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

The microstructural conversion mechanism in an alpha' martensite starting microstructure during hot deformation (at 973 K (700 A degrees 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 alpha' starting microstructure at 973 K (700 A degrees C)-10 s(-1) and at a height strain of 0.8, it is observed that the alpha' starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 mu m 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 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 (alpha + beta) 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 (alpha + beta) to the acicular alpha' martensite one. This behavior of the alpha' martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and twins, and the high-speed grain fragmentation along with subgrain formation in the alpha' starting microstructure during the initial stage of deformation. (C) The Minerals, Metals & Materials Society and ASM International 2013