Microstructure evolution and nano-hardness modulation of rapidly solidified Ti-Al-Nb alloy

Microstructure evolution, the formation of B2 (ordered BCC) phase and its relation to the nanomechanical properties of the rapidly solidified Ti68Al32, Ti63Al32Nb5 and Ti58Al32Nb10 alloys were investigated by the drop tube technique. For the Ti68Al32 alloy droplets, the solidified microstructures are composed of alpha(2)-Ti3Al phase, when the droplet diameters (D) are ranging from 226 to 1100 mu m. With the decrease of droplet diameters, the microstructural characteristics of alpha(2) phase transform from coarse dendrites to the fully equiaxed grains. Notably, at D < 226 mu m, the formation of alpha(2) phase is suppressed, resulting in the retention of metastable alpha-Ti phase. For the Ti68-xAl32Nbx (x = 5, 10) alloys, the increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the alpha(2) grains. Upon further decreasing the droplet diameter, the microstructure evolves from dendrite dendrites to equiaxed dendrites, and the B2 phase precipitates from the core of alpha(2) dendrites to alpha(2) grain boundaries, which ascribes to Nb-segregation from the dendrite core area to the grain boundary via L ->beta and beta ->alpha, respectively. Nanoindentation test reveals that the hardness of alpha(2) phase first increases and then decreases with the decrease of droplet diameters, which corresponds to the microstructure morphology, phase constitution, grain refinement of the alloys. Meanwhile, the additions of Nb exhibit enhanced properties of alpha(2) phase compared with those with low or without Nb additions, which can effectively modulate the hardness of these alloys. (C) 2020 Elsevier B.V. All rights reserved.