Improvement of the creep lifetimes and microstructural stability of beta-solidifying gamma-TiAl by cold crucible directional solidification

摘要

The cast and cold crucible directional solidification (CCDS) Ti-44AL-6Nb-lCr alloys were crept at 800 degrees C under 170-230 MPa. The creep lifetimes of CCDS specimens significantly increase comparing with the cast specimens. A novel nano-scale gamma/beta lamellar structure is found at microsegregation zone in CCDS alloy, which exhibits the higher microstructural stability. The omega phases precipitated in beta matrix with accumulating Nb and rejecting Cr. The small precipitated phase exhibits a higher specific surface area, which promotes the cavity and crack nucleation/growth at the interface of precipitated phase. The granular and lathy precipitated phases easily fall off during creep, which contributes to the crack propagation. The CCDS technique can improve creep lifetimes by optimizing the microstructure, promoting the formation of the beta/gamma lamellae, eliminating the horizontal colony boundaries and reducing the proportion of colony boundaries.