Microstructural stability, phase transformation and mechanical properties of a fully-lamellar microstructure of a Mo-modified high-Nb γ-TiAl alloy

摘要

Microstructural instability modes, phase transformation and mechanical properties evolution of a fine-grained fully-lamellar Ti-44Al-6Nb-1Mo-0.3(B, Y) (at.%) alloy were investigated systematically. During isothermal exposure at application temperatures, dramatic discontinuous precipitation of y and β_0 (ω_0) grains proceeded along boundaries of prior lamellar colonies, i.e. via a transformation of L(α_2/γ) → γ + β_0 (ω_0). Meanwhile, the internal lamellar structures degraded by α_2-lath dissolution and simultaneous γ-lath coarsening. Furthermore, thermal loading was found to contribute to the above decomposition processes. However, almost no β_0 and β_0 (ω_0) grains formed in the interiors of lamellar structures. The precipitation of equilibrium phases γ, β and ω_0 abided by normal crystallographic orientation relations with their respective parent phases. The present microstructural decomposition was considered to be essentially induced by thermodynamic instability of α_2 laths and high interfacial energy of nanoscaled lamellar structures. Finally, it was demonstrated that this micro-structural degradation caused both high-temperature strength and room-temperature ductility to deteriorate.