A shortened process of tri-modal microstructure developing in Ti-6Al-4V alloy via electropulsing-induces grain spheroidization

Abstract

A shortened process combining electropulsing treatment (EPT) and conventional heat treatment (HT) has been applied to develop a tri-modal microstructure in Ti-6Al-4V alloy. The tri-modal microstructure produced by this process has good overall mechanical properties (tensile strength ∼1110 MPa, elongation ∼22.7%). The rapid development of the tri-modal microstructure is attributed to the EPT production of low aspect ratio (LAR) V-poor martensite, which results from the EPT limiting the growth of high-temperature β grains and the diffusion of elements. The α grains decomposed from LAR martensite during the HT process retain a low aspect ratio morphology which can be easily spheroidized. LAR α grains do not undergo phase transformation at two-phase temperature (950 °C) but follow the cylinderization mechanism of spheroidization that relies more on the geometry of the grains than on the high density of dislocation substructures. In this study, the effects of the three types of grains on material strength and ductility are analyzed in detail.