GAT-2017 (Gamma Alloys Technology - 2017): Microstructure Development and Directional Solidification
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

Wednesday 2:00 PM
March 1, 2017
Room: Pacific 17
Location: Marriott Marquis Hotel

Session Chair: Pierre Sallot, Safran Tech; Ulrike Hecht, ACCESS

2:00 PM  Invited
Gamma Alloy Materials-Process-Microstructure Combinations for Greater Service Temperatures: Young-Won Kim1; Sang-Lan Kim2; 1Gamteck LLC; 2Gamtech LLC
    Currently, gamma alloys in specific alloy-microstructure forms are in service or about to be implemented in select aero-engines as LPT blades and in select automotive engines as turbocharger rotors and exhaust valves. Their service temperatures are limited to 750°C for LPT blades and around 900°C for turbocharger rotors. Greater service temperatures has been desired by the users and manufacturers for many years, and in spite of over one decade of efforts by both the materials RD community and industry, the limitations have remained virtually the same. The community’s inability in realizing the importance of systematic integration of accumulated data, knowledge and information (DKI), is the key reason for the lack of advances. This presentation assesses current alloys’ engineering microstructures, and identifies the application-specific alloy-processing-microstructure combinations that can be used at greater service temperatures up to 870°C for aero-engine rotational components and up to 1000°C gas temperatures for automotive engine components.

2:25 PM  
Solidification of TiAl Alloys with Low Contents of Si: Antoine Paris1; Mikael Perrut1; Dominique Daloz2; Anne Denquin1; 1Onera; 2Université de Lorraine
    Small additions of silicon in cast TiAl alloys are known to improve significantly their creep resistance at high temperature. However, silicon can undergo segregation during solidification, which may lead to TiAl+Ti5Si3 eutectic structures. As-cast microstructures of Si-doped TiAl alloys will be presented, focusing on the microsegregation in a Ti-47Al-2Zr-0.3Si (at%) alloy. SEM-EDS maps of as-cast microstructures have been acquired and quantitatively studied, obtaining a large amount of chemical data. Hence, the chemical segregations of each element and their cross-correlations have been measured. These results are compared to Scheil-Gulliver calculations. The relevance and precision of the method are finally discussed.

2:45 PM  
Microstructure Evolution of Ti-45Al-8.5Nb-(W, B, Y) Alloy during Continuous Cooling and Thermal Aging: Jieren Yang1; Bei Cao1; Xuyang Wang1; Rui Hu1; Lin Song1; Jinshan Li1; 1Northwestern Polytechnical University
    Continuous cooling widely occurs in hot working processes including casting, forging and welding, et al. The phase transformation behaviour of Ti-45Al-8.5Nb-(W, B, Y) alloy during continuous cooling under various cooling rates of 5 K/min, 20 K/min, 100 K/min, 700 K/min and water-quenching (WQ) was investigated in this study. Results indicate that the microstructure is highly dependent on the cooling rate and thermal history. At low cooling rate, full lamellar microstructure with the average colony size of 150 μm forms and few massive phases exsit at colony boundaries. With the increase of cooling rate, the lamellar morphology gradually weakens and more non-equilibrium phases such as massive B2 and γ emerge. A mass of lath martensite was obtained after WQ. Further, the thermal stability in intermediate temperature range of these samples obtained by different cooling processes was studied.

3:05 PM  
High-energy Synchrotron Radiation Investigation of the Massive Transformation in a Ti-Al-Nb-Ta Alloy: Marcus Willi Rackel1; Andreas Stark1; Gleb Dovzhenko1; Florian Pyczak1; 1Helmholtz-Zentrum Geesthacht
    In this study the cooling rate dependent massive transformation α to γm was investigated in a Ti-45Al-4Nb-4Ta (at.%) alloy by in situ high-energy synchrotron X-ray diffraction (HEXRD) experiments. The experiments were performed with five different cooling rates between 2–200 K/s. Only in situ experiments enable the continuous monitoring of phase evolution during heating and cooling with high frame rates up to 10 Hz and allow to analyze the high temperature phase distribution unambiguously, because they are not overlaid by post process phase transformations. In particular the transformation start temperature, the critical cooling rate, and the undercooling required for the massive transformation was directly investigated. As one result a CCT-diagram was calculated and a convoluted microstructure was successfully established using a moderate cooling rate followed by annealing in the two phase field α2+γ. The microstructure development of the specimens was characterised by additional EBSD and TEM investigations.

