John Lewandowski, Case Western Reserve University
Svea Mayer, Montanuniversitaet Leoben
Soumya Nag, GE Global Research
Hiroyuki Yasuda, Osaka University
GE Global Research
Metal Technology Co., Ltd.
National Science Foundation
PM06.01: Titanium Aluminides I
Monday AM, November 26, 2018
Hynes, Level 1, Room 104
8:30 AM - PM06.01.01
Development and Application of Titanium Aluminides
Wilfried Smarsly1,Helmut Clemens2,Svea Mayer2
MTU Aero Engines1,Montanuniversität Leoben2Show Abstract
Intermetallic Titanium Aluminid based alloys are considered for high-temperature aero and automotive engine applications. The advantage of this class of innovative high-temperature materials is their low density in combination with good strength and creep properties up to 850°C. A drawback, however, is their limited damage tolerance at room temperature, which is reflected in a low plastic fracture strain and fracture toughness. Advanced engineering Titanium Aluminid alloys are complex multi-phase materials which can be processed by ingot or powder metallurgy. Engine components can be manufactured by casting as well as additive manufacturing, e.g. electron beam melting. Each ingot production process leads to specific microstructures which can be optimized by thermo-mechanical processing, e.g. isothermal or hot die forging and subsequent heat treatments. Thermo-mechanical processing can provide balanced mechanical properties, i.e. a minimum ductility at room temperature as well as sufficient creep strength at elevated temperature. In order to achieve this goal, the knowledge of the occurring solidification processes and phase transformation sequences is essential. Therefore, thermodynamic calculations were conducted to predict the phase diagram of engineering TiAl alloys. After experimental verification, these phase diagrams provided the basis for the development of heat-treatments. To account the influence of deformation and kinetic aspects sophisticated ex- and in-situ methods have been employed to investigate the evolution of the microstructure during thermo-mechanical processing. For example, in-situ high-energy X-ray diffraction was conducted to study dynamic recovery and recrystallization processes during hot-deformation tests. Novel high-strength Titanium Aluminid based alloys, such as TNM alloys have been developed in the last decade to meet the advanced requirements of aero engines.These alloys are characterized by a high content of β-stabilizing alloying elements, such as Nb and Mo. Because Nb and Mo represent the decisive alloying elements, this alloy family, based on the γ-TiAl phase, has been named “TNM alloys” in order to distinguish them from the well-known and even stronger “TNB alloys” which rely on a high Nb concentration and small additions of B and C. At room temperature, strength levels > 1000 MPa can be achieved in advanced TiAl alloys by appropriate thermo-mechanical processing and subsequent heat treatments. It is important to note that also high temperature properties, such as creep resistance, were considerably improved, e.g. by implementation of precipitation hardening, which further extend the application range of Titanium Aluminid based alloys.
9:00 AM - PM06.01.02
Design Approaches and Achievements of Novel Wrought TiAl Alloys for Jet Engine Applications
Masao TakeyamaShow Abstract
Innovation of structural materials is urgently being required for contribution to worldwide issues on energy, environment, and high performance jet engine development with larger thrust-to-weight ratio is one of them, since more than 30,000 new airplanes are to be produced by 2030.
A five-year National project of “Structural Materials for Innovation (SM4I)” in Cross-ministerial Strategic Innovation Promotion Program (SIP) starting from 2014 is currently going on in Japan. In this project of SM4I, a focus is placed on innovative structural materials applicable for jet engines, where author at Tokyo Tech is committed to as a technical leader and take responsibility for alloy design and development of TiAl alloys in collaboration with other universities (Hokkaido Univ, Osaka Univ.) and industries (Kobe Steel, Ltd and IHI Co.). In this talk, design approaches and achievements for the development of wrought TiAl alloys to be used for LPT and HPC blades are presented. First, we built up a new database for phase diagram calculations in multi-component systems of the alloys, including both substitutional and interstitial elements. In case of Ti-Al-M1-M2 quaternary systems, for example, we optimize the interaction parameters to calculate the phase diagram in good agreement with experimentally determined phase diagram, by taking into considerations of temperature, aluminum and M1/M2 concentration dependencies in the four phases of β-Ti, α2,-Ti3Al, α-Ti and γ-TiAl phases. Based on the phase diagram calculations, we successfully proposed model alloys with excellent hot workability even at higher strain rate and room temperature ductility of more than 1%. It should be noted that an introduction of bcc b-Ti phase and microstructure design using a unique phase transformation pathway of β+α→α→β+γ in the multi-component systems makes it possible to have excellent properties in both process and service temperatures.
We revealed the alloy with lamellar microstructure decorated by β/γ duplex microstructure at the lamellar colony boundaries show better crack initiation and propagation resistance than that with fully lamellar microstructure. The detailed microstructure control method using the phase transformations and toughening mechanism will be presented.
Part of this study was carried under the research of SIP in JST (Japan Science and Technology Agency).
9:15 AM - PM06.01.03
Phase Transformation and Microstructure Evolution during Continuous Heating of an Intermetallic β-homogenized Ti-43Al-7Mo (at.%) Alloy
Petra Erdely1,Peter Staron2,Andreas Stark2,Thomas Klein1,3,Helmut Clemens1,Svea Mayer1
Montanuniversitaet Leoben1,Helmholtz-Zentrum Geesthacht2,Now with: Materials Center Leoben Forschung GmbH3Show Abstract
Intermetallic γ-TiAl based alloys provide promising engineering properties for lightweight high-temperature applications. Hierarchical structures that comprise microstructural constituents on the nanometer scale have received much attention in recent years, as research suggests that they can further improve the performance of these alloys. In the present work, a ternary Ti-43Al-7Mo (at.%) model alloy is investigated. This β/γ alloy can be homogenized in the β single-phase region at 1450 °C, while water quenching prevents further large-scale phase transformations. Subsequent reheating to temperatures above 500 °C provokes the formation of fine γ particles on the sub-micron level. As these particles appear homogeneously distributed throughout the βo matrix and form a continuous network after long annealing times, the microstructure of the alloy can, thus, be substantially refined. However, the exact mechanisms of this phase transformation are still not fully understood. The present work addresses these mechanisms through the application of a variety of characterization techniques. In a dilatometer setup at a synchrotron radiation source, β-homogenized specimens were heated at various constant rates ranging from 10 to 400 K/min. In situ high-energy X-ray diffraction was used to study the structural changes in the prevalent phases. The combination of in situ small-angle X-ray scattering and ex situ atom probe tomography allowed the analysis of the early stages of the precipitation process by characterizing the nanometer-scale precipitates in terms of size, shape and chemical composition. The final microstructure of selected heat-treated specimens was analyzed using scanning electron microscopy. In sum, the experiments offered deep insights into the functioning and the kinetics of the thermally activated γ growth sequence in a β-homogenized Ti-Al-Mo alloy. The detailed understanding of this phase transformation is an essential prerequisite for the design of refined microstructures in β/γ alloys without the necessity for prior hot working.
9:30 AM - PM06.01.04
Improvement of Mechanical Properties of TiAl Alloys Fabricated by Electron Beam Melting though Microstructure Control
Ken Cho1,Masahiro Sakata1,Takumi Fukuoka1,Jong Yeong Oh1,Hiroyuki Yasuda1,Mitsuharu Todai2,Takayoshi Nakano1,Ayako Ikeda3,Minoru Ueda4,Masao Takeyama5
Osaka University1,Institute of Niihama National College of Technology2,National Institute for Materials Science3,Metal Technology Co. Ltd.4,Tokyo Institute of Technology5Show Abstract
TiAl alloys are candidate materials for low pressure turbine blades of a jet engine due to their low density and excellent strength at high temperatures. Electron beam melting (EBM) is one of the suitable manufacturing processes for the TiAl alloys. Because it is possible to fabricate 3D products easily by adding layer-on-layer of materials. We found the TiAl alloys fabricated by EBM have unique layered microstructure consisting of equiaxed gamma grains regions (gamma bands) and duplex-like regions perpendicular to building direction. This layered microstructure is formed by repeated thermal effect from the melting pool.
