Effect of texture and mechanical anisotropy on flow behaviour in Ti–6Al–4V alloy under superplastic forming conditions

Abstract

The dependency of anisotropic flow behaviour on crystallographic texture is investigated in Ti–6Al–4V alloy at 750 °C and 900 °C under constant strain rates of 10−2, 10−3 and 2 × 10−4 s−1. The evolution of microstructure and crystallographic texture during these test conditions has been studied using electron backscatter diffraction (EBSD). Anisotropic flow stress behaviour was observed at 750 °C irrespective of the applied strain rate. The maximum flow stress at this temperature was recorded for samples with their lengths perpendicular to the rolling direction (RD), which had <0001>//Transverse Direction (TD) ± 20°, Basal TD texture. The presence of a banded microstructure appeared to be the prime reason for the anisotropic tensile behaviours at lower temperatures. However, at the higher temperature of 900 °C isotropic deformation was achieved disregarding sample orientations, i.e., parallel or perpendicular to the RD. Rachinger grain boundary sliding along α-β boundaries, accommodated by intragranular slip, was seen to contribute towards the total strain in samples perpendicular to the RD deformed under 2 × 10−4 s−1 strain rate. As such, Rachinger grain boundary sliding is the dominant deformation mechanism in the direction perpendicular to the RD at 900 °C. On the other hand, although exhibiting isotropic flow behaviour, the same texture is not observed for the samples parallel to the RD at 900 °C under the same strain rate (2 × 10−4 s−1). Thus Rachinger grain boundary sliding is not thought to be the dominating deformation mechanism for this sample orientation and potentially Lifshitz sliding is active. It is concluded that despite not having a strong effect on flow behaviour, microstructural texture determines the mechanism (i.e., Rachinger, Lifshitz) by which the superplastic deformation is driven.