纳米晶金属室温自回复性研究

Nanocrystalline (nc) metals always have very poor tensile ductility, which puts sand in the wheels of their molding and application as structural materials. Nc metals have a unique deformation mechanism which is inaccessible for their coarse-grained counterparts. The Frank-Read sources cease to work in nc metals. Such tiny grain can’t contain any dislocation sources, or even dislocation debris. Alternatively, the geometrically necessary dislocations located in grain boundary (GB) can operate as dislocation sources instead of Frank-Read source. The involved dislocations are emitted from GB, and then are absorbed by the opposite GB. In this process, none dislocation interaction happens, and thus none dislocation debris is left inside the nanograin. Meanwhile it is widely reported that GB activities are intensively involved into the plastic deformation of nc metals. Based on that, we proposed the research on the self-recovery of nc metals in room temperature. Its existence heavily depends on whether the GB activities, including sliding\immigration\rotation and coalescence and so on, can match the dislocation moments and smooth the bombardment as dislocations are emitting or being absorbed. If they do, the dislocation density and the stress concentration can be relieved automatically during deformation. That is really helpful to enhance the ductility and the molding of nc metals. Thus, in this proposal, the possibility of being and the condition will be researched according to the deformation mechanism and the mechanical properties of nc metals. Firstly, in situ high resolution transmission electron microscopy observation of the deformation process of nc gold film under compression will be completed on a AFM in TEM holder made by Nanofactory. We will pay more attention on the GB sliding and grain rotation, and the sliding rate and the rotation rate will be evaluated according to this experiment. This experiment can uncover whether GB activity is fast enough to relieve the stress concentration at GB induced by dislocation emitting and absorption. A successive stress relaxation experiment will be implemented on nc copper and nickel. The evolution of the mobile dislocation density and the dislocation velocity will be evaluated by this experiment. And the results will be compared with their coarse-grained counterparts to decide how much dislocation debris can accumulate inside the nanograin. Nc coppers and nickels with different grain size will be rolled at room temperature with different rolling rate. Its hardness and the XRD Full-Width of Half Height will be carefully detected during rolling. These results will be compared with its coarse-grained counterparts. So the existence condition can be determined according to this experiment.

纳米晶金属的低塑性严重限制了它的成型性,阻碍了它在工程方面的大规模应用.本项目针对这一问题,根据纳米晶金属独特的变形机制(晶粒中无位错源,变形过程中位错由晶界发射,并消失于对面晶界而不在晶粒内部形成位错残余;晶界运动参与塑性变形)提出了纳米晶金属室温自回复性的研究。它的存在性主要依赖于变形过程中位错滑移与晶界运动的相互协调而自动消除形变过程中的位错塞积和应力集中。它的存在显然能够大大提高纳米晶金属的塑性及成形性。因此，本项目中将从微观变形机制和宏观力学试验两方面探讨其存在的可能性和条件。在微观变形机制方面，将通过纳米晶金的透射电镜原位压缩试验观察晶粒旋转，晶界滑移的存在性，评估其速率及其与位错滑移的匹配性；通过对纳米晶铜的应力释放试验确定可动位错密度和位错滑移速度随应变的演变，评估位错的排空性。在宏观试验方面，将考察纳米晶铜冷轧过程中显微硬度和XRD半高宽的演变，确定自回复性的存在条件

纳米晶金属的低塑性严重限制了它的成型性,阻碍了它在工程方面的大规模应用。本项目针对这一问题,针对纳米晶金属独特的变形机制开展研究。利用透射电镜原位观察发现，纳米晶粒内部位错源稀缺,变形过程中位错是由晶界发射。并且位错湮灭不容易发生，位错是消失于对面晶界而不在晶粒内部形成位错残余。晶界滑移和晶粒旋转等晶界运动大规模参与纳米晶金属的塑性变形。因此纳米晶金属的塑性变形能力依赖于变形过程中位错滑移与晶界运动的相互协调性。只有当位错运动在晶界处产生的位错塞积和应力集中完全被晶界运动所平衡时，断裂才不会发生。本项目还通过在不同温度下对不同晶粒尺寸的纳米晶铜、纳米孪晶铜进行应力释放和蠕变实验，发现纳米晶金属材料的塑性变形是和时间、应力水平和温度相关的量。我们建立模型描述了他们之间的关系。通过将模型与实验数据拟合，我们得到模型中相关热力学参数的数值。比较这些数值我们发现，温度对塑性变形的影响是通过应力相关的变面激活能产生的。在位错变形机制中，流变应力越高，激活体积越小。模型还阐述了应变速率名感性指数和应力指数、激活体积之间的关系。我们发现，应变速率名感性指数实际上是通过两种途径影响应力和应变速率之间的关系：1降低热激活过程中的激活体积和激活能；2改变绝热过程中的应力指数。只用当激活激活体积为零时，应变速率名感性指数才是应力指数的倒数。