In this study, the influence of the layered microstructure on mechanical properties of the TiAl alloys was investigated focusing on the morphology of the gamma bands. To control the layered microstructure, the alloys were fabricated by EBM at various conditions. Moreover, some of the as built alloys were subjected to heat treatments including hot isostatic pressing (HIP) treatments at various temperatures. The width of the gamma bands was affected not just by the electron beam scan speed but also by the heat treatments. The room temperature ductility of the alloys increased with increasing the width of the gamma bands and reached a maximum at approximately 3%. The alloys with suitable morphology of the gamma bands show also excellent fatigue strength. These results indicate that the alloys with the unique layered microstructure have a great potential for aerospace applications.
9:45 AM - PM06.01.05
Micro-Mechanical Response and Texture Evolution of Ultrafine-Grained Titanium Aluminide Processed by High-Pressure Torsion
Megumi Kawasaki1,Jae-Kyung Han1,Xi Li2,3,Rian Dippenaar2,Klaus-Dieter Liss4,3,2
Oregon State University1,University of Wollongong2,Australian Nuclear Science and Technology Organisation3,Guangdong Technion - Israel Institute of Technology4Show Abstract
The present study applies a strategy of grain refinement on a γ-based Ti-45Al-7.5Nb intermetallic compound through high-pressure torsion (HPT) processing for 5 and 10 turns under a compressive pressure of 6.0 GPa at room temperature. Successful grain refinement was introduced in the duplex microstructure to have ultrafine laths with thicknesses of 40-100 nm after 10 HPT turns. Exceptional hardness is recorded after the severe plastic deformation and the X-ray diffraction analysis confirmed the occurrence of phase transformation from γ-TiAl to α2-Ti3Al in the alloy. The close investigation demonstrated that hardness and texture evolution vary gradually from the sample surfaces to the core, thereby exhibiting a heterogeneous microstructure. Macroscopic plastic flow is estimated thorough the micro-mechanical responses by the nanoindentation technique for the TiAl alloy. This study demonstrates the promising feasibility of HPT processing for improving essential mechanical properties in the TiAl.
10:30 AM - PM06.01.06
Industrial Recycling of Valuable TiAl Revert
GFE Metal & Materials GmbH1Show Abstract
Low Pressure Turbine blades made of TiAl alloys are being used in three civil aircraft engine families (GEnx, PW1000GTF, LEAP). The production process of TiAl semi-finished parts is based on three technological pathways:
VAR ingot metallurgy, VAR Skull Melter homogenization and subsequent investment casting to oversized components followed by mechanical machining
VAR ingot metallurgy, VAR Skull Melter homogenization and subsequent centrifugal casting in permanent moulds to small sized feed stocks for either forging and mechanical machining or direct mechanical machining
Plasma Arc Cold Hearth Melting to small sized ingots for direct mechanical machining
In both processing routes b) and c), a substantial amount of valuable TiAl revert is being generated during the different processing steps. An industrial recycling pocess based on Vacuum Induction Skull Melting with subsequent centrifugal casting according to the technological pathway b) has been developed and commissioned at GfE. Appropriate revert preparation technologies prevent a detectable impurity pick-up from previous processing steps even for multiple use of revert. The corresponding revert preparation and revert conversion technology is approved and validated for the production of pre-materials for components for aircraft engines. The presentation addresses the origin of revert and the appropriate revert preparation technology. The recycling process via Induction Skull Melting is being introduced. Resulting products are indistinguishable from products resulting from the virgin production route via VAR Skull Melting with regard to chemical composition and microstructure.
11:00 AM - PM06.01.07
Introduction of YSK-TiAl, Novel Wrought Alloy with High Strength and Advanced Productibility
Keiji Kubushiro1,Yutaro Ota1,Yohei Sakakibara1,Shin Usui1,Satoshi Takahashi1,Kotaro Tagawa1,Masanobu Baba1
IHI Corp1Show Abstract
To evaluate forgeability for wrought TiAl alloy (YSK alloy : Ti-43Al-9(V, Nb)-0.2B), greeble test and compression test were carried out. As a result, it was confirmed that deformation at (β+α) two-phase region promoted recrystallization of α phase and recrystallization was easily developed as grain size was smaller. In addition, (β+α) two-phase region in YSK alloy was larger than the others. Hot-forging was performed on the condition revealed by above tests, and then mechanical properties of heat-treated samples were investigated. Tensile strength and creep strength of YSK alloy were equal to or higher than the other alloys reported so far.
DKth at R.T was about 10 MPa√m. As to forgeability, extrusion, upset, and hot-die-forging at high speed succeeded with existing facilities. At last, more than 200 blades were manufactured by the established method.
11:15 AM - PM06.01.08
New Atomic Relaxation Mechanism Discovered in Nano-Lamellar γ-TiAl
Jose San Juan1,Leire Usategui1,Maria No1,Thomas Klein2,Svea Mayer3,Helmut Clemens3
Universidad del Pais Vasco1,Materials Center Leoben Forschung GmbH2,Montanuniversität Leoben3Show Abstract
Two different families of γ-TiAl intermetallics are already flying as blades of the low-pressure turbine in some engines of commercial aircraft, and considerable effort is being devoted to improve their performances. Here we focus on the family named TNM+, which is based on the TNM alloy but containing specifically designed amounts of C and Si in order to optimize the performances. The goal is to increase the creep resistance allowing to extent the working temperature range, and to this purpose a fully nano-lamellar microstructure was developed through thermal treatments.
In the present work we study at high temperature the mobility of defects controlling the mechanisms responsible for creep. However, the study is approached through mechanical spectroscopy by measuring the internal friction (IF) spectra between 300 and 1350oC. On one hand, the high temperature background (HTB) of the IF is closely related to creep behavior , and on the other hand, the relaxation peaks of the IF spectra offer valuable information about the atomic diffusion mechanisms involved in creep [2,3]. In particular, the developed nano-lamellar microstructure allow us to discover and analyze a new relaxation peak, which remains hidden in between the IF peak associated to Ti diffusion in α2-Ti3Al phase  and the HTB. We have measured the IF spectra at different frequencies and performed a deep analysis to decompose the IF spectra into the different contributions and isolate the relaxation corresponding to each individual atomic mechanism. This way we measured the activation energy of the hidden relaxation, Ea=3.7 eV, which is attributed to the Aluminum diffusion in γ-TiAl phase. Moreover, a new atomic relaxation mechanism is proposed to explain the characteristic of the observed IF peak. Finally, the importance of the discovered relaxation is discussed in terms of its relationship with the atomic diffusion processes involved in creep.
11:30 AM - PM06.01.09
Density Functional-Based Kinetic Monte Carlo Approach to Ni Pipe Diffusion
Luke Wirth1,Amir Farajian1,Christopher Woodward2
Wright State University1,Air Force Research Laboratory2Show Abstract
Pipe diffusion along dislocations in metals can occur at faster rates than in bulk regions, expediting creep and contributing to device failure. We use density functional theory methods to calculate the vacancy-mediated diffusion coefficient along a <1 -1 0> screw dislocation in FCC nickel. A lattice Green’s function technique is used to accurately configure the atomic structure surrounding the dislocation core. Vacancy hop rates within this region are calculated using principles of transition state theory with activation energy barriers and vibrational properties obtained from our ab initio framework. A kinetic Monte Carlo model then uses these rates to describe mass transport within the region on representative timescales at various temperatures.
PM06.02: Silicides and Ultra-High Temperature Alloys
Monday PM, November 26, 2018
Hynes, Level 1, Room 104
1:30 PM - PM06.02.01
Alloy Design of Refractory BCC-T2 Silicide-B2 Aluminide Multicomponent-Multiphase Alloys
Seiji Miura1,Satoshi Takizawa1,Ken-ichi Ikeda1
Hokkaido University1Show Abstract
Refractory-metal based materials have attracted attentions for many years because of the increasing demand for high temperature components. In order to obtain such materials having superior properties for high temperature use, multicomponent-multiphase alloys are required. Authors have conducted studies on the mechanical and physical properties of BCC solid-solution phase, M5Si3-T2 silicide phase and B2 aluminide phase, together with phase equilibrium among these constituent phases.
