Multiscale analysis of high impact toughness in a novel low-cost titanium alloy

This study systematically investigates the deformation behavior of novel low-cost Ti-6Al-4V-0.4Fe-0.6Mo alloy with varying microstructures under dynamic impact loading. The microstructures with equiaxed and bimodal were fabricated by controlling the temperatures (880°C, 900°C and 920°C) and cooling rates (air cooling, AC and furnace cooling, FC). Charpy impact testing results show that the 920FC sample, featuring large equiaxed α phases (average size: 8.2 μm), α lamellae (thickness: 1.5 μm), and retained β-phases, exhibits the highest impact toughness of ∼62.5 J/cm 2 . Compared with 880FC (∼45.3 J/cm 2 ), 900FC (∼42.1 J/cm 2 ) and Ti-6Al-4V alloys with equiaxed microstructures (∼50 J/cm 2 ), the impact toughness is improved by 38%, 48% and 25%, respectively. Moreover, the impact toughness of the 920AC sample (∼47.6 J/cm 2 ) is 20% higher than that of 880AC (∼39.5 J/cm 2 ) and 900AC samples (∼39.5 J/cm 2 ). Microstructural analyses results show that both 920AC and 920FC samples exhibit low α phase aspect ratios, high α phase spheroidization proportion, and large α grain sizes. The 920FC sample has a percentage of twin boundaries as high as 58.7%, which is the core mechanism for its superior impact toughness over the 920AC sample. Interestingly, we found that twinning nucleation in both α and secondary α phases (α s ) is induced by stacking faults and the abundant stacking faults form martensite 9R phase. The coarsening and growth of twins are affected by dislocation slip. In addition, the calculated results based on the Yu Rui-huang electron theory show that Fe and Mo weaken the inhibitory effect of V on dislocation slip and provide a low-energy path for twin boundary migration.