Strain rate- and temperature-dependent mechanical properties of Ti-6Al-4V in dynamic compression: hardening and softening behaviour analysis using strain energy-based method

Studies have shown that the deformation of Ti alloys is due to the competition between hardening and softening effects under dynamic loading. However, there are limited indicators of this behaviour throughout the complete stress–strain process. This study aims to quantify the impact of strain rate and temperature on the hardening/softening behaviour of Ti-6Al-4V using a split-Hopkinson pressure bar system over a range of 2000 to 7000 s⁻¹ strain rates and temperatures from 25 to 800°C. Firstly, this study proposes an evaluation index of material hardening/softening behaviour based on the complete stress–strain curve and energy evolution characteristic. Further, the dynamic mechanical properties of Ti-6Al-4V are investigated through the analysis of the stress–strain relationship and fracture morphology. Finally, the hardening/softening index is calculated and analysed. The findings revealed that the fracture surface of the impact specimen displayed dimple-like and smooth features, that are significantly influenced by both temperature and strain rate. The stress–strain curves demonstrated that Ti-6Al-4V exhibits remarkable strain-rate strengthening, plastic increasing, and strain work hardening behaviour. The hardening/softening index B_r decreases with an increase in strain rate. For specific strain rates of 3000, 5000 and 7000 s⁻¹, B_r increases as the loading temperature rises from 25 to 400°C, but decreases when the loading temperature is increased to 600°C. At a strain rate of 2000 s⁻¹, B_r increases monotonically until the loading temperature reaches \(\sim\)800°C. These observations are found to be related to the microstructural evolution at varying temperatures and strain rates.