Kinetically Induced Fine Secondary α-Ti Phase Formation in a Novel As-Cast Titanium Alloy

The formation of fine secondary hexagonal close packed (HCP) alpha-Ti precipitates provides major strengthening in a new Ti-6Al-5Fe-0.05B-0.05C (mass fractions in pct) cast alloy. The phase transformation mechanisms from the body-centered cubic (BCC) beta-Ti matrix to fine alpha phase in this new alpha-beta titanium alloy were investigated experimentally and computationally using CALculation of PHAse Diagram (CALPHAD)-based thermodynamic and kinetic models. The discrete distribution of alpha precipitates was observed in the as-cast alloy with evidence of strong Fe partitioning. Two main size groups of alpha precipitates and the Fe partitioning were characterized using scanning electron microscopy, scanning transmission electron microscopy, and synchrotron-based small-angle X-ray scattering techniques. A hypothesis of Fe-partitioning driven alpha precipitate nucleation and growth was validated by precipitation simulation using TC-PRISMA with customized thermodynamic and kinetic descriptions. These results suggested a new titanium alloy design route involving high-mobility elements (enhancing fine secondary alpha precipitates) and demonstrated the capability of CALPHAD-based modeling in titanium alloy design.

Automated summary

This study investigates the phase transformation and strengthening mechanism in a new α-β titanium alloy. It validates the hypothesis of iron-driven nucleation and growth of α precipitates. The findings have significant implications for titanium alloy design.