Deformation mechanisms during severe plastic deformation of a Cu-Ag composite
Deformation mechanisms during severe plastic deformation of a Cu-Ag composite
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DOI：
10.1016/j.jallcom.2016.11.085
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发表时间：
2017-02-25
影响因子：
6.2
通讯作者：
Pippan, R.
中科院分区：
文献类型：
--
作者：
Kormout, K. S.;Ghosh, P.;Pippan, R.
以元素铜和银粉为原料，采用常温高压扭转工艺制备了一种Cu-37at%Ag复合材料。最初的微米级粉末颗粒直接在高压扭转工具中压缩，随后变形到不同的应变水平。通过扫描、透射电子显微镜和同步辐射X射线等测试手段详细研究了组织演变过程，并通过显微硬度和纳米压痕测试研究了组织演变与力学性能的关系。HPT工艺导致了铜和银的择优取向形成层状复合组织。随着外加应变的增加，铜、银片层不断变薄，同时在铜、银片层中形成超细晶组织。当片层间距达到小于片层内部相应晶粒尺寸的值时，发生进一步的片层变薄，导致复合材料的硬度显著增加。当板层间距小于50 nm时，变形开始集中在150-300 nm的宽剪切带中，令人惊讶的是，没有表现出软化。相反，新剪切带的稳定形成有助于将片层结构转变为纳米晶等轴组织，并使片层基质向剪切面旋转。这一过程导致了合金的进一步细化和硬度的增加，直到获得恒定的硬度水平。同步辐射X射线和透射电子显微镜相结合的分析表明，在达到饱和显微硬度后，剪切带中发生了铜和银的机械混合，这可以归因于剪切带中容纳的巨大应变。由于剪切带的局部化变形，即使在很高的应变下，合金的组织和化学均一化也不能实现。最终的显微组织由嵌入在残留纳米片层基质中的纳米单相过饱和区组成。(C)2016爱思唯尔B.V.保留所有权利。
A Cu-37 at%Ag composite was produced by high-pressure torsion processing of elemental Cu and Ag powders at room temperature. The initial micrometer-sized powder particles were compressed directly in the high-pressure torsion tool and subsequently deformed to different strain levels. The microstructural evolution was studied in detail by scanning and transmission electron microscopy and synchrotron X-Ray measurements, and related to the mechanical properties by microhardness and nanoindentation measurements. The HPT process led to an alignment of Cu and Ag into a lamellar composite microstructure. With increasing applied strain the Cu and Ag lamellae were continuously thinned and simultaneously an ultrafine-grained microstructure was formed in the separate Cu and Ag lamellae. When the lamella spacing reached values lower than the respective grain sizes inside the lamellae, a further lamella thinning occurred causing a significant hardness increase of the composite. At lamella spacings below 50 nm deformation started to localize in 150-300 nm broad shear bands, which surprisingly exhibited no softening. Instead, the steady formation of new shear bands aided to transform the lamellar structure into a nanocrystalline equi-axed microstructure and additionally rotated the lamellar matrix towards the shear plane. This process led to an additional refinement of the alloy and a hardness increase until a constant hardness level was obtained. Combined analyses by synchrotron X-ray and transmission electron microscopy measurements revealed that, after reaching the saturation microhardness level, mechanical mixing of Cu and Ag occurred in the shear bands, which can be attributed to the enormous strains accommodated in the shear bands. Due to the localized deformation by shear bands, structural and chemical homogenization of the alloy was not achieved even at very high applied strains. The final microstructure was composed of nanocrystalline single-phase supersaturated regions embedded in a residual nano-lamellar matrix. (C) 2016 Elsevier B.V. All rights reserved.