中文摘要：

对工业纯钛（TA2）在液氮介质中机械孪晶随应变量的变化规律以及孪晶对晶粒尺寸的依赖性进行了研究。结果表明：在静拉伸过程中孪晶分数随应变量的增加而增加，孪晶的形成主要在均匀塑性变形阶段，尤其在塑性变形的初期，颈缩后孪晶分数增加缓慢。孪晶形貌的演化规律为：在变形的初始阶段生成孪晶的尺寸比较大，在随后的塑性变形中又发生破碎，最终形成一些孪晶密集的区域。低温下纯钛的塑性变形方式为孪生和滑移共同作用。粗晶粒（55μm）和细晶粒（18μm）的纯钛在室温和低温下的拉伸实验结果表明，晶粒的粗化没有降低纯钛的塑性，低温下粗、细晶粒纯钛的塑性均比室温下的高。这种现象与纯钛低温下活跃的孪生密切相关。

英文摘要：

Twinning behavior in tensile deformation of commercial pure titanium as well as its grain size dependence were investigated. The microstructure after deformation was observed by use of optical microscope and transmission electron microscope, and the twin fraction was obtained as well. The results show that twins appear mainly at the stage of strain hardening and twin fraction increases with increase of the plastic deformation. The size of twins at early stage of plastic deformation is larger than that at latter stage, however, larger twins will break under large deformation, which forms some zones with high density of twins together with small twins. The microstructures also show that twinning cooperates with slip in the deformation of titanium at cryogenic temperature. Commercial pure titanium with grain size of 18μm and 55 μm were obtained after annealed at 650℃ and 800 ℃, respectively. The tensile properties show that ductility of titanium increases with increase of grain size from 18 μm to 55 μm at room temperature and - 196℃, and twinning seems to become more common in coarsening grains. All the results imply that higher ductility in coarsening grain is related to the mechanisms of twinning induced plasticity.