Authors have conducted the investigation on the effects of various additive elements on the strength and deformability of BCC solid solution. As (Nb, Mo)5(Si, B)3 phase dispersion in Nb-Mo BCC solid-solution matrix has been studied by various researchers, we also try to understand the phase stability of T2 phase in alloys. Although silicide phases including (Nb, Mo)5(Si, B)3 show superior oxidation resistance, we have focused on B2-aluminide coating because B2-NiAl coatings have been used for commercial Ni-based superalloys. To realize a three-phase alloys composed of refractory BCC, T2 silicide and B2 aluminide, phase equilibrium in ternary, quaternary and higher-order phase diagrams are experimentally investigated. Among them, brittle compounds such as Nb(Ni, Al)2 Laves phase form at the interphase boundary between NiAl and Nb-based alloys during high temperature heat-treatment. To avoid the formation of Laves phase, both the B2 phase composition and the BCC alloy composition were optimized based on the phase diagrams such as Nb-Al-Pd and Nb-Mo-NiAl, then a composition area at which BCC solid-solution phase equilibrates with B2 aluminide was found in Nb-Mo-Ni-Al-Pd quinary system.
By the systematic investigation, it was found that the atomic size ratio of constituent elements is still an important key to understand the stability of ternary Laves phases including Al. For a further understanding of the substitution behavior of elements in the Laves phase, electronic structure calculations based on the density functional theory (DFT) was performed on the alloy system including Si.
This work was supported by the Advanced Low Carbon Technology R&D (ALCA) program of the Japan Science and Technology Agency (JST).
2:00 PM - PM06.02.02
Role of Mo Solid Solution on Ultrahigh-Temperature Tensile Creep Deformation of MoSiBTiC Alloy
Kyosuke Yoshimi1,Shiho Kamata1,Shunichi Nakayama1,Sojiro Uemura2,Sadahiro Tsurekawa2,Gunther Eggeler3,Kouichi Maruyama1
Tohoku University1,Kumamoto University2,Ruhr-University Bochum3Show Abstract
A Mo-Si-B-based alloy reinforced by TiC (65Mo-10Ti-5Si-10C-10B (at.%)) shows high creep resistance estimated over the rupture time of 1000 h under 137 MPa at 1350°C and relatively good room-temperature fracture toughness over 15 MPa(m)1/2. The excellent mechanical properties arise from the high strength of Mo5SiB2 (T2) and TiC, the good ductility of Mo solid solution, and the microstructural configuration of these phases. Under creep deformation, the resistance of Mo solid solution would be one of key factors controlling the overall creep strain of the material and the interfacial sliding between these phases would be the other factor. In this paper, the role of Mo solid solution on ultrahigh-temperature tensile creep deformation is addressed for the MoSiBTiC alloy. The microstructural continuity of Mo solid solution was analyzed in terms of the percolation probability. It was clarified by the percolation probability analysis that both the Mo solid solution and brittle phases (T2 and TiC) were not 100%-continuous in the microstructure. EBSD and TEM observations for ruptured specimens presented dynamic recrystallization in Mo solid solution due to heavy plastic deformation during creep but much less plastic deformation in the brittle phases. These results represent that the creep strain was given by the dislocation creep of Mo solid solution and the interfacial sliding between the phases. The apparent activation energy of creep was estimated to be about 560 kJ/mol from the Arrhenius plot of the logarithm of the minimum creep rate against inverse temperature. The value is much higher than the activation energy of the self-diffusion of Mo. This strongly suggests that the rate-controlling process of the creep is governed by the bulk diffusion in Mo solid solution rather than interfacial diffusion. However, it is unlikely to be caused by Mo self-diffusion because of its lower activation energy, but it might be caused by impurity diffusion, for example, of interstitial (I)-substitutional (S) pairs. The role of Mo solid solution on ultrahigh-temperature tensile creep deformation of the MoSiBTiC alloy will be further discussed with experimental data.
2:15 PM - PM06.02.03
Density Reduced Moss-(Mo,V)3Si-(Mo,V)5SiB2 Alloys
Julia Becker1,Ulf Betke1,Manja Krüger2
Otto von Guericke University1,Forschungszentrum Jülich GmbH2Show Abstract
In terms of preserving resources and reducing environmental impacts, improving the efficiency of turbines for power plants and aircraft engines is an increasingly important research subject. Potential high performance materials are Mo-Si-B alloys. Consisting of a molybdenum solid solution (Moss) phase and two intermetallic phases Mo5SiB2 (T2) and Mo3Si those alloys present balanced room temperature fracture toughness, high temperature creep strength and oxidation performance. In this work vanadium as a lightweight element with a density of 6.1 g/cm3 has been identified as a potential alloying partner which can entirely be solved in the Moss phase as well as in the Mo3Si and Mo5SiB2 phase. To identify the role of vanadium in terms of strengthening the solid solution phase different Mo-XV (X = 5…50 at.%) alloy compositions were produced and evaluated by means of microhardness measurements. Additionally, quantitative values for solid solution hardening were determined by the approach of Labusch. Compared to other alloying concepts, e.g. Mo-Ti, vanadium affects a more balanced strength – ductility relation at room temperature, i.e. a slightly reduced strength but increased plastic deformability.
In the next step, potential Mo-V-Si-B materials which provide a reduced density by about 20% as compared to reference alloy Mo-9Si-8B were investigated. Different alloy compositions were produced by powder metallurgy to observe the effects of V as a solute in the respective phases. The microstructure of the bulk Mo-40V-9Si-8B was characterized via SEM and XRD. From Rietveld refinements the preferred V sites in the lattices of the present phases were derived. The mechanical behavior was determined by microhardness measurements as well as constant displacement tests in the compressive mode between room temperature and 1100 °C. Three point-bending with notched samples as well as compressive creep tests reveal a high fracture toughness and acceptable creep strength of this new type of alloys. Therefore, the first results show a high potential as a lightweight version of Mo-Si-B alloys for structural applications.
2:30 PM - PM06.02.04
A Phase Field Study on Script Lamellar Pattern of MoSi2/Mo5Si3 Eutectic
Chuanqi Zhu1,Yuichiro Koizumi2,Akihiko Chiba1,Kyosuke Kishida3,Haruyuki Inui3
Tohoku University1,Department of Materials Science and Engineering, Osaka University2,Center for Elements Strategy Initiative for Structure Materials (ESISM), Kyoto University3Show Abstract
MoSi2-based alloys and composites have been considered as promising candidates for high temperature structural application. Directionally solidified (DS) MoSi2/Mo5Si3 composites have a script pattern in which discontinuous Mo5Si3 rods inclined to the growth direction are embedded within MoSi2 matrix. Since a deeper understanding of pattern formation is crucial to the microstructure design for property optimization, a phase field model based on Multiphase-Field framework has been constructed to elucidate the responsibility of continuous nucleation on solid-liquid interface for the discontinuity of this pattern, which has a close relation to material toughness. Under solidification conditions with various growth rates, three dimensional computation results of microstructures reproduced the characteristics of script pattern observed by scanning electron micrography (SEM). In addition, the simulation results show good agreement in length scale of lamellar spacing with experimental images and analytical solutions obtained by Jackson-Hunt approach.
2:45 PM - PM06.02.05
Printability of Mo-Si-B Alloys via Additive Manufacturing
Janett Schmelzer1,Silja-Katharina Rittinghaus2,Andreas Weisheit2,Martin Stobik3,Jörg Paulus4,Karl Gruber4,Egbert Wessel5,Manja Krüger5,Christoph Heinze6
Otto-von-Guericke-University Magdeburg1,Fraunhofer – Institut für Lasertechnologie2,NANOVAL GmbH & Co.KG3,Dr. Kochanek Entwicklungsgesellschaft4,Institut für Energie- und Klimaforschung (IEK-2)5,Siemens AG6Show Abstract
Current research on high temperature metallic materials focuses on Mo-Si-B alloys which are candidates for novel turbine materials. For structural applications, an important phase field in the ternary Mo-Si-B system is located between the Mo solid solution phase (Moss) and the silicides Mo5SiB2 (T2) and Mo3Si (A15), which is known as the so-called ‘‘Berczik-triangle’’. Near-eutectic Mo-Si-B alloys from this three-phase region exhibit outstanding creep properties, even above temperatures of 1100 °C, as well as a good oxidation resistance. However, ingot processing (IM) of this class of materials is challenging due to the high melting point of Mo-Si-B materials being typically > 2000 °C (eutectic Moss-Mo3Si-Mo5SiB2 alloys ~ 2000 °C). Different multi-step powder metallurgical processes (PM) were typically used in the past to produce dense Mo-Si-B samples under laboratory conditions.
