Effect of High-Temperature Structure and Diffusion on Grain-Boundary Diffusion Creep in fcc Metals
Effect of High-Temperature Structure and Diffusion on Grain-Boundary Diffusion Creep in fcc Metals
复制标题
高温结构和扩散对面心立方金属晶界扩散蠕变的影响
DOI：
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发表时间：
2003
期刊：
影响因子：
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通讯作者：
V. Yamakov
中科院分区：
文献类型：
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作者：
P. Keblinski;V. Yamakov
高能扭曲和倾斜面心立方钯双晶的分子动力学模拟揭示了一种普遍的、类液体的、各向同性的高温扩散机制，其特征是具有相当低的自扩散活化能，且与边界类型或取向错误无关。中能晶界在最高温度下表现出相同的行为；然而，在较低温度下，扩散机制变得各向异性，具有更高的、与取向错误相关的活化能。我们的模拟表明，高温下较低的活化能是由结构转变引起的，从低温下的固体边界结构到高温下的液体状结构。我们证明这种转变的存在对纳米晶面心立方金属的扩散蠕变具有重要影响。特别是，我们的模拟表明，在没有晶粒生长的情况下，仅包含高能晶界的纳米晶微观结构表现出稳态扩散蠕变，其蠕变速率与 Coble 蠕变公式给出的定量一致。值得注意的是，高温蠕变速率的活化能与表征高能双晶晶界中普遍高温扩散的活化能相同。
Molecular dynamics simulations of high-energy twist and tilt bicrystals of fcc palladium reveal a universal, liquid-like, isotropic high-temperature diffusion mechanism, characterized by a rather low self-diffusion activation energy that is independent of the boundary type or misorientation. Medium-energy grain boundaries exhibit the same behavior at the highest temperatures; however, at lower temperatures the diffusion mechanism becomes anisotropic, with a higher, misorientation-dependent activation energy. Our simulations demonstrate that the lower activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure at low temperatures to a liquid-like structure at high temperatures. We demonstrate that the existence of such a transition has important consequences for diffusion creep in nanocrystalline fcc metals. In particular, our simulations reveal that in the absence of grain growth, nanocrystalline microstructures containing only high-energy grain boundaries exhibit steady-state diffusion creep with a creep rate that agrees quantitatively with that given by the Coble-creep formula. Remarkably, the activation energy for the high-temperature creep rate is the same as that characterizing the universal high-temperature diffusion in high-energy energy bicrystalline grain boundaries.