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

Abstract

Microstructure evolution, the formation of B2 (ordered BCC) phase and its relation to the nano-mechanical properties of the rapidly solidified Ti 68 Al 32 , Ti 63 Al 32 Nb 5 and Ti 58 Al 32 Nb 10 alloys were investigated by the drop tube technique. For the Ti 68 Al 32 alloy droplets, the solidified microstructures are composed of α 2 -Ti 3 Al phase, when the droplet diameters (D) are ranging from 226 to 1100 μm. With the decrease of droplet diameters, the microstructural characteristics of α 2 phase transform from coarse dendrites to the fully equiaxed grains. Notably, at D < 226 μm, the formation of α 2 phase is suppressed, resulting in the retention of metastable α-Ti phase. For the Ti 68-x Al 32 Nb x (x = 5, 10) alloys, the increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α 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 α 2 dendrites to α 2 grain boundaries, which ascribes to Nb-segregation from the dendrite core area to the grain boundary via L→β and β→α, respectively. Nanoindentation test reveals that the hardness of α 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 α 2 phase compared with those with low or without Nb additions, which can effectively modulate the hardness of these alloys. • TEM images confirmed that the formation of α 2 phase is suppressed, resulting in the retention of metastable α-Ti phase. • The increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α 2 grains. • B2 phase precipitates from the core of α 2 dendrites to α 2 grain boundaries with the decreases of droplet diameter • The addition of Nb can effectively modulate the hardness of Ti 3 Al–Nb alloys.