Formation mechanism of ultrafine α+β structure in Ti-6Al-4V alloy during β→α_m→α+β continuous phase transformation

Although massive transformation has been increasingly observed in titanium alloys, the decomposition mechanism of the massive transformation product (α_m) is still unclear. Here, the β→α_m→α+β continuous phase transformation in Ti-6Al-4V alloy during cooling was studied. We found that massive transformation occurs at a moderate cooling rate below 291.5 °C/s, forming irregular featureless α_m. The thermodynamically unstable α_m is supersaturated with β stabilizers, and it tends to decompose into ultrafine α+β structure during the continuous cooling at ever-lower cooling rates and temperatures. In the initial stage of α_m decomposition, misfit dislocations appear as a predecessor and split α_m into strips. Accompanied by the elemental redistribution, misfit dislocations are accommodated and replaced by β phase. Both the formation and decomposition of α_m follow the typical Burgers orientation relationship.