纳米多晶金属薄膜微结构及其演化与热激活机制的关联性研究

Generally, plastic deformation in metallic materials could be considered as a thermally activated process. Therefore, quantitatively determination of thermal activated parameters, such as activation energy, activation volume and strain rate sensitivity, have been widely used to characterize the mechanical properties of nanocrystalline metals. However, the special microstructural features in nanocrystalline metals, which are quite different from their bulk counterparts, have not been considered as a crucial factor in conventional thermally activated thoery, which may affect characterizing mechanical properties significantly. Then, it might be some inadequate interpretations of the mechanical properties of nanocrystalline metals in using conventional thermally activated mechanism, which relies on the concepts of coarse grained metals. Therefore, the relevance of these concepts used in nanocrystalline metals must be questioned, considering the restricted mean free path of dislocations, and the extremely large volume of grain boundaries. Here, by focusing on several typical face-centered cubic nanocrystalline metal thin films, we try to establish and examine the characterizing microstructure in the nanocrystalline metals through pre-strain and low-temperature annealing; and try to establish the internal relevance between specific features of microstructure in nanocrystalline metals and those thermally activated parameters, i.e., strain rate sensitivity and activation volume. More importantly, we mainly focus on the applicability of the conventional thermally activated mechanism, which is established in coarse grain metals, in characterizing the mechanical properties of nanocrystalline metals; and eventually, establish the thermally activated mechanisms which can be more accurately describe the underlying plastic deformation of nanocrystalline metals then the conventional one. We expect the achievements derived fron this project coulg also be used in corresponding bulk nanocrystalline metals with face-centered cubic lattice structure.

金属晶体材料的塑性变形是一个热激活过程。因此，热激活参量的量化表征被广泛应用于纳米多晶金属力学性能的研究。然而，建立在粗晶金属变形行为基础之上的传统热激活理论并未考虑纳米多晶金属特有的微观结构及力学行为特征。因此，将传统热激活理论直接应用于纳米多晶金属塑性变形行为的研究方法是值得商榷的。基于此，针对面心立方纳米多晶金属薄膜，本项目在量化考察热激活参量在粗晶金属塑性变形中变化规律的传统方法基础上，通过预变形及低温热处理等方法构造并系统刻画纳米多晶金属中的若干特征微结构体系，尝试建立应变率敏感系数及热激活体积等热激活参量与纳米多晶金属特征微观结构的内在关联，研究方法具有较强的创新性和可操作性。从新的角度探讨传统热激活理论在纳米多晶金属特有微观结构体系下的适用性及评价方法，研究热激活参量在纳米多晶材料塑性变形中的物理意义。本项目的开展对热激活理论在块体纳米多晶金属的应用亦有重要借鉴意义。

本项目首次对应用传统热激活理论及其表征参量分析纳米多晶金属塑性变形的研究方法提出了“错位比较”的观点。我们认为在纳米多晶材料特异的微观结构体系下，将实验测得的热激活参量与建立在粗晶金属变形行为基础之上的传统热激活参量的具体化数值相比较是值得商榷的。因此，本项目拟在全面的描述相应纳米多晶金属材料微观组织结构的基础之上，力求通过综合分析纳米多晶金属中激活体积及应变率敏感性系数随力学参量及特征微观组织参量变化的趋势，尝试建立基于面心立方纳米多晶金属材料特异微观结构体系下的热激活理论，主要结果如下。首先，通过制备体积分数相同，但层数不同的CuTa/Cu多层膜，系统的研究了CuTa/Cu多层膜的本征尺寸效应和 Cu层厚度相同但体积分数不同时的多层膜的力学性能。认为在非等调制比的晶体/非晶多层膜中，其力学性能将由调至比，添加层厚度和界面结构三者共同决定。其次，在非共格的Cu/Ta金属纳米多层膜中并没有发现应变速率敏感性具有强烈的尺寸效应。这是由于忽略了晶界和界面本身结构的不同。实际上，相比与无序的晶界，界面上会规则排列大量的失配位错。这一研究系统的对比了纳米尺度下晶界和界面在塑性变形中的不同作用，揭示了多层膜这类及含有晶界又含有界面材料应变速率敏感性的尺寸效应。最后，通过模拟研究发现，Al2Cu 合金中(110)[001]的滑移系是最适合开动的，其能垒较低，位错核较宽，能在较低温度较小应力下开动；[001]伯氏矢量在（200）滑移面上的活动能力次之，位错核较窄。由于其层错能较高，两个不全位错在运动的时候会收缩。（110）滑移面上的[-111]位错在低温低应力下也很难开动，较高温度下有一定塑性，这也印证实验上在单晶Al2Cu中发现的高温蠕变现象。从理论的结果可以看出，实验及工程上应该尽量设计或生长出利于[001]的滑移系开动的晶体取向结构, 才有可能为提高Al2Cu的室温塑性创造条件。