超细晶材料在冲击载荷下的剪切局部化行为

Ultrafine-grained (UFG) materials have extensive prospects for engineering application due to their excellent mechanical properties. However, the grain size decrease reduces their strain hardening ability and makes UFG materials more susceptible to deformation instability such as shear localization. Currently, the phenomena of shear localization under impact loading are still under controversy and such study on UFG material is more inadequate. It has been determined that the shear localization behavior is decided by external environments (such as strain rate and temperature) and the material itself (such as crystalline structure, grain size and texture). In this study, we will choose metals with typical crystalline structures (face centered cubic (FCC), body centered cubic (BCC) and hexagonal close packed (HCP)) and produce UFG materials by equal channel angular pressing method (ECAP). The compressive and shear mechanical properties under a range of temperatures as well as strain rates will be systematically studied. The shear localization behavior will be analyzed and its generating conditions will be clarified. The criterion for shear localization under dynamic loading will be investigated. The microstructure evolution of shear band for UFG metals will also be investigated. The mechanism of shear banding and its dependence on crystalline structure and grain size will be revealed. The results of this project will be instructive for understanding the deformation and fracture behavior of UFG materials and promotive for their engineering applications, especially under impact environments.

超细晶材料由于其优异的力学性能而有着广泛的工程应用前景。然而晶粒尺寸的减小也导致其应变硬化能力降低，使其更容易产生变形失稳，发生剪切局部化。当前人们对金属材料在冲击载荷下剪切局部化现象的研究还存在争议，对于超细晶材料则更加缺乏深入研究，但可以肯定的是材料的剪切局部化行为受到外部载荷条件（如应变率、温度等）和材料本身（如晶格结构、晶粒尺寸、织构等）的影响。本课题选取具有典型晶格结构（FCC、BCC、HCP）的金属，利用ECAP方法制备超细晶材料，通过系统的力学实验获得其在不同温度、不同应变率下的压缩和剪切性能；研究冲击载荷下适合超细晶材料的剪切失稳判据，明确剪切局部化的产生条件；研究超细晶材料剪切局部化产生的微观机制，明确其对于晶格结构、晶粒尺寸等的依赖性。本课题的研究成果对于深入理解超细晶材料的变形破坏行为，推动其在工程中的应用，尤其是冲击环境下的应用具有重要意义。

根据计划书要求，本项目针对超细晶金属材料在冲击载荷下的变形局部化行为开展了深入研究，主要工作和成果包括以下几个方面：（1）通过大塑性变形法成功制备了超细晶钛、镁合金以及铜等不同结构的超细晶材料，并对其进行了微观组织表征；（2）获得了不同结构的多种超细晶材料在冲击载荷下的拉伸、压缩、剪切力学性能，丰富了这一领域的研究数据；（3）搭建了力-热-变形一体化测试平台，在国际上首次实现了绝热剪切局部化过程中的力学响应、温度变化和变形场的同时测量；（4）对超细晶及粗晶金属材料绝热剪切局部化产生条件和准则进行了深入研究，实验发现温度升高并不是绝热剪切带产生的原因，而是其导致的结果。这一结果推翻了长期以来人们对于绝热剪切带成因的认识。

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