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

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 (α_p) and soft β transformation microstructure (β_t). During crack propagation, large plastic deformation occurs within soft β_t region, and the deformation of β_t was constrained by surrounding hard α_p, resulting in kink deformation of β_t and local plastic deformation of α_p. In addition, {10-12}<-1011> tensile twins with both high and low Schmid factor were activated in α_p by the sever kink deformation of β_t and impact energy. Furthermore, some α_p 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 α_p's grain boundary in EM. Moreover, the coordinate deformation ability between α_p and β_t was weak due to the high content of α_p, resulting in a significant reduction of impact toughness. For LM1 and LM2, although {10-12}<-1011> tensile twins and {10-11}<-1012> 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.