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 nano-mechanical 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 α₂-Ti₃Al phase, when the droplet diameters (D) are ranging from 226 to 1100 μm. With the decrease of droplet diameters, the microstructural characteristics of α₂ phase transform from coarse dendrites to the fully equiaxed grains. Notably, at D < 226 μm, the formation of α₂ phase is suppressed, resulting in the retention of metastable α-Ti phase. For the Ti_68-xAl₃₂Nb_x (x = 5, 10) alloys, the increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α₂ 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 α₂ dendrites to α₂ 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 α₂ 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 α₂ phase compared with those with low or without Nb additions, which can effectively modulate the hardness of these alloys.