Hot tensile properties, microstructure evolution and fracture mechanisms of Ti-6Al-4V alloy with initial coarse equiaxed phases

Hot tensile experiments of a Ti-6Al-4V alloy with coarse equiaxed phases are implemented at the temperatures of 800-900 degrees C and strain rates of 0.0005-0.005 s(-1). All the flow stress curves at the tested conditions show the similar material flow characteristics, i.e., the flow curves include three different stages (elastic, softening and localized necking stages). For a given true strain, the true stresses are large at high strain rates and low deformation temperatures. Also, the coarse equiaxed alpha phases transform to the bimodal microstructures during hot tensile, which attributes to the dynamic spheroidization of alpha phases. The transformation process includes the formation of sub-structure, fragmentation of sub-boundary, penetration of beta phase, and inversion of misorientation angle. Because a high deformation temperature can promote the growth of sub-boundary and the penetration of beta phase into lamellar alpha phases, the fraction of equiaxed alpha phases significantly increases with raising the deformation temperature. The fracture features at all the tested conditions show the primary ductile fracture, especially at the high deformation temperatures and low strain rates.