Symposium F.SF07—Processing Structure–Property Relationship of Advanced Intermetallic-Based Alloys for Structural and Functional Applications

Polytypism is a special case of polymorphism when the two polymorphs differ only in the stacking of identical two-dimensional sheets or layers. The polytypes are characterized by a stacking sequence with a given repeating unit along a directional axis (c-axis) and are theoretically possible to have endless permutations of the sequences. Among these polytypes, the crystalline systems composed of close-packed (CP) layers have especially attracted attention on their fundamental and technological properties for many years. While these polytypes are often experimentally observed in wide-bandgap semiconductors, SiC systems are particularly known to show several hundred polytypes [1]. Since some SiC polytypes located around the ground state are energetically degenerated with about ΔT = 2K [1], it is difficult to control the phase stability during the growth of single crystals with the desired stacking polymorphism. For this reason, a variety of physical perspectives are being investigated in order to establish better single crystal growing techniques [2, 3]. In addition, the long period stacking ordered (LPSO) Mg alloys with light weight, high specific strength, and high heat resistance are also recently drawing attention as a metallic system with similar polytypism to that in SiC [4, 5].

We have proposed a computational method coupled with three theoretical tools (PGA: polytype generation algorithm; FPC-DFT: first-principles calculations based on the density functional theory; and ANNNI: axial next-nearest-neighbor Ising model), which can make us possible to efficiently investigate the structural energetics for diverse nonequivalent polytypes [1]. In our recent study [5], we have systematically evaluated the static energetics of metallic polytypes for a wide variety of elements based on the same computational method proposed by the authors [1]. For all these elemental systems, the atomistic geometry, energetics, and electronic structure for all polytypes with up to the periodic stacking length of L=13 (165 kinds of polytypes in total) have been carefully calculated based on the DFT within the GGA [5]. Using the ANNNI model extracted from the GGA calculations, we have shown from the perspective of static structural energetics that the interlayer interaction among CP layers is one of the important factors for the polytype formation [5].

Not only the static energetics but also the dynamical aspect generally affects the polytype selection during the crystal growth processes [2, 3]. It is, therefore, important to elucidate the polytype formation mechanism from the viewpoint of dynamics associated. Since the molecular dynamics (MD) simulations have become a powerful computational tool to understand and predict the structural, thermodynamic and dynamic properties of materials, the direct MD observation of dynamical aspects for the polytype growth is expected to greatly advance our understanding on the polytype selection. While the state-of-the-art potentials are often fitted to many databases including a large set of atomic configurations calculated by FPC-DFT [6], we have found that the potential models proposed so far cannot describe the accurate polytypes energetics. In the present study, we are, therefore, developing an Embedded Atom Method (EAM) type potential [6] which can be used for the dynamical simulations of metallic polytype structures focusing on the lanthanum (La) system. The basic properties and transferability of the EAM type potential model constructed in the present work will be discussed.

[1] K. Moriguchi, et al., J. Mater. Res. 28, 7 (2013).
[2] K. Kusunoki, Doctoral thesis: Nagoya University, 10725 (2014).
[3] K. Moriguchi et al., NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT 117, 56 (2017).
[4] E. Abe et al., Phil. Mag. Lett, 91, 690 (2011).
[5] T. Miyakawa and K. Moriguchi, 236th ECS Meeting Abstracts, Vol. MA2019-02, 2417 (2019).
[6] K. Moriguchi and M. Igarashi, Phys Rev B 74, 024111 (2006) and references therein.

Hot dipped Zn-Al alloy coated steel is widely used in the field of automobile. It is well known that the coated layer is mainly composed of the Fe₂Al₅ intermetallic compound. This compound has been considered to be the orthorhombic η phase (space group Cmcm). On the other hand, our group has recently found that the layer phase often crystallizes into highly ordered atomic arrangement most likely termed as the η^’’’ phase even during short-term dipping. Besides, recent researches address there exists several ordered phases similar to the η^’’’ phase with the framework structure of η phase^[1]. According to Burkhardt’s study^[2], the η phase is composed of a full occupied framework structure (4 Fe and 8 Al atoms) and partially occupied chains along the c-axis (6 Al sites). The c-axis chain is considered to allow chemical tenability and various kinds of atomic orderings; the formation of various kinds of ordered phases is considered to be responsible for how Fe and Al atoms order in the c-axis chain sites. However, the crystal structures of some of these ordered phases are still controversial and phase equilibrium among them are not consolidated.
In this study, we have examined the phase equilibrium among these phases and refined their crystal structures. Experiments are carried out with scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, transmission electron microscope, scanning transmission electron microscope and synchrotron X-ray diffraction. There are some invariant reactions : the η phase is found to dissolve into the η^’’’ and η^m phases at around 350 °C while the η phase has been thought to be stable at low temperature. Some of the other phases are found to have highly ordered long-period structures with periodic antiphase boundaries.

Reference: [1] H. Becker et al., Intermetallics, 93, (2018) 251-262.
[2] U. Burkhardt et al., Acta Cryst, B50, (1994) 313-316.

The local inhomogeneity such as defect, surface, interface, and nanostructure often assumes an important role for developing sophisticated materials since the functionalities of materials usually originate from the spatial inhomogeneity in the associated systems. In this work, among the many phenomena related to inhomogeneity in materials, we will report our computational research on the phenomenon called "Polytypism". Polytypism is a special case of polymorphism where two polymorphs differ only in the stacking sequences of the same two-dimensional sheets or layers.

Many crystalline compounds can be considered to be composed of one or more structural units. When these units can be stacked in different ways to form stable or metastable phases, the resulting phases are known as polytypes. Among the polytypes for these compounds, the silicon carbide (SiC) systems have been the most attractive and motivated tremendous amounts of theoretical and experimental investigations on their fundamental and technological properties for many years. Many SiC polytypes located around the ground state have been found to be energetically degenerated with energy of about ΔT = 2K [1]. It is, therefore, very difficult to control the phase stability during the growth of a single crystal in order to obtain the desired stacking polytype in SiC systems. For this reason, a variety of physical perspectives are being investigated in order to establish better single crystal growing techniques [2]. In addition, the long period stacking ordered (LPSO) Mg alloys with light weight, high specific strength, and high heat resistance are also recently drawing attention as a metallic system with similar polytypism to that in SiC [3]. For both systems, the physical mechanism behind polytype selection is not yet fully understood and remains challenging issues to be solved.

In this situation, we have proposed a computational method coupled with three theoretical tools (PGA: polytype generation algorithm; FPC-DFT: first-principles calculations based on the density functional theory; and ANNNI: axial next-nearest-neighbor Ising model), which can make us possible to efficiently investigate the structural energetics for diverse nonequivalent close-packed(CP) polytypes [1, 4]. In the present work, the static energetics of a wide variety polytypes for 17 kinds of metallic elements (M=Be, Mg, Sc, Y, Co, Zn, Cu, Ag, Au, Ni, Pd, Pt, Al, Ca, Sr, Ba, and La) is systematically evaluated based on the same computational method proposed by the authors [1, 4]. For all these elemental systems, the atomistic geometry, energetics, and electronic structure for all polytypes with up to the periodic stacking length of L=13 (165 kinds of polytypes in total) have been carefully calculated based on the DFT within the GGA. The equilibrium theories based on the ANNNI model that is well known in the field of statistical mechanics have played an efficient role in the study of the origins of polytypism [5, 6]. In this work, the ANNNI model including interactions up to the third-nearest neighbor layer with the four-spin term [6] is adopted for the systems considered. Using the ANNNI model extracted from the GGA calculations, we will describe the inter-layer interactions, the phase diagrams of ANNNI model with interactions up to third-nearest neighbor, and the stability of stacking faults for each element. The relation to polytypism in the metallic systems considered will be also discussed.

[1] K. Moriguchi, et al., J. Mater. Res. 28, 7 (2013).
[2] K. Moriguchi et al., NIPPON STEEL & SUMITOMO METAL TECHNICAL REPORT 117, 56 (2017).
[3] E. Abe et al., Phil. Mag. Lett, 91, 690 (2011).
[4] T. Miyakawa and K. Moriguchi, 236th ECS Meeting Abstracts, Vol. MA2019-02, 2417 (2019).
[5] E. Rodriguez-Horta, E. Estevez-Rams, R. Lora-Serrano and R. Neder, Acta Cryst. A73, 377 (2017) and references therein.
[6] J. J. A. Shaw and V. Heine, J. Phys.: Condens. Matter 2, 4351 (1990).

Mo–Si–B-based alloys have been developed as ultrahigh temperature material candidates for high-pressure turbine blades. Recently, Hasemann et al. reported that a Mo–Si–B alloy with Mo_ss + T₂ + Mo₃Si eutectic prepared by directional solidification (DS) has substantially improved creep resistance compared with a powder metallurgy (PM) processed alloy. It was found that a homogenous microstructure with preferentially orientated grains through DS can ameliorate mechanical performance, however, the effect of DS on the microstructure evolution of Mo–Si–B alloy is still not clear. In this study, phase constitution and microstructure of as-cast and directionally solidified near-eutectic Mo–Si–B alloys are investigated to clarify the microstructure evolution caused by DS.
Alloys with the nominal composition of Mo–17Si–7B (at.%) corresponding to the Mo_ss + T₂ + Mo₃Si eutectic point were prepared by arc-melting (as-cast alloy) and crucible-free zone melting (directionally solidified alloy) in an Ar atmosphere. The microstructure of the as-cast alloy consisted of primary Mo_ss dendrites and two eutectics of Mo₃Si + T₂ and Mo_ss + Mo₃Si + T₂ in cellular or dendritic growth, while the directionally solidified alloy was almost composed of a uniform and well-aligned Mo_ss + Mo₃Si + T₂ eutectic phase in cellular growth. The electron backscatter diffraction (EBSD) results showed that in the as-cast alloy only <100> orientation of the T₂ phase preferentially grew along the solidification direction while the other Mo_ss and Mo₃Si phases had no apparent orientation. However, the <001> growth direction of Mo₃Si phase parallel to the solidification direction became much stronger by the directional solidification. The coupled growth of <001>Mo_ss//<100>T₂//<001>Mo₃Si in the directionally solidified alloy presumably had an effect on the mechanical properties because of the texture. No habit relationship between the phases was found in both alloys possibly because the lattice parameters of constituent phases are greatly different and the system cannot minimize interfacial energy by atomic arrangement.

The self-accommodation microstructure, which is the martensitic microstructure of the shape memory alloy, is constructed by a plate-like martensite variant. Recently, formation process of the self-accommodation microstructure during the martensitic transformation have been reported in the several shape memory alloy systems. In TiNi shape memory alloy, specific variant pairs including {11−1} type-I twinning interface are frequently observed in the initial stage of martensitic transformation. In this study, we refer the variant pair that formed in the initial stage of martensitic transformation as initial stage microstructure (ISM). This result suggests that there is a selection rule for the self-accommodation microstructure in the initial stage of martensitic transformation. The selection rule of ISM is important for understanding self-accommodation microstructure because it represents a mechanism of variant selection in the absence of the influence of other variants. In the present study, we have performed evaluations of energy barrier of the formation of ISM in order to elucidate the factors that determine the selection rule of ISM in TiNi shape memory alloy.
The energy barrier of the formation of ISM is the sum of the interfacial energy of the twin interface included in ISM and the elastic strain energy for the ISM formation. In the TiNi shape memory alloy, 8 types of twin orientation relations and corresponding variant pairs exist. It is considered that the variant pair with the smallest energy barrier is selectively formed as ISM. The interfacial energy of twin interface was evaluated by the first-principles calculation using VASP (Vienna Ab initio Simulation Package) code. The elastic interaction energy to form a variant pair was evaluated by a micro-mechanics proposed by Eshelby.
Since interfacial energy of {11−1} type-I twin and elastic interaction energy of the variant pair including {11−1} type-I twin are relatively small among the twin interfaces considered, it is supposed that the energy barrier of the variant pair including {11−1} type-I twin is smaller than others. These results indicate suggests that the variant pair with a small energy barrier is selectively formed as the initial stage of martensitic transformation.

Transient liquid phase bonding (TLPB) has been used as a bonding process that joins dissimilar materials by employing an interlayer that provides a lower melting process. It shows the potential to be used in high temperature electronics (operated or exposed over 200°C), where there are no clear lead (Pb)-free solder options available. During the reflow process for electronics components, the interlayer melts and the constituents of the interlayer and its joining materials diffuse each other, subsequently making a homogenous intermetallic bond via isothermal solidification. Once formed, it results in a higher melting point than the bonding temperature. This research investigates such a TLPB system made of a nickel (Ni) bonding surface and a bismuth (Bi) interlayer, which can serve as a Pb-free solder alternative for high-temperature electronics. Upon melting of Bi on Ni, intermetallic phases (Bi₃Ni, BiNi) nucleate and grow by consuming a Ni layer on the substrate or the die side. Intermetallic reactions and microstructure developments were studied under various reflow conditions including temperature and time on three sets of the sandwiched assemblies: 1) Si die Sputtered Ni (1 µm-thick)/Bi/Si die; 2) Si die/Bi/Ni foil (0.5 mm-thick); 3) Ni foil/Bi/Ni foil. As such bonds constitute the structural joints in electronic packaging, mechanical properties of the intermetallic bonds and their long-term microstructural developments under high temperature aging and thermal cycling (-55°C to 200°C) were investigated by shear testing and cross-sectional microstructure analyses of the sandwiched samples. It was shown that temporal evolution of two intermetallic phases composed of faster Bi₃Ni and slower BiNi, along with the remaining Bi, are responsible for mechanical performances of TLPB. This presentation will also discuss about optimal microstructure development for the TLPB, which is critical to offset the defects and brittleness of the intermetallic phases.

