4:1 train-split procedure. The predictive power of many models was 80% or higher, estimated by ��² value on the test data.

1. Li S, Kattner UR, Campbell CE (2017) Integrating Mater Manuf Innov 6:229–248. https://doi.org/10.1007/s40192-017-0101-8
2. Jones SR, Carley S, Harrison M (2003) Emerg Med J 20:453. https://doi.org/10.1136/emj.20.5.453
3. Zhang S, Zeng L, Zhao D, et al (2022) Mater Sci Eng A 839:142836. https://doi.org/10.1016/j.msea.2022.142836
4. Senanayake NM, Carter JLW (2020) Integrating Mater Manuf Innov 9:446–458. https://doi.org/10.1007/s40192-020-00195-z
5. Senanayake N, Mukhopadhyay, S, Carter JLW (2020) Superalloys 2020. Seven Springs, PA, p 10
6. Kuehmann CJ, Olson GB (2009) Mater Sci Technol 25:472–478. https://doi.org/10.1179/174328408X371967

Ni-based superalloys are a superior class of structural materials used in aircraft turbines and power plants due to their excellent high-temperature strength, creep resistance, and corrosion resistance. Antiphase boundaries (APBs) are planar defects within the Ni₃Al precipitates that contribute to the strengthening of these superalloys, which motivates a better understanding for how chemical heterogeneity affects the APB energy. The APB energy varies with not only solute chemistry and concentration, but also sublattice site preference in the ordered (L1₂ structure) Ni₃Al precipitates. In order to systematically study these effects and optimize the APB energy in Ni₃Al intermetallics, we leverage high-throughput calculations to generate a large dataset of APB energies and we analyze what these data reveal about influential properties in superalloy design [1].

First, we use a thermodynamic model implemented in PyDII [2] combined with density functional theory (DFT) calculations to predict the site preference of alloying additions to Ni₃Al. We employ more high-throughput DFT calculations to calculate the APB energy for over 100 ternary Ni₃Al-based alloys and we generate a set of physically-motivated descriptors to build machine learning (ML) models for the APB energy from these data. The ML models not only give accurate predictions of the APB energy, but they also provide insight into the correlations between different descriptors and the APB energy, thus demonstrating how our automated, data-driven approach recovers known physics [1]. We discuss how this methodology allows us to intelligently screen for promising superalloy chemistries through validation with commercial superalloys that are far more compositionally complex than our training set.

[1] Enze Chen, et al. “Modeling antiphase boundary energies of Ni3Al-based alloys using automated density functional theory and machine learning,” npj Comput. Mater. 8, 80 (2022).
[2] Hong Ding, et al. “PyDII: A python framework for computing equilibrium intrinsic point defect concentrations and extrinsic solute site preferences in intermetallic compounds,” Comput. Phys. Commun. 193 (2015).

Intermetallic alloys show a dramatic increase in their work hardening rate (WHR) when ordered. For example, L1₂-ordered Ni₃Fe has a ~50% greater WHR than f.c.c. disordered Ni₃Fe. Starting in 1962, antiphase boundary (APB) tubes, which are a defect unique to deformed ordered alloys, were theorized to explain this phenomenon. APB tubes have since been identified via transmission electron microscopy (TEM). Theories abound as to how ABP tubes increase the strengthening of intermetallics, but there is little experimental evidence in favor or against these theories. Understanding the structure-property relationship between APB tubes and strengthening is important as ordered alloys are common strengthening components in structural superalloys, e.g. Ni₃Al-type precipitates in Ni-based superalloys.

In this work, experiments performed on B2-ordered FeAl and L1₂-ordered Ni₃Al show that APB tubes do no impact the WHR of ordered intermetallics. Single crystals of B2-ordered FeAl were deformed and the microhardness was measured with APB tubes present and then after the APB tubes were annihilated (with no additional changes in the deformation microstructure). The hardness did not change, showing that APB tubes do not affect the strengthening of FeAl. In L1₂-ordered Ni₃Al, the exact annihilation temperature of APB tubes was not already known. So, single crystals of L1₂-ordered Ni₃Al were lightly deformed and observed in the TEM. In-situ TEM heating was performed to determine the APB tube annihilation temperature. The annihilation temperature was then compared to the existing literature on the mechanical properties of Ni₃Al and it was determined that APB tubes do not affect the strengthening of Ni₃Al. APB tubes may be more accurately viewed as evidence of the motion of the complex superdislocation interactions that are the source of the high WHR in intermetallics, rather than the source of the strengthening themselves.

