Hot deformation behaviour of Ti-6Al-4V alloy with a transformed microstructure: a multimodal characterisation

Abstract

Hot deformation behaviour of Ti-6Al-4V with transformed microstructure was investigated for very high deformation. Hot compression tests were carried out up to 80% height reduction at 950°C and strain rate of 0.1 s−1. The true stress–strain curve exhibits an anomalous flow behaviour with distinctly different characteristic zones. The flow stress reaches a peak in ‘zone 1’ followed by softening in ‘zone 2’ and further anomalously increases in ‘zone 3’ after a true strain of 0.9. The flow behaviour was further investigated by conducting interrupted hot compression tests of 20% and 50% height reduction in zone 2. Multimodal characterisations such as OM, EBSD, XRD, TKD and TEM analyses were employed for this study. The TKD and TEM analysis reveals that the α-laths structure (under compressive loading) kinks, bends and fragments, resulting in flow stress softening in the transformed microstructure. The bending and breaking coexist and lead to the cascading effect phenomena resulting in globular microstructure. Further, an increase of flow stress at very high strain in zone 3 is attributed to the formation of lamellar morphology. The apparent lamellar morphology holds the Burger orientation relationship where (0001)α∥(110)β and ⟨112¯0⟩α∥⟨111⟩β and it deviates as lamellar morphology weakens. Interestingly, at very high deformation, dynamically recrystallised transformed microstructure shows nano-twinning, unlike the static recrystallised transformed microstructure. These outcomes are only possible when the multimodal characterisation technique is applied, since the volume fraction of β-phase is very low and it is distributed as a very thin layer between α-laths.