Eutectic composites composed of intermetallics and/or ceramics with very high eutectic temperatures have been considered as attractive materials for ultra-high temperature structural applications because of their high melting temperature, high structural stability and good mechanical properties. Recently, we have systematically investigated microstructure-property relationship of MoSi₂/Mo₅Si₃ eutectic composites with very high eutectic temperature of 1900 °C and have revealed that their high-temperature strength and room-temperature fracture toughness can be improved simultaneously by optimization of microstructure and interface properties through controlling growth condition and ternary/quaternary alloying. These previous results open a new possibility to develop novel structural materials endowed with excellent high-temperature strength and good room-temperature fracture toughness by utilizing interphase boundaries. In the present study, we focused on NbSi₂/Nb₅Si₃ eutectic composites (eutectic temperature ~1900 °C for binary) as a new target system and investigated microstructure variation of directionally solidified (DS) ingots of the eutectic composites as a function of ternary Mo addition. When Mo is alloyed more than 2 at. %, DS ingots are conformed to composed of C40-(Nb,Mo)Si₂ and D8_m-(Nb,Mo)₅Si₃, which indicates that the D8_m (high temperature) to D8_l (low temperature) phase transformation that occurs in binary Nb₅Si₃, is suppressed by the Mo addition. A fine eutectic microstructure of the so-called script-lamellar type, which is similar to those observed in MoSi₂/Mo₅Si₃ DS eutectic composites, is confirmed to develop for the Mo-alloyed NbSi₂/Nb₅Si₃ DS eutectic composites. Crystallographic orientation relationships between C40 and D8_m phases as well as their preferential growth directions are identified by electron backscattered diffraction (EBSD) in scanning electron microscopy and transmission electron microscopy (TEM).

Nano-indentation method is a powerful tool to evaluate the mechanical properties of each phase in materials consisting of multi-phase microstructures. If the mechanical properties of each phase can be evaluated correctly, then we can predict the mechanical properties of bulk materials having different amounts of the phases, which would allow us to construct the design principle for materials integration (MI) to solve invers problems. However, there are a number of factors affecting the properties by this method, since the machine is so sensitive to the specimen itself such as surface conditions, as well as the machine itself such as specimen set-ups and tip of the indenters. In this study, titanium aluminide alloys were selected to evaluate the mechanical properties of the constituent phases since the alloys consists of three phases of b-Ti (bcc), a₂-Ti₃Al (D0₁₉ with hcp based) and g-TiAl (L1₀ with fcc based). In addition, we recently revealed that the introduction of an appropriate amount of the b phase is effective in improving the fracture toughness as well as tensile properties. Thus, it is very important to build up the mechanical property database for each phase in equilibrium each other in order to construct the construction of MI platform for inverse problems. The alloy studies are Ti-Al-Cr ternary alloys. These alloys were homogenized first in the b single phase region, and then heat treated in the three-phase region at 1173 K to reach the equilibrium state, based on our phase diagram study. The specimens were mechanically polished, followed by electro-polishing. Before and after the indentation test, the microstructure and orientation analysis of each phase were performed by FE-SEM and EBSD. Nano-indentation tests were performed to evaluate the Young’s modulus and hardness using a continuous stiffness measurement (CSM) method with a frequency of 110 Hz using a Berkovich type indenter made of diamond. The advantage of the CSM method allows us to evaluate the properties continuously as a function of indentation depth. The results were compared with those obtained from the Oliver-Pharr method. Both properties are found to be sensitive to the indentation depth and sampling distance, depending on the phases as well as orientation. The details of the appropriate conditions will be discussed, in conjunction with the O-P method. 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.

FeCr-σ with a crystal structure of tP30 (ordered tetragonal phase) is one of the TCP phases and often observed in heat resistant materials. This phase is considered as a harmful phase to deteriorate mechanical properties of the materials. However this phase could be a strengthening phase, just like Laves phase, if the phase stability can be increased by alloying. In the previous study, we have found that the lattice parameters of both a and c become either larger or smaller by the addition of M in solution, depending on the size of atomic radius of M relative to those of Fe (r_Fe=1.274 Å) and Cr (r_Cr=1.282 Å) as usual, except Fe addition. In case of the excess addition of Fe in solution, the a axis becomes smaller whereas the c axis becomes larger. We attributed the unusual anisotropic lattice parameter change to the site occupancy change in five sub-lattice sites (M1, M2, M3, M4, M5) of the σ phase, based on the Rietveld analysis; the occupancy of Cr atom in M5 sub-lattice site increase with increasing excess Fe atoms. In this study, we picked up non-transition element Si as the third element since it has larger atomic radius (r_Si=1.322 Å) and is known as strong σ stabilizer, and investigated the lattice parameter change with Si in solution.
The alloys studies are Fe-(50-x/2)Cr-xSi (at. %) where x=5, 10. They were prepared by arc melting to 30g button ingot, and homogenized in the σ single-phase region. Phase identification was conducted by powder XRD techniques and the lattice parameters were calculated from obtained XRD profiles. The obtained XRD profiles were further analyzed by means of Rietveld method to determine the structural change of σ phase as well as site occupation of the elements. Microstructures were examined by SEM and hardness was measured by micro Vickers hardness machine. We confirmed that both alloys with 5 and 10 at.% Si exhibit σ single phase with almost the same hardness of 12 GPa. It is interesting to note that, just like Fe addition, the anisotropic behavior of the lattice parameters was obtained; the a axis decreases whereas the c axis increases with increasing in Si content. However, Rietveld analysis suggested that this anisotropic behavior is different from that in Fe case related to the site occupancy change of M5 sub-lattice site. The detailed mechanism of this anisotropic behavior and the crystal structure change will be discussed in conjunction with the c/a ratio and unit cell volume of the σ phase.

We demonstrate how widely used high-throughput first-principles calculation frameworks for novel phase discovery can be augmented to enable phase stability predictions above absolute zero and yield informative phase diagrams at a fraction of the computational cost of traditional methods. The approach is based on a combination of (i) the special quasirandom structures formalism, extended to account for possible short-range order, (ii) carefully selected lattice dynamics calculations (iii) efficient molecular dynamics modeling of liquid phases that leverage high-quality experimental thermodynamic for elemental liquids and (iv) the CALPHAD approach for free energy modeling. As an example, we investigate the Ni-Re system, where earlier calculations predicted the presence of novel D0₁₉ and D1_a phases. We find that, in spite of their stability at 0K, these intermetallic compounds become unstable at high temperature. These findings also prove helpful in guiding the search for alloys that provide cost effective substitutes for rhenium as a structural material.

High temperature materials, such as superalloys, are often covered with metallic coatings in order to protect the substrate from oxidation and to extend its working life. The phases present in the coatings, partly determined by the alloy composition, play an important role in the material properties. Among them, several are beneficial for the properties while some phases are detrimental during service. Co and Ni are the major elements in the metallic coatings. The appropriate amounts of Al, Cr and Y are added to improve their overall performance and to meet the demanding industrial requirements. In general Al-Co-Cr-Ni alloys are adopted as the main coatings for superalloys and small amounts of Y are added to improve oxide scale adherence. However, it is very difficult to predict how phase stabilities are affected by the addition of each element in multi-component alloys, without performing computational simulations.
A reliable thermodynamic database containing Al, Co, Cr, Ni and Y has been developed to support the design of CoNiCrAlY coatings, following the CALPHAD approach. All binary and ternary subsystems are included in the database. Most of the ternary subsystems, due to the lack of consistent thermodynamic description in the literature, were assessed by our group, namely Al-Co-Ni [1], Al-Cr-Ni [2], Co-Cr-Ni [3] and several ternary systems containing yttrium, such as Al-Co-Y, Al-Cr-Y, Al-Ni-Y and Co-Ni-Y (paper in preparation).
The new database was used to simulate a few commercial alloys used as metallic coatings protecting gas turbine blades. The calculated phase compositions were then confronted with the available experimental data from the literature for the following coatings: PWA270 (Ni-21%Co-18%Cr-12.7%Al-0.17%Y), PWA286 (Ni-21.1%Co-17.1%Cr-12.7%Al-0.61%Y) at 1000°C [4] and the Co-based coating Co-28.6Ni-21.2Cr-15.6Al-0.4Y at 1150°C [5] (in at%). The good agreement between calculated and experimental data was evident, therefore, the present database can be reliably used to predict phase equilibria in alloy coatings already in use or to support the design of new alloys with improved performance. In particular, to find the temperature range in which undesired phases (e.g. σ) are stable and investigate the influence of each element on the phase stability.

[1] Y. Wang, G. Cacciamani, Experimental investigation and thermodynamic assessment of the Al-Co-Ni system, Calphad Comput. Coupling Phase Diagrams Thermochem. 61 (2018) 198–210. doi:10.1016/j.calphad.2018.03.008.
[2] Y. Wang, G. Cacciamani, Thermodynamic modeling of the Al-Cr-Ni system over the entire composition and temperature range, J. Alloys Compd. 688 (2016) 422–435. doi:10.1016/j.jallcom.2016.07.130.
[3] G. Cacciamani, G. Roncallo, Y. Wang, E. Vacchieri, A. Costa, Thermodynamic modelling of a six component (C-Co-Cr-Ni-Ta-W) system for the simulation of Cobalt based alloys, J. Alloys Compd. 730 (2018) 291–310. doi:10.1016/j.jallcom.2017.09.327.
[4] M. Hasegawa, M. Iwashita, Y. Kubota, P. Dymáček, F. Dobeš, Microstructure evolution under high temperature deformation of CoNiCrAlY bond coat alloy, Mater. Sci. Eng. A. 756 (2019) 237–247. doi:10.1016/j.msea.2019.04.030.
[5] O. Zubacheva, Plasma-Sprayed and Physically Vapor Deposited Thermal Barrier Coatings: Comparative Analysis of Thermoelastic Behavior Based on Curvature Studies, RWTH Aachen University, 2004. http://sylvester.bth.rwth-aachen.de/dissertationen/2004/203/.

Mg-TM(transition metal)-RE(rare earth) ternary alloys containing a precipitation phase with a long-period stacking-ordered (LPSO) structure have received considerable attentions as a new light-weight structural material endowed with high strength and good ductility simultaneously. Such attractive mechanical properties are believed to be caused by heavily kinked structure of platelet precipitates of Mg-LPSO phase developed during hot extrusion process. Formation of the kinked structure in HCP metals with a relatively high c/a ratio has been interpreted generally with the dislocation-based kink band formation mechanisms such as the Hess and Barrett model, Frank-Stroh model, and Gilman model, which considered different trigger mechanisms for the activation of basal slip when the loading axis is (nearly) parallel to the basal planes, i.e., elastic buckling, stress concentration caused by pre-existing or pre-introduced defects, and a slight tilt of basal plane, respectively. However, the actual controlling factors for the kink-band formation in Mg-LPSO phases have not been fully clarified yet. In the present study, we investigated compression deformation behavior of Mg-LPSO single crystals as a function of loading axis orientation, specimen shape and stacking sequence of the LPSO structure (18R and 14H-type) by utilizing micropillar compression methodology in order to clarify the dominant controlling factors for the kink-band formation in Mg-LPSO phases.

In the Seebeck effect in p-type semiconductor, the voltage can be generated by the diffusion of excited career in high temperature side to low temperature side in impurity level. However, careers in a valence band rediffuse to high temperature side because of electric potential difference, and the value of voltage drop at equilibrium. On the other hand, the block of rediffusing career can generate larger the value of voltage. In this study, we combined two different fermi level of p-type semiconductors, the light boron doped silicon and the heavy boron doped silicon, in order to form internal electric field in the interface with Spark Plasma Sintering. This process has formed the barrier of energy for prevention of career rediffusion. We found that the synthesized sample’s voltage increased in comparison with the light boron doped silicon and the heavy boron doped silicon. It is indicated that voltage can increase by forming internal electric field, with the sample combined different fermi level p-type semiconductors.

Shape memory alloys are able to stand large deformations and to recover their shape afterwards. Depending on the temperature, the shape recovery could take place after heating or by the so-called pseudoelastic behavior. These particular properties make these smart materials good candidates for applications such as damping or actuation. Cu-based shape memory alloys are commercially attractive due to their relatively low cost compared to Ni-Ti alloys. Multiple research efforts have focused on expanding their potential applications by adding features that can improve the material. Porous shape memory alloys were developed in an effort to combine the shape recovery and energy dissipation properties with reduced density and weight. However, most polycrystalline Cu-based shape memory alloys are prone to intergranular fracture both in compact and porous samples. Reducing the material grain size could decrease the stress concentration at grain boundaries and help to avoid fracture. This can be achieved by alloying with other elements or by selecting an appropriate manufacturing method. As a possible fabrication route, we used powder metallurgy starting from a small particle size.
Among Cu-based shape memory alloys, Cu-Al-Ni alloys are an alternative with good properties and low cost. In this work, we explored the fabrication of porous Cu-14Al-3Ni by conventional powder metallurgy, starting from mechanically pre-alloyed powder and a mixture of the elemental metal powders. For preparing the pre-alloyed powder, we employed a high-energy Fristsch Pulverissette 7 planetary mill, processing under Argon at 700 rpm for 3 hours. The resulting powder was analyzed by means of X-Ray Diffraction (XRD). The XRD pattern showed no presence of Al or Ni while the Cu peaks were shifted to lower angles indicating the solid solution of both Al and Ni in the structure. The Cu peaks were also significantly widened due to the strain resulting from the high-energy milling. Some peaks of γ-Cu₉Al₄ were detected, as well as small CW peaks resulting from the milling media contamination. The other precursor was prepared by hand mixing the elementary powders in an agate mortar. Both precursors were mixed with 1 wt.% mixture of PVA and PVB as a binder and, after drying, 30 vol.% ammonium bicarbonate was added as space holder. Cylindrical samples were prepared by uniaxial cold pressing at 700 MPa. After sintering at 1000 °C, the samples were solution-treated for 24 h at 900 °C under Ar. After the treatment, they were water-quenched to obtain the martensitic phase.
The samples prepared with the milled powder were investigated by XRD. After sintering, the phases detected were α, γ-Cu₉Al₄ and AlNi. The heat treated samples consisted of the shape memory phase 18R accompanied by α and AlNi. The presence of AlNi in the sintered sample suggests that the alloy resulted short of both elements and consequently the solution treatment was performed in the α-β two-phase field. After quenching, α remained as a secondary phase. Meanwhile, the samples prepared with the hand mixed precursor consisted of the 18R and 2H shape memory phases.
X-ray microtomography images of the porous samples were acquired with an Xradia Micro XCT-200 microscope. The samples prepared with the pre-alloyed precursor presented internal cracks indicating that the presence of the brittle γ-Cu₉Al₄ phase reduced the powder compressibility. The lower density prevented their mechanical characterization. On the other hand, samples prepared with the hand mixed precursor were well sintered allowing their complete characterization. Compression uniaxial tests were performed and the shape memory behavior was corroborated. Samples recovered 100% from a 7% strain and 98.56% from 11.5% strain when heated to 140 °C. The obtained grain size is about 30 µm.

