纳米晶金属卸载塑性变形行为及其机制的研究

When conventional coarse-grained polycrystalline metals deform plastically, dislocation nets produced by dislocation interactions on cross-slip planes would limit motion of dislocations and result in strain hardening. Upon unloading, dislocation structures remain stable, thus coarse-grained metals exhibit uauslly elastic unloading behavior. However, for nanocrystalline metals, dislocation sliding occurs on limited slip plans and does not form dislocation nets, dislocatons are in high stress state. Upon unloading, dislocation activity does not cease and thus produce significant unloading plastic deformation effect. Due to different initial deformed structures and stress state, dislocation activity and grain boundary deformation process of nanocrystalline metals during unloading would differ considerably that occurred in loading stage. Studying unloading behavior of nanocrystalline metals and relevant dislocation and grain boundery mechanisms can enhance our understanding for plastic deformation nature and also provide the basis for reasonable application of nanocrystalline metals. In this project, the series compression cycling and nanoidentation tests will be performed to examine the unloading plastic deformation behaviors of nanocrystalline Cu and Ni and their relationship to material grain size, deformation rate and loading/unloading mode and temperature. By combinating the results of dislocation stress field components determined by incremental unloading tests, the dislocation density expression developed based on relevant dislcation theoreis，the transmission electron microscopy observations, the thermal activation paratemeters measued by using plastic deformation dynimic expression, unloading plastic deformation behaviors of nanocrystalline Cu and Ni will be explained theoretically.

传统粗晶多晶体金属塑性变形时，交叉滑移系上位错相互作用产生的位错网会限制位错的运动并导致应变硬化，卸载时位错结构仍保持稳定，所以，粗晶金属总表现为弹性卸载。但对于纳米晶金属，位错活动局限在少数滑移面上，不形成位错网，位错处于高应力状态，卸载时位错活动不会停止，将导致明显的卸载塑性变形效应。由于不同的初始变形结构和应力状态，纳米晶金属卸载时的位错活动和晶界变形过程明显不同于加载阶段。研究纳米晶金属卸载塑性变形行为和相关位错和晶界变形机制可深化对纳米晶金属塑性变形本质的认识，为纳米晶金属的合理使用提供依据。本项目开展系列压缩循环和纳米压痕实验，检验纳米晶铜和镍的卸载塑性变形行为及其与材料晶粒尺寸、加/卸速率和模式、温度等的关系；利用增量卸载实验确定应力场，基于相关位错理论建立位错密度模型，利用塑性变形动力学表达式确定热激活参数，进行透射电镜观察，对纳米晶铜和镍卸载塑性变形行为进行理论解释。

传统粗晶多晶体金属塑性变形时，交叉滑移系上位错相互作用产生的位错网会限制位错的运动并导致应变硬化，卸载时位错结构仍保持稳定，所以，粗晶金属总表现为弹性卸载。但对于纳米晶金属，位错活动局限在少数滑移面上，不形成位错网，位错处于高应力状态，卸载时位错活动不会停止，将导致明显的卸载塑性变形效应。由于不同的初始变形结构和应力状态，纳米晶金属卸载时的位错活动和晶界变形过程明显不同于加载阶段。研究纳米晶金属卸载塑性变形行为和相关位错和晶界变形机制可深化对纳米晶金属塑性变形本质的认识，为纳米晶金属的合理使用提供依据。.本项目在纳米铜上开展了压缩循环实验，在纳米铜、纳米镍、纳米镍-铁合金、纳米级奥氏体不锈钢和纳米钽上开展了系统的纳米压痕保载蠕变实验，这些实验显示纳米晶金属及合金显示明显的卸载或加载后塑性变形效应，其效应强烈依赖加载阶段的位错结构(密度)或加载应变或应力速率和纳米晶金属及合金的显微结构(晶粒尺寸和晶界结构)。通过从压缩增量卸载实验和蠕变实验得到热激活参数(激活体积、应变速率敏感性和激活能)的测定,建立了位错密度概率模型。研究发现，导致卸载塑性变形效应的本质是纳米金属中缺少足够的位错钉扎点，同时高密度的晶界即是位错的源，也是位错的阱，位错结构不能有效存储，卸载时会迅速在晶界处释放，从而导致明显的塑性变形效应。.为进一步检验这种效应，在纳米晶金上开展高分辨电镜原位观察，表明纳米金属位错和晶界活动活跃，具有极高的本征塑性，进一步证实了卸载塑性变形效应的位错和晶界机制。通过在纳米和超细晶纳铜以及不同晶粒尺寸的奥氏体不锈钢上开展不同速率拉伸实验，利用位错结构和变形表面形态的系统电镜观察，这些实验进一步证实了位错和晶界机制在控制纳米金属的应变硬化及应变局部化过程的独特作用。