Impact toughness and deformation modes of Ti-6Al-4V alloy with different microstructures

Abstract The impact toughness and deformation modes of Ti-6Al-4V alloy with different microstructures, i.e., equiaxed microstructure (EM), bimodal microstructure (BM), fine lamellar microstructure (LM1) and coarse lamellar microstructure (LM2), were studied and revealed in detail. The impact toughness of BM was determined as 85 J/cm2, which was about 70%, 42% and 35% higher than that of EM, LM1 and LM2, respectively. BM possesses a composite-like microstructure containing hard primary α grain (αp) and soft β transformation microstructure (βt). During crack propagation, large plastic deformation occurs within soft βt region, and the deformation of βt was constrained by surrounding hard αp, resulting in kink deformation of βt and local plastic deformation of αp. In addition, {102} tensile twins with both high and low Schmid factor were activated in αp by the sever kink deformation of βt and impact energy. Furthermore, some αp grains near the fracture surface were crushed significantly by impact loading. As a result, the synergistic effect of these aforementioned factors can effectively dissipate impact energy, correspondingly enhancing the impact toughness of Ti-6Al-4V alloy. By contrast, the crack mainly propagates along the αp’s grain boundary in EM. Moreover, the coordinate deformation ability between αp and βt was weak due to the high content of αp, resulting in a significant reduction of impact toughness. For LM1 and LM2, although {102} tensile twins and {101} compression twins were activated, the impact toughness was still lower than that of BM due to little plastic deformation in LM1 and heterogeneous deformation in LM2. These results indicated that the coordinated deformation ability of the microstructure plays an important role in impact toughness of the alloy.