综述了剧烈塑性变形引起的块体纳米金属材料的结构和力学性能演变. 以电化学沉积法制备的 fcc 结构纳米晶 Ni-20%Fe (质量分数)合金为研究对象, 通过对其进行不同应变量的高压扭转实验, 系统分析了变形引起的结构和力学性能演变. 结构表征结果表明: (1) 变形引发纳米晶 Ni-Fe 合金晶粒旋转, 实现晶粒长大. 同时, 晶粒长大过程伴随着位错密度、孪晶密度的演变; (2) 存在一个最有利于变形孪晶生成的晶粒尺寸范围(45~100 nm), 在这个晶粒尺寸范围之外, 去孪晶起主导作用使原有的生长孪晶或变形孪晶消失; (3) 位错密度是影响位错与孪晶反应的新的影响因素. 当发生孪晶的晶粒内位错密度低时, 位错可完全穿过孪晶界, 部分穿过孪晶界, 或被孪晶界吸收; 发生孪晶的晶粒内位错密度高时, 大量位错缠绕并堆积在孪晶界附近, 形成应力集中, 破坏孪晶界原有的共格性. 为释放局部应力, 将从孪晶界的另一侧发射不全位错形成层错和二次孪晶; (4) 在塑性变形导致的晶粒长大过程中, 原先偏聚于消失了的晶界上的 C 和 S 沿残留晶界扩散并继续偏聚于晶界上. 结构与力学性能关系结果表明: 随着应变量的增加, 应变强化、应变软化交替出现. 位错密度对硬度的演变起主导作用, 其它结构演变(如孪晶密度的变化和晶粒尺寸变化)对硬度的演变起次要作用.

Severe plastic deformation techniques including high-pressure torsion and equal channel angular pressing have been widely used to refine coarse-grained materials to produce nanocrystalline and ultrafine-grained materials, or manipulate the microstructure of nanocystalline materials for superior mechanical properties. This pa-per overviews severe plastic deformation induced structural and mechanical property evolutions on bulk nanocrystalline metals, mainly in a nanocrystalline Ni-20%Fe (mass fraction) alloy with a face-centred cubic (fcc) structure processed by high-pressure torsion to different strain values. The structural evolution and mechanical property evolution at different strain values were studied. Comprehensive characterizations on structural evolution during deformation indicate that: (1) grain growth occurred via grain rotation, and is accompanied with changes in dislocation density and twin density; (2) there is a significant grain size effect on deformation induced twinning and de-twinning. There exists an optimum grain size range for the formation of deformation twins. Outside of this grain size range the de-twinning process will dominate to annihilate existing twins; (3) different types of dislocation-twin boundary (TB) interactions occurred during deformation. Dislocation density plays an important role in dislocation-TB interactions. In a twinned grain with a low dislocation density, a dislocation may react with a TB to fully or partially penetrate the TB or to be absorbed by the TB via different dislocation reactions. In a twinned grain with a high dislocation density, dislocations tangle with each other and are pinned at the TBs, leading to the accumulation of dislocations at the TBs and raising the local strain energy. In order to release the stress concentration, stacking faults and secondary twins formed by partial dislocation emissions from the other side of the TB; (4) atom probe tomography investigation reveals that C and S atoms, which are the major impurities in the Ni-Fe alloy and segregated at grain boundaries (GBs) of the as-deposited material, migrated from disappearing GBs to the remaining GBs during high-pressure torsion. Investigation on structure-hardness relationship of the Ni-Fe alloy reveals that: strain hardening and strain softening occurred at different deformation stages. Dislocation density evolution plays a major role in the hardness evolution, while other structural evolutions, including twin density and grain size evolutions, play minor roles in the hardness evolution. © Copyright.