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

对工业纯钛（CP Ti）和可冷加工富β（α+β）钛合金SP 700在不同条件下进行单向扭转和循环反扭转试验，制备了试样。通过对试样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，我们得到了以下结果：首先，关于商业纯钛CP Ti：（1）在循环回扭过程之后，即使是具有方形横截面的试样也显示出相对线性的脊，并且在外观上没有太大的变化。因此，该过程能够将样品恢复到其原始形状。(2)循环回扭过程没有改变表面粗糙度很大。该工艺能够比单向扭转工艺更多地降低表面粗糙度。(3)循环回扭过程和随后的再结晶退火形成梯度结构， ...更多信息 晶粒尺寸在远离中心的方向上逐渐减小，而硬度在该方向上增加。(4)单向扭转和循环回扭工艺提高了拉伸强度。循环反向扭转工艺能够比单向扭转工艺更多地增加拉伸强度，因为可以使特定的扭转角更大。(5)在高温拉伸试验中，循环回扭处理的试样表现出更大的应变速率敏感性指数m（m=0.28）在923 K的变形温度比冷轧试样。因此，在任何应变速率下，反扭处理试样的总延伸率都比冷轧试样的总延伸率高10%~ 20%。对于SP 700钛合金：（6）循环反扭处理对退火试样的硬度没有显著影响，但提高了其抗拉强度。回扭过程导致样品的表面和中心结构向周边伸长。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