Combined Effects of High Undercooling and Large Cooling Rate on the Microstructure Evolution and Hardening Mechanism of
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CONTROLLING the microstructure evolution and phase transition by activating metallurgical mechanisms is of a major interest in materials development and is a practical method for improving Ti-Al alloys.[1–4] Optimization of Ti-Al alloys has aroused much interest in the past decades due to their unique microstructure evolution and phase transition mechanism as well as potential applications as structural materials in the aerospace and vehicle industries.[5–7] Therefore, conventional casting, heat treatment, powder metallurgy, and other processes are used to study the microstructure and mechanical properties of Ti-Al alloys.[8–13] The improvement of mechanical properties, such as the yield strength, plasticity, and hardness, depends on the
Z.C. LUO and H.P. WANG are with the School of Physical Science and Technology, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted October 14, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
optimized microstructure, including the composition and distribution of the phases, grain size, etc.[14–19] Based on microstructure changes and grain refinement effects, a variety of studies on microstructure evolution and phase transitions have been implemented in a large number of alloy systems under different undercooling or various cooling rate conditions.[20–24] However, studies on the combined effects of high undercooling and large cooling rate on rapid solidification mechanism in Ti-Al alloys are still limited, and there is an urgent need to understand the relationship between the mechanical properties and corresponding microstructure of the rapidly solidified alloys. Of all the mechanical properties, hardness is widely considered due to its importance for applications.[25,26] Moreover, the hardness of Ti-Al alloy is inferred to be affected by the microstructure characteristics, including the dendrite growth manner, dendrite sizes, phase compositions and phase distribution. The microstructure characteristics are determined by experimental design whose adjustment can be used as a practicable method to significantly improve the mechanical properties. Novel microstructure characteristics benefitting from rapid solidification techniques may offer a
II.
EXPERIMENTAL PROCEDURES
An ultrahigh vacuum arc melting furnace was used to prepare the master alloys using 99.999 pct Ti and 99.999 pct Al, and each master alloy had a mass of approximately 1 g. The rapid solidification experiments of Ti-(47, 50, 54) at. pct Al alloys were performed using a 3 m drop tube. We first evacuated the drop tube to 104 Pa and then filled it back with a mixed gas of argon and helium in a volume ratio of 1:1 to 105 Pa. Each sample was placed in a F 16 mm 9 150 mm quartz tube that had a F 1 mm 9 1 mm orifice at the bottom. The samples were overheated to 100 K to 200 K above their liquidus temperature by a high frequency power supply whose radio frequency was approximately 50,000 Hz. Then, a high pressure argon jetting gas was used to eject
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