GAT-2017 (Gamma Alloys Technology - 2017): Processing-Microstructure-Property Relationships
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 8:30 AM
March 1, 2017
Room: Pacific 17
Location: Marriott Marquis Hotel

Session Chair: Fritz Appel, Helmholtz-Zentrum Geesthacht; Juraj Lapin, IMMM, Slova Academy of Science

8:30 AM  Invited
Control of Microstructure and Mechanical Property and Superplasticity for High Nb-TiAl Alloy Sheet: Junpin Lin1; Yongfeng Liang1; Laiqi Zhang1; Guojian Hao1; Xiangjun Xu2; 1University of Science and Technology Beijing; 2Zhongyuan University of Technology
    High Nb-TiAl alloys have been paid more and more attentions because of attractive properties such as high melting points, good oxidation resistance, high specific strengths and modulus, etc. High Nb alloying becomes the optimum method to raise the service temperature of TiAl alloy. Further potential components for aero engine exhaust nozzles and honeycomb structures are based on high Nb-TiAl sheet and foil material. Large scale high Nb-TiAl sheet was successfully fabricated by hot pack rolling direct from cast ingot. By control of rolling temperature, the typical microstructure of TiAl alloy can be obtained, such as DP, NG and NL, and the mechanical test shows that the NL microstructure with fine colony size exhibits excellent room and elevated temperature mechanical properties. The NG microstructure has outstanding superplasticity, and the elongation reaches 824% at 1050℃ and high strain rate. The strengthening/toughning mechanism of the sheet materials is discussed in this work.

8:55 AM  Invited
Methodological Discussion on Enhancing the Temperature Tolerance of TiAl Alloys: Ji Zhang1; Xiwen Zhang1; Jing Zhu2; 1China Iron and Steel Research Institute Group; 2Tsinghua University
    Light-weight gamma TiAl alloys are expected to be the substitutes for Ni-based superalloys, whereas so far not competitive enough because of the high cost and limited application temperature range. The efforts to increase the high temperature strength retention of TiAl alloys have produced many successful examples. Among those, developing higher Nb containing alloys and minor additions of C seem more likely to play the role in the application extension of TiAl alloys. This presentation will assess the effectiveness and drawback of the both approaches based the creep performance and related thermal microstructure stability. It has been noticed that the TiAl-Nb alloys are not preponderant in long-term creep resistance since their lamellar structures are even less stable than those in conventional TiAl-C alloys. Therefore, the influence of Nb or Al segregation on microstructure stability and the effects of adding small amount of C in TiAl-Nb alloys are examined and discussed accordingly.

9:20 AM  Invited
Microstructure and Mechanical Properties of In-situ TiAl Matrix Composites Reinforced with Ti2AlC Particles: Juraj Lapin1; 1Institute of Materials and Machine Mechanics, Slovak Academy of Sciences
    In-situ TiAl matrix composites reinforced with Ti2AlC particles were prepared by induction melting and casting into graphite moulds. Various content of Ti2AlC particles in the composites was achieved by alloying of the master Ti-Al-Nb alloys during melting. The casting resulted in a homogenous distribution of coarse primary Ti2AlC particles in the samples. Bimodal distribution of Ti2AlC particles was achieved by heat treatments resulting in precipitation of secondary Ti2AlC particles in a lamellar (TiAl)+α2(Ti3Al) matrix. Ageing experiments were carried out to study microstructure stability of the as-cast microstructure at temperatures from 850 to 1000 °C. Instrumented Charpy impact tests were performed to determine the effect of the Ti2AlC particles on fracture toughness. Compression and tensile properties were measured as a function of temperature up to 1000 °C. Creep test were carried out at temperatures from 850 to 1000 °C to evaluate the effect of the Ti2AlC particles on the creep deformation.

9:45 AM  
Gamma Alloy Process-Microstructure Combinations vs. Deformation and Fracture at Ambient as well as Elevated Temperatures: Young-Won Kim1; Sang-Lan Kim1; 1Gamteck LLC
    Gammalloys generate numerous microstructures that sensitively depend on alloy composition, solidification path, processing route (casting vs.. wrought), post-processing heat treatment cycle and aging/stabilization. Their mechanical behavior varies widely dependent on the microstructure type as well as its detailed features, including phase distribution. The microstructures can be grouped into four typical groups: duplex, nearly lamellar (NL), fully lamellar (FL) and XD lamellar (XDL). The first three can be produced in cast as well as wrought forms, while XDL is in cast forms only, and their microstructure-property relationships are too complex to be quantified. However, the properties (tensile, fracture, creep, fatigue, DKth) of well-controlled wrought duplex and wrought FL materials can be described at least semi-quantitatively as a function of microstructure. Since other microstructure types are mixtures or variations of the two typical microstructures, the relationships can be estimated to predict their properties. The above interrelations will be assessed and discussed.

