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

Dual heat treatment (near-beta heat treatment + (alpha+beta) 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-beta and (alpha+beta) heat treatments determines the content of secondary lamellar alpha phase in the final tri-modal microstructure. During near-beta heat treatment, the volume fraction of equiaxed alpha phase decreased sharply to 8% -9%, and equiaxed a 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 alpha phase at the initial stage of (alpha+beta) 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 alpha phase increased to about 20%. Tertiary lamellar a phase precipitated from high temperature beta phase together with residual beta phase to form transformed beta matrix during subsequent air cooling. Eventually, tri-modal microstructure is formed, and its mechanical properties meet the requirements of aviation forgings. (C) 2019 Elsevier B.V. All rights reserved.