Microstructure evolution and grain fragmentation induced by the co-activation of slip and [formula omitted] nanotwin during cryo-forging of a Ti-6Al-4V alloy

Abstract

The nanostructured titanium alloy usually exhibits superior mechanical properties than the coarse-grained titanium alloy. In the present work, the α-grain fragmentation induced by slip/nanotwin co-activation during cryo-forging was studied in a bimodal Ti-6Al-4V alloy, in which the nanosized (022‾1‾) deformation twin was revealed. The results show that the lamellar α-grains fragmented to small globular grains after 25% compression at 0.1 mm/s and 0.5 mm/s. The high strain rate and large deformation increased the dislocation density and flow stress, therefore strengthened the deformation texture and promoted the grain fragmentation. The (0001) <112‾0> basal slip system was regarded as the dominant slip mode during cryo-forging, which activated more intensely in lamellar α-grains than that in equiaxed α-grains, and was promoted as the deformation ratio and stain rate increased. In addition, the activation of basal slip system was inhomogeneous in lamellar α-grains. The partial lattice rotation caused high angle grain boundary (HAGB) formation and grain fragmentation. On the other hand, the (022‾1‾) nanosized deformation twins appeared within fragmented α-grains after 25% cryo-forging at 0.5 mm/s, in which the (011‾1) planes between matrix and twin existed a ∼27° misorientation. The (022‾1‾) deformation twins contributed to grain fragmentation by twin boundary grooving and twin-twin intersection. This work demonstrated an available route to achieve refined or gradient grained structure in Ti-6Al-4V alloy by slip/twinning co-activation.