Microstructures of the ternary Co-Al-W system consist of two phases, an ordered Co₃(Al,W) phase ( L1₂ crystal structure, usually designated as the γ’ phase) and a face centered (fcc) Co-rich solid solution. There has been a significant research activity in the development of Co-based superalloys where Co₃(Al,W) is the strengthening phase with an initial expectation that high-temperature mechanical properties of Co-based superalloys would outperform those of Ni-based alloy. This was related to the fact that precipitation of the γ’ phase in the γ matrix occurs coherently and forms cuboidal precipitates like in Ni-based superalloys with the advantage that both the melting temperature and the elastic stiffness are higher for Co than for Ni. However, in spite of the expectation, the high-temperature creep strength of Co-based superalloys which were so far developed are modest, only comparable to those of Ni-based superalloys of the first generation. A lower phase stability of γ’ phase, which causes a low high-temperature strength was suggested to be responsible for this disappointing find. In fact, the addition of elements that increase the γ’ phase stability (L1₂-stabilizer) is reported to improve the creep properties of the Co-based alloys. The γ’ high-temperature strength is also expected to be improved in these alloys but nothing is known at present about how a higher high-temperature strength of the γ’ phase is achieved by L1₂-stabilizer additions.
In the present study, we investigate the compression behavior of L1₂-compounds Co₃(Al,W) with Ni and Ni+Ta additions, which are known as L1₂-stabilizers, in a temperature range from room temperature to 1000^oC, in order to elucidate the effects of stability of the L1₂ phase on the mechanical properties. Emphasis is placed on the deformation microstructures of these L1₂ compounds through detailed transmission electron microscopy (TEM) investigations, paying special attention to how the anomalous increase in yield stress (often referred to as ‘yield stress anomaly’ in L1₂ compound research) occurs in these L1₂ compounds.
The result shows that the addition of Ni and Ta increases the high-temperature strength of the γ’ phase. The strength increase is shown to be more significant as the amount of additions of these elements and thereby the stability of the L1₂ phase increases. The reduction of the onset temperature of yield stress anomaly (YSA-onset) due to the increased complex stacking fault (CSF) energy and the increased anti-phase boundary (APB) energy is considered to account for the strength increase at high temperatures. The increased strength of the L1₂ phase due to a higher phase stability thus partly accounts for the improved creep strength of Co-based superalloys upon alloying with Ni and Ta.

High- and medium-entropy alloys (HEAs/MEAs) have attracted extensive interest for their extraordinary and promising mechanical performance, such as excellent combination of strength and ductility that increase at cryogenic temperatures and exceptional fracture toughness. Systematic work has revealed that the equiatomic Cr-Co-Ni MEA exhibits remarkably higher strength and ductility than the Cr-Mn-Fe-Co-Ni and other subsets. The improved strengths have motivated intensive detailed research to explore their micro-scale origins via experimental approaches and theoretical calculations. These results have implied that the type and bonding of atoms, instead of number of elements, are more important for solid solution hardening in HEAs. The high strength is believed to be related to the severe and ubiquitous crystal lattice distortion, such as the mean-square atomic distortion (MSAD) model. The excellent tensile ductility of the Cr-Co-Ni MEA is generally attributed to the high propensity of deformation twinning resulting from its low stacking fault energy. Thin layers of the hexagonal close packed (HCP) structure are formed in association with deformation twins in the Cr-Co-Ni MEA. This indicates that not only TWIP (twinning induced plasticity) but also TRIP (transformation induced plasticity) are responsible for the excellent tensile ductility of the Cr-Co-Ni MEA as in the case of high-Mn steels. On the other hand, the local chemical inhomogeneity derived from preferred bonding between specific atoms, usually termed short-range ordering (SRO), is widely believed to be another reason for its peculiar properties. The effect of SRO on the atomic-scale structures have been investigated extensively in theoretical calculations. In experimental work, a few studies have indicated that the formation of SRO can act as additional obstacle to dislocation motion, while no measurable effect of SRO on strength are claimed in other studies. However, it is well known that in polycrystals grain size and boundaries induce scattered results of mechanical response. Therefore, single crystals are of prime important to investigate the critical resolved shear stress (CRSS), stacking fault energy (SFE) and twinning shear stress as well as the possible effects of SRO on these parameters. In the present study, we investigate the plastic deformation behavior of bulk single crystals of the equiatomic Cr-Co-Ni after various heat treatment, in order to experimentally deduce materials parameters (such as CRSS for slip, twinning shear stress as well as SFE) and the contribution of SRO in Cr-Co-Ni MEA. Single crystals of Cr-Co-Ni were grown from a polycrystalline rod with an optical floating-zone furnace. The single crystals were annealed at 1473 K for 168 hours as the beginning materials. Then, the specimens were further annealed at 573-1373 K for various duration to introduce SRO. Tensile and Compressive tests were conducted at room and liquid nitrogen temperatures. A careful tilting observation along various zones were conducted to get the diffraction information by transmission electron microscopy, and the results were compared with those obtained by theoretical simulation for atomic structures with various degrees of SRO. Synchrotron X-ray diffraction was also applied to characterize the atomic-scale structures.

