一维应变压缩下TC4钛合金绝热剪切研究

 利用内径为57 mm的压缩气炮，在撞击速度为0.2~1.2 km/s（相应的靶中压力为3~15 GPa）范围内进行对称碰撞实验，以研究TC4（Ti-Al6-V4）钛合金在一维应变冲击压缩条件下的绝热剪切现象。对回收得到的受冲击样品，在扫描电镜（SEM）下进行细观金相分析。结果指出，一维应变冲击压缩条件下，TC4钛合金中绝热剪切带产生的对称碰撞速度阈值为500 m/s（相当于样品中的压力为5.87 GPa）；主剪切带与冲击方向约为45°角，带上有圆形和椭圆形两种孔洞且随碰撞速度的增大而增多和长大，这是典型的韧性损伤特征。随碰撞速度增大，产生与主剪切带成15°角的支剪切带。这些与理论预言相符。X射线能谱分析结果指出，剪切带内材料发生了(α+β)→β相的转变，是典型的相变带。剪切带的温度估算与实验提供的信息吻合。

A STUDY ON ADIABATIC SHEAR BANDING OF TC4-TITANIUM ALLOY UNDER PLANAR SHOCK COMPRESSION

Investigations for adiabatic shear banding (ASB) of a TC4-titanium alloy under planar shock compression were performed by using a compressed gas gun of 57 mm bore diameter, and symmetric impact technique. The stresses in the target, from 3 to 15 GPa, were designed above the Hugoniot elastic limit, σ_HEL=2.85 GPa, of the target material. Microstructural features of the recovered sample were examined metallographically by using scanning electron microscope. Results indicated: (1) ASB emerges only if the stress in target is not less than 5.87 GPa (or impact velocity 500 m/s), a stress far above σ_HEL. It implies that the ASB formation, resulted from heterogeneous energy deposition, is merely a stage in the material yielding process. Results from X-ray diffraction analysis indicated that α→β phase transition occurred in ASB, and also in matrix material but with weaker degree and rather dispersive manner; (2) The angle between ASB and shock compression direction is about 45°, consistent with the theoretical prediction of maximum shear direction for isotropic medium, which is just the case of TC4-titanium alloy; (3) Spherical and elliptic voids appeared in ASB and increased both in number and in size with increasing shock stress in target, showing the typical features of the dynamic damage for ductile materials; (4) A few ASB branches appeared at stress of 15 GPa, with an angle of about 15° inclined to the main ASB; and (5) microstructure of ASB, characterized by the appearance of tiny and lengthy crystallites with preferred orientation parallel to the bounds of ASB, is significantly different from that of the matrix material. Accordingly, we believe that a melting and recrystallization process happened in ASB during its development. Calculations for ASB temperature and the melting temperature at corresponding shock stress showed that the above arguments are reasonable.