Grain Rotation Accommodated GBS Mechanism for the Ti-6Al-4V Alloy during Superplastic Deformation

Abstract

An investigation of flow behavior and the deformation mechanism for Ti-6Al-4V alloy during the superplastic deformation process is presented in this paper. Constant strain rate tensile tests were performed at 890–950 °C and strain rates of 10−2, 10−3, and 10−4/s. Then, surface observation by Optical Microscope (OM), Scanning Electron Microscopy (SEM), and Electron Back-scattered Diffraction (EBSD) was applied to obtain the microstructure mechanism. With pole figure maps (PF) for α-phase, obvious texture gradually changed in the main deformation direction. For the titanium alloy, the evolution of texture in deformed samples was attributed to grain rotation (GR). Significant grain rearrangement occurred between grains after deformation. A complete grain rotation accommodated grain boundary sliding (GBS) deformation mechanism is proposed, which can explain texture evolution without grain deformation.

Dynamic Softening and Hardening Behavior and the Micro-Mechanism of a TC31 High Temperature Titanium Alloy Sheet within Hot Deformation

“…It should be noted that the recrystallized grains grew up significantly under the strain rate of 0.001 s −1 ; therefore, the softening effect caused by DRX decreased with the decreasing strain rate and the flow stress exhibited dynamic hardening. Similar dynamic hardening was also reported during the superplastic deformation of the TC4 titanium alloy [ 24 , 34 ]. It can be seen from Figure 8 a–f that the amount of the recrystallized grains without LAGBs inside increased and the average grain size decreased with the increasing strain rate.…”

“…It is well known that the sliding resistance of the α/β phase interface is lower than that of the α/α interface and the β/β interface [ 40 , 41 ]. Therefore, the increase in the proportion of the α/β phase interface which is attributed to the rise of the volume fraction of β phase is beneficial to grain boundary sliding, and the deformation consistency is also improved simultaneously [ 22 , 24 , 41 ]. At the same time, the recovery and recrystallization at 950 °C was more efficient than those at 850 °C, which led to a lower dislocation density near the phase and grain boundaries [ 18 , 39 , 40 ].…”

“…The deformation energy was higher around the grain boundaries than that at other sites [ 11 , 28 ]; therefore, most of the recrystallized grains nucleated near the grain boundaries. The deformation time of the sample deformed at a strain rate of 0.1 s −1 was too short for the growth of new recrystallized grains [ 24 ]. Therefore, the flow stress exhibited dynamic softening under the higher strain rate conditions.…”

“…When the strain was further increased to 0.7, the mean value of KAM was 0.716 and the reduction was not significant. It can be seen from Figure 9 a,e that the dislocation density was at a lower level with a strain rate of 0.001 s −1 and the strain of 0.5 and 0.7, which would hinder the nucleation of dynamic recrystallization [ 24 ]. Therefore, the main deformation mechanism may have been grain boundary sliding and the grain size increased gradually [ 20 , 24 ].…”

“…When the deformation occurred at the β single-phase region, the cross slip and climb of dislocations easily occurred. The research on the hot deformation mechanisms of dual-phase titanium alloys has mainly concentrated on the TC4 titanium alloy [ 24 ]. At present, there are few reports on the hot deformation behavior and microstructure evolution of the new developed TC31 dual-phase high temperature titanium alloy.…”