Multi-component alloys with a single-phase microstructure (generally known as medium/high entropy alloys) have recently emerged as a new class of alloys. In particular, body-centered cubic (bcc) solid solution alloys exhibit superior strength retention up to high temperatures but low toughness at ambient temperature. This issue is closely related to the ductility control of conventional ferritic stainless steels with FeCrNi solid solutions at various temperatures. To fundamentally understand the roles of solute elements in plastic deformation of multi-component bcc FeCrNi alloys, we investigated the strain rate dependence of flow stress in single-crystals of bcc Fe-Cr-Ni alloys with different solute Cr and Ni contents by micropillar compression tests in terms of thermal activation of dislocation motion.
Fe-Cr binary alloys (with 18% and 40% Cr) and Fe-18%Cr-Ni ternary alloys (with 1% or 2% Ni) were used in the present study. These alloy ingots were hot-rolled to approximately 4 mm thickness and solution-treated at 1300 °C for 1800 s, followed by water quenching. The solution-treated alloy samples exhibited fully recrystallized microstructures with the bcc single-phase. High-index orientations for the favorable activation of a single slip system were selected for the compression directions (corresponding to the crystallographic directions normal to the sample surface). We used a Focused Ion Beam (FIB) to fabricate cylindrical micropillar specimens with different diameters (d) of approximately 2 and 5 µm on the sample surface of the solution-treated alloys. We performed the micropillar compression tests at a wide range of initial strain rates (~10^-5s^-1 to ~10^-2s^-1) using a load-controlled mode at ambient temperature. The measured values of flow curves were used to calculate the strain rate sensitivity (m). Afterward, the slip traces of the compressed micropillars were observed to identify the activated slip system. Transmission electron microscope (TEM) to evaluate the dislocation morphologies before and after the micropillar compression test.
In the Fe-18%Cr binary alloy, single-crystal micropillars with d = 2 µm exhibited a relatively high strain-rate sensitivity (m = 0.12) of stress required for the slip initiation. The m value became lower (0.04) in the large-sized micropillars with d = 5 µm. The m value was equivalent to one of pure iron. The added solute Ni element in bcc solid solutions reduced the m value in both specimen sizes. The Fe-18%Cr-2%Ni ternary alloy exhibited low m values below 0.01. All compressed micropillars showed the predominant activation of the [111] (12-3) single slip system. These results suggested the sluggish kinetics of kink-pair nucleation and motion of screw dislocations interacted with solute Ni atoms. The Fe-40%Cr binary alloy showed higher yield strength and much lower m values than the Fe-18%Cr alloy, regardless of the specimen size. The compressed micropillars of the Fe-40%Cr alloy micropillars exhibited the slip activation on multi-planes (from (110) to (11-2)). These results suggest the interaction of dislocations on multi-planes might be enhanced by high-concentrated Cr solute element in bcc solid solutions. The roles of solute Ni and Cr elements in the thermal activation process of dislocation motion will be discussed in terms of the activation volume of plastic deformation.

