GAT-2017 (Gamma Alloys Technology - 2017): Surface Protection with Panel Discussion and Oral Posters
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 2:00 PM
February 27, 2017
Room: Pacific 17
Location: Marriott Marquis Hotel

Session Chair: Al Sommer, Del West Engineering; Laiqi Zhang, Univ. of Science and Technology Beijing

2:00 PM  Invited
The Role of Surface Protection for High Temperature Performance of TiAl Alloys: Michael Schütze1; 1DECHEMA-Forschungsinstitut
    In the temperature range where TiAl alloys are currently used in jet engines and automotive industries surface reaction with the operating environment is not a critical issue yet. Surface treatment may, however, be needed in order to provide improved abrasion resistance. Currently development routes aim at a further increase of operation temperatures in gas turbines up to 800°C and higher, in automotive applications even up to 1050°C. In this case oxidation rates may reach levels where significant metal consumption of the load-bearing cross section can occur. Another possibly even more critical issue can be high temperature induced oxygen and nitrogen up-take into the metal subsurface zone with subsequent massive ambient temperature embrittlement. Solutions for these problems are based on a deliberate phase change of the metal subsurface zone by diffusion treatments and by using effects like the halogen effect for a change of the oxidation mechanism at high temperatures. The paper will discuss the recent development in this area and the requirements for future work.

2:25 PM  
Effect of Surface Condition on the RT Tensile Properties and Oxidation Resistance of TiAl Alloys: Bochao Lin1; Renci Liu1; Qing Jia1; Yuyou Cui1; Rui Yang1; 1Institue of Metal Research
    RT tensile properties and oxidation resistance during thermal exposure of wrought TiAl tensile specimens with different surface conditions were investigated. Ground tensile specimens of extruded Ti-46Al-2Cr-2Nb-0.15B alloy were subjected to mechanical polishing and electro-chemical polishing to get different surface conditions. Surface morphology, tensile properties, and oxidation behavior during thermal exposure and tensile properties after thermal exposure of specimens with different surface conditions were compared. Results show that both mechanical polishing and electro-chemical polishing reduce the surface roughness and improve the RT ductility but the latter is more effective. The oxidation resistance at 1073 K of the mechanical polished and electro-chemical polished specimens was improved compared to ground specimens, with electro-chemical polished specimens having the smallest mass change which may be due to ‘halogen effect’ caused by HClO4 in the electrolyte. RT ductility decreased after thermal exposure, while the drop is smallest for electro-polished specimens and largest for ground specimens.

2:45 PM  
Mechanical Properties and Environment Induced Embrittlement of a High Nb Containing TiAl Alloy: Tiebang Zhang1; Zeen Wu1; Hongchao Kou1; Jinshan Li1; 1Northwestern Polytechnical University
     Mechanical properties in different environments, including oxygen, vacuum, air and H2 environments, of a high Nb-containing TiAl alloy with the composition of Ti-45Al-8.5Nb-(W, B, Y) have been investigated in this work. Fracture behavior in different environments are evaluated and correlated with microstructure of the experimental alloy. The fracture strength and elongation at break of Ti-45Al-8.5Nb-(W, B, Y) in different environmental medium are 598.4MPa/4.18% (O2), 539.6 MPa /3.67% (vacuum), 499.1 MPa /2.77% (air) and 441 MPa/2.27, respectively. Fractography analysis indicates that the experimental alloy exhibits a typical transgranular cleavage fracture in each environmental media. Based on the results of microstructure investigations, it is found that cracks initiation and propagation always take place on boride/matrix and the β phase/matrix interfaces of Ti-45Al-8.5Nb-(W, B, Y) in all environmental conditions. It is also found that the microstructure morphology and the volume fraction of γ phase play an important role on the environmental embrittlement of the experimental Ti-45Al-8.5Nb-(W, B, Y) alloy.

3:05 PM Panel Discussion Topic 1 (Surface Engineering) : Al Sommer (Del West Engineering) and Michael Schuetze (DECHEMA)

3:40 PM Break

3:55 PM  
Observation of Modulated Structure in High Nb-containing TiAl Alloy by Synchrotron Radiation and Electron Microscopy: J. Sun1; 1Shanghai Jiaotong University
    High Nb-containing TiAl Alloys with modulated structure have been exhibited a promising combination of room temperature ductility and creep resistance. The orthorhombic phase has been identified as one of the constituents of modulated lamellae, nevertheless there is no general consensus on its crystalline structure (B19/O phase) and formation paths. In this work, a high Nb-containing TiAl Alloy with composition of Ti-45Al-8.5Nb (at.%) has been prepared and then subjected to aging at temperatures from 823 to 923 K. The microstructure has been investigated by synchrotron radiation X-ray and electron microscopy. The results have shown that the orthorhombic phase within modulated lamellae is O phase rather than B19, which precipitates from the parent α2 phase during aging. The O phase can be considered as further ternary ordering of the α2 phase, which is consistent with theoretical prediction of phase transformation based on a subgroup relation between phases in TiAl-Nb alloys.

4:15 PM  
Origin of Enhanced Ductility of TiAl Alloys: A Hybrid Study on the Deformation Behavior of Gamma Phase in TiAl Alloys Using In-situ Transmission Electron Microscopy Experiments and Molecular Dynamics: Seong-Woong Kim1; Seung-Hwa Ryu2; Jaemin Kim2; Young-Sang Na1; Seung-Eon Kim1; Jong-Taek Yeom1; Andrew Minor3; 1Korea Institute of Materials Science (KIMS); 2KAIST; 3Lawrence Berkeley National Laboratory
    An in-situ transmission electron microscopy study was conducted at room temperature to understand an underlying mechanism on room temperature ductility of TiAl alloys. Also, molecular dynamics simulation was conducted to calculate the stacking fault energy of TiAl alloys and to show which deformation mode is dominant. From in-situ straining transmission electron microscopy experiments, it was revealed that deformation mode is different between the TiAl alloys with/without room temperature ductility. The γ lamellar of TiAl alloys having room temperature ductility was deformed by slip. However, the γ lamellar of TiAl alloys without room temperature ductility was deformed by deformation twin. The difference in deformation mode was explained by stacking fault energy of the TiAl alloys which was calculated by molecular dynamics. Furthermore, the role of lamellar orientation of tensile direction on deformation behavior was examined using Schmid factor of each orientation. The important microstructural factors for room temperature ductility was suggested.

4:35 PM  Student
Determination of the Isothermal Sections of the Ti-Al-Nb Ternary System at 1300 ˚C and 1400 ˚C: Shuai Xu1; Yong Xu2; Xiangjun Xu3; Jianping He1; Yongfeng Liang1; Junpin Lin1; 1University of Science and Technology Beijing; 2Shandong Jianzhu University; 3Zhongyuan University of Technology
    The phase equilibria in the Ti-Al-Nb ternary system was investigated by a combination of scanning electron microscopy (SEM), powder X-ray diffraction (XRD) and electron probe micro analysis (EPMA). Isothermal sections at 1300 ˚C and 1400 ˚C were constructed based on experimental data from 90 quenched alloys. The phase equilibria among the following intermetallics and solid-solution phases are described: γ-TiAl, β, α, σ-Nb2Al, δ-Nb3Al and ε-(Ti1-xNbx)Al3. The range of α, σ and β phases expand and the range of γ and ε phases shrink comparing with the calculated results. With the increase of temperature, the site of the tie-triangle of α+β+γ moves to higher Nb content, which is different with the calculated results. The range of the tie-triangle of σ+β+γ is also different with the calculated results.

4:55 PM Concluding Comments Briefing of Selected Posters