Both cast and wrought titanium aluminide alloys have been studied for more than two decades because of attractive properties that include low density, high specific strength, high specific stiffness and oxidation resistance up to about 700°C. Electron beam melting provides another processing approach to produce net shape components, although little work has been conducted to examine processing-microstructure-property relationships. This work examines as-deposited γ-TiAl (Ti-48Al-2Cr-2Nb) specimens made by Arcam AB and post-processed materials. Mechanical behavior studies on as-deposited, HIPed, and HIPed + Vacuum heat treaded conditions included Vickers micro-hardness, compression, fracture toughness and fatigue crack growth testing. In addition, microstructural details were investigated over a range of scales using various microscopy tools. The presentation will summarize this evolving work on the characterization of AM γ TiAl and provide some comparison to other conventional (e.g. as-cast, wrought) γ-TiAl alloys.

Electron Beam Melting has proven to be a technology capable of producing high quality Titanium Aluminide blades and is currently being applied to additively manufacture Low Pressure Turbine airfoils for the GE9x: the largest, most powerful and fuel-efficient commercial aircraft engine in its class, which incorporates GE’s most advanced technologies developed over the last decade.
While providing several benefits such as being capable to process a highly reactive alloy such as TiAl, producing sound parts and reducing development time, this additive manufacturing technology has also posed many challenges related to its industrialization and scale-up.
This article is aimed at giving an overview of the main technical areas that had to be addressed to advance EBM from a lab scale to a mass production environment and to guarantee material and process control when it comes to part certification.
One of the main areas of attention was to identify Key Process Variables, determine the associate tolerance band and their impact on desired characteristics of finished parts, such as mechanical (e.g., static strength, fatigue, fracture), metallurgical, physical, or chemical properties.
A significant part of this effort was focused on gaining a good understanding of the defects that can be generated by the additive process and that are different from traditional manufacturing routes such as casting and forging. Additionally, the impact of such defects on material properties, their detectability and acceptance limits are key items to be considered.
Process quality control is another crucial focus area, including part-to-part, build-to-build, machine-to-machine variations, machine qualification, powder handling and correlation between batch coupons and parts. Destructive and non-destructive tests and statistical process control are also part of this effort.
As of now more than 10000 blades have been produced and most certification engine tests have been completed, which makes a great contribution to pushing further the additive technology development and usage.

Intermetallic TiAl alloys that exhibit low density, high strength at high temperatures and excellent oxidation resistance are used for low pressure turbine (LPT) blades of an aero engine. In recent years, the alloys containing the β phase have attracted much attention due to their good mechanical properties at operation temperature of LPT blades. In this study, rectangular rods of β-containing Ti-Al-Cr alloy were fabricated by electron beam melting (EBM) which is one of the suitable manufacturing processes for the TiAl alloys. Surface morphology, internal defect, microstructure and mechanical properties of these rods were investigated focusing on input energy density of the EBM process. We found that the microstructure of the samples is strongly influenced by the input energy density. The rods fabricated at low energy densities contain fine α₂/γ lamellar structure with the β/γ cells discontinuously precipitated at the lamellar colony boundary. On the other hand, the α₂/β/γ mixed structure composed of the α₂/γ lamellar and (β+γ) structures can be obtained at high energy densities. The variation in microstructure is caused by the difference in solidification process and temperature distribution in the vicinity of the melt pool during the EBM process. We also found that the tensile properties of the alloys depend strongly on the volume fraction of the α₂/γ lamellar structure, the β/γ cells and the b phase. The strength of the alloys prepared under the optimum condition demonstrate at room temperature and 1023 K is higher than that of Ti-48Al-2Cr-2Nb alloy and comparable to that of Ti-Al-Nb-Mo-B (TNM) alloy.

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).

TiAl/Ti2AlNb dual alloys exhibit potential applications on aerocraftengines, but manufacturing of such alloys needs to achieve an appropriate gradient transition between these two alloys. Here, we deposited -TiAl alloy on a AlNb substrate using the direct metal deposition (DMD) technique and analyzed their graded zone near the interface. Our results show that the transition zone mainly consists of three characteristic layers: the first layer possesses a small amount of and phases dispreading in the matrix. In the the second layer, directional dendrites ( phases as dendrite core and phases as dendrite arms) are developed in matrix. The third layer exhibits equiaxial (+) colony with fully lamellar microstructure, which is similar to the microstructure of the -TiAl alloy. We also preformed nanoindentation tests on different phases and layers to revel their mechanical properties. The relationship between nanoindentation results and characteristic microstructure has been identified.

Key words: intermetallics; direct metal deposition; microstrcture; nanoindentation; mechanical properties.

A five-year National project of “Structural Materials for Innovation (SM⁴I)” in Cross-ministerial Strategic Innovation Promotion Program (SIP) starting from 2014 in Japan ended in 2018, and the following another 5-year project of “Materials Integration for Revolutionary Design System of Structural Materials” in the SIP II Program has started in 2018. For both programs I have been committed to as the technical leader and take responsibility for the alloy design and development of TiAl alloys, in collaboration with universities and industries. In the first program, the focus was placed on the design/process principles of innovative wrought and cast TiAl alloys to be used for LPT blades, based on the thermodynamics and kinetics of multi-components titanium aluminide systems, and we have successfully proposed and developed new alloys. In the second program, however, we have focused on construction of inverse design principle for innovative TiAl alloys with required properties available for both MIM (metal injection molding) and AM (additive manufacturing) processes, in collaboration with Kobe Steel, Ltd., Osaka Yakin Kogyo Co., Ltd, Mitsubishi Heavy Industries Aero Engine and Osaka University. The degree of freedom of the microstructure design in MIM and AM processes is much wider than that in wrought and cast processes, but attention has to be paid to impurity levels of oxygen and carbon picked up by processing routes of powdering, injection molding and sintering in case of MIM. In this talk, we first show you how oxygen affects the phase equilibria among β-Ti, α-Ti (α₂-Ti₃Al) and γ-TiAl phases, and eventually affects the microstructures created through the phase transformations, based on our phase diagram studies. You have to be careful for alloy development under powder processes if you use currently existing thermodynamic databases. None of them works. The microstructure can significantly be changed from the fully α₂/γ lamellae to the β/γ grained, depending on oxygen level, even the major alloy elements as well as the heat treatment routes are the same. Current progress on this project, especially how important the b phase plays in role in improving mechanical properties for both MIM and AM, based on our novel customized powders, will be presented. A part of this study has been carried under the research of SIP II in JST (Japan Science and Technology Agency).

Intermetallic TiAl alloys based on the γ-TiAl phase are already used as engineering lightweight high-temperature materials in aircraft and automotive engines. Thereby, they partly substitute the twice as heavy Ni-base superalloys. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines and engine parts used in racing sport applications. All the applications mentioned above require balanced mechanical properties, i.e. certain ductility at room temperature as well as defined creep strength at elevated temperatures. In the framework of this presentation the alloy design strategies, which have been applied for the development of so-called process adapted TiAl alloys will be explained. Besides the considerations which have led to the selected alloying elements, the heat treatments conducted subsequent to conventional hot-forging and additive manufacturing are discussed. In this context it will be shown that a combination of computer-aided alloy design and novel characterization techniques, e.g. in-situ high-energy X-ray and neutron diffraction, has accelerated both alloy and process development as well as the understanding of this class of alloys sustainably.

Hot isostatically forged TiAl turbine blades made of TNM-B1 are commercially used in aircraft engines, as they offer significantly lower weight than the traditional nickel-based blades while exhibiting similar strength. Like other TiAl alloys, TNM-B1 displays high peak stress followed by a strong softening behavior (i.e. stress reduction) during hot deformation. This softening can be used to accelerate the deformation process by reducing the processing time and in turn the costs for TNM-B1 parts. In order to avoid increased damage during the accelerated process, a pre-heat treatment (HT) for the hot isostatically pressed material (HIP) is required. To simulate the accelerated forming process, hot compression tests were performed with a DIL805A/D/T dilatometer from TA Instruments (New Castle, Delaware, USA) with different strain rates (0.0013, 0.005, 0.01 and 0.05) and temperatures (T=1150, 1175 and 1200°C). Deformation of the heat-treated state revealed lower flow stress (in both, peak stresses and steady state stresses) and fewer voids compared to the HIP state. The compression test data were used to develop material and temperature specific strain rate profiles based on a material model presented in [Stendal, J.A et al., Applying machine learning to the phenomenological flow stress modeling of TNM-B1. Metals 9 (2019) 220]. Subsequently, hot compression tests were performed with different strain rate profiles (starting strain rates 0.0013 and 0.0052) for the HIP and the HT state. The results were evaluated with regard to their microstructure, deformation, and damage behavior. A reduction of the processing time for all tested strain rate profiles by factors 2-3 could be achieved compared to constant strain rates. Furthermore, the results indicated that the deformation with strain rate profiles (compared to constant strain rates) did not significantly change the resulting microstructure or damage tolerance of the HT state.
Maybe also HEXRD results of the strain rate profiles can be presented to show in detail that the accelerated forming process can lead to the same results as the constant forming process.

In modern aero engines, γ-TiAl alloys can be used as a replacement for Ni-base super alloys for certain components such as low-pressure turbine blades, due to γ-TiAl alloys having about half the density of superalloys. Nevertheless, due to the relative novelty of γ-TiAl, not the whole variety of processes available for established materials has been fully investigated. One possibility for a joining or repair process is diffusion bonding. Diffusion bonding can be used to repair components by replacing damaged or worn sections without significantly modifying the microstructure. For Ni-base super alloys, diffusion bonding and brazing are well-established repair processes that are used to extend the life cycle of turbine blades. For γ-TiAl alloys, such repair processes are not commercially available.
In the current work diffusion bonding of two commercially used γ-TiAl alloys, Ti-48Al-2Nb-2Cr (4822) alloy and Ti-43.5Al-4Nb-1Mo-0.2B (TNM) alloy, (all compositions in at.%), was as a possible repair technique. The microstructure developed during diffusion bonding and the corresponding mechanical properties after bonding have been determined at room and elevated temperatures. Furthermore, the use of different surface treatments before bonding, such as wire cutting, grinding, drilling and turning as well as various geometric arrangements were examined.
For successful diffusion bonding temperatures above 1000°C had to be appliedto produce sound jointsracke in both alloys. The TNM alloy showed changes in the substrate microstructure (e.g. lamellar coarsening) after bonding at high temperatures close to the γ-solvus temperature. In contrast, there seemed to be no significant microstructural changes in the substrate of the cast 4822 alloy, except for some grain growth at and across the bonded joint.
By optimising the diffusion bonding parameters, it was possible to reach tensile strengths and ductilities in diffusion bonded samples that were comparable to those of the base material. Through optimisation of the surface treatment, it was also possible to produce solid joints for various geometric arrangements and improve the overall quality of the joint.

TiAl-alloys are a typical example that advanced modern lightweight materials have an increasingly complex chemical composition and phase constitution. Accordingly, a sophisticated and complete characterisation of the material is necessary to fully comprehend their behaviour and properties for knowledge-based alloy and process development. Unfortunately, many processing methods for TiAl as well as their operation itself take place at high temperature making their direct monitoring and characterisation difficult. In addition, it is also of interest to investigate the material behaviour if other external loads as for example an applied stress are present in parallel.
In the contribution examples for the direct monitoring and characterisation of the processing of TiAl alloys using in situ specimen environments and high energy X-ray diffraction at synchrotron sources are shown. The processes can be rather simple as for example a heat treatment at a certain annealing temperature where the phase composition and its development are monitored (e.g. [1]). But, also complex processing scenarios like hot forming [1,2] or a transient liquid phase bonding process [3] can be directly investigated by those methods. In the former case the texture development during the hot forming is observed while in the latter case the solidification of the joining zone and the subsequent phase transformations are investigated to understand the microstructure formation.
Simple heat treatment, hot forming, transient liquid phase bonding and many other processes for TiAl alloys have in common that phase constitution, texture or microstructure react very sensitively to changes of applied temperature and/or stress. They normally change significantly when the stress is removed or the specimen is cooled down at the end of the processing step. Due to these reasons it is not possible to unambiguously reconstruct the situation at processing temperature and/or under stress based on subsequent material characterisation at room temperature or in absence of the mechanical load. In situ characterisation as presented here is the only means to aquire the full picture and get insight into crucial process details under processing conditions.

References:
[1] Pyczak, F., Rackel, M., Gabrisch, H., Halici, D., Lorenz, U., Schreyer, A., Stark, A., In-situ monitoring of thermal treatments and hot deformation of a g-TiAl alloy by high energy X-ray diffraction, 123HIMAT-2015 Proceedings (2015) 323-326.
[2] Stark, A., Rackel, M., Tchouaha Tankoua, A., Oehring, M., Schell, N., Lottermoser, L., Schreyer, A., Pyczak, F., In situ High-Energy X-Ray Diffraction during Hot-Forming of a Multiphase TiAl Alloy, Metals 5 (2015) 2252-2265.
[3] Hauschildt, K., Stark, A., Burmester, H., Tietze, U., Schell, N., Müller, M., Pyczak, F., Phase Transformations in the Brazing Joint during Transient Liquid Phase Bonding of a g-TiAl Alloy Studied with In Situ High-Energy X-Ray Diffraction, Materials Science Forum 941 (2018) 943-948.

In order to clarify the effect of microstructure factor on the fracture toughness in β phase containing γ-TiAl based alloys, the room-temperature fracture toughness of these alloys have been investigated, because the room-temperature fracture toughness is a quite important material property for jet engine application materials. In addition, it is essentially important to understand the relationship between microstructure factor and alloy properties for not only predicting alloy properties from microstructure feature but also solving the inverse problem which means to propose the optimal microstructure feature from the required properties.
The microstructure of alloys studied were intentionally controlled from the nearly α₂-Ti₃Al/γ lamellae to β-Ti/γ grained, through the β/γ lamellae using the cellular reaction of α₂+γ → β+γ in Ti-Al-Cr ternary system alloys. The fracture toughness was evaluated by an instrumented three-point bend test using a chevron-notched square-bar specimen with 30 mm in a span length under the cross-head speed of 0.05 mm/min. The K_IC was calculated using a maximum force in the load-displacement curve on the bend tests and the shape factor of chevron-notched specimen. Some of the specimens were also tested under the higher test speed. The results were compared with the ΔK_th and Paris slope those obtained by compact tension (CT) test.
We found that the existence of the β-Ti phase is effective in increasing the fracture toughness. Based on these results, methodology to predict the fracture toughness is discussed.
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.

