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

对纯钛（CP Ti）和冷加工富β (α+β)钛合金SP700在不同条件下分别采用单向扭转和循环反扭转工艺制备试样。通过对样品进行室温和高温拉伸试验、硬度试验、表面粗糙度测量和结构观察，得到以下结果：首先，对于市产纯钛CP Ti:(1)经过循环反扭过程后，即使是方形截面的样品也呈现出相对线性的脊状，外观变化不大。因此，这个过程能够将标本恢复到原来的形状。(2)循环反扭过程对表面粗糙度影响不大。该工艺比单向扭转工艺更能降低表面粗糙度。(3)循环反扭过程和随后的再结晶退火形成了一个梯度结构，其中晶粒尺寸在远离中心的方向上逐渐减小，硬度在远离中心的方向上逐渐增大。(4)单向扭转和循环反扭转均提高了抗拉强度。由于扭转的特定角度可以更大，因此循环反扭转过程比单向扭转过程更能提高拉伸强度。(5)高温拉伸试验中，循环反扭处理试样在变形温度为923 K时的应变率敏感性指数m （m=0.28）高于冷轧试样。因此，在任何应变速率下，后扭处理试样的总伸长率都比冷轧试样高约10% ~ 20%。(6)循环反扭过程对退火试样的硬度变化不大，但对试样的抗拉强度有一定的提高。后扭转过程导致试样的表面和中心结构向周长方向拉长。少

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