Dhal A, Panigrahi S K, Shunmugam M S
Manufacturing Engineering Section, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, 600036, India.
Sci Rep. 2019 Jul 23;9(1):10683. doi: 10.1038/s41598-019-46957-4.
During microforming of conventional materials, specimen and microstructural length-scales are close to each other. This leads to an abnormal deformation behavior of the material and reduces microformability. Engineering ultrafine-grained (UFG) microstructure in the material is a possible solution. However, micro-scale deformation behavior of UFG material is not fully understood. Present work attempts to comprehensively investigate the micro-scale deformation of four distinctly engineered microstructures: UFG with residual dislocations and elongated grains, UFG free of residual dislocation with equiaxed grains, bimodal-grained and coarse-grained. The deformation behavior is captured via micro-scale uniaxial tensile test and micro-deep drawing operation. Micro-cups generated from UFG material with equiaxed grains show excellent surface quality, form-accuracy and minimal process scatter. Postmortem microscopy of the formed micro-cups attributes this improved microformability to the activation of grain boundary-mediated plasticity in the material which results in synergetic grain migration and rotation. Presence of residual dislocations and elongated grains hinders the grain migration and rotation leading to strain localization and thinning. In case of bimodal and coarse-grained material, cross-slip based deformation mode progressively dominates over grain migration and rotation, which results in a reduction in microformability due to the influence of size-effect.
在传统材料的微成形过程中，试样和微观结构的长度尺度彼此接近。这导致材料出现异常变形行为并降低微成形性。在材料中设计工程超细晶粒（UFG）微观结构是一种可能的解决方案。然而，UFG材料的微观尺度变形行为尚未得到充分理解。目前的工作试图全面研究四种经过不同设计的微观结构的微观尺度变形：具有残余位错和拉长晶粒的UFG、无残余位错且晶粒等轴的UFG、双峰晶粒和粗晶粒。通过微观尺度单轴拉伸试验和微深冲操作来捕捉变形行为。由具有等轴晶粒的UFG材料制成的微杯具有优异的表面质量、形状精度和最小的工艺分散性。对形成的微杯进行的事后显微镜检查将这种改善的微成形性归因于材料中晶界介导的塑性激活，这导致了协同的晶粒迁移和旋转。残余位错和拉长晶粒的存在阻碍了晶粒迁移和旋转，导致应变局部化和变薄。在双峰和粗晶粒材料的情况下，基于交滑移的变形模式逐渐超过晶粒迁移和旋转，由于尺寸效应的影响，这导致微成形性降低。
