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lidongze

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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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1樓 2012-03-05 16:12:11

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楠楠monkey

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lidongze(金幣+30, 翻譯EPI+1): 2012-03-08 23:50:12

大體積超細(xì)晶（UFG）金屬和合金由劇烈塑性變形法（SPD）制備，特別是通過等徑角擠壓法（ECAP）展現(xiàn)其優(yōu)越的機(jī)械性能。相對于傳統(tǒng)晶粒度（CG）材料，對非常細(xì)粒度散裝材料潛在應(yīng)用：循環(huán)變形和疲勞特性的重視是至關(guān)重要的�，F(xiàn)在回顧，在過去的十年里，對非常細(xì)粒度散裝材料的疲勞性能的研究將被討論。ECAP制備的主要的散裝UFG材料在將為是較小的成都，還被認(rèn)為是真正意義上的納米材料。
討論將集中在簡單材料、更復(fù)雜的合金和結(jié)構(gòu)材料上。經(jīng)發(fā)現(xiàn)，總應(yīng)變疲勞壽命圖中疲勞性能的介紹特別適合在高循環(huán)疲勞（HCF）和低循環(huán)疲勞（LCF）范圍內(nèi)，將強(qiáng)大的UFG和更可塑的CG材料作比較。總體上，HCF材料的疲勞強(qiáng)度通過晶粒細(xì)化而賢者增強(qiáng)，尤其是HCF狀態(tài)。然而，同時(shí)，LCF性能被強(qiáng)烈硬化的微觀不穩(wěn)定性削弱。低韌性UFG材料，作為循環(huán)軟化疲勞引起的晶粒粗化（通過相應(yīng)溫度的動(dòng)態(tài)再結(jié)晶）和大規(guī)模剪切帶的證明。
這些影響都經(jīng)過批判性的討論，同時(shí)也討論了ECAP-加工使用的路線的影響，材料純度，和疲勞測試模式。通過在一個(gè)中等強(qiáng)度的損失花費(fèi)下，溫和的退火處理的補(bǔ)救將被討論。將呈現(xiàn)循環(huán)變形和疲勞性能的模型實(shí)例，簡要概述一些今后的研究方向。

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2樓2012-03-07 13:42:39