The introduction of a one-step processing route for this type of material via additive manufacturing (AM) or 3D printing represents an important innovation that will allow the production of complex bulk materials with net shape geometries (e.g. turbine blades). This work shows the feasibility of printing pre-alloyed near-eutectic Mo-Si-B powder materials via laser metal deposition (LMD). Therefore, Mo-Si-B powder was manufactured via gas atomization (GA) process out of solid raw materials meeting the requirements for AM regarding flowability and particle size. The specific challenge is the ultra-high melting point of this type of alloys, accompanied by problems of interlayer bonding and defects that may occur during cooling. Compact multiphase Moss-Mo3Si-Mo5SiB2 builts containing low porosity could be manufactured. For further understanding of the microstructural evolution powder particles after GA were investigated and detailed analyses of the resulting microstructure were carried out. For purpose of comparison with PM and IM Mo-Si-B alloys first results of mechanical tests, e.g. hardness, compressive strength and creep response, are presented.
3:30 PM - PM06.02.06
Room Temperature Deformation of Transition-Metal Silicides Investigated by Micropillar Compression Method
Kyosuke Kishida1,Haruyuki Inui1
Kyoto University1Show Abstract
Transition-metal (TM) silicides have received considerable attention as promising structural materials for ultra-high temperature applications that can replace the currently used Ni-based superalloys because of their very high melting temperature above 2000 °C, good mechanical properties and good oxidation resistance. Extensive studies using bulk single- and poly-crystals have revealed that most TM silicides can plastically deform only at high temperatures above 1000 °C. The activation of various deformation modes have been reported so far, however, some ambiguity in the identification of operative deformation modes still remains mostly because of severe oxide formation on the specimen surface as well as dislocation climb, which is inevitable in the case of high temperature experiments. Recently, the micropillar compression method first introduced by Uchic et al. has been widely recognized as a new attractive technique to investigate the mechanical response of not only pure metals but also various hard and brittle materials at size-scales of tens of micrometers or less. Recently, we have applied the micropillar compression method to single crystals of various TM silicides such as TMSi2 (TM=Mo, Nb, Ta, V, Cr), TM5Si3 (TM=Mo, Nb, Ti) and T2-Mo5SiB2 as a function of specimen size and loading axis orientation. For most of the TM silicides tested, plastic flow was observed if the specimen size is reduced to micron meter size. For T2-Mo5SiB2 phase, three different slip systems were identified to be operative at room temperature. The values of critical resolved shear stress (CRSS) for the three slip systems in T2-Mo5SiB2 are extremely high all exceeding 2 GPa. The CRSS value for each slip system increases with the decrease in the specimen size, following the inverse power-law relationship with an exponent much smaller than those reported for FCC and BCC metals. The dissociation scheme and glide plane (actual atomic layers) of the identified dislocations in T2-Mo5SiB2 were investigated both experimentally through atomic-resolution scanning transmission electron microscopy imaging of their core structures and theoretically by first-principles calculations of the relevant generalized stacking fault energy curves.
4:00 PM - PM06.02.07
Mo Base Alloys Feasibility and Optimization for the High Temperature Turbine Blades Application
Olha Popovych1,Hanna Tsybenko1,Konstantin Naumenko1,Manja Krüger2
Otto von Guericke University Magdeburg1,Research Center Jülich2Show Abstract
Application of the single-crystal Ni base superalloys for turbine blades at temperatures of up to 90 % of their melting point already reached the limit of their development. Because of the beneficial physical and mechanical properties at high temperatures, Mo base alloys are very promising to substitute Ni base alloys. These materials possess high temperature strength and excellent creep resistance as well as acceptable fracture toughness. Nevertheless, the replacing of Ni base superalloys in turbine applications is still a difficult problem. This study is focused on FE calculations of the deformation of a simple turbine blade made of Mo based alloys under the typical loading conditions in comparison to conventional turbine blade materials, aimed to evaluate the feasibility of Mo base alloys.
The investigations of Mo base alloys displayed the best combination of the high temperature properties for alloys containing Si and B, specifically for the alloy family with a three-phase microstructure: ductile molybdenum solid solution phase and the two intermetallic phases Mo3Si and Mo5SiB2. However, an extensive knowledge about the physical and mechanical properties of each phase is not achieved, yet. In this study, the creep properties of the individual phases are determined and applied for the estimation of the creep properties of the alloy in general. This provides a possibility for the advanced alloy design.
4:15 PM - PM06.02.08
Phase Field Simulation of Spontaneous C11b-MoSi2/D8m-Mo5Si3 Eutectic Reaction in Directional Solidification
Yuichiro Koizumi1,Chuanqi Zhu2,Toshihiro Yamazaki2,Akihiko Chiba2,Koretaka Yuge3,Kyosuke Kishida3,Haruyuki Inui3
Osaka University1,Tohoku University2,Kyoto University3Show Abstract
C11b-MoSi2/ D8m-Mo5Si3 eutectic alloys with script lamellar structure  have been proposed as one of the candidates for materials of next-generation gas turbine operated at extremely high temperatures above 1700 °C. The refinement of script lamellar structure and the modification the properties of lamellar interface are believed to improve the toughness at room temperature and creep resistance at high temperature, which is the key to the practical application. In this study, we have developed a phase-field model for examining the effects of various factors on the geometry of the script lamellar structure formed during directional solidification on the basis of the previously developed model for simulating lamellar structure formation in C11b-MoSi2/C40-NbSi2 [2, 3]. The model can take into account the elastic strain energy and the relaxation of strain by structural ledges. It is also possible to include interfacial segregation of ternary elements to the model. When spontaneous decomposition from liquid to C11b-phase and D8m-phase was assumed, inclined lamellar structures were formed under limited conditions, and the lamellar spacing decreased with increasing cooling rate as expected. It is suggested that the nucleation of D8m-Mo5Si3 is important for more quantitatively precise reproduction of script lamellar structure. The effects of nucleation will be presented by Zhu et al. elsewhere. Reference:  K. Fujiwara et al. Intermetallics 52 (2014) 72-85.  T. Yamazaki et al. Intermetallics 54 (2014) 232-241,  T. Yamazaki et al. Comp. Mat. Sci. 108 (2015) 358–366.
4:30 PM - PM06.02.09
Microstructural Study of a Nb-Si Based Alloy Through the Different Steps of a Powder Metallurgy Route
Virgil Malard1,2,Stefan Drawin1,David Neumeyer3,Jean-Philippe Monchoux3,Anne Denquin1,Alain Couret3
ONERA1,Paris-Saclay University2,CEMES/CNRS3Show Abstract
Niobium silicide intermetallic alloys are good candidates for applications as low-pressure turbine blades in aircraft engines for service temperatures between 800°C and 1000°C. The Nb-Si based alloys are multiphase materials constituted by a niobium solid solution Nbss, silicide phases of the Nb5Si3-α and / or Nb5Si3-β (tetragonal structure) and / or Nb5Si3-γ (hexagonal) type, sometimes Nb3Si type, depending on the heat treatments applied or the alloying elements added. The solidification structure is generally dendritic with more or less complex eutectic cells. Microstructure control during solidification and processing steps is essential to obtain good mechanical properties. This is difficult by conventional casting, knowing that forming processes (forging, etc.) require high temperatures for these materials which often contain more than 50 vol.% of intermetallic phases. Powder Metallurgy (PM), thanks to the microstructural homogeneity it offers, and to easier shaping processes, is a manufacturing process that deserves to be studied.
This communication reports the study of an alloy with composition 43Nb-25Ti-3Mo-3Cr-6Al-20Si (at.%). The microstructure of this alloy has been followed throughout a typical PM cycle. It is detailed for the following steps: (1) cast ingot to be atomized; (2) powders produced by inert gas atomization, of [40-100] μm and [100-200] μm particle size; (3) bulk samples obtained by Spark Plasma Sintering (SPS) at 1385°C, 1520°C and intermediate temperatures; (4) bulk SPS samples with additional heat treatment (1500°C, 100 h). Quasi-static compression tests as well as creep tests, at 800°C and 1000°C, are presented.
