Combined Effects of High Undercooling and Large Cooling Rate on the Microstructure Evolution and Hardening Mechanism of Rapidly Solidified Ti-Al Alloys

Abstract

Controlling the microstructure evolution of Ti-(47, 50, 54) at. pct Al alloys by combining the effects of high undercooling and large cooling rate has been investigated. The calculated undercooling and cooling rates of the three component alloy droplets increase as power functions with the decreasing alloy droplet diameter D. With decreasing D, the microstructure of Ti-47 at. pct Al alloy evolves from dendritic dendrites to equiaxed dendrites, and the main microstructure of Ti-50 at. pct Al alloy transforms from a lamellar microstructure to a non-lamellar microstructure, while that of Ti-54 at. pct Al alloy appears as an interweaving microstructure. The α2 phase with a superlattice structure and the α2n phase with a nonsuperlattice structure are confirmed by transmission electron microscopy, and the large difference of Al contents leads to formation of the α2 phase and the α2n phase. Atomic images show that the interface of the α2n phase and γ phase is coherent. The α2n phase precipitates from the γ phase during cooling. The combined effects of high undercooling and large cooling rate suppress the transformation of the α phase to (α2 + γ) phases. The Young’s modulus first increases and then decreases with decreasing D, while the nanohardness is controlled by the combined factors of the microstructure morphology, phase composition, phase ratio, and grain refinement. As for the microhardness, grain refinement dominates the hardening of the Ti-54 at. pct Al alloys, while the combined factors dominate the hardening of Ti-(47, 50) at. pct Al alloys.