An Overview of Plasticity in Nanoscale Composites
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An Overview of Plasticity in Nanoscale Composites J.D.Embury and C.W.Sinclair Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario, Canada Introduction In the past two decades there has been great activity in the area of nanoscale composites. This has included enormous effort in the areas of epitaxial structures for microelectronics applications, organometallic systems, coatings [1], layered metallic structures and drawn in-situ composites. A great deal of progress has been made in the development of controlled fabrication methods including sputtering, electrodeposition and crystallization of amorphous structures. Also, attention has been given to the integration of ultrafine scale structures into the design of many engineering applications from high field magnets operating at cryogenic temperatures [2] to future gas turbines [3]. These developments emphasize the need to explore, at a fundamental level, the progress associated with plasticity of ultrafine scale structures. The processes of plasticity can be explored at the macroscopic, mesoscopic, and microscopic levels in order to delineate those aspects of the mechanical response which are characteristic of ultrafine scale materials. Clearly, it is important to emphasize that there can be competition between plasticity and damage and fracture events and between competitive processes of plasticity and that these are dependent on the characteristic length scale of the structures. A classical system which reflects the competition of plasticity and fracture is the system Fe-Fe3C. This was explored in the seminal work of Langford [4] illustrated in figure 1. This indicates that Fe3C in the form of particles 1-10 µm in thickness is brittle but when the scale is reduced to 50 nm the Fe3C is ductile and can undergo extensive plastic flow.
Figure 1: Langford’s compilation of the data in the literature for the scale dependent ductile to brittle transition of cementite in pearlitic steels (from Ref. 4) B1.1.1
There are two important consequences of these observations. The first is that normally brittle phases may, when embedded in a ductile matrix, undergo plastic flow and thus exhibit a size dependent ductile-brittle transition. The second is that even complex structures which have limited numbers of slip systems may be able to co-deform with a matrix capable of general plasticity. In addition to the competition between plasticity and fracture there is a scale dependant competition between deformation mechanisms and this can be studied via the utilization of the deformation mechanism maps described by Frost and Ashby [5]. There is a need to explore the behavior of ultrafine scale structures over a range of temperatures and strain rates in order to develop these scale dependant maps in a quantitative manner. If we turn to a mesoscopic view of plasticity of ultrafine scale materials, the problem is essentially to examine the compatibility of flow between the constituent phases. This can be considered in terms of load transfer to an elasti
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