The microstructures are compared with respect to the nature and morphology of the phases, the chemical homogeneity and the porosity. The main detected phases are Nbss (containing Ti, Al, Mo, Cr), two forms of the Nb5Si3 silicide (α or β, and γ; Nb is mainly substituted by Ti) and TiN, probably formed during ingot melting. Depending on the state of the material, the observations show, for the silicides, different sizes, chemical compositions and morphologies, as well as a variable microstructural homogeneity. The local silicon content can create hypoeutectic zones and lead to the primary solidification of Nbss. The microstructure of partially densified samples shows the sintering mechanism by Nbss diffusion at the interparticle contact points. The SPS process for niobium silicide based alloys offers good prospects for obtaining a fine and homogeneous microstructure, conferring good mechanical properties on this type of intermetallic material.
This work has benefited from State support managed by the ANR under the "Investissements d'Avenir" program through the MATMECA project (reference ANR-10-EQPX-37) and from ANR funding through the SYNOPSIS project (reference ANR-15-CE08-0042).
4:45 PM - PM06.02.10
Characterization Crystal Phase and Thermoelectric Properties of Thin Film SrSi2
Kodai Aoyama1,Takao Shimizu1,Hideto Kuramochi2,Masami Mesuda2,Ryo Akiike2,Yoshisato Kimura1,Hiroshi Funakubo1
Tokyo Institute of Technology1,Tosoh Corporation2Show Abstract
Silicon based alloys with good thermoelectric property such Si-Ge, Mg-Si and Mn-Si system have been widely investigated not only bulk form but also thin film form. α-SrSi2 is also a promising candidate as a thermoelectric material because it consists of abundant nontoxic elements and a good thermoelectric power factor near the room temperature was reported by Hashimoto et al . However, the number of researches is limited compared with former widely investigated silicide and there are no reports in film form. In this study, we firstly prepared α-SrSi2 thin films on insulating substrates and measured their thermoelectric properties.
Thin films of Sr-Si system were deposited on c-Al2O3 substrate by using RF magnetron sputtering method at various deposition temperature and total pressure.
Constituent phases strongly depend on the deposition temperature. The films deposited below 400oC consisted of amorphous phase. Metastable SrSi2 phase with CaSi2 structure was obtained between 500 and 600oC and finally stable α-SrSi2 above 700oC.
Metastable SrSi2 phase with CaSi2 structure showed low power factor below 10 μW/(m K2) for the temperature range of 100-400oC. On the other hand, α-SrSi2 show good thermoelectric power factor beyond 700 μW(m K2) at room temperature. This much value is larger than observed value of Mg2Si (111) one-axis-oriented films prepared by the same deposition process, maximum 130 μW/(m K2) at 300oC. The present result shows that α-SrSi2 is one of the promising candidates as thin film thermoelectric materials.
K. Hashimoto et al., J. Appl.Phys. 102 (2017) 063703.
John Lewandowski, Case Western Reserve University
Svea Mayer, Montanuniversitaet Leoben
Soumya Nag, GE Global Research
Hiroyuki Yasuda, Osaka University
GE Global Research
Metal Technology Co., Ltd.
National Science Foundation
PM06.03: Titanium Aluminides II
Tuesday AM, November 27, 2018
Hynes, Level 1, Room 104
8:30 AM - PM06.03.01
Productionization of Gamma Titanium Aluminides for Aerospace Applications
Although the benefits of titanium aluminides for high temperature applications were well conceived and significant research and development activities were conducted in the past four decades, they remained as developmental materials due to barriers associated with melting, processing, scale-up, and affordable productionization. Demanding requirements of efficient aero-engines and extensive risk reduction demonstrations paved the path for commercial introduction of gamma titanium aluminides, the single most attractive current application is for low pressure turbine blades in aero-engines replacing conventionally cast nickel superalloys. This talk provides an overview of recent progress, productionization challenges, and opportunities for improvements.
9:00 AM - PM06.03.02
A Study of the Creep Properties at High Temperatures of the as-spsed IRIS-TiAl Alloy
Alain Couret1,Jean-Philippe Monchoux1
This presentation aims at describing the creep properties at high temperatures 700°C-850°C of the IRIS-TiAl alloy (Ti-48Al-2W-0.08B) densified by Spark Plasma Sintering (SPS). SPS is a technic of powder metallurgy for which the densification is due to the simultaneous application of a direct pulsed electric courant of high intensity and of a uniaxial pressure on a graphite assembly containing the powder. This process allows achieving original microstructures with enhanced properties due to its rapid processing cycle as well as non-textured, homogeneous alloys resulting from the use of the powder metallurgy route.
The as-spsed IRIS-TiAl alloy exhibits a fine microstructure made of small lamellar colonies (40 µm) which are surrounded by single phased borders made of γ grains and containing some precipitates of β0 phase. Creep curves and creep results obtained between 700°C and 850°C under various stress levels will be presented. The microstructures of crept samples were studied by transmission electron microscopy. Firstly, it will be shown the activation of pure climb of  dislocations in (001) planes which was never reported in the literature. Secondly, the displacement mode of ordinary dislocations will be investigated with a special attention to the role of climb.
These results will be discussed to determine what is controlling the creep properties of the IRIS alloy and to explain the origin of the very high creep resistance of this alloy.
9:15 AM - PM06.03.03
Microstructure Design for Enhancement of Room-Temperature Ductility in Multi-Component TiAl Alloys
Ryosuke Yamagata1,Yotaro Okada1,Hirotoyo Nakashima1,Masao Takeyama1
Tokyo Institute of Technology1Show Abstract
TiAl alloys are required high toughness and ductility for jet-engine applications. In previous studies, we clarified that introduction of the β-Ti / γ-TiAl duplex (DP) microstructure at the α2-Ti3Al/γ lamellar colony boundaries improves the stress intensity factor range threshold ΔK and decreases the Paris exponent m. Microstructure analysis revealed many slip lines exist in γ phase in DP region. This suggested that it is possible to enhance room-temperature ductility through controlling DP microstructure. Therefore, in this study, microstructure factors to enhance room-temperature ductility was investigated in multi-component TiAl alloys.
Multi-component TiAl alloys were used in this study. The microstructure was controlled by phase transformation of α → β + γ based on our phase diagram studies. Room-temperature tensile tests were conducted with a strain rate of 3 × 10-4 s-1 using an Instron-type universal testing machine. In order to removed surface strain that was introduced during machining, tensile test specimens were firstly electrical-polished. Microstructures observation was carried out using field-emission scanning electron microscopy.
The room-temperature ductility changes from 0.1 % to 1.4 % with increasing of volume fraction of γ phase from 5 % to 80 %. However, γ single phase alloy did not show any ductility, less than 0.1 %. Microstructure analysis revealed that the key of factors for enhancement of room-temperature ductility are the large amount of γ phase (>70%) in DP region and the existence of β phase. The other techniques and mechanisms will be discussed in presentation. This study was supported by Strategic Innovation promotion Program (SIP) in Japan.
9:30 AM - PM06.03.04
Microstructural Evolution in Mo-Alloyed Al-Rich Titanium Aluminides
Reinhold Wartbichler1,Frank Stein2,Martin Palm2,Helmut Clemens1,Svea Mayer1
Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben1,Max-Planck-Institut für Eisenforschung GmbH2Show Abstract
Titanium aluminides based on the intermetallic γ-TiAl phase are well-established lightweight structural materials within the temperature range from 600 to 800°C due to their high specific (creep) strength, low density and sufficient oxidation resistance. Increasing the aluminum content lowers the density even further while improving the oxidation behavior and leads to the formation of Al-rich intermetallic phases, such as r-TiAl2. Although for an alloy with a given chemical composition of Ti-60at.%Al desired lamellar microstructures of γ-TiAl and r-TiAl2 can be adjusted, the occurrence of metastable phases, such as h-TiAl2 and Ti3Al5, impairs the formation of an equilibrium state and embrittles the alloy. The effects of additional alloying elements, like Mo and Nb, on the microstructural evolution of Ti-60Al are mostly unknown and were investigated in the course of this work for 1 and 3 at.% Mo, respectively, and 1 at.% Mo and 4 at.% Nb. The sample material was manufactured by vacuum arc remelting. Several heat treatments were carried out to make a proper comparison to the binary alloy system. Samples were investigated via scanning electron microscopy to visualize the evolving microstructures. X-ray diffraction was performed to identify the occurring phases. Electron probe micro analysis was executed to measure phase compositions and solubility limits. To investigate transition temperatures of stable as well as metastable phases differential thermal analysis and differential scanning calorimetry was performed. The as-cast state revealed process-related defects and cracks. Mo has segregated in all specimens during solidification, whereas Nb had not. Homogenization was possible via holding the samples at 1400°C for one hour followed by furnace cooling. Subsequently, heat treatments of 200 hours at 1000°C and 500 hours at 800°C were performed, followed by water quenching. The addition of Mo and Nb impaired the microstructural evolution and lowered the amount of r-TiAl2, but lamellar structures of γ-TiAl and r-TiAl2 were still obtainable in case of 1 at.% Mo. However, none of the specimens reached an equilibrium state. The range of the γ-TiAl + r-TiAl2 two-phase field region decreased with increasing Mo content as well as the stability of the embrittling Ti3Al5-phase. Transition temperatures were determined and a quasi-binary phase diagram was generated, which visualizes the γ-stabilizing effect of Mo by increasing the range of the γ single phase field region.
