GAT-2017 (Gamma Alloys Technology - 2017): Poster Session
Sponsored by: TMS Structural Materials Division, TMS: Titanium Committee
Program Organizers: Young-Won Kim, Gamteck LLC; Wilfried Smarsly, MTU Aero Engines AG; Junpin Lin, University of Science and Technology Beijing; Pierre Sallot, Safran Tech; Paul Withey, Rolls-Royce; Al Sommer, Del West Engineering, Inc; Rui Yang, Institute of Metal Research CAS; Florian Pyczak, Helmholtz-Zentrum-Geesthacht; Dennis Dimiduk, BlueQuartz Software, LLC

Monday 6:00 PM
February 27, 2017
Room: Hall B1
Location: San Diego Convention Ctr

Session Chair: Lin Song, Northwestern Polytechnical University; Todor Stoyanov, ACCESS; Jieren Yang, Northwestern Polytechnical University

F-29: Effect of Crack Location, Size and Shape on the Machanical Behavior of TC4/TiAl Welded Joints: Chengli Dong1; 1AECC/BIAM
    The elastoplasticity constitutive model was employed to study the effect of crack location, size and shape on the mechanical behavior of TC4/TiAl welded joints. The cracks were located in different zones, for example, only TC4 and only TiAl and the interface of TC4/TiAl welded joint. The crack shape contained U-type and V-type and C-type. The size and shape of the plastic zone and J integral was studied in the loading process.

F-30: Flow Stress Behavior of Ti-45Al-12Nb Alloy with Ultrafine Grains during Hot Compression Deformation: Hua Chen1; Xue Bo Gong1; 1Changchun University of Technology
    The flow stress behaviors of Ti-45Al-12Nb titanium alloy during hot compression deformation are investigated. The functional characteristics are measured by thermal simulator Gleeble-1500D, in the deformation temperature range of 1100–1200░C, at constant strain rate ranging from 0.0001 s−1áto 0.01s−1. The influence of temperature and strain rate on the flow stress behaviors are discussed and constitutive equation is derived. The experimental results reveal that Ti-45Al-12Nb alloy with ultrafine grains is sensitive to strain rate and temperature, and all the true stress–strain curves show the characteristics of strain hardening (at the true strain lower than 0.15) . At the same temperature, the peak stress decreases with the decrease of strain rate. Under constant strain rate, the peak stress shows a clear trend of declining with increasing deformation temperature. The activation energy for the hot deformation is 693.55kJ/mol by calculating, which is higher than that for self-diffusion of TiAl alloy.

F-31: Influence of Hot Processing Parameters on Dynamic Recrystallization Behavior of Ti-47Al-2Nb-2Cr Alloy: Lianxi Hu1; Zhipeng Wan1; Yu Sun1; 1Harbin Institute of Technology
    The Ti-47Al-2Nb-2Cr alloy exhibits several excellent mechanical properties for the applications at elevated temperature. The mechanical properties of the deformed components rely on the homogeneity and volume fraction of dynamically recrystallized microstructure. In order to investigate the effect of the hot deformation temperature in combination with the strain rate on the dynamically recrystallized behavior of Ti-47Al-2Nb-2Cr alloy, series of compression tests over a wide range of temperature (1223-1423K) and strain rate (0.001-0.1s-1) were performed. In the present work, the dynamic recrystallization kinetic model which presents the correlation between processing parameters and dynamically recrystallized microstructure was established based on the flow stress behavior. Microstructure characterization along with grain orientation spread analysis indicated that the altered temperature and strain rate led to change in microstructure such as the grain refinement and dynamic recrystallization volume fraction.

F-32: Study on the Lamellar Boundary Orientation of Ti-46Al-8Nb Alloy with Various Growth Rate: Jongmoon Park1; Ho Seung Jang1; Seongwoong Kim2; Seungeon Kim2; Younghwan Hong3; Myunghoon Oh1; 1Kumoh National Institute of Technology; 2Korea Institute of Materials Science; 3Suwon Science College
    In this study, phase transformation phenomena at the high-temperature field were investigated by using designed ternary Ti-Al-Nb alloy. Examination of dendritic morphologies in arc-melted button ingot could reveal the crystallography of the primary solidification phase. The pseudo-ternary phase diagram of Ti-Al-Nb system was constructed by DTA. On the basis of the pseudo phase diagram completed in the study, the Ti-46Al-8Nb(at%) alloy, which was directionally solidified by using a floating zone type DS apparatus at the various growth rate. It has been found that the lamellar orientation was aligned nearly 0 or 45 degrees to the growth direction. However, when the directional solidification was performed at a growth rate of 90mm/h, the lamellar boundary orientation of DS ingot was nearly perpendicular to the growth direction. These results mean that lamellar boundary orientation of DS ingot has been affected by the growth rate as well as composition.

