Internal-variable analysis of high-temperature deformation behavior of Ti–6Al–4V: A comparative study of the strain-rate-jump and load-relaxation tests

The high-temperature deformation mechanisms of Ti–6Al–4V with either a fine or coarse alpha particle size were quantified using an internal-variable theory. For this purpose, strain rate jump tests (SRJT) and load relaxation tests (LRT) were conducted at 700, 800, and 900 °C to determine the strain rate sensitivity and to establish constitutive behavior. Stress–strain rate plots obtained by both SRJT and LRT were in good agreement with the theoretical predictions based on the activation of grain-matrix deformation and particle/grain-boundary sliding (P/GBS). The relative contribution of the two mechanisms varied with the microstructure, temperature, and strain rate, which affected the flow stress and strain rate sensitivity of the alloys. A clear difference in the strain rate sensitivity was observed depending on the experimental method. In all cases, the SRJT values were higher than those from the LRT. The discrepancy in strain rate sensitivity could be attributed to a variation in prestrain between the two methods. This variation resulted in microstructural differences, such as the fraction of alpha/beta interfaces and the misorientation of alpha grain boundaries, and hence affected the contribution of P/GBS to the overall deformation.