Development of a Technique for Controlling the Gradient Structure of Titanium by the Severe Cyclic Back-torsion Process

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We produced specimens by applying a one-way torsion process and a cyclic back-torsion process to commercially pure titanium (CP Ti) and cold-workable β-rich (α+β) titanium alloy SP700 under various conditions. We obtained the following results from specimens by performing room-temperature and high-temperature tensile tests, hardness tests, surface roughness measurements, and structural observations:Firstly, concerning the commercially pure titanium CP Ti :(1) After the cyclic back-torsion process, even specimens having square cross-sections exhibited comparatively linear ridges and exhibited no great changes in appearance. The process is thus capable of restoring specimens to their original shape.(2) The cyclic back-torsion process did not change the surface roughness greatly. This process is capable of reducing the surface roughness more than the one-way torsion process.(3) The cyclic back-torsion process and subsequent recrystallization annealing formed a gradient structure in which … More the crystal grain size gradually decreased in the direction away from the center, whereas the hardness increased in this direction.(4) The one-way torsion process and cyclic back-torsion process increased the tensile strength. The cyclic back-torsion process is capable of increasing the tensile strength more than the one-way torsion process since the specific angle of twist can be made greater.(5) In the high-temperature tensile test, the cyclic back-torsion processed specimen exhibited a greater strain-rate sensitivity index m (m=0.28) at a deformation temperature of 923 K than the cold-rolled specimen. Accordingly, the total elongation of the back-torsion-processed specimen was approximately 10% to 20% greater than that of the cold-rolled specimen at any strain rate.Concerning the titanium alloy SP700 :(6) The cyclic back-torsion process did not greatly change the hardness of the annealed specimen, but it did increase its tensile strength. The back-torsion process caused the surface and central structures of the specimens to elongate toward the perimeter. Less