9:45 AM - PM06.03.05
Tribological Properties of γ-Based TiAl Alloys Under High Temperature Sliding Wear Conditions
Lukas Mengis1,Christoph Grimme1,Mathias Galetz1
DECHEMA Research Institute1Show Abstract
Due to their peerless combination of material properties at high temperatures, intermetallic titanium aluminides have already started to be implemented as a turbine blade material in the low pressure section of the turbine engine.
As turbine blades are attached to the turbine disk by a specific dovetail connection, metallic surfaces are inevitably in direct contact under harsh mechanical loads as well as thermal conditions. In this regard, friction and wear issues always occur and can strongly affect the overall lifetime of the components.
Within the scope of this study a basic analysis of the friction and wear properties of two γ–based TiAl alloys, namely TNM-B1 and GE 48-2-2, has been conducted using a high temperature pin-on-disk apparatus. Hereby, tests were performed covering a wide temperature range (up to 800°C) under variation of the counterpart material to simulate conditions that are relevant for todays turbine engines.
As oxidation plays a vital role in terms of high temperature sliding wear conditions being the decisive factor for the specific wear mechanism and rate, the basic oxidation behavior of the chosen TiAl alloys has to be additionally investigated. Isothermal exposures in air were conducted to determine the respective influence of the exposure time (up to 1000h) as well as the temperature.
Conclusively, wear losses/rates were calculated and changes in the occurring wear mechanisms were identified using a profilometer as well as different metallographic techniques to depict differences in the overall tribological performance of both substrates.
10:30 AM - PM06.03.06
Accelerated Design of Gamma-TiAl Alloys by High Throughput Calculation
Fan Zhang1,Jun Zhu1,John Foltz2
CompuTherm1,ATI Specialty Alloys & Components2Show Abstract
Gamma TiAl alloys are gaining acceptance as a lighter weight alternative to nickel-based superalloys in certain high temperature applications. Further successful adoption of these alloys into new aero-engine components requires additional improvements to temperature capability and property balances. Comprehensive understanding of composition-processing-microstructure-property correlation is essential in aiding the selection of suitable alloy composition and the control of processing conditions. Although simulations tools have been developed for such a purpose, trial-and-error approaches are still widely used in alloy design and process optimization.
In this work, we will present our work in the development of a simulation tool that can be used to accelerate the design and development of gamma TiAl alloys. In particular, we developed a thermodynamic database for TiAl-based alloys using the CALPHAD (CALculation of PHAse Diagram) approach. This database contains 16 components which covers most of the major and minor alloying elements for TiAl-based alloys. We have also developed a high throughput calculation (HTC) in Pandat software through which calculations at numerous alloy compositions can be performed and alloy compositions that satisfy user-defined criteria can be searched through data mining of the simulated results. In this presentation, we will demonstrate how we can use HTC function and the thermodynamic database we have developed for TiAl alloys to understand the effect of various alloying elements on the properties of TiAl alloys and therefore identify the alloy compositions with good potentials for certain applications.
11:00 AM - PM06.03.07
Computational and Experimental Phase Diagram Study in Ti-Al-Nb-Cr Quaternary System
Hirotoyo Nakashima1,Masao Takeyama1
Tokyo Institute of Technology1Show Abstract
The thermodynamic modeling has been performed in order to reproduce the change in phase equilibria among β-Ti, α-Ti, α2-Ti3Al and γ-TiAl phases within the temperature range from 1573 K to 1073 K in Ti-Al-Nb-Cr quaternary system. Two ternary subsystems of Ti-Al-Nb and Ti-Al-Cr have been assessed with special attention to the α2 stabilizing effect against α in Nb and α stabilizing effect against α2 in Cr. Then those two systems have been combined to calculate quaternary phase diagram. In all the temperature investigated in the present study, the negative interaction exists among three elements of Al-Cr-Nb and Cr-Nb-Ti to stabilize the β against the α(α2) and γ phases. Thus the ternary interaction parameters have been introduced and its Nb/Cr ratio, Al content and temperature dependence were optimized. The experimental validation of the thermodynamic database at the lower temperature of 1273K ~ 1073 K will be discussed both in terms of the phase transformation pathway and the change in volume fractions of the constituent phases.
11:15 AM - PM06.03.08
Titanium Aluminides Under Evolution—In Situ and Real-Time Information Revealed by Neutron and Synchrotron X-Ray Diffraction
Klaus-Dieter LissShow Abstract
The microstructural evolution and transformation behavior of titanium aluminides is a complex interplay in alloy design, process development and under operating conditions. In-situ neutron and synchrotron X-ray diffraction deliver unique and complementary insight into the material’s response to high temperature, deformation and extreme conditions. Neutrons illuminate a larger bulk volume and reveal quantitative phase abundance, bulk texture, lattice parameter changes and other ensemble averaged quantities. In contrast, fine-bundled high-energy X-rays deliver reflections from a number of individual grains. For each constituting phase, their statistics and behavior in time reveal information about grain growth or refinement, subgrain formation, static and dynamic recovery and recrystallization, slip systems, twinning, etc. Features will be presented on selected examinations on titanium aluminides, especially to characterize phase evolution and crystallographic changes upon heating and during plastic deformation, and severe conditions.
11:30 AM - PM06.03.09
Formation of Orthorhombic Intermediate Temperature Phases in Different Commercial and Experimental γ-TiAl Alloys
Florian Pyczak1,2,Marcus Rackel1,Heike Gabrisch1,Uwe Lorenz1,Andreas Stark1
Helmholtz-Zentrum Geesthacht1,Brandenburgisch Technische Universität Cottbus-Senftenberg2Show Abstract
γ-TiAl alloys for aero engine blades can be produced by forging but the inherent brittleness of TiAl makes this difficult. A means to facilitate forging of γ-TiAl alloys is to stabilise the ductile soft disordered β phase at forging temperature. This is achieved by the addition of β stabilising elements as for example Nb, Mo or V and was successfully implemented in the TNM alloy. In the ideal case the β phase may be present at forging temperature but can be dissolved fully at service temperature where its presence is unwanted. Unfortunately, often not only the two phases γ and α2 are found at service temperature but additional stable or metastable phases are present. In an experimental Ti-42Al-8.5Nb alloy an orthorhombic phase constituent was identified which had the crystal structure of the O-phase and is a transformation product of α2-phase at temperatures below 650 °C. To unambiguously identify the phase and its transformation path high-energy X-ray diffraction (HEXRD) measurements at varying temperatures using in-situ specimen environments were performed. Based on literature knowledge the occurrence of O-phase is only expected for significantly higher niobium and lower aluminum contents.
Initially it was supposed that this O-phase is a feature of this very special experimental alloy composition. Nevertheless, screening tests over a wide variety of alloy compositions proofed that the O-phase can be found in a number of commercial as well as experimental alloys depending on composition and heat treatment history. In general it can be stated that alloy compositions with a combined content of β stabilising elements of 6 at.% or more and Al contents of 46 at.% or lower form O-phase to a greater or lesser extent. Minor alloying elements, such as boron or carbon exhibited no measureable effect on O-phase formation.
The results can be valuable for the development of β stabilised TiAl alloys and the understanding of the long term behaviour of TiAl parts as the possibility of O-phase formation in the medium temperature range was until now disregarded for such alloys.
