The reverse transformation mechanism of β phase and its stability of Ti-6Al-4V alloy fabricated via laser powder bed fusion

Abstract

The laser powder bed fusion manufactured Ti-6Al-4V alloy predominantly consists of α' martensite, resulting in excellent strength but limited ductility, hinders widespread application. Therefore, it is crucial to achieve the decomposition of α' martensite and improve the ductility. However, the reverse transformation behavior and its mechanism of the β phase, as well as its stability remain unclear. Our findings demonstrate that the reverse transformation behavior of the β phase follows a diffusion-controlled process. The decomposition of α' martensite into α + β initiates at approximately 550 ℃, while the transformation from α to β phase occurs around 870 ℃. Increasing temperature leads to an increase in reverse-transformed β phase. The retention of these reverse-transformed β phases at room temperature also depends on their stability, which exhibits a significant correlation with V element partitioning. Additionally, this study reveals a critical temperature (840 ℃) at which the maximum volume fraction (18.6 %) of retained β phase is obtained, exhibiting exceptional mechanical properties, particularly the uniform elongation surpassing those observed in an as-printed microstructure of LPBF-ed Ti-6Al-4V alloy.