The effect of microstructure on localized melting at separation in Ti-6Al-4V tensile samples

Abstract

The phenomenon of localized melting during the separation process of tensile samples has been found in a number of different titanium alloys. In this study, the effect of microstructure on the mechanism responsible for localized melting found on the fracture surfaces of Ti-6Al-4V tensile samples has been investigated. Tensile samples with three different microstructures were strained to fracture at quasi-static applied strain rates, and the resulting fracture surfaces were investigated and characterized using stereo scanning electron microscopy. Selected fracture surfaces were also nickel-plated and sectioned to reveal the subsurface microstructures and to get an idea of the nature of deformation in the region of the fracture surface. Samples which had an equiaxed α microstructure as a result of mill annealing and heat treating showed large regions of concentrated shear and considerable localized melting. By contrast, samples with a finea plate colony microstructure displayed much higher fracture surface tortuosity and no localized melting. The connection between large-scale shear and localized melting implies that a shear instability, such as adiabatic shear, is quite likely a necessary factor for achieving the melting temperature at final separation.