Formation and evolution of tri-modal microstructure during dual heat treatment for TA15 Ti-alloy

Abstract

Dual heat treatment (near-β heat treatment + (α+β) heat treatment) provides an effective way for the acquisition of a tri-modal microstructure in TA15 titanium alloy, and this kind of microstructure is found to possess excellent comprehensive performance. The temperature difference between near-β and (α+β) heat treatments determines the content of secondary lamellar α phase in the final tri-modal microstructure. During near-β heat treatment, the volume fraction of equiaxed α phase decreased sharply to 8%–9%, and equiaxed α phase with grooves on the surface broke up due to phase transformation and then spheroidized. Martensite was formed after water quenching, and it decomposed into a large number of secondary lamellar α phase at the initial stage of (α+β) heat treatment. Afterwards, parts of the decomposed lamellae dissolved, and the rest grew through the merging of adjacent lamellae. In the meantime, the content of equiaxed α phase increased to about 20%. Tertiary lamellar α phase precipitated from high temperature β phase together with residual β phase to form transformed β matrix during subsequent air cooling. Eventually, tri-modal microstructure is formed, and its mechanical properties meet the requirements of aviation forgings.