Intermetallic materials typically change their deformation behavior from brittle to ductile at a certain temperature called the brittle-ductile transition temperature (BDTT). This specific temperature can be determined by tensile or bending test results conducted at different temperature steps. In order to find the effect of strain rate on the BDTT, these experiments should be repeated at different strain rates. In order to reduce the amount of specimens for finding the BDTT a new methodology is studied. Experiments with particular loading strategies were performed on a fully lamellar Ti-48Al-2Nb-0.7Cr-0.3Si alloy in order to find the BDTT by using a single specimen. For this methodology cyclic loading is applied isothermally under strain control. The same loading block is then repeated on the specimen at different temperatures. The change in plastic strain with increasing temperature is then used for determining the BDTT of the specimen. In the next step a study is conducted to find the effect of strain rate on the BDTT also by using a single specimen. The same methodology as for finding the BDTT is used, but the cyclic loading was applied at different strain rates in a temperature step. Additional dwell times at constant total strains were introduced to identify the BDTT at very low plastic strain rates. The BDTT’s of the TiAl alloy at different strain rates determined by this new methodology are compared in an Arrhenius plot from which the activation energy for brittle-ductile transition can be found.

Microstructures of β-solidifying TiAl alloys typically consists of β-Ti, α₂-Ti₃Al, and γ-TiAl three phases. We have revealed that fatigue crack growth resistance and ductility could improve by introduction of β phase. However, in order to evaluate and estimate the properties of a bulk material more quantitatively, it is essential to measure the properties of each phase. Nano-indentation method is appropriate to evaluate properties of individual phase in multi-phase material. In this study, Young’s modulus and hardness of β, α₂, and γ phases have been investigated using nano-indentation in Ti-Al-Cr system.
The alloys were heat-treated based on our phase diagram study. Young’s modulus and hardness were measured by nano-indentation tests with continuous stiffness measurement mode. Crystallographic orientation analysis was performed before measurements with electron backscatter diffraction technique in order to identify the effect of orientation on the properties.
In Ti-44Al-4Cr (at.%) equilibrated at 1373 K, the Young’s modulus and hardness of all phases vary depending on the crystallographic orientation. In β and γ phases with (001) orientation, the Young’s modulus is 136 and 146 GPa respectively, and the hardness is 7.0 and 3.0 GPa at room temperature. The hardness of β phase is more than twice as much as that of γ phase. β phase is harder than γ phase even at 1073 K although β phase is often considered to be soft at high temperature.
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.

Phase equilibria among β-Ti, α-Ti, α₂-Ti₃Al and γ-TiAl phases has been examined experimentally in order to reveal the effect of interstitial carbon and oxygen in Ti-Al binary and Ti-Al-M (M: β stabilizing elements) ternary systems. Alloys with carbon/oxygen content from 0.13 (base) to 1.5 at. % were prepared by vacuum arc melting into button ingots. Those ingots were homogenized at the β or α single-phase region, and then equilibrated at temperature range from 1573 K to 1073 K for up to 2 months. Chemical compositions of the metal elements in the phases present in equilibrated samples were analyzed by conventional wavelength dispersive spectroscopy (WDS). Carbon and oxygen contents in phases were analyzed by Soft X-ray Emission Spectroscopy (SXES).
In Ti-Al binary system, both carbon and oxygen partition into the α (or α₂) against γ phase, and the α+γ two-phase region expands toward both lower and higher Al content side. Oxygen content in γ phase in equilibrium with a is clearly smaller than that of carbon, and thus the partition coefficient of oxygen between α/γ is almost twice higher. In Ti-Al-M ternary systems, interstitial elements also partition into the a (or α₂) against γ and β phases. By adding interstitials into the ternary alloys, the β+α+γ three-phase coexisting region shift towards the higher M content side. At the same time, the terminal composition of the β phase increases with increasing interstitial content in the alloy, especially in case of oxygen. Detailed difference between the effect of carbon and oxygen will be discussed.
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.

Development of next-generation TiAl-based alloys for high-temperature, high-strength applications such as turbine blades relies on computational methods like CALPHAD modelling for alloy and microstructure design. To achieve the goal of further increased service temperatures, an exact knowledge of phase equilibria and microstructures is essential and plays an important role in determining the high-temperature mechanical properties of respective alloys. Therefore, a dataset of reliable experimental results is needed to improve the accuracy of the modelling. The Ti–Al–Nb system is the essential system for alloys such as TNM, TNB or 4822 which are currently used for low pressure turbine blades. However, there are only few results in the literature about the Ti–Al–Nb system regarding phase equilibria below 1000°C, and there is a variety of different and contradicting experimental results for temperatures of 1000°C and above. Based on these data, reliable predictions from CALPHAD modelling is difficult. For further development of TiAl-based alloys the existing CALPHAD database should therefore be improved. Another factor complicating the matter is the not fully understood formation of ternary O- (Ti₂NbAl) and ω- phase (Ti₄NbAl₃) from (βTi) and α₂ (Ti₃Al), respectively, which depend on a variety of factors. Since these phases are important for the mechanical properties of these alloys, research presented here aims to clarify the uncertainties in phase equilibria in Ti-Al-Nb alloys. The research is carried out within the scope of the EU-funded project ADVANCE which is part of the Clean Sky 2 research initiative. The aim of the project is to improve an existing database for TiAl-based alloys for speeding up the development of next-generation alloys based on titanium aluminides. The project is carried out in collaboration with Helmholtz-Zentrum-Geesthacht (Germany), Montanuniversität Leoben (Austria), and Thermo-Calc Software (Sweden).

Titanium aluminide alloys (TiAl) are widely recognized for their potential as aero engine components due to their combination of similar service temperature and lower density than conventional metallic systems as for example Ni-base superalloys. However, an improved understanding of the microstructure formation and phase transformation during the production of TiAl parts is necessary to adapt the process parameters for hot forming. Fortunately, an appropriate tuning of the alloy composition can provide good deformation capabilities. For instance, the addition of Zirconium (Zr), a β-stabilizing element, lowers the phase transformation temperatures of the ‘softer’ disordered α and β phases and has other additional beneficial effects e.g. solution strengthening of TiAl alloys. In order to determine appropriate TiAl-Zr compositions thermodynamic calculations were performed using Thermo-Calc Software (‘TCTI1 v1’ database). Based on these, samples with selected compositions were produced by arc melting and afterwards heat treated for chemical homogenization. The thermodynamic behavior during heating was investigated by differential scanning calorimetry (DSC) using a NETZSCH DSC 404 F3 Pegasus. Additionally, high-energy synchrotron X-Ray diffraction (HEXRD) was performed in-situ to observe the phase changes during heating. Both measurement methods reveal that thermodynamic calculations based on currently available databases for TiAl-Zr alloys can only show tendencies how phase transitions change with increasing or decreasing Zr content, but do not allow accurate predictions of transition temperatures.

Due to their good oxidation resistance and excellent specific mechanical properties at elevated temperatures, intermetallic Titanium Aluminides are candidate materials to replace Ni-base alloys in high temperature turbine applications. One promising example of Titanium Aluminides is Ti-48Al-2Cr-2Nb (at-%) which is already used for low-pressure turbine blades. Although its duplex microstructure is stable up to a temperature of 850 °C, the alloy is limited to temperatures below 700 °C since oxidation rates increase dramatically at higher temperatures. One possibility to reduce oxidation and increase the operation temperature are oxidation protective coatings. However, these coatings are typically very brittle and therefore unfavourable for fatigue loading. In this study, the influence of oxide layers and oxidation protective coatings on the fatigue lifetime of Ti-48Al-2Cr-2Nb at 750 °C are investigated. Stress-controlled fatigue tests were conducted on coated and uncoated samples that were pre-oxidised for 300 h at 850 °C as well as on untreated samples. The results indicate that for low stress amplitudes the lifetimes for coated, uncoated pre-oxidised and untreated samples are comparable. Above a certain stress amplitude the S-N-curves for coated and uncoated-pre-oxidised samples are much flatter than for untreated material. I. e. lifetimes of coated and uncoated pre-oxidised materials decrease much more with increasing stress amplitude than lifetimes of untreated samples. The behaviour is discussed based on cyclic deformation analysis and damage observations and explanations are proposed.

High temperature creep behavior of TiC-added Mo-Si-B alloy was studied in the temperature range of 1400 - 1600°C and the stress range of 75 – 300 MPa. Creep strain under constant applied stress control was measured by directly reading the change in specimen gage length using CMOS image sensors. 4 stages were observed on the creep curves. That is, I. instantaneous strain, II. primary creep, III. secondary creep with the minimum creep rate and IV. tertial creep with acceleration. The minimum creep rates showed a good linear relationship with applied stress in the Norton plot, and the obtained stress exponent, n, was about 3.5, indicating power-law creep. The minimum creep rates also showed a good linear relationship with inverse temperature in the Arrhenius plot, and the obtained apparent creep activation energy, Q_c, was about 497 kJ/mol. The value is in good agreement with the activation energy of Mo self-diffusion (488 kJ/mol), strongly suggesting that Mo self-diffusion is the rate-controlling process of creep deformation of the MoSiBTiC alloy under the conditions. Instantaneous strain varied from 10^-4 to 10^-2 depending on stress and temperature. To reach a 10^-2 instantaneous strain, some mechanisms in addition to an elastic strain should be considered to operate. Careful SEM observations indicated that micro-cracking in coarse T₂ phase and interfacial separation appeared to occur during the increase in instantaneous strain. Those weakening mechanisms would cause the large instantaneous strain. From the perspective of high temperature applications, the time for creep strain to reach 1% is one of the guidelines for development. Therefore, suppressing the occurrence of those weakening mechanisms through microstructure control as well as the increase in strain-hardening coefficient of Mo solid solution is important for improving the high temperature creep strength of the MoSiBTiC alloy especially in a small creep strain.

High-temperature structural materials require not only high solidus temperatures and acceptable densities, but especially good creep and oxidation resistance. The improvement in efficiency of high-temperature applications requires the development of alternates to Ni-based superalloys with operating temperatures higher than 1100 °C and densities lower than 8.5 g/cm³. Ti macro-alloyed Mo-Si-based alloys have shown great potential due to their high solidus temperatures in the range of 1800 °C and considerably reduced densities down to 6 to 7 g/cm³, while possessing a competitive creep resistance compared to state-of-the-art single crystalline Ni-based superalloys [1]. However, the oxidation behaviour is typically characterised by catastrophic mass loss at temperatures below 1000 °C caused by volatilisation of MoO₃, known as ‘pesting’ [1]. In order to achieve pesting-stability in Mo-Si-Ti alloys, a threshold of Ti content of 43 at% needs to be exceeded [2]. The attained fine-scaled microstructures are either fully eutectic (alloy Mo-20Si-52.8Ti (at%) comprising Mo solid solution Mo_SS and (Ti,Mo)₅Si₃ [1]) or eutectic-eutectoid (alloy Mo-21Si-43.4Ti, comprising Mo_SS, (Ti,Mo)₅Si₃ and (Mo,Ti)₅Si₃ [2]) and possess a solidus temperature-dependent creep resistance. Thus, the creep behaviour is determined by the Ti to Mo ratio, while the Ti content is pre-defined in order to ensure pesting-stability. With the aim of improving the creep resistance of those alloys, several alloy design approaches are pursued: (i) adjustment of the Mo content accordingly to the required Ti threshold, (ii) variation of the microstructural length scale including artificially coarsened microstructures and (iii) potential strengthening phases like Mo_SS and (Mo,Ti)₅Si₃. It is found that both targets, pesting-stability and creep resistance are met by an intermediate alloy Mo-21.6Si-44.2Ti comprising a eutectic-eutectoid matrix with a small volume fraction of primary solidified (Mo,Ti)₅Si₃ (< 5 vol%). Moreover, the alloy Mo-26Si-40Ti with almost 30 vol% primary solidified (Mo,Ti)₅Si₃ was created, which reveals a further improved creep resistance at 1200 °C and still provides pesting-stability.
[1] D. Schliephake, A. Kauffmann, X. Cong, C. Gombola, M. A. Azim, B. Gorr, H.-J. Christ, M. Heilmaier, Intermetallics. 2019, 104, 133-142.
[2] S. Obert, A. Kauffmann, M. Heilmaier, Acta Materialia. 2020, 184, 132-142.

Mo-Si-B alloys are high-performance materials that exhibit excellent mechanical properties at ultra-high temperatures (> 1200°C), thus enabling their use in demanding load scenarios. Especially ternary Mo-Si-B alloys, which consist of a Mo solid solution matrix with intermetallic phases (Mo₃Si and Mo₅SiB₂), offer the potential to replace the currently used nickel base superalloys, whose operating temperature is limited to a maximum of 1150°C.
However, the production of such Mo-Si-B alloys is difficult due to the very high melting temperatures of over 2000 °C and the complex solidification behavior. A possible way of production is a multi-stage powder metallurgical process. The development of a single-stage process for the production of Mo-Si-B alloys represents a new challenge.
Additive manufacturing processes such as the powder bed-based Laser Powder Bed Fusion (L-PBF) and the powder nozzle-based Directed Energy Deposition (DED) offer unique new possibilities for near-net-shape production with targeted adjustment of fine-grained microstructures or chemically graded materials.
Initial findings show that it is possible to produce near-eutectic Mo-Si-B alloys using the L-PBF process. These alloys show excellent creep properties and reduced brittle-ductile transition temperatures compared to other Mo-Si-B alloys with a silicide matrix.
Besides the mechanical properties at elevated temperatures, oxidation resistance is another decisive factor for the use of Mo-Si-B alloys. Current work is being carried out to investigate the behavior under cyclic oxidation conditions. The mass loss curves of the additively manufactured Mo-Si-B alloys show that, compared to nickel-based alloys, comparatively high mass losses occur, which can be explained by the microstructures formed by additive processing. However, the AM alloys show an improved oxidation resistance compared with powder metallurgically produced Mo-Si-B alloys having a solid solution matrix.

