Deformation-induced grain refinement and amorphization in Ti-10V-2Fe-3Al alloy

The microstructural evolution and grain refinement mechanisms of a Ti-10V-2Fe-3Al alloy, β-solution quenched and cold forged (CF) to strains of 0.1, 0.35, and 1.2 have been investigated using optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The results showed that the stress-induced martensitic transformation became a predominant deformation mode in the metastable Ti-10V-2Fe-3Al alloy during cold forging. These martensites α″ repeatedly divided the original β parent phase into a large number of micron-sized blocks when the forging strain was 0.1. Shear bands were observed to traverse α″/β lamellae and resulted in a significant grain refinement of the β phase, as the forging strain increased to 0.35. The degree of grain refinement inside shear bands was higher than the outside. Nanocrystalline and amorphous structures were produced in local areas of the original β phase, when the forging strain rose to 1.2. This dramatic grain refinement in the metastable Ti-10V-2Fe-3Al alloy could be attributed to the stress-induced martensitic transformation promoting the initiation and growth of shear bands across α″/β lamellae. More dislocations were produced and accumulated inside grains to accommodate plastic deformation. The crystal structure was collapsed and an amorphous structure was formed as soon as the dislocation density was accumulated to a critical value of 10 ¹⁴/cm ². Moreover, some of the reverse martensitic phase transformation, α″→β, was observed to contribute to grain refinement of Ti-10V-2Fe-3Al alloy as well.