F-33: Vacuum Brazing of Ti-48Al-2Cr-2Nb: Yusheng Cai1; Renci Liu1; Dong Liu1; Yuyou Cui1; Rui Yang1; 1Institute of Metal Research, Chinese Academy of Sciences
    Ti–48Al–2Cr–2Nb alloy was vacuum brazed with Ti–Zr–Cu–Ni based amorphous foil and the influence of brazing welding parameters on the microstructure and tensile properties of the joints was investigated. Four zones of the joint were identified and the phase constituents of each zone were characterized. For a welding time of 30 minutes, the room temperature tensile strength of the joint reaches its peak value of 362 MPa at 950 oC. At this optimum brasing temperature the room temperature tensile strength of the joint reaches its peak value of 404 MPa at 45 minutes.All the brazed specimens fractured at the welding joint.The fracture surface varied with the brazing welding parameters and the cracks initiated at the fusion zone where the volume fraction of (Ti, Zr)2(Cu, Ni) phase is relative high.

F-34: Joining Process of Gamma-TiAl and Structural Steel with Insert Metals by Friction Welding: Myunghoon Oh1; Jongmoon Park1; Kiyoung Kim2; Kyoungkyun Kim2; Ho Seung Jang1; Younghwan Hong3; 1Kumoh National Institute of Technology; 2Asan Friction Welding Co., Ltd; 3Suwon Science College
    In this study, the microstructure and related mechanical properties of friction welded TiAl(superior characteristic high temperature & specific strength) and SCM440(alloy steel for machine structural use) were investigated. The friction welding was performed using a servo-motor type friction welding machine for control the several variables. Experimental variables were changed in order to bond the test pieces TiAl-Cu-SCM440, TiAl-Ni-SCM440.Scanning electron microscopy (SEM) was used to observe microstructural evolution taking place during friction welding.It was found that several variable (upset, upset time, rpm, force, etc) changes could be caused by different heat-affected zone size, welding flash shape, microstructure and resulted in hardness changes in welded zone. It is necessary to prevent the formation of the intermetallic compound layer. In order to increase bond strength, we used insert metals (Cu, Ni). As a result, we could confirm to change with type and thickness of insert metal.

F-35: Microstructure and Mechanical Properties of Powder Metallurgy Ti-22Al-25Nb Alloy Fabricated by Hot-pressing Sintering: Yu Sun1; Heng Zhang1; Siqiu Wang1; Lianxi Hu1; 1Harbin Institute of Technology
    In the present investigation, vacuum hot-pressing sintering processing was employed to fabricate Ti-22Al-25Nb alloy in a temperature range of 1000oC-1150oC with a pressure of 40MPa for 1h. Microstructure and phase composition examinations were conducted by applying optical microscopy, scanning electron microscope and X-ray diffraction analysis. Tensile tests of this alloy were studied at room temperature and 650oC, respectively. The results indicated that the main phases of samples are B2 and O with a small amount of α2 phase distributed along the grain boundaries. The yield strength and elongation of the hot-pressed samples were 737.90MPa and 5.607% at room temperature, respectively, while the yield strength reduced to 522.75MPa and the elongation reached 16.66% at 650oC. The alloy sintered at 1100oC exhibited better comprehensive mechanical properties.

F-37: Microstructural Evolution and Evaluation of Mechanical Behaviors of the Cast Ti-Al-Mo-Nb-(B, Mm) Alloys: Kwang Soo Choi1; Joon Sik Park1; S. Yi2; Fan Zhang3; Y. B. Song4; 1Hanbat National University; 2Kyungpook National University; 3CompuTerm, LCC; 4Agency for Defense System
    Cast alloys of Ti-Al-Mo-Nb-(B,Mm) have been investigated to identify phase stability and corresponding strength levels. When the alloys were heat treated, γ / β (or B2) / α2 phase combinations were identified. When Mm was added to the alloys, Mm components (La and Ce) were found and the phase was distributed through the specimen. The alloys with Mm showed enhanced compressive yield strength at 800 C°. At the same time, when the alloys with B showed fine (~20Ám) lamellar colonies in as cast forms, which leads to increased ductility. The phase distribution for each alloy was measured as a function of temperature (800 ~ 1300 C) to show the phase stability variation of γ / β (or B2) / α (or B2) / α2. The microstructure evolution is related to the phase distribution and also used to explain the resulting mechanical properties.