TiC is a promising carbide for ultrahigh temperature applications not only as a dispersion-strengthening phase but also as structural components. One of the biggest problems with this phase is its poor fracture toughness at room temperature. Interestingly, TiC phase has a wide compositional range away from stoichiometry like intermetallics, which can influence its elastic and plastic properties. Therefore, it is important to understand changes in the elastic and plastic properties of TiC phase in relation to off-stoichiometry and structural point defects. In this work, the effects of off-stoichiometry and Mo substitution on the elastic and plastic properties of TiC phase at room temperature were investigated for Mo-Ti-C ternary alloys in the Mo/TiC two-phase region. Multiple alloys on the same tie lines at 2073 K were made based on the Mo-Ti-C ternary phase diagram. The elastic moduli of the TiC phases with different terminal compositions in the two-phase region were estimated from the elastic moduli of the alloys obtained experimentally based on the rule of mixture by the Voigt model. The Young’s modulus and shear modulus of the TiC phase increase with Mo substitution, but decrease from off-stoichiometry toward the metal-rich side. The change in the elastic moduli of the TiC phase is more effective by the off-stoichiometry than by the Mo substitution. The increase in Young's modulus and the decrease in shear modulus may improve fracture toughness since the increase in yield stress and the improvement of plastic deformability increase the energy absorbed during crack propagation. The TiC phase in the Ti-C binary system, which has the lowest shear modulus and Young’s modulus, exhibits better toughness at room temperature than other TiC phases. The decrease in shear modulus appears to be a more important factor in improving the toughness of the TiC phase at room temperature.

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 the feasibility of printing pre-alloyed near-eutectic V-9Si-5B powder material via directed energy deposition (DED) as a method for additive manufacturing. Tailored V-9Si-5B powder material was produced by means of a gas atomization process. A novel setup for the DED experiments was developed and an overview of the production parameters for manufacturing of crack-free specimens is given. The microstructural evolution of the three-phase V-9Si-5B alloy is described by means of SEM, EBSD and STEM analyses during the entire process chain, i.e. the gas atomization of the powder material, the consolidation via DED and the heat treatment of the compacts. First mechanical tests demonstrate the high hardness and the competitive creep resistance of the AM V-9Si-5B material in comparison to other three-phase V-based alloys.
[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.

Additive manufacturing (AM) of Mo-Si-B alloys is very challenging due to their high melting temperatures above 2000 °C and their possible oxide formation reactions during processing. Additionally, multi-phase Mo-Si-B alloys are brittle up to ~ 950 °C, which is critical in terms of possible crack formation during fast cooling in AM processes.
In this presentation we will demonstrate that laser-based AM can be applied to produce dense Mo-Si-B bodies using tailored gas atomized powders. Therefore, the alloys Mo-9Si-8B, Mo-13.5Si-7.5B and Mo-16.5Si-7.5B were selected, which have different concentrations of silicide phases. We show how the modified processes Laser Powder Bed Fusion (LPBF) and Direct Energy Deposition (DED) lead to homogeneous and crack-free bulk materials with a low porosity of < 1%. The microstructure of these materials will be compared to microstructures derived from powder metallurgical (PM) processes and the properties of the AM and PM materials will be comparatively assessed. It will be demonstrated that the hardness, the brittle-ductile-transition-temperature (BDTT) and the creep response of AM Mo-Si-B materials are quite competitive with PM Mo-Si-B materials.

Transition-metal disilicides, TMSi₂ (TM = Nb, Cr, Ta and V) with the hexagonal C40 structure have received considerable attention as base materials of a new-class of ultra-high temperature structural materials for more than three decades because of their high melting points, attractive mechanical properties at high temperatures and relatively low density. In these hexagonal C40 silicides, only basal slip has been confirmed to be operative at high temperatures above 400 °C in pristine single crystalline specimens. Among these four C40 silicides, CrSi₂ exhibits quite peculiar slip behavior of synchroshear-type, while the other three silicides deforms with normal slip. Recently, we have investigated room-temperature plastic deformation behavior of various hard and brittle intermetallics, such as TM₅Si₃ type silicides, laves phase, sigma phase, by micropillar compression methodology and have confirmed the dislocation activations in most of these hard and brittle intermetallics even at room temperature in the micropillar form. In the present study, room-temperature deformation behavior of single crystals of various transition-metal disilicides TMSi₂ (TM = V, Cr, Nb and Ta) with the C40 structure was investigated by the micropillar compression method as a function of the loading axis orientation and specimen size in order to clarify the details of operative slip systems and to evaluate their CRSS values at room temperature. Dissociation schemes of the identified dislocations were investigated by transmission electron microscopy imaging of dislocation microstructures and atomic-resolution scanning transmission electron microscopy imaging of their core structure. Although plastic flow is observed only at high temperatures for most TMSi₂ in the bulk form, basal slip was confirmed to be operative at room temperature in the four C40-TMSi₂ in the micropillar form. The CRSS value for each slip system exhibits ‘smaller is stronger’ behavior following an inverse power-law relationship with the power-law exponents depending on slip systems and materials. Deformation mechanisms of these TMSi₂ at room temperature will be discussed based on the comparison with those previously obtained for bulk-sized single crystals at elevated temperatures.

Due to their low density in combination with a high melting point, vanadium-based alloys provide great potential as a new type of structural lightweight material for high temperature applications. Based on the promising V-Si-B system, the microstructural evolution and room temperature deformability in different binary eutectic V_SS (V solid solution)-V₃B₂ alloys were determined. In particular, it was investigated how the binary eutectic trough (V_SS-V₃B₂) is affected by alloying with silicon and how the resulting phase fractions affect the compression properties at room temperature. These aspects where systematically investigated for the first time.
Different binary V-B and ternary V-Si-B alloys were produced by conventional arc-melting. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) methods were used to examine the as-cast microstructures as well as to identify phases and to determine phase fractions. Microhardness measurements and room temperature compression tests were carried out to determine the mechanical properties in dependence of the respective phase fractions.
The results were compared with the current literature in order to estimate the contributions of the various phases to the resulting compressive strengths and to re-evaluate the information available on solidification processes in the V-Si-B system. The results indicate that the V_SS phase controls the plastic deformability in the V_SS-V₃B₂ eutectic microstructure whereas the intermetallic V₃B₂ phase acts as second phase strengthener. With increasing silicon addition, the eutectic composition is shifted to lower boron concentrations. Compared to the calculated liquidus projection of the V-Si-B system, the present experimental results also suggest a much larger binary eutectic trough than recently reported in the literature.

The solidification behavior of arc-melted alloys in the V-rich portion of the ternary V-Si-B alloy system has been experimentally investigated. A detailed microstructure analysis of several as-cast alloys based on SEM observations, EDS/WDS and EBSD measurements was carried out. As a result, different solidification reactions in the V-rich V-Si-B system were identified. On the two binary boundary systems V-Si and V-B those reactions were attributed to primary V_SS, V₃Si, V₅Si₃, V₃B₂ and VB. Furthermore, a primary solidification reaction within the ternary system was attributed to V₅SiB₂. As a result, the new data allows a reconstruction of the liquidus surface of the V-Si-B phase diagram. Furthermore, a V_SS-V₃Si-V₅SiB₂ ternary eutectic reaction was identified and its composition was investigated experimentally. The V-based ternary eutectic is isomorph to the well-known Mo-Si-B system and the Mo_SS-Mo₃Si-Mo₅SiB₂ ternary eutectic. However, the V-based ternary eutectic has a solid solution character since V_SS forms the major phase as compared to the intermetallic character of the Mo-base eutectic with the Mo₃Si phase as the major phase. This will have advantages considering low temperature deformability and fracture toughness.
By carrying out this work special attention had been paid to the eutectic reactions in the binary V-Si and V-B systems as well as in the ternary V-Si-B system. Thus, the binary eutectic compositions of V_SS-V₃Si and V_SS-V₃B₂ had been re-investigated carefully, since they represent important anchor points for the liquidus projection in the ternary system.
Besides the microstructure evolution, room temperature compression tests were carried out to get first insights in the alloys compressive yield stress and deformability. Silicon was found to act as a strong solid solution strengthener without embrittling the V_SS phase, even at its maximum Si solubility of about 7 at.%.

In this work we consider the design of alloys that can mimic the excellent combination of room temperature and high-temperature mechanical properties displayed by the refractory elemental metal rhenium, but with reduced cost. Experimental investigations of the deformation mechanisms and microstructures in this material highlight the important role of {11-21} deformation twinning, and high-resolution electron microscopy combined with first-principles calculations offer insights into the origin of the dominance of this twin system. These considerations lead to a design principle that trades off average valence electron count and cost considerations, identifying a promising pool of candidate substitute alloys in the Mo-Ru-Ta-W quaternary system. We demonstrate how this design principles is combined with a computational thermodynamics model of phase stability, based on high-throughput ab initio calculations, to further narrow down the search and lead to the identification of alloys that maintain rhenium’s desirable hcp crystal structure. The predictions of this modeling are validated through comparisons to known binary phase diagram sections and corroborated by experimental synthesis and structural characterization demonstrating multi-principle-element hcp solid-solution samples selected from a promising composition range. Additional theoretical calculations and initial mechanical properties measurements will be presented to assess the potential performance of these alloys for high temperature applications.

In the search for single-phase high entropy alloys (HEAs), intermetallic HEAs have been largely neglected. Intermetallic HEAs are often components of multi-phase HEAs that have good structural properties, so, it is critical to understand their deformation microstructures. This work shows that the HEA Al_8.8Co_18.1Cr_21.7Fe_20.2Ni_31.1, which is the L1₂ phase of the AlCoCrFeNi_2.1 eutectic system, can be cast as a single ordered phase. The deformation microstructure was examined in the transmission electron microcope using weak-beam dark-field imaging on samples compressed to ~3% plastic strain. Deformation occurred on the {111}<110> slip system and stacking faults, dipole nets, and large macrokinks were observed. The stacking fault energy was estimated and dissociated dislocations identified through careful tilting experiments. Tensile tests were performed on the alloy in both the disordered and strongly ordered states. The difference in tensile properties between the two is thus due to features unique to the ordered state, such as antiphase boundary (APB) tubes. Additionally, magnetization curves were taken with a vibrating sample magnetometer to measure the the change in magnetization with APB tube formation. The formation of APB tubes changes the local chemical environmnt, including where ferromagnetic elements sit in realtion to one another. The formation of APB tubes in ordered Al_8.8Co_18.1Cr_21.7Fe_20.2Ni_31.1 is compared to other binary alloys, with a focus on Ni₃Fe. This work was supported by the U.S. Department of Energy, Basic Energy Sciences, under award #DE-SC0018962.

For high-temperature structural applications, e.g. compressor blades in steam turbines, novel sustainable and heat resistant materials are required. Fe–Al based alloys show significant longevity due to their outstanding corrosion and wear resistance additionally to their low density and good mechanical properties.
Fe–Al based alloys are becoming more interesting for industrial applications, not at least because they are an economically efficient alternative to replace Cr-steels and Ni-based superalloys in specific applications. Moreover, standard processing techniques can be applied for their large-scale production [1, 2].

Through recent developments, especially the creep resistance could be improved by different alloying concepts [2]. E.g., incoherent precipitates such as borides or Laves phase or coherent precipitates can significantly enhance the mechanical properties.

In Fe–Al–Ta the formation of the stable hexagonal C14 Laves phase is preceded by the formation of the coherent metastable L2₁ Heusler phase, which improves strength and creep resistance up to 750°C. Through thermomechanical processing, heat treatment or doping, microstructures with fine scaled and homogeneously distributed Laves phase precipitates inside the Fe–Al matrix and at grain boundaries were obtained [3].

For further optimization, Ta was replaced by Nb in the present research.
In the Fe–Al–Nb system, the controlled distribution of the stable Laves phase within the Fe–Al matrix and at grain boundaries could also be achieved when the formation of the incoherent Laves phase is preceded by the formation of the coherent metastable Heusler phase.
Further investigations focus on the morphology of Laves phase precipitates at grain boundaries and their coarsening.

[1] C. G. McKamey, Iron aluminides, Phys. Metall. Process. Intermet. Compd. Springer (1996) 351-391
[2] M. Palm, F. Stein, and G. Dehm, Iron Aluminides, Annu. Rev. Mater. Res. 49 (2019) 297-326
[3] P. Prokopčáková, M. Švec, and M. Palm, Microstructural evolution and creep of Fe–Al–Ta alloys, Int. J. Mater. Res. 107 (2016) 396-405

Reactions at interfaces between different solid (crystalline or amorphous) materials are critical processing steps for a variety of applications including microelectronics, superconducting magnet wires, coatings on turbine blades, and reactive materials.[1] Due to the highly non-equilibrium nature of interfaces, phase formation at these interfaces is complicated and difficult to predict. It is well established experimentally that the first phase to form through an interfacial reaction need not be the most stable phase predicted by the phase diagram. As an example, DSC and TEM experiments in the Ni/Al multilayer system have suggested that the formation of intermetallic phases can be suppressed by a high heating rate, [2, 3] or by reducing the interspacing between the layers.[4, 5] Different heating rate or interspacing lead to different composition gradient across the interface, which strongly affects the thermodynamics and kinetics of the nucleation process.

We studied homogeneous nucleation of the non-stoichiometric NiAl-B2 intermetallic compound from deeply undercooled amorphous melts with an imposed composition gradient.[6] Data from MD simulations permits quantification of the nucleation rate of the intermetallic phase and indicates that nucleation proceeds in a polymorphous mode. The nucleation rate decreases with increasing gradient, and nucleation is completely suppressed above a critical gradient, as suggested by Desré and co-workers[7]. Based on an argument of crystal nucleus stability, a simple estimate provides good prediction of the critical gradient. A modified classical nucleation model parameterized with thermodynamic and kinetic quantities calculated independently predicts the nucleation rates and their variation with the imposed gradient, matching the MD results very well. This model can be used to compute the nucleation rates for different intermetallic compounds at an interface for the prediction of the phase formation sequence.

References:
1. Adams, D.P., Reactive multilayers fabricated by vapor deposition: A critical review. Thin Solid Films, 2015. 576: p. 98-128.
2. Trenkle, J.C., et al., Phase transformations during rapid heating of Al/Ni multilayer foils. Applied physics letters, 2008. 93(8): p. 081903.
3. Grapes, M.D., et al., A detailed study of the Al3Ni formation reaction using nanocalorimetry. Thermochimica Acta, 2017. 658: p. 72-83.
4. Edelstein, A.S., et al., Intermetallic phase formation during annealing of Al/Ni multilayers. Journal of Applied Physics, 1994. 76(12): p. 7850-7859.
5. Blobaum, K.J., et al., Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation. Acta Materialia, 2003. 51(13): p. 3871-3884.
6. Yi, et al., Predict homogeneous crystal nucleation rate from melts with a steep composition gradient, (to be submitted).
7. Desré, P.J. and A.R. Yavari, Suppression of crystal nucleation in amorphous layers with sharp concentration gradients. Physical Review Letters, 1990. 64(13): p. 1533-1536.

