Superplasticity and microstructural characteristics of ultra-fine grained Ti-6Al-4V alloys obtained through protium treatment

To study the structural characteristics and superplasticity of ultra-fine grained alpha+beta type Ti-6Al-4V titanium alloys obtained through protium treatment (protium absorption-quenching-hot rolling-protium desorption), observation of the structure and each grain orientation using a scanning (SEM) and a transmission electron microscope (TEM), and superplastic tensile tests in vacuum were carried out. The temperature and initial strain rate dependences of flow stress and elongation in ultra-fine grained Ti-6Al-4V alloy were investigated. The structural changes after testing were also observed using an optical microscope. Experimental results are as follows: according to the observation of ultra-fine grained materials using TEM, most grains were between 0.3 and 0.5 mum with high angle boundaries. When the superplastic properties of the materials with each grain size were investigated, the temperature at which ultra-fine grained materials showed maximum elongation was lower than those of coarse-and fine-grained materials. The ultra-fine grained material exhibits a huge elongation more than 9000%, the higher value than any metallic material, at the temperature of 1123 K and a strain rate of 1 x 10(-3) s(-1). Observation of microstructure in the coarse-grained material after tensile testing revealed that many voids appeared at the early stages of deformation. In the case of ultrafine grained materials, voids don't appear until growing into coarse grain. This is considered to be one of the factors explaining the huge superplastic elongation.