As one of the major approaches in harvesting and converting solar energy, concentrated solar power (CSP) offers advantages in cost-effective energy storage and the capability to provide electricity when the sun is absent. Following Carnot’s theorem, the CSP systems prefer higher operation temperature for higher conversion efficiency. However, currently-employed tubing materials cannot work at temperatures exceeding 873 K due to mechanical, corrosion, and oxidation limitations. To solve this challenge, high entropy alloys (HEAs) are being considered due to their novel mechanical properties especially at high temperatures. In this study, we present microstructural characterization, tensile strength, and creep resistance of FeMnNiAlCr high entropy alloys, with different Mn content (x = 32.9 – 42 at. %) at temperatures of 873-1023 K. The effect of recrystallization, which is achieved by cold-rolling and annealing, on the microstructures and mechanical properties were also studied. Both as-cast and recrystallized alloys exhibit high strength, good creep resistance, and long-term stability at high temperature. In-situ deformation experiments were conducted in the transmission electron microscope to examine the deformation mechanisms. The results suggest the potential of FeMnNiAlCr HEAs as a cost-effective candidate for CSP systems.

The thermodynamic and kinetic features of the Al-Ce alloy system enable the intermetallic volume fraction of intermetallic particles to be increased significantly, while remaining processable by conventional and advanced manufacturing processes. Here, we present recent work to establish Al-Ce alloys for casting, wrought, and additively manufactured alloys that leverage high intermetallic fraction for enhanced properties. Thermodynamic features are combined with thermophysical and structural properties to fashion alloys with property sets tailored to intended applications. Furthermore additional alloying elements can induce complex changes in phase distribution and strength, both of which are also a function of solidification rate. Promising Al-Ce-X-Y and sometimes -Z candidates for particular applications will also be discussed.
This research was sponsored by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy and Advanced Manufacturing Office. Work performed at LLNL under contract DE-AC52-07NA27344

Most of the geometrically close-packed (GCP) compounds are A₃B type, whereas a very few A₂B type GCP compound exists. The Ni₂Cr (oP6) is one of them. According to the Ni-Cr phase diagram, this phase is a Kurnakov-type compound and is stable up to 867 K. Recently, our systematic phase diagram study on Ni-Cr-Mo ternary system experimentally reveals that a single-phase region of Ni₂(Cr, Mo) with oP6 crystal structure exists as an island with compositions around Ni-15Cr-15Mo (at.%), up to about 1073 K, approximately 200 K higher than the binary Ni₂Cr compound. The reason why Mo in solution in Ni₂Cr phase stabilizes the phase is bonding energy between Cr and Mo in Cr site should be negative, because thermodynamical calculation shows that modifying ternary-interaction parameter L_Cr,Mo:Ni to negative is the most strongly contributes to reproduce the experimental phase diagram in other thermodynamical parameters, e.g. end-members and interaction parameters among binary and ternary elements. This result is very attractive from both fundamental and engineering viewpoints. The Ni₂Cr phase could be an alternative strengthener to Ni₃Al-γ' phase in Ni-based alloys, if we can further increase the phase stability. In this study, then, in order to verify the mechanism of the increase in phase stability of Ni₂Cr compound by Mo addition, effect of 5^th and 6^th group elements on the phase equilibria at elevated temperatures were systematically examined. Thermodynamic calculations to evaluate the phase stability of this phase is also conducted, by taking the interaction parameters among the three elements into account. Two series of alloys were prepared: series I with nominal compositions of Ni-15Cr-15(Mo, M) (at.%) by replacing Mo with M by every 5 at.% and series II with those of Ni-15Cr-15Mo-5M by addition of 5at.%M to Ni-15Cr-15Mo. These alloys were prepared by arc melting in argon atmosphere with non-consumable tungsten electrode as 35 g button ingots. These ingots were firstly cold rolled by 40% in height, and solution treated at 1473 K for 24 h, followed by a water quench, in order to eliminate the micro segregation. Then, they were equilibrated at 1173-973 K for up to 1000 h. It was found that, in any alloys containing M, the observed Ni₂(Cr, Mo) single phase region in the ternary system was reduced, even in the case of V where Ni₂V is also the A₂B type GCP compound with the oP6 structures. Instead, the stable A₃B-type GCP phases of Ni₃V-γ" (D0₂₂), Ni₃Nb-δ (D0_a) and Ni₃Ta-δ (D0_a) appears in the quaternary systems, indicating that the M elements apparently destabilize the oP6 phase with respect to the A₃B-type GCP phases. The A₂B type GCP phase has B-B bonding unlike A₃B type, so the total bonding energy should always be smaller for A₂B than A₃B (Ω_A-B < 0). The thermodynamically stable existence of the Ni₂(Cr, Mo) phase, despite the thermodynamic existence of the Ni₃Mo-δ phase in the binary system, must be caused by not only the interaction among ternary elements but also size effects. Based on these results, together with the thermodynamic calculations on the observed A₂B and A₃B type GCP phases, elements which do not form A₃B type GCP phase would possibly be effective in increasing the stability of Ni₂Cr. The size effect as well as atomic scale considerations on the difference in phase stability between A₃B and A₂B type GCP phases will also be touched in detail. A part of this research was supported by JST-Mirai Program Grant Number JPMJMI21E7, Japan.