PM06.04: Titanium Aluminides III and Superalloys
Tuesday PM, November 27, 2018
Hynes, Level 1, Room 104
1:30 PM - PM06.04.01
Influence of Solidification Condition on Boride Morphology in Cast TiAl Alloys
Institute of Metal Research CAS1Show Abstract
Boron additions are indispensible in gamma TiAl alloys designed for cast applications because borides of different structures, shape, and size or their mixtures limit grain growth during solidification, subsequent solid state phase transformation, and during hot isostatic pressing. Some forms of borides, such as very long ribbons, especially when concentrated and entangled, may degrade mechanical properties. In this talk, we attempt to classify the conditions for the formation of such borides according to alloy composition, cooling rate and solidification sequence. Experimental evidence will be presented from both cast turbine blades and wedge-shape samples with step thickness. Corresponding tensile test data will be discussed.
2:00 PM - PM06.04.02
Phase Equilibria among β/α2/α/γ Phases and Phase Transformations in Ti-Al-Cr System at Elevated Temperatures
Ali Shaaban1,2,Hideki Wakabayashi1,Hirotoyo Nakashima1,Masao Takeyama1
Tokyo Institute of Technology1,Central Metallurgical Research and Development Institute (CMRDI)2Show Abstract
Designing of wrought TiAl alloys opens the window for a wide range of applications and it will not be exclusive to be applied as low-pressure turbine materials but also as high-pressure compressor materials. β-Ti phase in TiAl alloys allows excellent hot workability during processing and excellent mechanical properties in service temperatures. In other words, microstructure control using β-phase is a key to develop high-toughness wrought γ TiAl alloys. This can be accomplished by understanding the phase transformations involving β-phase. Effects of group Vth (V, Nb) and VIth (Cr, Mo) elements as β-stabilizers on phase equilibria of TiAl alloys above 1473 K were extensively studied by our group. It was found that V and Nb stabilize α2 against α. Also, the change in three-phase coexisting region β+α+γ that exists above 1473 K to that of β+α2+γ occurs not just by the ordering transformation α→α2 (2nd order phase transformation) but by a transition peritectoid reaction β+α→α2+γ (1st order phase transformation) at a temperature between 1453 K (the congruent temperature of α→α2) and 1400 K (the eutectoid reaction temperature of α→α2+γ) in Ti-Al binary system with decreasing temperature. Thus this phase transformation allows a unique transformation pathway for α→α2+β in the ternary systems. However, it has not been clarified yet that the addition of group VIth elements (Cr, Mo) stabilizes either α or α2. Thus, in this study, the effect of Cr addition to TiAl alloy on the phase equilibria among the four phases and phase transformation pathways within the temperature range of 1473 K~1073 K were investigated using several alloys in the composition of interest. In between 1473 K-1373 K, the slope of β/α tie-line in the three-phase coexisting region of β+α+γ remains basically unchanged where the Al content in β-phase is much lower than that in α-phase. However, this slope drastically rotates in a clockwise direction and the Al content between the two phases becomes nearly equal, in between 1373 K and 1273 K and below the eutectoid reaction temperature in the binary system (1400 K). This is a strong indication that thermodynamically α-phase exists even below the 1400 K, i.e. addition of Cr stabilizes α against α2 and the three-phase coexisting region of β+α2+γ at lower temperatures is formed through a ternary eutectoid reaction (α→β+α2+γ) with decreasing temperature. This three-phase tie-triangle moves towards lower Al content in phase diagram. This suggests that Cr addition results in increase of the volume fraction of γ-phase with decreasing temperature even in alloys with low Al content. In between 1173 K and 1073 K, no further shift was observed in the tie-triangle meanwhile it expands towards high Cr content in β-phase. Based on these information, a novel technique for developing a new wrought γ-TiAl with excellent workability during processing at elevated temperatures and excellent toughness in service conditions, could be proposed.
2:15 PM - PM06.04.03
A Dislocation-Scale Characterization of the Evolution of Deformation Microstructures on a Bulk TiAl Alloy
Antoine Guitton1,2,Hana Kriaa1,2,Julien Guyon1,2,Emmanuel Bouzy1,2,Nabila Maloufi1,2
Université de Lorraine – CNRS – Arts et Métiers ParisTech – LEM31,Labex Damas - Université de Lorraine2Show Abstract
Developing new materials and understanding how they deform is the main challenge of engineers in order to follow and predict the fast evolutions of our society. For instance, in a framework of energetic cost reductions, titanium aluminide (TiAl) alloys have attracted considerable attention due to their unique combination of properties such as high specific strength and stiffness, good creep properties and resistance against oxidation and corrosion, which make them suitable candidate materials for high temperature applications . However, TiAl alloys are brittle at Room Temperature (RT), i.e. below their brittle-to-ductile transition temperature, which lies between 800°C and 1000°C . Furthermore, their complex microstructures (multiphase, different types of microstructures, specific dislocation mechanisms…) with several impacts at different scales are puzzling the materials science community. Despite intense research, literature suffers from a lack of understanding of their elementary deformation mechanisms and the precise role of microstructures .
In order to address these questions, we report here, an original and an innovative approach bringing the necessary information, thus allowing linking the multiscale aspects of the mechanical behavior of TiAl alloys at RT. Particularly, we bring new breakthrough on the evolution of deformation microstructures at RT in the vicinity of interfaces in γ phase of a dual-phase bulk TiAl alloy. Plastic deformation is induced locally by µN-nanoindentation. The evolution of the microstructures is characterized comprehensively by accurate Electron Channeling Contrast Imaging (aECCI) before and after deformation . aECCI is a non-destructive groundbreaking procedure offering the ability to provide, inside a SEM, TEM-like diffraction contrast imaging of sub-surface defects (at a depth of about one hundred of nanometers) on centimetric bulk specimen with still unsurpassed resolutions . Defects, such as dislocations, can be characterized by applying the TEM extinction criteria . All features help to explain the poor ductility of the TiAl-based alloys at RT. Accommodation of the deformation is reported and a scenario is proposed .
 – Y. Kim, D. Dimiduk, JOM 43, (1991).
 – C. Zambaldi, PhD thesis, Aachen (2010).
 – A. Guitton, H. Kriaa, E. Bouzy, J. Guyon, N. Maloufi, Materials 11, 2 (2018)
 – H. Kriaa, A. Guitton, N. Maloufi, Scientific reports 7, (2017).
 – H. Kriaa, A. Guitton, N. Maloufi, Submitted
2:30 PM - PM06.04.04
Relationship Between the Microstructure and Atomic Relaxation Processes in the Last Generation of TiAl Intermetallics
Maria No1,Leire Usategui2,Thomas Klein3,Svea Mayer4,Helmut Clemens4,Jose San Juan2
Universidad del Pais Vasco (UPV/EHU)1,Universidad del Pais Vasco2,Materials Center Leoben3,Montanuniversität Leoben4Show Abstract
Engineering structural intermetallics exhibit outstanding stability at high temperature. Among the different families of intermetallics, γ-TiAl alloys are good candidates for propulsion systems in aero and automotive industries. Recently a third and a fourth generation bearing Nb and Mo in well-balanced quantities and small amounts of C and Si (TNM and TNM+ alloys) have been developed. Adequate thermal treatments of these alloys give a lamellar arrangement of α2/γ colonies that exhibits a good creep resistance at high temperatures, which has been studied by tensile creep experiments and mechanical spectroscopy /1,2/. A microstructural analysis before and after the mechanical spectroscopy experiments should give important information about the mechanisms responsible for the observed behaviour and this is the approach used along this study.
In the present work two lamellar alloys were studied: Ti-43Al-4Nb-1Mo-0.1B (at%) (TNM alloy) and Ti-43Al-4Nb-1Mo-0.1B-0.3C-0.3Si (at%) (TNM+ alloy) with a final aging thermal treatment at 1123K and 1173K respectively. Mechanical spectroscopy measurements between 850K and 1225K show the diffusion of several solute atoms at different temperatures. The activation parameters were determined by tensile creep and mechanical spectroscopy experiments.
The regions of discontinuous precipitation surrounding the lamellar colonies were characterized by scanning electron microscopy (BSE, EDX, EBSD) technique. The lamellar α2/γ colonies and the small precipitates observed inside the lamellae were characterized by transmission electron microscopy (BF, DF, microdiffraction, nanodiffraction, STEM-HAADF, EDX) with a Titan Cubed 80-300KV working a 200KV. A good correlation between the mechanical spectroscopy results and the small precipitates characterized by TEM-STEM-EDX has been established through the corresponding models of precipitation and mechanical properties.
