Charles C.F. Kwan

Strain incompatibility and its influence on grain coarsening during cyclic deformation of ARB copper

It is well known that the characteristic length scale in ultra-fine grained and nanocrystalline metals has a significant effect on the mechanical behaviour. The inhibited ability to accommodate imposed strain with conventional dislocation mechanism has led to the activation of unconventional deformation mechanisms. For one, grain coarsening at shear bands has been observed to occur within metals with sub-micron grain size upon cyclic deformation. Such grain coarsening is often linked to the observed cyclic softening behaviour. The purpose of this study was to investigate the relationship between strain localisation associated with shear banding and the observed deformation-induced grain coarsening in ultra-fine grained metals. The investigation was carried out using ultra-fine grained, oxygen-free high conductivity copper processed by accumulative roll-bonding. A close relationship between strain localisation and deformation-induced grain coarsening was revealed. As strain localisation is not only found at shear bands, but also at other places whereby heterogeneous microstructure or geometric discontinuity is present, hence the present study bears a general significance. Such strain localisation sites may also include a hard constituent embedded in a relatively ductile matrix, micro-crack tips and artificial notches. The stress concentration at these sites provides a high input of strain energy for grain boundary motion leading to grain coarsening. Furthermore, when the grain size is very small, the stress gradient leading away from the stress concentration sites is also believed to increase the driving force for grain boundary migration within the affected regions.

On the Cyclic Deformation Response and Microstructural Mechanisms of ECAPed and ARBed Copper - an Overview

With the increase of interest in using ultra-fine and nano-grained metals for structural purposes, the need to build on the knowledge pool regarding the response and behaviour of those metals under a mechanical load becomes more vital. However, it is well known that, especially for this type of materials such as the ECAPed and ARBed materials, the thermo-mechanical history affects the mechanical behaviour of the product strongly. Although ECAP and ARB are different techniques under the category of severe plastic deformation, similarities in their cyclic deformation response is observed from time to time. Specifically, the microstructural mechanisms involved in accommodating cyclic plastic strain in these two types of materials is seemingly comparable. The similarities arise from the similar microstructures in the majority of the volume of the bulk. In this report, the cyclic deformation response, and the related microstructural mechanisms of ECAPed copper will be discussed first and those of ARBed second. A comparison between ECAPed copper and ARBed copper will then be performed. Furthermore, the differences due to the unique features of ARBed material will be discussed. Lastly, the reasons behind the observed similarities in cyclic deformation behaviour and the related micro-mechanisms for metals process with the two different techniques will also be explored.

Cyclic Deformation of Ultra-Fine Grained Commercial Purity Aluminum Processed by Accumulative Roll-Bonding

Accumulative Roll-Bonding (ARB) is one of the more recently developed techniques capable of producing bulk ultra-fine grained (ufg) metals. There are still many aspects of the behavior of ufg metals that lacks an in-depth understanding, such as a generalized view of the factors that govern the cyclic deformation mechanism(s). This study aims to advance the understanding of the cyclic deformation behavior of ufg metals through the systematic investigation of ARB processed aluminum upon cyclic loading. It was found that the cyclic softening response often reported for ufg metals is largely influenced by the microstructure stability as the cyclic softening response is facilitated by grain coarsening which becomes inhibited with highly stable microstructure. On one hand, shear bands resembling braids of dislocations trespassing multiple grains have been observed to operate for the accommodation of the imposed cyclic strain in cases where grain coarsening is largely restricted. On the other hand, it was found that the microstructure stability can be overcome at higher applied cyclic plastic strain levels, leading to grain coarsening and thus a cyclic softening response. The findings in this study have further confirmed that the cyclic softening behavior found in many ufg metals, which may be detrimental in practical applications, can be inhibited by improvements in the microstructure stability.

Progression of Late Stage Abnormal Grain Growth of Electroformed Nanocrystalline Ni Without the Addition of Grain Refiner

Nanocrystalline (nc) metals are very attractive research materials due to the many beneficial properties the nc structure offers. However, the thermal stability of the nc structure is often of concern due to its high energy state. One curious phenomenon, often reported in nc Ni, is the occurrence of a late stage abnormal grain growth (AGG) which results in the appearance of abnormal-sized grains with faceted grain boundaries. Previously, it was reported that such an AGG stage was brought on by a critical S concentration at certain boundaries. On the contrary, a late stage AGG was observed in electroformed nc Ni without intentionally added additives, including those containing S. The objective of this study is to further elucidate on the initiation and growth of these faceted boundaries. The progression of this AGG stage in electroformed Ni has been systematically investigated by utilizing optical and electron microscopy. Clearly, the late stage AGG did not require a critical S concentration to occur in this case. Moreover, these faceted boundaries were found to be highly stable once its planar nature is fully developed suggesting that the previous focus on the rapid migration of these faceted boundaries may be misplaced.