Dislocations are the lattice defects that are mainly responsible for plastic deformation of crystalline materials through their glide motion. The glide motion of dislocations on a slip system generally takes place on a crystallographic plane between two adjacent atomic layers in the case of most metallic materials with relatively simple crystal structures such as face-centered cubic (FCC) and body-centered cubic (BCC) metals. However, the dislocation glide process in materials with complex crystal structures has not been fully clarified yet, because it can be very complicated often involving multiple atomic planes during dislocation glide. One of the dislocation glide models predicted to operate in complex materials is the zonal dislocation model, in which non-uniform atom shuffling is predicted to occur in multiple atomic planes called shear zone. The zonal dislocation model was introduced by M.L. Kronberg to describe possible glide motion of dislocations of the {110}<001> slip in β-U with the tetragonal Ab structure (identical to D8_b structure of σ-FeCr). However, it has never been experimentally verified to occur. This is because β-U is one of the typical radioisotopes, which prevents from further detailed experimental study of its dislocations. Another reason is that the brittleness of the sigma-phase arising from the complex crystal structure has made impossible for plastic flow to occur by dislocation glide. Recently, we have systematically investigated fundamental deformation behavior of hard and brittle materials by utilizing micropillar compression method and have successfully identified the activation of {110}<001> slip in σ-FeCr at room temperature. In the present study, we investigated dislocation core structures of {110}<001> slip in σ-FeCr by atomic-resolution scanning transmission electron microscopy and confirmed that <001> dislocations glide on {110} are of the zonal-type but the thickness of the shear zone was different from that predicted by Kronberg. New atomic shuffling model in the shear zone was proposed based on the experimentally observed dislocation core structure.

In recent, laser powder bed fusion (L-PBF) has emerged as one of the most representative metal additive manufacturing techniques capable of producing metallic components by using a scanning laser beam to selectively melt consecutive bedded-powder layers. The L-PBF process enables not only the manufacturing of complex geometrical forms but also the fabrication of Al alloys with superior mechanical properties. The L-PBF manufactured Al–2.5Fe (mass%) binary alloy exhibits a high tensile strength of approximately 300 MPa due to the fine morphology of Al₆Fe metastable phase (orthorhombic, oC24). The strength level slightly decreases after long-term exposure at elevated temperatures, indicating the feasibility of employing lightweight heat-resistant materials. However, increasing Fe alloy content significantly reduces L-PBF processability due to the formation of coarsened Al₁₃Fe₄ stable phase. It is therefore required to stabilize the fine Al₆Fe phase by third alloy elements for further strengthening without a loss of L-PBF processability for Al–Fe alloys. The present study was undertaken to investigate the effect of selected third elements (Mn or Cu) on the microstructure and mechanical properties of the Al–Fe binary alloy additive manufactured by the L-PBF process.
In this study, attempts of L-PBF processing using the gas-atomized Al–2.5Fe–2Mn or Al–2.5Fe–2Cu (mass%) ternary alloy powder with an average particle size of about 20 mm were made. Rectangular alloy samples were fabricated using a metal AM ProX DMP 200 machine (3D Systems, USA) under a wide range of laser scan speed (0.6 ~ 1.4 m/s) and laser power (102 ~ 204 W). The applied laser-scanning hatch distance, powder-bed thickness, and beam focus size were 0.1 mm, 0.03 mm, and ~0.1 mm, respectively. The results of measuring the sample density provided the optimum laser parameter sets for the manufacturing of both alloy samples with high relative densities above 99 %, indicating high L-PBF processability for the Mn or Cu added Al–Fe alloy powders. The L-PBF manufactured samples exhibited microstructure consisting of a number of melt pools in which regions locally melted and rapidly solidified due to scanning laser irradiation. The added Mn element was partitioned into the refined Al₆Fe phase, resulting in the formation of Al₆(Fe, Mn) phase with an orthorhombic structure (oC24). A certain amount of solute Mn (approximately 0.8 mass%) was detected in the a-Al matrix. Numerous nanoscale particles of Al₆(Fe, Mn) phase were homogeneously dispersed in the α-Al matrix, whereas relatively coarsened Al₆(Fe, Mn) phases with a cellular morphology appeared localized along melt-pool boundaries. Similar microstructures containing the Al₂₃CuFe₄ phase (orthorhombic, oC24) were observed in the L-PBF processed Al–2.5Fe–2Cu alloy samples. These results indicate the added third element could stabilize the refined Al₆Fe phase formed in rapid solidification by the L-PBF process. The L-PBF processed Al–2.5Fe–2Mn and Al–2.5Fe–2Cu alloys exhibited higher strength of approximately 360 MPa than the Al–2.5Fe binary alloy and an adequate tensile ductility of approximately 10 % in total elongation at ambient temperature. The high-temperature strength of the L-PBF manufactured ternary alloys will be presented.

