Role of growth rate on microstructure evolution, element distribution and nanohardness of phases in directionally solidified multiphase high-Nb TiAl alloy

Abstract

A multiphase TiAl alloy with nominal a composition of Ti–43Al–5Nb-3.5Cr–1Zr (in at. %) was fabricated using cold crucible directional solidification equipment at multiple growth rates. Effect of growth rate on the microstructure evolution, element distribution and mechanical property of phases in TiAl alloy were experimentally and statistically studied, especially the mechanism that affects the phase nanohardness. Multiphase microstructures consisting of α 2 /γ lamellar colonies and blocky γ phases as well as strip-like B2 phases were investigated through SEM and EDS identifications. With the increase of growth rates, the volume fractions of B2 and blocky γ phase increased accordingly from 7.3% to 13.3% and 4.6% to 12.6%, respectively. Furthermore, refinement of colonies can also be observed, which is attributed to phase interface migration driven by the change in growth rate. The content of Cr and Zr in the B2 phase is positively correlated with the growth rate, because the increase of growth rate will increase the effective distribution coefficient and make the solidification process deviate from the equilibrium state. Nanoindentation test results showed that nanohardness of B2 and blocky γ phases changed along with the growth rates owing to the enrichment or barrenness of alloying elements. The increase of Zr in blocky γ phase will increase its nanohardness, while the increase of Cr in B2 phase will lead to a decrease in its nanohardness, which is ascribed to lattice distortion caused by atomic substitution.