Low-temperature superplastic deformation mechanism in Ti–6Al–4V alloy processed by friction stir processing

Abstract

The rolled Ti–6Al–4V alloy plates with ~2.3 mm in thickness were processed by Friction stir processing (FSP), and ultrafine (~0.57 μm) and equiaxed microstructure was successfully obtained in the stir zone (SZ). Superplastic tensile tests of this microstructure were conducted in the strain rate range of 1 × 10−4-3 × 10−3 s−1 with the tensile testing temperature of 550–600 °C and an extremely excellent low-temperature superplasticity (LTSP) with an elongation of 1400% was obtained at 3 × 10−4 s−1 and 600 °C. Such a large elongation was owing to the presence of a large proportion of high angle grain boundaries (90.7%) and random crystallographic orientation in the SZ of the friction stir processed (FSPed) Ti–6Al–4V alloy. Furthermore, the precipitated β phase during tensile deformation also made a contribution to the enhancement of LTSP. In addition, a slight grain growth from initial 0.57 μm in the FSPed microstructure to 1.07 μm in the gauge section of the tensile fractured sample at 3 × 10−4 s−1 and 600 °C proved the excellent microstructure stability of the FSPed Ti–6Al–4V alloy. The grain boundary sliding (GBS) was recognized as the dominant superplastic deformation mechanism. The grain rotation (GR) and dynamic recrystallization (DRX), especially the discontinuous dynamic recrystallization (DDRX), acted as the coordination mechanisms that could relax the stress concentration generated during GBS, and thereby promoted the continuous operation of GBS and the achievement of the enhanced LTSP.