Effect of accumulative hot rolling on the microstructure and mechanical properties of dual-phase titanium alloy consolidated via SPS sintering

Abstract

The improvement of microstructure and mechanical properties of Ti alloys is of great significance for their industrial application. In this study, a developed multi-step hot deformation process was employed to regulate the grain boundaries (GBs), hetero-phase boundaries (PBs) and dislocation structure for dual-phase titanium alloy. The effects of accumulative thermal deformation ratios from 33% to 75% (2–8 cycles) on the microstructure and mechanical properties have been investigated. It is found that the sintered sample shows a typical Widmanstätten structure without obvious pores and micro-cracks. However, with the increase of the deformation ratio, for the hexagonal close packed (HCP) Ti, the crystal orientations of {011‾0} and {1‾21‾0} were gradually transformed into {0001}, with the change of high-angle grain boundaries (HAGBs) to low-angle grain boundaries (LAGBs). The density of stored geometrically necessary dislocations (GNDs) density raised dramatically from 1.95 × 1014 m−2 to 6.5 × 1014 m−2, and various types of dislocations have been characterized. For the β phase, in the thermal deformation process, high density α-precipitates with an average thickness below 20 nm had formed with Burgers relationship of {110}BCC//{0001}HCP and <1‾11 >BCC//<21‾1‾0 >HCP. Finally, the yield strength of the dual-phase titanium alloy increased dramatically from 881 MPa to 1178 MPa, and the ductility can still maintain at above 8.5%. The enhanced strength was mainly contributed by the increase of geometrically necessary boundaries (GNBs) and nano-lamellae boundaries (NLBs). Therefore, this strategy of combining hetero-phase boundaries (PBs) and dislocation engineering could open up new avenues to designing strong and ductile titanium matrix materials.