Additive manufacturing is one of highly efficient processes to fabricate TiAl low pressure turbine blade. In the present study, large triangular prisms and small cylindrical rods of Ti-48Al-2Cr-2Nb alloys were prepared by electron beam powder bed fusion (EB-PBF) to examine the effect of product shape on the microstructure and mechanical properties. The microstructure after EB-PBF depends on the process parameter such as beam current and scanning speed. Under an optimum condition, peculiar banded structure composed of duplex-like structure and equiaxed γ grains (γ band). Such microstructure is homogeneously formed in cylindrical rods 10 mm in diameter. On the other hand, in the case of the triangular prisms, the microstructure vary from area to area. In the thick area, the banded structure is formed, while typical lamellar structure composed of the α₂ and γ phases can be seen in the thin area. It is also noted that hardness distribution was also inhomogenous depending on the microstructure. In EB-PBF process, an electron beam moves showing a snake pattern. In the thin area, the beam moves forth and back quickly, resulting in an increase in temperature. This leads to the formation of the lamellar structure. In order to get homogeneous microstructure throughout the triangular prisms, the process parameter is changed from area to area. As a result, homogeneous microstructure consisting of the banded structure was formed and the hardness distribution of the triangular prisms also became homogeneous.

In recent years, TiAl alloys containing the β phase are expected to expand the application scope of the alloys due to their superior mechanical properties at high temperatures. Electron beam powder bed fusion (EB-PBF) that is one of the additive manufacturing processes for metallic materials has attracted much attention as a fabrication process of TiAl alloy parts for aerospace applications with complex geometries. Furthermore, it is possible to obtain unique microstructures by repeated and rapid fusion and solidification during the EB-PBF process, resulting in excellent mechanical properties. In the present study, relationships between microstructure and mechanical properties of β-containing TiAl alloys prepared by the EB-PBF process were investigated focusing on the influence of the input energy density. Specimens fabricated at low energy densities contain ultrafine lamellar structure composed of the α₂ and γ phases, together with the β/γ cells discontinuously precipitated at the lamellar grain boundary. The ultrafine lamellar grains and the β/γ cells are effective in increasing strength and ductility of the alloys, respectively. In addition, we developed new heat treatment processes to control the morphology of the microstructure. The alloys subjected to optimum heat treatments exhibit higher strength and larger ductility at 1023 K, compared with the cast alloys.

This work was supported by Council for Science, Technology and Innovation(CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Materials integration” for revolutionary design system of structural materials” (Funding agency: JST).

