Activation of homogeneous precursors for formation of uniform and refined α precipitates in a high-strength β-Ti alloy

The influence of heating rates and interrupted aging treatments on α precipitation in the β-quenched Ti-6Cr-5Mo-5V-4Al alloy has been investigated. Microstructure observations reveal that slow heating rates and low temperature pre-aging produce much refined and uniform distributions of intragranular α precipitates in comparison to aging using fast heating rates at a high temperature only. For interrupted aging treatments, the nucleation rate and refinement of α are gradually enhanced with increasing pre-aging time, but this is dependent on the final aging temperature: at high final aging temperatures, the distribution of α precipitates is similar to single aging at the same temperature. The kinetics of phase transformation was analyzed by thermal mechanical analysis (TMA) under the framework of the Johnson–Mehl–Avrami (JMA) theory. Coupled with microstructural observations, it is suggested that epitaxial growth of pre-existing α phase formed during heating to the aging temperature is the governing mechanism under slow heating rate conditions, while for interrupted aging treatments, the mechanism for intergranular α precipitation is consistent with that of fast heating rate single temperature aging treatments. For interrupted aging, the formation of uniform and refined α precipitates essentially relies on activation of homogeneous precursors associated with chemical partitioning and structural modulation which provide a strong driving force for nucleation of α.