Change in mechanical properties and microstructure of ARB processed Ti during annealing

Ultrafine grained commercial purity titanium (CP-Ti) was fabricated by accumulative roll-bonding (ARB) process up to 6 cycles at ambient temperature. The microstructure was composed of the equiaxed grain structure having a mean grain size of 90nm and the lamellar boundary structure having a mean lamellar spacing of 70 nm. The specimen ARB processed by 6 cycles were subsequently annealed at various temperatures for 1.8ks. After annealing at 400 degrees C, the ARB specimen showed the partially recrystallized microstructure composed of recrystallized grains with grain size of approximately 0.5 mu m and the recovered ultratine structure. After annealing at 500 degrees C, the microstructure was filled with the equiaxed recrystallized grains having a mean grain size of approximately 2 mu m. The mechanical properties of the ARB processed and subsequently annealed specimens were investigated by tensile test. The tensile strength decreased and the total elongation increased continuously with increasing the annealing temperature. It was found that the tensile strength decreased linearly with increasing the total elongation in a strength-ductility balance plot, which was significantly different from the cases of Al and the interstitial free (IF) steel where the strength-elongation balance showed a trade-off relationship. The result indicates that ultrafine grained Ti has an excellent strength-ductility balance compared with Al and IF steel.