3:25 PM  Invited
Study on Preparation of Larger Size TiAl Ingot with Oriented Lamellar Microstructure: Jun Shen1; 1Northwestern Polytechnical University
    Our research group put forward the "seed + non-contact electromagnetic shaping and directional solidification” technology, for the manufacture of large size (diameter 20-50mm) TiAl alloy ingots with directional lamellar microstructures oriented along axial. We have done some research on microstructure control, component segregation under different parameters, especially in different diameter, during directional solidification of TiAl alloy, also analysed mechanical properties and fracture mechanism. At present, we have prepared TiAl alloy ingots with directional lamellar microstructures in diameter of 20mm, length of 80-140mm by using this technology. Compared with the non-oriented microstructure, the results show that the plasticity and strength of the lamellar microstructure oriented along axial are significantly improved, and the room temperature ductility is especially significant, the elongation up to 4-9%.

3:50 PM Break

4:05 PM  Invited
High Temperature Mechanical Properties of Polysynthetic Twinned TiAl-Nb Alloys: Zhixiang Qi1; Guang Chen1; Yingbo Peng1; Gong Zheng1; 1Nanjing University of Science and Technology
    The mechanical properties of TiAl-Nb PST single crystals were investigated at high temperatures. The results showed that BDTT of the TiAl-Nb single crystals was in the range of 950℃ to 1000℃. Because of Nb’s enrichment in γ phase, mechanical twinning formed easily during deformation, which turned out to be benefit for the improvement of high temperature strength. The α2/γ and γ/γ interfaces are also responsible for the excellent strength by impeding the dislocations slip. The strengthening effect of these interfaces is in the order of twins/matrix>α2/γ>γ/γ. At 1000℃, the twins become instable and dislocations found exist in the twin boundary and cause the superplastic phenomenon. For the reason that the nucleation of twins is much higher than that for the growth of twins, the stress-stain curve of the PST single crystals at 1000℃ showed oscillations phenomenon, another factor is the cyclic depinning–repinning process from the effect of dislocations and twins.

4:30 PM  Invited
Microstructure and Mechanical Properties of TiAl Alloys Prepared by Cold Crucible Directional Solidification: Ruirun Chen1; Jingjie Guo1; Hongsheng Ding1; Hengzhi Fu1; 1Harbin Institute of Technology
    Cold crucible directional solidification (CCDS) is a newly developed technique, which combines the advantages of the cold crucible and continuous melting. It can be applied to directionally solidify reactive, high purity and refractory materials. This paper describes the principle of CCDS and its characteristics; the measured and numerical calculated results of the flow field and temperature field during CCDS of Ti based alloys. Ti6Al4V, TiAl alloys and high Nb-containing TiAl alloys have been directionally solidified with different cold crucibles. The cross-sections of the cold crucibles include round, near rectangular and square with different sizes. Effects of solidification velocity and temperature gradient on microstructure and mechanical properties of TiAl alloys have been studied. Tensile testing results show that the elongation of directionally solidified Ti6Al4V can be improved to 12.7% from as cast 5.4%. The strength and the elongation of the directionally solidified Ti47Al2Cr2Nb and Ti44Al6Nb1.0Cr2.0V are 650 MPa/3% and 602.5 MPa/1.20%, respectively.

4:55 PM  
Seeded Growth of Ti-46Al-(3~10)Nb PST Crystals: Hao Jin1; Ronghua Liu1; Yuyou Cui1; Quangang Xian1; Dongsheng Xu1; Rui Yang1; 1Institute of Metal Research, Chinese Academy of Sciences
    Mechanical properties of TiAl-Nb alloys could be improved significantly if PST crystals of these alloys with lamellae parallel to the crystal growth direction could be obtained. This paper summarizes our recent work on growth of Ti-46Al-(3~10)Nb alloy utilizing a Ti-43Al-3Si seed in an optical floating zone furnace. For all amount of addition, the lamellar orientations of Ti-46Al-xNb alloys were successfully controlled. As to Ti-46Al-3Nb alloy, stray grains tend to form after about 40mm. For alloys with Nb addition large than 5%, the main grain could occupy a dominant position in the whole bar. A model was construct to explain the results based on phase diagrams and thermodynamics. For alloys with Nb addition less than 10%, room temperature tensile elongations of 10% were typically obtained. As to Ti-46Al-10Nb alloy, B2 phase may be the reason for its low ductility. The highest elongation of 18% has been obtained for Ti-46Al-8Nb alloy.