Recent global events have been negatively affecting the air transport market exposing the great need for operational excellence and efficiency due to optimal utilization of materials [1]. Better aircrafts with more efficient engines will push materials to the limit, and as a vital alloy for turbine discs, Alloy 718, a Ni-based superalloy, must have its properties improved in order to withstand higher temperatures and workloads. Direct ageing is a common processing route where solution annealing between forging and ageing is omitted, producing yield strength increases up to 10% in comparison to conventional ageing [2]. Several mechanisms are responsible for this behaviour, such as changes in γ-matrix grain size, δ phase fraction, dislocation density, and mainly the configuration of γ' and γ" precipitates [3]. An investigation regarding the nucleation and growth of these precipitates during the early stages of precipitation during direct ageing was published recently [4] and it has found that the presence of dislocations enabled the formation of γ" precipitates during air cooling from forging and an accelerated growth during direct ageing. A comparison between the evolution of the nanoscale precipitates during direct ageing of materials water quenched after forging and conventionally aged materials is still lacking. In this study through the use of atom probe microscopy and TEM in-situ heating, a holistic analysis of the precipitation of γ' and γ" is given and the effects on the mechanical properties are investigated.
[1] M. Hader, https://www.rolandberger.com/en/Point-of-View/COVID-19-How-we-will-need-to-rethink-the-aerospace-industry.html. Accessed 10/06/2020
[2] C. Krueger, The Minerals, Metals and Materials Society, 1989
[3] F. Theska, A. Stanojevic, B. Oberwinkler, S.P. Ringer, S. Primig, Acta Mater. 156 (2018) 116–124.
[4] F. Theska, K. Nomoto, F. Godor, B. Oberwinkler, A. Stanojevic, S.P. Ringer, S. Primig, Acta Mater. 188 (2020) 492-503.

Strengthening of Ni-base superalloys are in principle designed using GCP (Geometrically Closed- packed) phase of Ni₃Al-γ' (L1₂), however game-changing microstructural design principle without relying on γ' phase will be needed for further development of the alloys. We are currently constructing a novel microstructure design principle, using thermodynamically stable TCP (Topologically Close-packed phase) or α-W for grain boundaries, together with GCP (Geometrically Close-packed phase) other than γ' phase for grain interiors, based on grain boundary precipitation strengthening mechanism.
One of the promising systems is Ni-Cr-Mo ternary system, where TCP phases of NiMo, μ and P together with GCP phases of Ni₃Mo, Ni₄Mo and Ni₂Cr (oP6) exist. We have clarified that oP6 phase becomes extremely stable by Mo in solution at over 1073 K which is stable at 873 K in Ni-Cr binary system. Thus, there exist three-phase regions of γ/NiMo/oP6 and γ/P/oP6, unlike the phase diagram calculated based on commercially available thermodynamic databases. Moreover, oP6 phase precipitates coherently with tweed-like morphology and possess an orientation relationship of {110}_γ//(100)_oP6, <001>_γ//[010]_oP6, just like precipitation behavior of γ' particles in ordinary Ni-based superalloys. In contrast, the TCP phases preferentially precipitate at the g grain boundaries. On the other hand, in Ni-Cr-W ternary system, g phase equilibrates with α-W phase. By partially replaced Mo by W, there arises another possibility of γ/α-W/oP6 three-phase region. Since W is the same transition metals of VIb group with Cr and Mo, oP6 phase stability is expected to increase by W.
In order to evaluate the possibility for utilizing a novel microstructure design principle for Ni-based alloys having TCP phase at grain boundaries and GCP phase other than γ' phase within grain interiors, in this study phase equilibria among γ/TCP or α-W/GCP oP6 phases in Ni-Cr-M (M: Mo, W) ternary and Ni-Cr-Mo-W quaternary systems, have been experimentally examined at elevated temperatures. Since it is not easy to clarify the phase diagram based on only experimental, we assess the thermodynamic model for each phase and optimize the interaction parameters to calculate the phase diagram in good agreement with experimentally determined phase diagram. Novel microstructure design using TCP or α-W and oP6 phases has been suggested based on the phase equilibria.

An LPSO (long period stacking ordered) phase appearing in Mg alloys has been attracting attention as a strengthening phase. The phase is composed of transition elements (TM) such as Zn, Ni, Cu and Al, together with rare-earth elements (RE) such as Y, Dy and Gd. Many investigations have been dedicated to understand the stability of LPSO phases, however, still the reason of the formation type of Mg alloys with LPSO phases are not well understood. In a previous study, the effect of substitution of TM elements in a Mg-Al-Zn-Gd quaternary system from Mg85Al6Gd9 to Mg85Zn6Gd9 was investigated and found that the formation type of Mg alloys changes from Type II -> Type I -> Type II ⁽¹⁾. For a further discussion, it is needed to investigate the substitution behavior of rare-earth elements. Especially there are several rare-earth elements which do not form LPSO phases. In the present study it is attempted to confirm the existence of stable phases around the LPSO to understand the substitution behavior of Mg-Zn-RE LPSO phase.
Quaternary alloys, the total weight of each sample is approximately 30 g, were prepared by melting Mg blocks (99.95 wt.% purity), Zn blocks (99.5 wt.% purity) and several RE metal blocks (99.9 wt.% purity) in a graphite crucible placed in a high-frequency furnace under argon atmosphere, and casting into a mild steel mold. All the alloys sealed in glass tubes under argon atmosphere were annealed for up to 1000 h at 673 K or 723 K and then quenched into water. Phases in the samples were identified by X-ray diffraction (XRD, PHILIPS, X’Pert-MPD), and by wavelength dispersive X-ray spectrometer (WDS) equipped with field emission electron probe microanalyzer (FE-EPMA, JEOL, JXA-8530F).
It was found that almost no Ce substitutes for Y in Mg-Zn-Y-Ce based LPSO, while almost no Y substitutes for Ce in the Mg₁₂Ce phase. These phases co-exist in Mg85Zn6Y4.5Ce4.5 alloy, suggesting that the stable Mg₁₂Ce is one of the reasons why the limited amount of Ce can be solved into the LPSO phase.

Reference
(1) Formation of LPSO Phases in As-Cast Mg-Al-Zn-Gd Quaternary Alloys, K. Masaoka, T. Yamada, T. Horiuchi, T. Itoi and S. Miura, Japan Institute of Metals, VOL.83, (2019)257-263, Materials Transactions, VOL.61 (2020) 849-855.

Acknowledgments
A part of this work was supported by JSPS KAKENHI for Scientific Research on Innovative Areas “MFS Materials Science” (Grant Numbers JP18H05482). A part of this work was conducted at Hokkaido University, supported by "Nanotechnology Platform" Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

Tribaloy T400 (CoCrMoSi) alloy is considered as a potential candidate in impeding the wear-assisted surface material loss from engineering modules operating at a temperature as high as 800 °C. In this report, a hybrid suspension-powder plasma spray technique has been employed to deposit TiC (d₅₀= 2.2 µm)-reinforced T400 (average powder size = ~10-45 µm) composite coating for such application. A uniform and adherent T400-TiC coating of thickness ~100 µm revealed in microstructural analysis infers favorable higher densification (ImageJ Pro analysis) characteristic to suspension plasma spray. Dendritic Laves CoMoSi/Co₃Mo₂Si and hard intermetallic Co₇Mo₆/Co₂Mo₇ phases of T400, along with the corresponding TiC phases, are confirmed in phase analysis. Synergistic reinforcement of TiC has further elicited an enhancement in elastic modulus (of ~184.5 GPa) by ~36%, and Vickers hardness (of ~11.5 GPa) by ~82% in comparison to that of T400 (~135.2 GPa, and ~6.3 GPa respectively); which unambiguously elucidates the consequent increase in plasticity index by ~26.7% and a drop in maximum displacement amplitude (h_max) by ~19.8% in T400-TiC. A subsequent deceased fretting wear contact diameter, hence an improved damage tolerance, in T400-TiC compared to T400 alone is noted from a contact area-based wear model postulated herein. Further, with synergistic cell viability in L929 fibroblast, TiC reinforcement in T400 is ascertained to accommodate heavy-duty in-situ load-bearing application with substantially improved micro-hardness via retaining the smaller grain size, and a subsequent fretting wear augmentation through carbide phase strengthening.

The systematic tuning of the lattice parameters to achieve improved kinematic compatibility between phases is an effective strategy for improving the reversibility, and lowering the hysteresis, of solid-solid phase transformations. We present an apparently paradoxical example in which tuning to near perfect compatibility leads to a high degree of irreversibility in oxides of ZrHfYNb, as manifested by violently explosive behavior. Tuning lattice parameters slightly away from near perfect compatibility gives an intermediate “weeping” behavior, in which the polycrystal slowly and steadily falls apart at the grain boundaries. Finally, tuning to satisfy a condition we term the equidistance condition results in reversible behavior with the lowest hysteresis in this system. We give evidence that these observations are explained by a more careful analysis of compatibility of the polycrystal. These results show an extreme diversity of behavior, from reversible to explosive, is possible in a chemically homogeneous system by manipulating conditions of compatibility.

Shape memory materials or superelastic crystalline solids have the capability to recover their original shape even after the large amount of deformation is applied. The shape recovery of these materials usually occurs through a reversible phase transformation between martensitic and austenitic phase, which is a shear process. However, their elastic performance is limited by the energetics and geometry of the phase transformation, limiting their application range. Therefore, it is desirable to seek out a new material that can overcome this limit through a completely different superelasticity mechanism. In this presentation, we report our first discovery of the giant superelasticity in a novel intermetallic compound SrNi₂P₂ via the double lattice collapse and expansion, which correspond to making and breaking P-P bonds in two different co-existing crystal structures.
In-situ uni-axial compression on [001]-oriented SrNi₂P₂ micropillars revealed superelasticity with a remarkably huge elastic limit over 17%, which is one of the highest elastic limits ever reported for crystalline solids, via sudden lattice collapse and expansion. This sudden structural collapse corresponds to the first order phase transition. The collapsed tetragonal phase transition, which results from making P-P bonds during the uni-axial compression, is the main mechanism of superelasticity and differs entirely from the shear process of martensitic-austenitic transition of conventional crystalline superelastic materials. Density Functional Theory (DFT) simulation, X-ray diffraction measurement, and High-Resolution Transmission Electron Microscopy analysis have confirmed that SrNi₂P₂ consists of two different crystal structures. DFT calculations of uniaxial deformation of each crystal structure showed that these structures are collapsed at different stresses, leading to double lattice collapse and expansion, which are also shown as double-steps in both loading and unloading curves in stress-strain data. No fatigue damage was observed after 10,000 cycles of superelastic deformation because a simple making and breaking bond mechanism is not associated with any shear-based plastic deformation process such as dislocation plasticity or deformation twinning. Therefore, SrNi₂P₂ is expected to be strongly fatigue-resistant. Up to our knowledge, this is the first report that shows the co-existence of two different superelastic ThCr₂Si₂-type structures in a single material and their unique double lattice collapse and expansion. By considering that ThCr₂Si₂-type structure is one of the most populous crystal structures (over 2500 compounds), therefore, our results are believed to make a significant impact in fundamental understanding of superelasticity of ThCr₂Si₂-structured intermetallic compounds.

The low symmetry martensite phase in Nickel-Titanium (NiTi) shape memory alloy (SMA) has a hierarchical microstructure with micro-scale crystallites, which themselves consist of nano-laminates with an internal twin structure. The martensite phase deforms inelastically though reorientation of the twin structure. Furthermore, structural features in the specimens such as cracks or notches interact with the microstructure, influencing the deformation of martensite. We explore this complex interaction between structure and microstructure during tensile loading in martensitic NiTi specimens, with machined notches [1, 2].

In this work, using multi-scale experiments and macro-scale modeling, we demonstrate that the deformation of martensite in NiTi at the macro scale in a notched specimen is largely determined by the structural features, and the hierarchical nature of the microstructure plays a smaller role. Digital image correlation is used to obtain macro (specimen)-scale strain fields while macro-scale modeling is used to isolate the effect of notch geometry from the inherent microstructural anistropy in the heterogeneous deformation field. At the micro-scale, in-situ, high-energy x-ray diffraction measurements at two spatial resolutions (3D x-ray diffraction microscopy and microdiffraction) indicate that the martensite microstructure continuously evolves during tensile loading and unloading. This spatial evolution of the martensitic microstructure is also spatially heterogeneous, with respect to the notch. This observation is counter to the intuition of martensite twins reorienting to large domains of the most favored crystallographic variant at larger loads. Finally, the nano-laminate twin structure persists despite the large imposed deformation and the presence of stress concentrating features such as a notch, primarily due to the non-uniaxial stress field induced by the notch in its vicinity. Nanoscale characterization (using TEM) reveals nanocrystallization of martensite close to the notch on the fracture surface, and a persistence of twins away from the notch on the fracture surface.
These results can guide modeling of fracture-related deformation phenomena in NiTi martensite phase. This persistence of a nano-laminate structure rather than coarsening into large domains of individual variants indicates that the deformation field around cracks/notches in martensite may be modeled for fracture-related phenomena with adequate accuracy using coarse-grained models, without a compelling need for computationally expensive martensite single-variant-scale models.

References:
[1] P. Paul et. al, In-situ, microscale characterization of heterogeneous deformation around notch in martensitic Shape Memory Alloy, Materials Science and Engineering: A, 771, pp 138605 (2020).
[2] P. Paul et. al, Heterogeneity and Inelasticity of Deformation in a Notched Martensitic NiTi Shape Memory Alloy Specimen, Acta Materialia (2020).

