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Bulk ultrafine-grained (UFG) metals and alloys prepared by techniques of severe plastic deformation (SPD), in particular by equal channel angular pressing (ECAP), exhibit exceptional mechanical properties.With respect to potential applications of this new class of very fine-grained bulk materials, the cyclic deformation and fatigue behaviour, relative to that of conventional grain size (CG) materials, is of crucial importance. In the present review, the research performed on the fatigue properties of very fine-grained bulk materials during the last decade will be discussed. Mainly bulk UFG materials prepared by ECAP will be considered and, to a smaller extent, also bulk truly nanocrystalline materials. The discussion will focus on simple materials and on more complex alloys and structural materials. Presentation of fatigue performance in total strain fatigue life diagrams has been found to be particularly suitable to compare the strong UFG and the more ductile CG materials in both the high cycle fatigue (HCF) and the low cycle fatigue (LCF) ranges. In general, the fatigue strength of UFG materials is enhanced considerably by grain refinement, in particular in the HCF regime. However, at the same time, the LCF performance is impaired by microstructural instabilities of the strongly hardened but less ductile UFG materials, as manifested in cyclic softening, fatigue-induced grain coarsening (by dynamic recrystallization at rather low homologous temperature!) and massive shear banding. These effects are discussed critically, also with respect to the effects of the route of ECAP-processing employed, the purity of the material – and the mode of fatigue testing. Remedies by mild annealing treatments which have been partially successful in improving the LCF strength by enhancing the ductility at the expense of a moderate loss of strength will be discussed. Examples of modelling of the cyclic deformation and fatigue properties will be presented, and some directions of future research will be outlined. |
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楠楠monkey
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lidongze(金币+30, 翻译EPI+1): 2012-03-08 23:50:12
lidongze(金币+30, 翻译EPI+1): 2012-03-08 23:50:12
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大体积超细晶（UFG）金属和合金由剧烈塑性变形法（SPD）制备，特别是通过等径角挤压法（ECAP）展现其优越的机械性能。相对于传统晶粒度（CG）材料，对非常细粒度散装材料潜在应用：循环变形和疲劳特性的重视是至关重要的。现在回顾，在过去的十年里，对非常细粒度散装材料的疲劳性能的研究将被讨论。ECAP制备的主要的散装UFG材料在将为是较小的成都，还被认为是真正意义上的纳米材料。 讨论将集中在简单材料、更复杂的合金和结构材料上。经发现，总应变疲劳寿命图中疲劳性能的介绍特别适合在高循环疲劳（HCF）和低循环疲劳（LCF）范围内，将强大的UFG和更可塑的CG材料作比较。总体上，HCF材料的疲劳强度通过晶粒细化而贤者增强，尤其是HCF状态。然而，同时，LCF性能被强烈硬化的微观不稳定性削弱。低韧性UFG材料，作为循环软化疲劳引起的晶粒粗化（通过相应温度的动态再结晶）和大规模剪切带的证明。 这些影响都经过批判性的讨论，同时也讨论了ECAP-加工使用的路线的影响，材料纯度，和疲劳测试模式。通过在一个中等强度的损失花费下，温和的退火处理的补救将被讨论。将呈现循环变形和疲劳性能的模型实例，简要概述一些今后的研究方向。 |
2楼2012-03-07 13:42:39