Superplasticity of Fine-Grained Titanium Allloys Obtained through Protium Treatment

To obtain fundamental data on superplasticity of fine-grained α+β type titanium alloys obtained through protium treatment, superplasticity tensile tests were carried out in vacuum, the strain-rate and temperature dependence of elongation and flow stress n fine-grained Ti-6Al-4V alloy were investigated. Then, changes in microstructure after the tensile test were observed using an optical and scanning electron microscope. Coarse-grained titanium alloy was also tested, and compared with fine-grained alloy. Experimental results are as follows:Fine-grained material having a grain size of 1μm to 3μm shows elongation of over 3700% at a testing temperature of 1123 K and 1173K, and a strain rate of 1×10-3S. When the maximum elongation of fine-grained material was investigated, the material showed a huge elongation of more than 8008% at a testing temperature of 1123K and a strain rate of 1×10-3S. The m-value of fine-grained material is much higher than that of coarse-grained material, with the maximum m-values being 0.80 and 0.64 respectively. These results indicate that it is not easy to occur a necking in fine-grained material. In the observation of structure after the superplasticity tensile test, coarse-grained material did not exhibit grain growth, but fine-grained material did.