10:05 AM Break

10:20 AM  Invited
Research Progress on Gamma TiAl Alloy Technology in NPU: Hongchao Kou1; Bin Tang1; Liang Cheng1; Zhigang Sun2; Jinshan Li1; 1State Key Laboratory of Solidification Processing, Northwestern Polytechnical University; 2Shaanxi Engineering Research Center for Advanced Materials and Solidification Processing
    Gamma TiAl alloy is becoming a competitive light-weight high temperature structural material in the aerospace field, and many efforts on component-specific processing received much attention in past decades. In this presentation, the studies of processing technology in TiAl alloys at NPU will be reviewed. Recently, the TiAl ingot production capacity and composition control level have been improved, and the cold crucible induction melting-counter gravity casting equipment has been used to produce TiAl blades and other castings. We also explored the wrought processing and forming technology by utilizing recrystallization, and the forging parts with fine grain and homogeneous microstructure have been fabricated. At last, the process on diffusion bonding and powder metallurgy of TiAl alloys are also introduced.

10:45 AM  
Microstructure-sensitive Computational Scheme for Fatigue Resistance of Gamma-TiAl TNM Alloys: Adrienne Muth1; Paul Kern1; Aaron Tallman1; Thomas Payne1; Don Shih2; Ben Smith2; David McDowell1; 1Georgia Institute of Technology; 2Boeing Research and Technology
    TiAl alloys show potential for elevated temperature components in aerospace applications, owing to comparable strength and creep resistance but reduced density relative to Ni-base superalloys, resulting in significant weight savings. Due to higher volume fraction of beta phase, addition of niobium and molybdenum to create TNM facilitates use of conventional forging and processing methods, which eases commercial restrictions on fabrication. Understanding process-structure-property relationships is critical to developing TNM alloys. In the spirit of Integrated Computational Materials Engineering, microstructure-sensitive computational modeling efficiently represents these relationships for stiffness, strength and fatigue resistance to augment costly empirical methods. A Python-scripted pipeline generates statistical volume elements capturing the morphology of multiphase TNM microstructures. Two crystal plasticity models are implemented for finite element analyses of generated microstructures to simulate macroscopic and local responses, considering model parameter and model form uncertainty. Deformation mechanisms are explored via Fatigue Indicator Parameters computed using ensembles of statistical volume elements.

11:05 AM  Invited
R-curve Behaviour of Different Nearly Lamellar Microstructures in an Intermetallic Ti-43.5Al-4Nb-1Mo-0.1B Alloy: Martin Schloffer1; Thomas Leitner2; Svea Mayer3; Helmut Clemens3; Jörg Esslinger1; Wilfried Smarsly1; Reinhard Pippan2; 1MTU Aero Engines AG; 2Erich Schmid Institute of Material Science, Austrian Academy of Sciences; 3Montanuniversität Leoben
    In this study the R-curve behaviour of different microstructures of a Ti-43.5Al-4Nb-1Mo-0.1B (at.%) alloy, was fracture mechanically tested. The focus was placed on the study of the crack growth behaviour of different nearly lamellar (NL) microstructures. The crack length was thereby measured in-situ with the potential drop technique from room temperature up to 700°C. Four NL microstructures were adjusted by different multi-step heat treatments. The crack propagation curve, as measured by the potential drop technique, is transferred into the Kitagawa diagram and is compared to the experimentally evaluated crack propagation achieved from short crack experiments. It was found that the fracture toughness depends on the structure of the colony boundaries and crack propagation on oxidation mechanism.

11:30 AM  
Mechanical Behavior and Microstructure Evolution of Fine-grained High Nb Containing TiAl Alloy under Isothermal Compression: Yudong Chu1; Jinshan Li1; Bin Tang1; Hongchao Kou1; 1Northwestern Polytechnical University
    Isothermal compression tests of a high Nb containing TiAl alloys with fine (α2+γ) microstructure and (β+γ) microstructure, Ti-43Al-8Nb-(W,B,Y), were carried out at the temperature 1000℃ with low strain rates 10-4S-1 and 10-3S-1. SEM, EBSD and TEM were employed to investigate the microstructure characteristic. The results show that the TiAl alloys with two different microstructures exhibit the same characteristics that, when the alloys were deformed at 10-3S-1, severe γ grain refinement occured and the fraction of low-angle grain boundary of γ grain significantly increased. When the alloys were deformed at 10-4S-1, γ grain was slightly refined and more uniform, grain boundary characteristics of γ grain was stable. Moreover, according to investigation by TEM, it is believed that the dislocation slip and twinning in the interior of initial γ grain was responsible for the deformation at 10-3S-1. The grain boundary sliding between γ grains was responsible for the deformation at 10-4S-1.

11:50 AM  Student
Fracture and Fatigue Crack Growth Behavior of Wrought Gamma Titanium Aluminide Ti-43Al-4Nb-1Mo in Different Microstructure Conditions: Matthew Dahar1; Thomas Podbesek2; Sesh Tamirisakandala2; John Lewandowski1; 1Case Western Reserve University; 2Alcoa Titanium & Engineered Products
    Ti-43Al-4Nb-1Mo (TNM) is a third generation wrought gamma titanium aluminide being used for high performance gas turbine engine low-pressure turbine blades. This study summarizes the room temperature tension, fracture, and fatigue crack growth behavior of TNM at various stages of processing. Tension and bend bar samples were machined from the longitudinal and transverse directions of as-cast, post-hot isostatic pressing (HIP), and forge plus heat treated conditions. Fatigue crack growth tests were conducted over a range of load ratio (R) values in order to determine the dependence of fatigue threshold, Paris slope, and stress intensity at overload (Kc) on R. Optical and SEM examinations were used to determine the microstructure, fracture path, and topography and compare mechanistic behavior of TNM in various conditions.