Titanium aluminide-based alloys have found use in the aerospace industry in applications such as low pressure turbine blades due to their light weight and high temperature properties. It is necessary to develop a deep understanding of the structure-property correlations of these materials in order to understand how to engineer their mechanical properties. To this end, we have used models at multiple length scales to explore the relationship between temperature, microstructure, and mechanical properties. At the atomic scale, we performed a critical evaluation of existing empirical potentials and found that potentials based on the Embedded Atom Method and the Modified Embedded Atom method were capable of replicating most (but not all) mechanical properties as compared to DFT and experiment. For example, all existing models tend to overestimate the thermal expansion coefficients in the α2-Ti3Al structure. At the lamellar structure, we developed a crystal plasticity model that explicitly accounts for the alternating lamellae of γ-TiAl and α2-Ti3Al. By phenomenologically including experimentally observed trends such as the temperature-dependent yield stress anomaly and deviations from Hall-Petch behavior, we were able to establish relationships between orientation dependent deformation, structural length scales such as lamellae thickness, and stress state.

Mg-based LPSO phase is known to contribute to increase in both of strength and ductility of Mg alloys. As a deformation mode in it, kink-band formation is recently focused. A kink band is a deformation band formed by the explosive generation of dislocations in a localized region and their subsequent alignment along a direction normal to the slip plane. However, details of its formation criteria have not yet been clarified. According to the study on the LPSO phase, its unique crystal structure which is constructed by the alternative stacking of soft and hard layers, called mille-feuille structure, are supposed as plausible factors to govern the formation of deformation kink bands.
To confirm these assumptions, we have examined the deformation behavior of several directionally solidified Mg-based and Al-based two-phase eutectic alloys with lamellar microstructure as a model material, such as Mg/Mg₁₇Al₁₂, Mg/Mg₂Yb and Al/Al₂Cu. Consequently, the formation of kink-bands was confirmed in all the alloys, as expected. The details of the deformation microstructure and accompanying deformation behavior will be discussed in the presentation.

Powder bed fusion (PBF) is a category of additive manufacturing (AM) applied to the fabrication of metal parts with various materials, including superalloys. Solidification by PBF is characterized by a high cooling rate of around 1 million K/s. Recent studies have revealed the very fine submicron-sized celluller structures associated with solute segregations between the cells in Ni-based superalloys fabricated by the PBF process. Such segregation is expected to affect the precipitation of the γ’- phase. In this study, we utilized phase-field simulation to investigate the precipitation of γ’-phase from the solute segregation formed by rapid solidification in PBF. The multiphase-field method was applied to the simulation of microstructural evolution in directional solidification under various solidification conditions focusing on solute segregation. A composition consisting of five major elements of Inconel738LC alloy was selected as the model. Initially, cellular structure with intercellular micro-segregation was generated by simulating directional solidification under a condition with a cooling rate of 10⁶ K/s, solidification rate of 10^-1 m/s, and temperature gradient of 10⁷ K/m, which is a typical solidification condition in PBF process. γ’ forming elements, (i.e. Al and Ti) segregated at the intercellular region. The growth of γ’ precipitates and the relaxation of the segregation occurred simultaneously, and there was no significant influence of the segregation on the precipitates in the subsequent aging at relatively high temperature. In contrast, the precipitation occurred preferentially at the intercellular segregation in the case of aging at relatively low temperature. This work was supported by Council for Science, Technology and Innovation, Cross-ministerial Strategic Innovation Promotion Program, “Materials Integration for revolutionary design system of structural materials” (Funding agency: JST) .

Revealing the possibility of transition among stress-induced martensitic phase during continuous cold deformation in metastable β titanium alloy, Ti-10V-2Fe-3Al (Ti-1023) is the main objective of this study. In general, martensitic transformations induced either by quenching or load application have a significant influence on the mechanical properties of metastable β-Ti alloys. It is known that martensitic transformation in metastable β-Ti is highly dependent on the alloy content and deformation. With the increase in β stabilizing elements, HCP-α' martensite transforms to orthorhombic-α" martensite upon quenching from a single β phase. However, deformation-dependent martensitic transformation involves the role of lattice strain and atomic shuffle. β to α" and β to α' transformation differ in terms of atomic shuffle displacement, wherein the former requires less displacement than the latter [4]. Therefore, β to α" transformation can be considered crystallographically incomplete as compared to β to α' transformation. At sufficiently high strain, it can be assumed that the formation of the α′′ is exhausted, and other stress-induced deformation mechanism may also activate, resulting in the possibility of α" to α' transformation. In the present investigation, Ti-1023 alloy was solution treated in the single β phase-field followed by water quenched. Solution-treated samples were unidirectionally cold rolled to 5%, 15%, 25%, 35% and 43% thickness reduction levels. The resulting microstructures are extensively analyzed by XRD and TEM analysis. XRD and TEM confirm the transformation of β to α′′ to α′. Lattice parameters ratios, as well as volumetric strain (%) calculations, provide indications about the transitions of α′′ to α′. Furthermore, the TEM results confirm the dominant presence of α′′ martensites at low cold rolling percentage compared to α′ martensites observed at high cold rolling percentage. It has been observed that the driving force for martensitic transformation is the combined effect of lattice strain induced by cold rolling and the different alloying elements influencing the atomic shuffle displacements. A correlation of the lattice strain and alloying elements with atomic shuffle displacements on the transformation of β to α′′ to α′ in Ti-1023 has been established.

