Effect of initial grain size on the microstructure and mechanical properties of vacuum solution treated Ti–6Al–4V alloy

Abstract

Ti–6Al–4V alloy warm rolled sheets with different initial grain sizes were obtained by warm rolling with different reduction rates. In order to improve its strength-ductility matching, it was treated by vacuum solution treatment (ST). By means of OM, SEM, EBSD, TEM, EPMA and XRD, the effects of different initial grain sizes of warm rolled sheet on the microstructure and mechanical properties of vacuum ST. The findings indicate that the initial grain sizes significantly influence the grain size, morphological distribution, and mechanical properties following vacuum ST. By warm rolling 60% combined with subsequent solution treatment (ST-60%), the grain size of the primary α phase (αp) and the β transformed (βt) structure is minimized, predominantly fine and equiaxed, with a dense distribution. This results in an optimal strength-ductility balance, with ultimate tensile strength, yield strength, and total elongation measuring 1283 MPa, 1194 MPa, and 10.5%, respectively. The volume fraction, grain size, morphological distribution, and the acicular alpha phase (αs) of αp and βt structure are among the factors influencing their mechanical properties. Furthermore, the solution strengthening of Aluminum (Al) in αp during the ST significantly increases the nano-hardness of αs compared to βt structure. This disparity leads to a plastic strain distribution between αp and βt structure, activating a substantial amount of Geometrically Necessary Dislocations (GND) near the βt/αp interface. This provides additional Hetero-Deformation-Induced (HDI) strengthening, thereby maintaining high strength and good ductility.