CuZr-based alloys are being investigated as potential shape memory alloys for use in high-temperature applications. Previous studies on bulk samples of these alloys have reported typical athermal martensitic transformations where the degree of transformation to martensite is independent of time, and is a function of temperature and strain only. Here we report that in sputter-deposited CuZr, CuZrNi and CuZrCo thin-film samples, the austenitic phase can be supercooled below the martensite finish temperature M_f and then transforms to martensite in an explosive process that converts the entire sample in microseconds. This explosive transformation behavior is characterized by the following observations: 1) The transformation takes place at a critical temperature below M_f, causing a sudden temperature spike. 2) The critical temperature has a narrow distribution and decreases slightly with cooling rate. 3) Whether the transformation occurs explosively depends on the temperature history of the sample above the austenite finish temperature A_f. If the sample is quenched immediately on heating above A_f, the martensite forms gradually on cooling below the martensite start temperature M_s; if the sample is allowed to dwell above A_f for a few seconds, the martensite forms explosively. We suggest that austenite supercooling in thin-film samples is due to a lack of heterogeneous nucleation sites, while the gradual transformation is facilitated by the presence of defects. Transmission electron microscopy of CuZrCo samples suggests that the defects may be residual martensite above the austenite finish temperature. Kinetics measurements using nanocalorimetry reveals that the reverse transformation of this residual martensite can be described by an interface mobility model that takes into account the free energy difference between the two phases.

Heterogeneous catalysis is dominated by two material properties, i.e. the electronic structure (ligand effect) and the arrangement of the atoms (geometric effect). A priori, any material offering altered electronic or structural properties is of interest for heterogeneous catalysis. While intermetallic compounds fulfil these criteria, they offer three advantages compared to substitutional alloys: i) stability, caused by the chemical bonding, which can prevent segregation or decomposition in reactive atmosphere, ii) a wide range of chemical potential of the involved (transition)metals and iii) peculiar combinations of electronic and crystal structure. Since intermetallic compounds are thermodynamically stable and available in a wide range of materials, covering nanoparticles, polycrystalline materials as well as large single crystals, they can serve as platform materials to address catalytic challenges.
The stability allows studying their intrinsic properties in many environments, while the different available chemical potentials can be used to adjust the redox properties to the needs of the reaction in question. Systematic study of the geometric effects in reactions is possible by exploiting the large variety of available crystal structures. The use of isostructural substitutional series or compounds with different valence electron concentration allows addressing the electronic effect (ligand effect) on a reaction. Since only the filling degree of the electronic structure is changes, while the crystal structure and thus the electronic scaffold is kept, the electronic effect can be studied with only minor influence of the geometric effect. Like the metallic elements and the substitutional alloys, intermetallic compounds possess electrical conductivity, allowing electrochemical studies. The versatility of intermetallic compounds created a vivid and active community and in the contribution, several examples will be presented to elaborate the possibilities - and the requirements - for catalytic studies on this class of materials.

Demand for high-performance permanent magnets for motors is increasing rapidly for applications such as wind turbine generators and motors in electric and hybrid cars. Sm-Co and Nd-Fe-B magnets are generally used for such challenging applications. While these rare earth (RE) magnets have the highest energy product (BH)_max of any material, they are not without problems such as corrosion and brittle fracture. Other important issues are that over 95% of REs are produced in China, and that there are no US-owned manufacturers of RE magnets. There has also been substantial price volatility with the RE elements. Finally, RE mining has been associated with severe environmental degradation. In this project we aimed to establish a low-cost method to produce bulk L1_o-structured NiFe, which has magnetic properties comparable to the best RE magnets. Replacing the latter in electric motors in electric/hybrid cars and wind turbines will not only lead to large cost reductions, and hence greater adoption of these technologies with associated energy reductions, but the reduction in RE metal production will also lead to substantial environmental benefits and reduction in energy usage in their production. The key issue is that the transformation from the high temperature f.c.c. phase to the L1o-structured phase occurs at around 600 K and, thus, is exceedingly slow. We attempted several paths to produce the transformation to the equilibrium room-temperature L1_o - structured, i.e. the incorporation of a large supersaturation of vacancies followed by annealing; cold rolling (including at 77 K) followed by conventional annealing, electro-pulse annealing or high magnetic field processing; and high pressure torsion processing followed by annealing. We also looked at carbon doping, which can lead to the formation of a martensitic phase upon quenching. It is quite difficult to determine if the L1_o phase has formed since the superlattice reflections are weak and the lattice parameters of the L1_o and f.c.c phases are both 0.3585 nm. Thus, both synchrotron XRD and neutron diffraction were used to identify the phases, while EBSPs/orientation imaging in a SEM was used to determine the extent of recrystallization. The results of these different processing methods will be outlined. Funding at Dartmouth was provided by the Irving Institute for Energy and Society, the Women in Science Program, the First-Year Research in Engineering program, and the Presidential Scholars program.

Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. Thermoelectric properties are sensitively affected by microstructure of materials which changes during high temperature fabrication processing and practical long term operations. Improvement of thermoelectric performance can be achieved by microstructure control. We focus on the formation of interfaces on a multi-scale according to the phase separation between the same ordered structure phases in two alloy systems. One is phase separation between half-Heusler structure TiNiSn–(Zr,Hf)NiSn according to the miscibility gap, and the other is that between anti-fluorite structure Mg₂Si–Mg₂Sn during coring microstructure formation. Objective of the present work is to understand the formation mechanism of interfaces by the phase separation, and evaluate the effects of interfaces on thermoelectric properties focusing on the electrical and thermal conduction.
Half-Heusler MNiSn (M = Hf, Zr, Ti) are excellent n-type thermoelectric materials. The present author’s group reported the phase separation between TiNiSn and ZrNiSn, and TiNiSn and HfNiSn from a random solid solution MNiSn phase. The existence of a miscibility gap was suggested at temperatures higher than 1273 K. The relaxed lattice misfit, evaluated using XRD measurement, is about 3.2% between TiNiSn and ZrNiSn, and 2.5% between TiNiSn and HfNiSn, respectively. These values are quite large compared with 0.6% between ZrNiSn and HfNiSn. Considering the elastically constrained phase interface, a quite large elastic energy can be the driving force of the phase separation. Sharp grain boundaries between Ti-rich and Zr-rich half-Heusler phases can be observed in solidified microstructure of (Ti,Zr)NiSn alloys using SEM. On the other hand, it is very hard to observe a constrained interface. Diffuse interface can be barely observed using TEM in a (Ti,Zr)NiSn alloy after the heat treatment, annealed in the two-phase field for phase separation. Excellent electrical power factor exceeding 5 mWm^–1K^–2 can be achieved while the lattice thermal conductivity is effectively reduced around 3 Wm^–1K^–1 in Ti-rich (Ti,Zr)NiSn alloy according to the effect of solid solution together with the effect of interfaces introduced by phase separation.
Microstructure development in the Mg₂Si–Mg₂Sn pseudo-binary system is characterized by the peritectic reaction. The Mg–Si–Sn ternary phase diagram was evaluated experimentally focusing on the reaction scheme and solidification sequence. It has been found that the gradient composition layered structure is formed in Mg₂(Si_x,Sn_1-x) alloys on a micro-scale during the solidification process. Dominant mechanism is the phase separation of Mg₂(Si,Sn) solid solution phase into Si-rich Mg₂Si(Si,Sn) and Sn-rich Mg₂(Sn,Si) phases during the coring structure development. Phase separation is occurred leaving phase interfaces to release the large elastic energy stored due to quite large lattice misfit. The constrained lattice misfit between Si-rich Mg₂Si(Si,Sn) and Sn-rich Mg₂(Sn,Si) phases has been evaluated about 5% based on the powder XRD measurements, and the semi-coherent interfaces with interfacial dislocation networks can be observed using TEM. Each layer of the gradient composition layered microstructure is consisting of nano- to micro-scale poly-crystal grains having a compositional gradient of Si/Sn composition ratio. The lattice thermal conductivity can effectively be reduced lower than 2 Wm^–1K^–1 in a Mg₂(Si_0.75,Sn_0.25) alloy which contains high density of phase interfaces of gradient composition layered microstructure. Note that the dominant mechanism for the reduction of thermal conductivity is attributed to the phono scattering based on the solid solution effect in Mg₂(Si_x,Sn_1-x) phase grains and layers.

Composite materials are conceptualized with the intent of obtaining a final material with improved characteristics. In thermoelectrics this has been achieved for organic-inorganic composites where conducting materials embedded in polymeric matrixes improved the thermoelectric properties. A completely different situation is the combination of two inorganic compounds. Although according to some studies a thermoelectric composite cannot have better properties than its constituents, a study from Bergman and Fel (1999) proved that the enhancement of the figure of merit of a composite material is possible under specific conditions. The purpose of this work is to show the effect of different kind of metal inclusions on the electronic and thermal property of Yb₁₄MnSb₁₁. Yb₁₄MnSb₁₁, with a zT of about 1.2 at 1200 K, is already the most performing p-type material for high temperature thermoelectric applications. Our intent is to further increase its thermoelectric performance by decoupling the electronic transport properties (σ, S). Transition metals are characterized by a metallic electrical conductivity, but some of them still show a relatively high Seebeck coefficient. Therefore, the selected metal inclusions are expected to provide a boost to the electrical conductivity, while affecting only marginally S. Another important aspect of compositing with metal inclusions is the improvement of mechanical properties and fracture toughness, a fundamental aspect in terms of device fabrication. To verify the validity of our assumptions, we synthesized samples with different inclusion density, always remaining largely below percolation limit. The samples have been chemically, electrically, thermally, and mechanically characterized and the obtained results will be shown and compared with the baseline of the Jet Propulsion Laboratory for the Yb₁₄MnSb₁₁ ATEC (Advanced ThermoElectric Converter).

BaCuSi clathrate samples for p-type thermoelectric materials were synthesized with various initial composition of Cu ratio. Cu ratio in the samples increased with initial composition Cu ratio. However, volume of BaCuSi clathrate phases in their samples decreased with the increase of initial composition of Cu ratio. Those phases cannot be detected their accurate electrical properties because of their small grain size. In this study, we synthesized the sample by using CZ method to obtain the larger phase. It was found that the Cu ratio in the CZ sample was smaller than the sample without CZ method. The CZ sample with a large grain size can be obtained with heavy Cu doping sample. The CZ sample successfully showed p-type properties.

Heavily-doped, p-type PbTe thermoelectric materials reach the heat-to-electricity conversion efficiency necessary for improved space power generation or waste heat recovery. However, unexplained brittle behavior at high electronic doping levels occurs in p-type PbTe, but not in n-type PbTe. This distinctively asymmetric trend restricts the thermoelectric community’s best-performing materials for practical use. Our mechanical and elastic property measurements on single crystal and polycrystalline PbTe contradict previously suggested explanations for embrittlement, such as bond stiffening from PbTe’s Σ band. We also show certain dopants break the previously established mechanical trend. Strain measurements through neutron powder diffraction show that ball milling can double the amount of strain in undoped PbTe beyond unstrained samples. Introducing extrinsic p-type dopants increases the strain by another factor of two and better maintains strain at high temperatures. We propose that increased dislocation strain from typical processing methods is linked to embrittlement. Further, by analyzing extrinsic doping efficiency, we note a link between strain and both extrinsic dopant defects and intrinsic compensating defects. Introducing dislocation strain, as is often done to reduce thermal conductivity and increase thermoelectric efficiency, thus appears to also be detrimental for mechanical properties in p-type PbTe. This study seeks to explore the link between these properties and offers routes to tune them through phase boundary mapping, choice of dopant, and intrinsic defect engineering.

Thermoelectric properties of the materials originate from thermal conduction by free charge carriers instead of by collective lattice vibrations (phonons). Understanding the scattering mechanism, or dissipation, of phonons allows the engineering of materials with higher coefficient of Seebeck [1], i.e. higher difference of electric potential created for a given thermal gradient. In skutterudite type of thermoelectric materials, dissipation of phonons are attributed to localized, low frequency and anharmonic vibrations of heavy ions of the lanthanide family introduced into the large voids of the crystalline structure. Since most of these materials can be synthesized in the form of high quality single crystals, we have the possibility of revisiting the phonon scattering mechanism through x-ray phase measurements via dynamic diffraction effects [2]. Structure factor phases are susceptible to the differences between the vibration amplitudes of the atoms (root mean square atomic displacements), or in other words, phase values are invariants with temperature only when all occupied sites of the unit cell have the same Debye-Waller factor.

Structure factor calculation in model structures revealed suitable Bragg reflections and x-ray energies to resolve the difference in atomic vibrations as a functions of temperature. It also revealed a giant resonante phase shift for the whole familiy of filled skutterudites RFe4P12 (R=Ce, La, Nd, Pr, Sm). By exploiting this resonant phase shift with synchrotron x-rays at different temperatures in CeFe4P12, we were able to demonstrated that rattling of Ce alone inside the icosahedral cage is not enough to explain phonon scattering and that the whole cage of 12 P is taking part of scattering mechanism.
[1] G. J. Snyder, E. S. Toberer. Complex thermoelectric materials. Nat. Mater. 7, 105-114 (2008).
doi = 10.1038/nmat2090
[2] S. L. Morelhão, C. M. R. Remédios, G. A. Calligaris, G. Nisbet. X-ray dynamical diffraction in amino acid crystals: a step towards improving structural resolution of biological molecules via physical phase measurements. J. Appl. Cryst. 50, (2017). doi: 10.1107/S1600576717004757

Both cast and wrought titanium aluminide alloys have been studied for more than two decades because of attractive properties that include low density, high specific strength, high specific stiffness and oxidation resistance up to about 700°C. Electron beam melting provides another processing approach to produce net shape components, although little work has been conducted to examine processing-microstructure-property relationships. This work examines as-deposited γ-TiAl (Ti-48Al-2Cr-2Nb) specimens made by Arcam AB and post-processed materials. Mechanical behavior studies on as-deposited, HIPed, and HIPed + Vacuum heat treaded conditions included Vickers micro-hardness, compression, fracture toughness and fatigue crack growth testing. In addition, microstructural details were investigated over a range of scales using various microscopy tools. The presentation will summarize this evolving work on the characterization of AM γ TiAl and provide some comparison to other conventional (e.g. as-cast, wrought) γ-TiAl alloys.

