Processing-Microstructure-Mechanical Property Paradigm in Hot Rolled Ti-6Al-4V Alloy

Abstract

Abstract The effects of hot rolling temperature and subsequent cooling at different cooling rates on the microstructure, texture, and mechanical properties of α + β titanium alloy Ti-6Al-4V were studied using a battery of structural and mechanical characterization tools. To this end, Ti-6Al-4V samples were subjected to a rolling reduction of 50 % below the β-transus (750°C and 850°C) and near the β-transus (950°C) temperature followed by water quenching and air cooling. Detailed electron back scatter diffraction provided information on the fraction, morphology, and orientation of the alpha and β-phases for different processing conditions and bulk texture analysis provided information on the orientation relationship at the macroscale. Mechanical properties, like hardness and modulus, were determined at different length scales using the Vickers microhardness experiment, instrumented microindentation, and nanoindentation. Hot rolling at sub β-transus temperatures (750°C and 850°C) leads to the equiaxed morphology of the α-phase and the absence of a Burgers orientation relationship (BOR) with the β-phase, whereas near the β-transus, hot rolling leads to the multi-variant lamellar morphology of the α-phase and a strong BOR with the β-phase for both the cooling conditions. Hot rolling at 850°C followed by water quenching showed an optimum combination of indentation hardness and modulus. This has been primarily attributed to the relatively low kernel average misorientation of the basal-prism-oriented grains compared with the prism-pyramidal oriented grains after indentation. The basal oriented grains are both elastically and plastically harder compared with the prism-oriented grains that are elastically soft but plastically hard. The orientation specific indentation hardness property is reflected in the microhardness property for sub β-transus deformed samples. A clear processing-microstructure-texture-mechanical property paradigm in the context of variant selection and distinct cooling rates for hot rolling of Ti-6Al-4V is established.