F-40: TiAl-based Intermetallic Alloy with Addition of Zirconium: Sangwoo Kim1; Hyouk-Chon Kwon1; Hyo-soo Lee1; 1Korea Institute of Industrial Technology
    TiAl-based alloys represent an important characteristic of high temperature structural materials. So, the Ti-Al based alloy have been used in various fields such as aircraft industry, chemistry industry and automobile industry. This paper describes the processing results of TiAl alloys melted by VAR. TiAl alloys, with 0~4 at% zirconium, were produced to replace their structure by a quasi-lamellar one. A evaluation of the respective zirconium partition among micro-constituents suggests that the more zirconium added, the more volume fraction of gamma phase increased. This study shows that the zirconium is the role of stabilizer of gamma. The morphology and structure of alloy with addition of zirconium were analyzed by using SEM and XRD. The oxidation behavior of specimens was assured by TGA. The research has shown that addition of zirconium to TiAl resulted in a slight increase in hardness and a significant improvement of the oxidation resistance.

F-42: Interficial Reaction between TiAl Alloy and Ca(Y)-doped BaZrO3 Crucible: Hao Zhang1; Mingyang Li1; Baotong Li1; Guangyao Chen1; Ziwei Qin1; Xionggang Lu1; Chonghe Li1; 1Shanghai University
    A homemade Ca(Y)-doped BaZrO3 crucible was used to melt TiAl alloys. The microstructure of the melt TiAl alloy and the interficial reaction between the Ca(Y)-doped BaZrO3 crucible and TiAl melt were analyzed by optical microscopy, scanning electron microscopy, X-ray diffraction and atomic emission spectrometry, respectively. The results demonstrate that the microstructure of TiAl alloys is largely composed of lamellar α2+γ with different orientations and small amount of bulk γ phase, the melts exhibit poor wettability on Ca(Y)-doped BaZrO3 crucible and no reaction layer is observed between them. No obvious element diffusion is observed between crucible and metal. This may imply that Ca(Y)-doped BaZrO3 is a promising candidate of materials of TiAl alloys.

F-43: Atom Probe Investigation of the Partitioning of Impurities in TiAl Alloy: Gong Zheng1; Zhixiang Qi1; Yingbo Peng1; Guang Chen1; Zhijun Ge1; 1Nanjing University of Science and Technology
    Some impurity elements’ distribution (O, N, C, Si) in the α2+γ lamellar structure of binary Ti-46.3Al and ternary Ti-45Al-8Nb alloys was characterized by using atom probe tomography. It is found out that O and N mainly partition to the α2 phase as interstitial atoms, but with the addition of Nb the concentration of O and N in the alloys rises a lot, especially in the γ phase, in which the O’s concentration rises from ~300 at. ppm to ~6000 at. ppm. C was also found to partition to the α2 phase, but not as clearly as O and N do. In addition, there is a tendency that C segregates at the interfaces, which indicates that the distribution of C is in between interstitial atoms and substitutional atoms. Si was found to segregate at interfaces just like the distribution of Nb, indicating the existence of Si in the alloys is substitutional.

F-45: Fine Structure of Ordinary Dislocation Dipoles and their Evolution in Deformed Gamma-TiAl via Atomistic Simulations: Yan He1; Zhao Liu1; Hao Wang1; Dongsheng Xu1; Rui Yang1; 1Institute of Metal Research, Chinese Academy of Sciences
    In intermetallic gamma-TiAl, dislocation dipoles in the form of faulted dipoles are frequently encountered in deformed samples, but their fine structure and transformation into point defects remain largely undocumented. In this work, narrow dipoles in gamma-TiAl are systematically investigated with atomistic simulations. Their configurations are unraveled by interatomic potentials, which indicates the stability of faulted dipoles over other forms. The results are compared with experimental observations. Molecular dynamics simulations indicate that at elevated temperature, dipoles transform into individual defects, e.g., vacancy clusters, stacking fault tetrahedra and interstitial loops, depending on dipole height and orientation. Via metadynamics methods, activation energies of the atomic processes therein are obtained and the lifetime of the deformation debris is estimated, which shows their stability on the experimental timescale. The influence of stable narrow dipoles on plastic deformation is discussed.

F-46: Hot Working Behavior and Microstructural Evolution of As-cast Ti-42Al-5.5Mn Alloy: Hao Xu1; Bo Chen1; Yingche Ma1; Lei Shu1; Kui Liu1; 1Institute of Metal Research
    the research focused on the hot deformation behavior of Ti-42Al-5.5Mn(at%) ingot produced by vacuum induction melting(VIM) and vacuum arc remelting(VAR). The experiment was performed by isothermal compression test at the temperatures between 1050-1250°C with the strain rates in the range of 0.001-10s-1. Three results concluded from the investigation: Firstly, the processing map based on strain-stress curves, which revealed the alloy can be hot worked from 1250°C-10s-1 to 1050°C-0.1s-1. Secondly, the constitutive equation was established to describe the flow behavior and yielded stress exponent of 2.71 and apparent activation energy of 596.7kJ/mole. Thirdly, the alloy exhibited flow instability at lower temperatures and higher strain rates, as predicted by the processing map. The surface fractures of corresponding specimens were in accordance with massive streamlined microstructure. Inversely, fine microstructure and β/γ globularization were generated by dynamic recrystallization(DRX) at higher temperatures and lower stain rates.