Electron Beam Melting has proven to be a technology capable of producing high quality Titanium Aluminide blades and is currently being applied to additively manufacture Low Pressure Turbine airfoils for the GE9x: the largest, most powerful and fuel-efficient commercial aircraft engine in its class, which incorporates GE’s most advanced technologies developed over the last decade.
While providing several benefits such as being capable to process a highly reactive alloy such as TiAl, producing sound parts and reducing development time, this additive manufacturing technology has also posed many challenges related to its industrialization and scale-up.
This article is aimed at giving an overview of the main technical areas that had to be addressed to advance EBM from a lab scale to a mass production environment and to guarantee material and process control when it comes to part certification.
One of the main areas of attention was to identify Key Process Variables, determine the associate tolerance band and their impact on desired characteristics of finished parts, such as mechanical (e.g., static strength, fatigue, fracture), metallurgical, physical, or chemical properties.
A significant part of this effort was focused on gaining a good understanding of the defects that can be generated by the additive process and that are different from traditional manufacturing routes such as casting and forging. Additionally, the impact of such defects on material properties, their detectability and acceptance limits are key items to be considered.
Process quality control is another crucial focus area, including part-to-part, build-to-build, machine-to-machine variations, machine qualification, powder handling and correlation between batch coupons and parts. Destructive and non-destructive tests and statistical process control are also part of this effort.
As of now more than 10000 blades have been produced and most certification engine tests have been completed, which makes a great contribution to pushing further the additive technology development and usage.

TiAl-alloys are a typical example that advanced modern lightweight materials have an increasingly complex chemical composition and phase constitution. Accordingly, a sophisticated and complete characterisation of the material is necessary to fully comprehend their behaviour and properties for knowledge-based alloy and process development. Unfortunately, many processing methods for TiAl as well as their operation itself take place at high temperature making their direct monitoring and characterisation difficult. In addition, it is also of interest to investigate the material behaviour if other external loads as for example an applied stress are present in parallel.
In the contribution examples for the direct monitoring and characterisation of the processing of TiAl alloys using in situ specimen environments and high energy X-ray diffraction at synchrotron sources are shown. The processes can be rather simple as for example a heat treatment at a certain annealing temperature where the phase composition and its development are monitored (e.g. [1]). But, also complex processing scenarios like hot forming [1,2] or a transient liquid phase bonding process [3] can be directly investigated by those methods. In the former case the texture development during the hot forming is observed while in the latter case the solidification of the joining zone and the subsequent phase transformations are investigated to understand the microstructure formation.
Simple heat treatment, hot forming, transient liquid phase bonding and many other processes for TiAl alloys have in common that phase constitution, texture or microstructure react very sensitively to changes of applied temperature and/or stress. They normally change significantly when the stress is removed or the specimen is cooled down at the end of the processing step. Due to these reasons it is not possible to unambiguously reconstruct the situation at processing temperature and/or under stress based on subsequent material characterisation at room temperature or in absence of the mechanical load. In situ characterisation as presented here is the only means to aquire the full picture and get insight into crucial process details under processing conditions.

References:
[1] Pyczak, F., Rackel, M., Gabrisch, H., Halici, D., Lorenz, U., Schreyer, A., Stark, A., In-situ monitoring of thermal treatments and hot deformation of a g-TiAl alloy by high energy X-ray diffraction, 123HIMAT-2015 Proceedings (2015) 323-326.
[2] Stark, A., Rackel, M., Tchouaha Tankoua, A., Oehring, M., Schell, N., Lottermoser, L., Schreyer, A., Pyczak, F., In situ High-Energy X-Ray Diffraction during Hot-Forming of a Multiphase TiAl Alloy, Metals 5 (2015) 2252-2265.
[3] Hauschildt, K., Stark, A., Burmester, H., Tietze, U., Schell, N., Müller, M., Pyczak, F., Phase Transformations in the Brazing Joint during Transient Liquid Phase Bonding of a g-TiAl Alloy Studied with In Situ High-Energy X-Ray Diffraction, Materials Science Forum 941 (2018) 943-948.

Intermetallic TiAl alloys based on the γ-TiAl phase are already used as engineering lightweight high-temperature materials in aircraft and automotive engines. Thereby, they partly substitute the twice as heavy Ni-base superalloys. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines and engine parts used in racing sport applications. All the applications mentioned above require balanced mechanical properties, i.e. certain ductility at room temperature as well as defined creep strength at elevated temperatures. In the framework of this presentation the alloy design strategies, which have been applied for the development of so-called process adapted TiAl alloys will be explained. Besides the considerations which have led to the selected alloying elements, the heat treatments conducted subsequent to conventional hot-forging and additive manufacturing are discussed. In this context it will be shown that a combination of computer-aided alloy design and novel characterization techniques, e.g. in-situ high-energy X-ray and neutron diffraction, has accelerated both alloy and process development as well as the understanding of this class of alloys sustainably.

In this work we consider the design of alloys that can mimic the excellent combination of room temperature and high-temperature mechanical properties displayed by the refractory elemental metal rhenium, but with reduced cost. Experimental investigations of the deformation mechanisms and microstructures in this material highlight the important role of {11-21} deformation twinning, and high-resolution electron microscopy combined with first-principles calculations offer insights into the origin of the dominance of this twin system. These considerations lead to a design principle that trades off average valence electron count and cost considerations, identifying a promising pool of candidate substitute alloys in the Mo-Ru-Ta-W quaternary system. We demonstrate how this design principles is combined with a computational thermodynamics model of phase stability, based on high-throughput ab initio calculations, to further narrow down the search and lead to the identification of alloys that maintain rhenium’s desirable hcp crystal structure. The predictions of this modeling are validated through comparisons to known binary phase diagram sections and corroborated by experimental synthesis and structural characterization demonstrating multi-principle-element hcp solid-solution samples selected from a promising composition range. Additional theoretical calculations and initial mechanical properties measurements will be presented to assess the potential performance of these alloys for high temperature applications.

The systematic tuning of the lattice parameters to achieve improved kinematic compatibility between phases is an effective strategy for improving the reversibility, and lowering the hysteresis, of solid-solid phase transformations. We present an apparently paradoxical example in which tuning to near perfect compatibility leads to a high degree of irreversibility in oxides of ZrHfYNb, as manifested by violently explosive behavior. Tuning lattice parameters slightly away from near perfect compatibility gives an intermediate “weeping” behavior, in which the polycrystal slowly and steadily falls apart at the grain boundaries. Finally, tuning to satisfy a condition we term the equidistance condition results in reversible behavior with the lowest hysteresis in this system. We give evidence that these observations are explained by a more careful analysis of compatibility of the polycrystal. These results show an extreme diversity of behavior, from reversible to explosive, is possible in a chemically homogeneous system by manipulating conditions of compatibility.

Heterogeneous catalysis is dominated by two material properties, i.e. the electronic structure (ligand effect) and the arrangement of the atoms (geometric effect). A priori, any material offering altered electronic or structural properties is of interest for heterogeneous catalysis. While intermetallic compounds fulfil these criteria, they offer three advantages compared to substitutional alloys: i) stability, caused by the chemical bonding, which can prevent segregation or decomposition in reactive atmosphere, ii) a wide range of chemical potential of the involved (transition)metals and iii) peculiar combinations of electronic and crystal structure. Since intermetallic compounds are thermodynamically stable and available in a wide range of materials, covering nanoparticles, polycrystalline materials as well as large single crystals, they can serve as platform materials to address catalytic challenges.
The stability allows studying their intrinsic properties in many environments, while the different available chemical potentials can be used to adjust the redox properties to the needs of the reaction in question. Systematic study of the geometric effects in reactions is possible by exploiting the large variety of available crystal structures. The use of isostructural substitutional series or compounds with different valence electron concentration allows addressing the electronic effect (ligand effect) on a reaction. Since only the filling degree of the electronic structure is changes, while the crystal structure and thus the electronic scaffold is kept, the electronic effect can be studied with only minor influence of the geometric effect. Like the metallic elements and the substitutional alloys, intermetallic compounds possess electrical conductivity, allowing electrochemical studies. The versatility of intermetallic compounds created a vivid and active community and in the contribution, several examples will be presented to elaborate the possibilities - and the requirements - for catalytic studies on this class of materials.

A five-year National project of “Structural Materials for Innovation (SM⁴I)” in Cross-ministerial Strategic Innovation Promotion Program (SIP) starting from 2014 in Japan ended in 2018, and the following another 5-year project of “Materials Integration for Revolutionary Design System of Structural Materials” in the SIP II Program has started in 2018. For both programs I have been committed to as the technical leader and take responsibility for the alloy design and development of TiAl alloys, in collaboration with universities and industries. In the first program, the focus was placed on the design/process principles of innovative wrought and cast TiAl alloys to be used for LPT blades, based on the thermodynamics and kinetics of multi-components titanium aluminide systems, and we have successfully proposed and developed new alloys. In the second program, however, we have focused on construction of inverse design principle for innovative TiAl alloys with required properties available for both MIM (metal injection molding) and AM (additive manufacturing) processes, in collaboration with Kobe Steel, Ltd., Osaka Yakin Kogyo Co., Ltd, Mitsubishi Heavy Industries Aero Engine and Osaka University. The degree of freedom of the microstructure design in MIM and AM processes is much wider than that in wrought and cast processes, but attention has to be paid to impurity levels of oxygen and carbon picked up by processing routes of powdering, injection molding and sintering in case of MIM. In this talk, we first show you how oxygen affects the phase equilibria among β-Ti, α-Ti (α₂-Ti₃Al) and γ-TiAl phases, and eventually affects the microstructures created through the phase transformations, based on our phase diagram studies. You have to be careful for alloy development under powder processes if you use currently existing thermodynamic databases. None of them works. The microstructure can significantly be changed from the fully α₂/γ lamellae to the β/γ grained, depending on oxygen level, even the major alloy elements as well as the heat treatment routes are the same. Current progress on this project, especially how important the b phase plays in role in improving mechanical properties for both MIM and AM, based on our novel customized powders, will be presented. A part of this study has been carried under the research of SIP II in JST (Japan Science and Technology Agency).

High temperature creep behavior of TiC-added Mo-Si-B alloy was studied in the temperature range of 1400 - 1600°C and the stress range of 75 – 300 MPa. Creep strain under constant applied stress control was measured by directly reading the change in specimen gage length using CMOS image sensors. 4 stages were observed on the creep curves. That is, I. instantaneous strain, II. primary creep, III. secondary creep with the minimum creep rate and IV. tertial creep with acceleration. The minimum creep rates showed a good linear relationship with applied stress in the Norton plot, and the obtained stress exponent, n, was about 3.5, indicating power-law creep. The minimum creep rates also showed a good linear relationship with inverse temperature in the Arrhenius plot, and the obtained apparent creep activation energy, Q_c, was about 497 kJ/mol. The value is in good agreement with the activation energy of Mo self-diffusion (488 kJ/mol), strongly suggesting that Mo self-diffusion is the rate-controlling process of creep deformation of the MoSiBTiC alloy under the conditions. Instantaneous strain varied from 10^-4 to 10^-2 depending on stress and temperature. To reach a 10^-2 instantaneous strain, some mechanisms in addition to an elastic strain should be considered to operate. Careful SEM observations indicated that micro-cracking in coarse T₂ phase and interfacial separation appeared to occur during the increase in instantaneous strain. Those weakening mechanisms would cause the large instantaneous strain. From the perspective of high temperature applications, the time for creep strain to reach 1% is one of the guidelines for development. Therefore, suppressing the occurrence of those weakening mechanisms through microstructure control as well as the increase in strain-hardening coefficient of Mo solid solution is important for improving the high temperature creep strength of the MoSiBTiC alloy especially in a small creep strain.

Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. Thermoelectric properties are sensitively affected by microstructure of materials which changes during high temperature fabrication processing and practical long term operations. Improvement of thermoelectric performance can be achieved by microstructure control. We focus on the formation of interfaces on a multi-scale according to the phase separation between the same ordered structure phases in two alloy systems. One is phase separation between half-Heusler structure TiNiSn–(Zr,Hf)NiSn according to the miscibility gap, and the other is that between anti-fluorite structure Mg₂Si–Mg₂Sn during coring microstructure formation. Objective of the present work is to understand the formation mechanism of interfaces by the phase separation, and evaluate the effects of interfaces on thermoelectric properties focusing on the electrical and thermal conduction.
Half-Heusler MNiSn (M = Hf, Zr, Ti) are excellent n-type thermoelectric materials. The present author’s group reported the phase separation between TiNiSn and ZrNiSn, and TiNiSn and HfNiSn from a random solid solution MNiSn phase. The existence of a miscibility gap was suggested at temperatures higher than 1273 K. The relaxed lattice misfit, evaluated using XRD measurement, is about 3.2% between TiNiSn and ZrNiSn, and 2.5% between TiNiSn and HfNiSn, respectively. These values are quite large compared with 0.6% between ZrNiSn and HfNiSn. Considering the elastically constrained phase interface, a quite large elastic energy can be the driving force of the phase separation. Sharp grain boundaries between Ti-rich and Zr-rich half-Heusler phases can be observed in solidified microstructure of (Ti,Zr)NiSn alloys using SEM. On the other hand, it is very hard to observe a constrained interface. Diffuse interface can be barely observed using TEM in a (Ti,Zr)NiSn alloy after the heat treatment, annealed in the two-phase field for phase separation. Excellent electrical power factor exceeding 5 mWm^–1K^–2 can be achieved while the lattice thermal conductivity is effectively reduced around 3 Wm^–1K^–1 in Ti-rich (Ti,Zr)NiSn alloy according to the effect of solid solution together with the effect of interfaces introduced by phase separation.
Microstructure development in the Mg₂Si–Mg₂Sn pseudo-binary system is characterized by the peritectic reaction. The Mg–Si–Sn ternary phase diagram was evaluated experimentally focusing on the reaction scheme and solidification sequence. It has been found that the gradient composition layered structure is formed in Mg₂(Si_x,Sn_1-x) alloys on a micro-scale during the solidification process. Dominant mechanism is the phase separation of Mg₂(Si,Sn) solid solution phase into Si-rich Mg₂Si(Si,Sn) and Sn-rich Mg₂(Sn,Si) phases during the coring structure development. Phase separation is occurred leaving phase interfaces to release the large elastic energy stored due to quite large lattice misfit. The constrained lattice misfit between Si-rich Mg₂Si(Si,Sn) and Sn-rich Mg₂(Sn,Si) phases has been evaluated about 5% based on the powder XRD measurements, and the semi-coherent interfaces with interfacial dislocation networks can be observed using TEM. Each layer of the gradient composition layered microstructure is consisting of nano- to micro-scale poly-crystal grains having a compositional gradient of Si/Sn composition ratio. The lattice thermal conductivity can effectively be reduced lower than 2 Wm^–1K^–1 in a Mg₂(Si_0.75,Sn_0.25) alloy which contains high density of phase interfaces of gradient composition layered microstructure. Note that the dominant mechanism for the reduction of thermal conductivity is attributed to the phono scattering based on the solid solution effect in Mg₂(Si_x,Sn_1-x) phase grains and layers.