Zhao Yonghao, Gu Yanglin, Guo Yazhou
Nano and Heterogeneous Materials Center, School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.
School of Aeronautics, Northwestern Polytechnical University, Xi'an 710072, China.
Nanomaterials (Basel). 2021 Feb 25;11(3):574. doi: 10.3390/nano11030574.
The conventional engineering stress-strain curve could not accurately describe the true stress-strain and local deformability of the necking part of tensile specimens, as it calculates the strain by using the whole gauge length, assuming the tensile specimen was deformed uniformly. In this study, we employed 3D optical measuring digital image correlation (DIC) to systematically measure the full strain field and local strain during the whole tensile process, and calculate the real-time strain and actual flow stress in the necking region of ultrafine-grained (UFG) Ti. The post-necking elongation and strain hardening exponent of the UFG Ti necking part were then measured as 36% and 0.101, slightly smaller than those of the coarse grained Ti (52% and 0.167), suggesting the high plastic deformability in the necking part of the UFG Ti. Finite elemental modeling (FEM) indicates that when necking occurs, strain is concentrated in the necking region. The stress state of the necking part was transformed from uniaxial in the uniform elongation stage to a triaxial stress state. A scanning electron microscopic (SEM) study revealed the shear and ductile fracture, as well as numerous micro shear bands in the UFG Ti, which are controlled by cooperative grain boundary sliding. Our work revealed the large plastic deformability of UFG metals in the necking region under a complex stress state.
传统的工程应力-应变曲线无法准确描述拉伸试样颈缩部分的真实应力-应变及局部变形能力，因为它是通过使用整个标距长度来计算应变的，假定拉伸试样是均匀变形的。在本研究中，我们采用三维光学测量数字图像相关技术（DIC）系统地测量了整个拉伸过程中的全应变场和局部应变，并计算了超细晶粒（UFG）钛在颈缩区域的实时应变和实际流动应力。随后测得UFG钛颈缩部分的颈缩后伸长率和应变硬化指数分别为36%和0.101，略小于粗晶钛的（52%和0.167），这表明UFG钛颈缩部分具有较高的塑性变形能力。有限元建模（FEM）表明，当颈缩发生时，应变集中在颈缩区域。颈缩部分的应力状态从均匀伸长阶段的单轴应力状态转变为三轴应力状态。扫描电子显微镜（SEM）研究揭示了UFG钛中的剪切和韧性断裂以及大量微剪切带，它们受协同晶界滑动控制。我们的工作揭示了UFG金属在复杂应力状态下颈缩区域的大塑性变形能力。