F-48: Gamma Phase Nucleation from Stacking Fault in TiAl Alloys: Chunyu Teng1; Yonghong Li1; Zhanyong Ren1; Dongsheng Xu2; Rui Yang2; 1China Aero-Polytechnology Establishment; 2Institute of Metal Research, Chinese Academy of Sciences
    The best combination of strength and stability in TiAl alloys can be obtained by forming structures with small lath thickness and high twin boundary frequency in the lamellae. Some experimental investigation show that introducing (complex) stacking fault in the matrix of the alpha2 phase may form the nucleation sites of lamellae. In order to understand how the (complex) stacking fault affecting gamma phase nucleation in TiAl alloys, a phase field model is introduced to simulate the gamma phase nucleation under the influence of different factors, such as (complex) stacking fault density, size and configuration, chemical driving force magnitude, etc.. The results show that a (complex) stacking fault can act as the preferred gamma phase nucleation site. The gamma phase nucleation energy barrier is reduced by the (complex) stacking fault dramatically. Therefore, the lamellar structure with small lath thickness and high twin boundary frequency is preferred.

F-51: Characterization of Thermal Deformation Behavior of a Novel Ti-47Al-Cr-2Mn-0.5Fe- 0.05Y Alloy: Xiaopeng Wang1; Fantao Kong1; Qin Sun1; Yu Zhang1; Shouzhen Cao1; Yuyong Chen1; 1Harbin Institute of Technology
     The high temperature deformation behavior of Ti-47Al-Cr-2Mn-0.5Fe- 0.05Y alloy was investigated by hot compression tests in the temperature range of 1050 to 1250 ℃ and strain rate range of 0.01 s-1 to 0.5 s-1. The hot compression results indicated that the Ti-47Al-Cr-2Mn-0.5Fe-0.05Y alloy exhibits good deformability at high temperature. The deformation activation energy of this alloy was 352.36 KJ/mol which was higher than the diffusion activation energy of Ti and Al. and this result meant that the main softening mechanism of thermal deformation was dynamic recrystallization. A thermal processing map of this alloy was developed on the basis of the flow stress and dynamic material model, and the optimum hot forging parameter for industrial productions was determined (1240℃,0.05s-1). Finally a crack-free Ti-47Al-Cr-2Mn- 0.5Fe-0.05Y alloy pancake was prepared by canned forging.

F-52: Microstructure and Mechanical Properties of High Nb Containing TiAl Alloy Sheets: Fantao Kong1; 1Harbin Institute of Technology
    Titanium aluminide alloys have been considered for application as high temperature structural materials due to their combination of low density, high specific strength, low diffusivity, good resistance against oxidation and corrosion, and high ignition resistance. Especially, high Nb containing TiAl alloys have improved tensile strength, creep strength as well as oxidation resistance than the ordinary TiAl alloys, have been focused on in recent studies. Up to now, high Nb containing TiAl alloys have not yet been widely applied because of their room temperature brittleness and limited workability. In this paper, the research results of vacuum arc melting, near Isothermal canned forging and hot-pack rolling of a high Nb containing TiAl alloy are showed. The microstructure evolution and mechanical properties of the high Nb containing alloy sheets prepared by hot-pack rolling methods were investigated.

F-55: Effect of Al Content on the Microstructure and Tensile Properties of Cast Ti-xAl-15Nb-1Mo Alloy: Liangliang Liu1; Dong Liu1; Yuyou Cui1; Rui Yang1; 1The Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS)
    In this study, the microstructure and tensile properties at room temperature and 600 oC of four cast Ti-xAl-15Nb-1Mo alloys were evaluated with Al contents varying from 15 to 23.5 at.%. The microstructure of the alloys and the volume fraction of B2, α2 and O phase were evaluated at the different Al level. As expected Ti-15Al-15Nb-1Mo contains no O phase but has the highest portion of the B2 phase, while Ti-23.5Al-15Nb-1Mo has the highest portion of the O phase but contains little B2 phase. No apparent effect of Al content on grain size was found. For Al content varying from 15 to 20.5 at.%, the strength decreases and ductility increases at both room temperature and 600 oC. When Al content increases from 20.5 to 23.5 at.%, the room temperature tensile strength and ductility do not change much, but the strength increases and ductility decreases at 600 oC.