Frequent Occurrence of Discontinuous Dynamic Recrystallization in Ti-6Al-4V Alloy with α′ Martensite Starting Microstructure

Abstract

The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deformation (at 973 K (700 °C)-10 s⁻¹) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deformation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s⁻¹ and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deformation, subgrain formation in martensite variants and the formation of new grains with high-angle boundaries at interfaces of martensite variants, and $\{ 10\bar{1}1\}$ twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deformation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and $\{ 10\bar{1}1\}$ twins, and the high-speed grain fragmentation along with subgrain formation in the α′ starting microstructure during the initial stage of deformation.