Microstructure and Mechanical Properties of Powder Metallurgical TiAl-Based Alloy Made by Micron Bimodal-Sized Powders
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Microstructure and Mechanical Properties of Powder Metallurgical TiAl-Based Alloy Made by Micron Bimodal-Sized Powders Yibo Ren, Ying Han, Shun Yan, Jiapeng Sun, Zhenxin Duan, Hua Chen, and Xu Ran Submitted: 1 August 2020 / Revised: 19 October 2020 / Accepted: 7 November 2020 Three powder metallurgical Ti-48Al-2Cr-2Nb compacts were prepared using spherical pre-alloyed powders, mechanically milled powders, and a mixture of the spherical pre-alloyed powders and the mechanical milled powders in a weight ratio of 1:4. Different microstructures corresponding to coarse grains, ultrafine grains, and bimodal-size grains, respectively, were obtained. The compact with a bimodal grain structure exhibits a good combination of high-yield compressive strength ( 1393 MPa) and improved compression ratio to fracture ( 13.9%) at room temperature due to the effects of back-stress and ductile c-TiAl singlephase layer generated near the ultrafine/coarse grain interface. At high temperatures, the compressive properties of the compact with the bimodal grain size distribution are sensitive to the temperature. A relatively high deformation resistance is achieved at 750 °C. At this temperature, the coarse grain region of the bimodal grain-sized microstructure undergoes more strain, and the dynamic recrystallization is promoted with increasing strain, improving the ductility. By contrast, the ultrafine grains in the bimodal grain size microstructure dominate the dynamic softening when the temperature is higher than 850 °C due to their accelerated dynamic recrystallization and easy grain boundary slip that are responsible for the good formability and the sharp decrease in deformation resistance of this alloy. Keywords
bimodal grain microstructure, mechanical property, powder metallurgy, TiAl alloy
1. Introduction Lightweight TiAl-based alloys have been considered to be attractive candidate materials for replacing nickel-based superalloys, titanium alloys, and other high-temperature alloys for aerospace structural and engine applications due to their high specific strength, high specific modulus, good corrosion resistance, and excellent high-temperature properties (Ref 1, 2). However, the microstructure defects caused by conventional casting such as composition segregation, shrinkage cavity and porosity, grain coarsening, and microstructure inhomogeneity can severely decrease the availability and workability of TiAlbased alloys (Ref 3, 4). Consequently, improvement of metallurgical quality is of great significance for the development of TiAl-based alloys. The powder metallurgy technique is an alternative process that facilitates the design of a broad spectrum of compositions, microstructure control, and property optimization, so as to efficiently avoid metallurgical defects in castings and easily achieve microstructure homogeneity (Ref 5–7). In previous Yibo Ren, Ying Han, Shun Yan, Zhenxin Duan, Hua Chen, and Xu Ran, Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University
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