[Introduction] The mechanical or magnetic properties of the ordered phases are closely related to the antiphase domain (APD) structures, such as the APD size and the characteristics of antiphase domain boundaries (APBs). For example, it has been reported that pseudoelasticity is caused in Fe₃Al by the interaction between APBs and dislocations ^[1]. Phase-field (PF) simulation has been proved to be an effective way of studying the formation and time-evolution of APD for ordered phases. Koizumi et al. ^[2] studied the ordering mobility in nearly stoichiometric Fe₃Al at constant temperatures by the PF method. The objective of this study is to develop PF simulations applicable to temperature variations for the APD growth in D0₃ ordered Fe₃Al alloy, aiming at the optimization of aging conditions for ordering in the future.
[Methods] PF simulations of ordering in Fe-28at.%Al alloy were performed for the cases of constant cooling rate and isothermal aging. The dimension of the simulation domain area was a cube with a size of 30 nm. Thermodynamic equilibrium at 873 K was assumed as the initial state for all simulations. The evolution of D0₃ APD during cooling to 673 K at rates of 0.1~1000 K/s and that during aging at constant temperatures of 673 K, 723 K, and 773 K for 1 h after cooling at 100 K/s were examined and compared with experimental results.
[Results] The simulation results for the case of constant cooling rate showed that the growth of the D0₃ APD takes place before the detectable long-range order (LRO) appears. Moreover, slower cooling rates resulted in larger APD sizes. On the other hand, although the ultimate LRO obtained by isothermal aging at a lower temperature is higher than that at a higher temperature, D0₃-LRO exhibited a significant increase earlier in aging at higher temperatures. Besides, the growth of D0₃ APD also begins earlier when aging at higher temperatures, which is due to the larger ordering mobility at the higher temperature. In conclusion, isothermal aging at higher temperatures after cooling at a higher rate is considered suitable for the fast growth of D0₃ APD in the Fe₃Al.
[References]
[1] H. Yasuda et al.: “Effect of ordering process on giant pseudoelasticity in Fe₃Al single crystals”, Acta Mater. 51 (2003) 5101–5112.
[2] Y. Koizumi et al.: “Solute and vacancy segregation to a/4<111> and a/2<100> antiphase domain boundaries in Fe₃Al”, Acta Mater. 56 (2008) 5861–5874.

Topological semimetals with multiple topological phases have gained renewed interest from the materials research community as new materials started to appear with tunable or concurrent quasiparticle excitations of different types.
Here, in this work, using Density Functional Theory (DFT) we studied a new topological semimetal SmAlGe, which has similar lattice structure to LaPtSi, a newly identified candidate [1] with multiple topological phases. The magnetic properties emerging from the 4f elections of Samarium in SmAlGe grant further scope to tune the topological states magnetically. First, we find that the material SmAlGe is thermodynamically more stable in the antiferromagnetic phase. All the further calculations are done on this ground state antiferromagnetic phase. Second, we analyse and report the splitting of 4f-orbital in the 12-fold coordination of Sm in presence of spin-orbit coupling. Finally, we study the topological properties of the material. Band-structure shown in the figure 1(b) reveals multiple Lorentz symmetry violating (Type-II) Weyl nodes very close to the fermi energy. Further, we studied the exotic surface states using a Wannier-based tight-binding Hamiltonian.
[1] Shi, Xianbiao, et al. "Topological states in the noncentrosymmetric superconductors LaPtSi and LaPtGe." Physical Review B 104.24 (2021): 245129.