/1/ J. San Juan, P. Simas, T. Schmoelzer, H. Clemens, S. Mayer, M.L. Nó. Acta Mater. 65 (2014) 338-350
/2/ T. Klein, L. Usategui, B. Rashkova, M.L. Nó, J. San Juan. H. Clemens, S. Mayer. Acta Mater. 128 (2017) 440-450
This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO), CONSOLIDER-INGENIO 2010 CSD2009-00013 project, as well as by the Consolidated Research GroupGIU17/071 from UPV/EHU. This work made use of the FIB and the TITAN Cubed microscope facilities of SGIKER from the UPV/EHU.
2:45 PM -
3:30 PM - PM06.04.06
The Effect of Oxidation on Microstructure and Phase Stability in the Subsurface Zone of Titanium Aluminides Exposed at High Temperatures
Mathias Galetz1,Anke Ulrich1,Lukas Menigs1,Alexander Donchev1,Ceyhun Oskay1,Diana Fähsing1
DECHEMA Research Institute1Show Abstract
Titaniumaluminides had been in the focus of research for more than three decades before they finally took off as part of modern aircraft engines in 2011. Due to their density of about 4 gcm-3 they offer a high specific strength since they weigh only the about the half in comparison to nickel-based alloys. Thus, a favorable advantage in efficiency, noise reduction and fuel consumption is achieved. Because of their properties they gained interest also for application temperatures beyond the present range of ~750°C. This limit is defined by the strong degradation of their oxidation resistance above that temperature. Beside oxidation resistance the mechanical properties are also affect, however, the origin of this phenomenon is still under debate. In addition, several authors reported an increase in oxygen and nitrogen concentration in the surface region of TiAl-alloys. It was shown that the loss of ductility can be restored by removal of the surface and subsurface layer following high temperature exposure. One issue related to that is, that the oxygen and nitrogen uptake within the alloy are not easy accessible.
Recently developed TiAl alloys and applications such as TMB® and TMB®+ aim for even higher application temperatures and improved hotworkability. Beside γ-TiAl and α2-Ti3Al, such alloys additionally contain the β/βo-phase, whose impact on the high temperature oxidation and mechanical properties after exposure is hardly investigated and shows the requirement of a deeper understanding of the subsurface changes in such alloys due to dissolved nitrogen and oxygen.
In this work, the impact of high temperature exposure on the subsurface microstructure of TMB as well as the resulting effect on the mechanical properties at room temperature are investigated. For the latter, 4-point bending tests are conducted on samples before and after oxidation. Oxidation tests are carried out at 900°C for varying times (24 h – 1000 h) in air. The evolution of the microstructural change (based on changes in phase fractions) in the subsurface zone is investigated over time. Based on EPMA and SEM the change of the microstructure in dependence of the distance from the surface is shown, quantified, and could be correlated to oxygen uptake. A mechanism for phase transition phenomena is proposed.
4:00 PM - PM06.04.07
Microstructure—Property Relationship of Vanadium Solid Solutions, Two-Phase and Three-Phase Alloys
Christopher Müller1,Georg Hasemann2,Manja Krüger2,1
Otto von Guericke University1,Forschungszentrum Jülich GmbH2Show Abstract
Due to the low density in combination with a high melting point, vanadium demonstrates a great lightweight potential for turbines in aircrafts or energy industry. Since vanadium as a structural material is in focus of research only recently, the effects of several alloying elements on the materials properties are not or insufficiently examined yet. Therefore, various binary V-X and ternary V-Si-X-systems, that frequently contain intermetallic phases, have been studied. By means of ingot metallurgy (arc-melting process), vanadium samples with different concentrations of alloying elements were manufactured. Resulting from this, single phase vanadium solid solutions (Vss), two-phase and three-phase alloys were produced. Microhardness measurements and compression tests were carried out to determine the mechanical properties in dependence on the alloying components. The combination between mechanical characteristics and microstructural investigations enables conclusions concerning the materials behavior and the efficiency of solid solution strengthening and second phase strengthening. Therefore, SEM (Scanning Electron Microscopy) and XRD (X-ray Diffraction) methods were used to examine the microstructure, to identify phases and to measure elements concentration in the respective phases. Results achieved within this study may help to assess the potential of novel vanadium-based structural materials regarding to high temperature applications.
4:15 PM - PM06.04.08
An Attempt to Design a New Class of Co-Based Superalloy with Improved Oxidation Resistance and Creep Property
Zhenghao Chen1,Norihiko Okamoto2,Haruyuki Inui1
Kyoto University1,Tohoku University2Show Abstract
Recently, a new ternary L12 (γ′) phase Co3(Al,W), coexisting with a fcc solid-solution phase (γ) based on Co, has been discovered. With precipitation strengthening by the γ′ phase, this class of Co-base superalloy exhibits an improved high-temperature strength, compared to that of conventional Co-based superalloy. However, the creep property in ternary alloy is still insufficient, due to the insufficient high-temperature strength and the lack of γ′ in volume fraction. Besides, the oxidation resistance is also an unresolved problem. In previous studies, the effect of some alloying elements on Co-base superalloys has been investigated. Ni, Ta and Ti are effective in improving the creep property by either increasing the γ′ high-temperature strength or the γ′ volume fraction. On the other hand, Cr and Si are proved to be effective in improving oxidation resistance, but providing a negative effect on creep property, by decreasing the γ′ solvus temperature, unfortunately. Thus, in the present study, we investigate the effect of co-addition of alloying elements Ni, Ta, Ti, Cr, Si on Co-base superalloys, attempting to find out a composition exhibiting both excellent creep property and sufficient oxidation resistance, simultaneously.
Ingots of (Co0.8, Ni0.2)-aAl-bW-xTa-yTi-8Cr-Si (at.%; a, b, x, y≥0) were prepared by arc melting. These ingots were homogenized at 1200 oC for 24 h in vacuum, followed by heat treatment at a sub-solvus temperature for 96 h. The γ′ solvus temperatures were determined by differential scanning calorimetry (DSC), and the microstructures were examined by scanning electron microscope (SEM). Single crystals with selected compositions exhibiting exclusive γ/γ′ two-phase cuboidal structure and reasonable γ′ solvus temperature were prepared by modified Bridgman technique, followed by heat treatment at 900 oC for 96 h. Creep tests were performed in tension under the conditions of 137 MPa/1000 oC and 428 MPa/900 oC. Oxidation resistance behavior at 1000 oC were investigated with cyclic oxidation test for 200 h (20 cycles).
Although Cr and Si alloying decrease γ′ solvus temperature drastically, substituting W (and Al) with Ta (or Ti) is an effective way to increase γ′ solvus temperature without precipitating any secondary phases (such as D019). Oxidation resistance in multicomponent alloys are indeed to be improved, compared to non-Cr, Si alloyed Co-7Al-8W-Ta-4Ti. However, Rupture time of tensile creep at the condition of 900 oC /428 MPa in 4Ta and 8Ti are extremely short, only few hours. 2Ta6Ti exhibits relatively high creep property at the condition of 1000 oC /137 MPa (103h in rupture), but still not sufficient (approximately one-third to Co-7Al-8W-Ta-4Ti). It seems that, unfortunately, improving oxidation resistance and creep property simultaneously in Co-based superalloy to a utility-level may come out to be difficult, unless new type of alloying elements that increase γ′ solvus temperature more effective than Ta would be discovered.
4:30 PM - PM06.04.09
Alloying of Cr-Base Alloys by Ternary and Quaternary Elements and Their Effect on Oxidation and Nitridation Resistance at High Temperatures
Anke Ulrich1,Ali Solimani1,Petra Pfizenmaier2,Uwe Glatzel2,Mathias Galetz1
DECHEMA-Forschungsinstitut1,University Bayreuth2Show Abstract
Due to their high melting points Cr-base alloys are future candidates for materials for high temperature applications beyond Ni-base superalloys. In addition, high Cr content alloys offer lower densities compared to the commonly used Ni-base superalloys. However, for a future successful application still an optimal alloy composition has to be defined with respect to the improvement of nitridation, oxidation, and mechanical properties at high temperatures. In this work, the influence of Si, Ge, Mo, and Pt alloying on the oxidation and nitridation resistance of Cr-rich Crss-Cr3Si alloys was investigated using thermogravimetric analysis at temperatures from 1050°C – 1350°C. The samples were additionally analyzed using EPMA, SEM, and XRD analysis. Based on the binary Cr91Si9