Defects in Materials: Their Characterization and Simulation
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DEFECTS IN MATERIALS: THEIR CHARACTERIZATION AND SIMULATION.
COLIN G. WINDSOR
National Non-Destructive Testing Centre, AEA Technology, B521.2, Harwell Laboratory, OXlI ORA, UK. ABSTRACT
Materials research does not necessarily need to eliminate defects, but rather to characterize them, and to understand and control their effects. In most cases chacterization of defects means making structural or dynamic measurements of their properties. To understand these measurements in order to predict material and defect properties outside the range of the measurements is a much harder problem. Ideally a theory is required. However in the materials examples considered in this review, point defects in uranium oxide, copper clusters in steel, grain boundary aggregations, and stress concentrations, a true analytic theory is beyond our capabilities. Here computer modelling is often able to make the progress needed. This review considers the complementary nature of experimental characterization and computer simulation in our understanding of defects in materials.
INTRODUCTION
Defects are a fact of life. As Dr Marshall said to our UK House of Commons Select Committee on Energy, "The central point is that you cannot make anything without having some defect in it. It may be very tiny or it may be very large, and you have to assess how important that defect is." Defects can be classified in many ways. Size is an obvious classifier, and will be used in this review, but there are many others. Dimensionality is a vital axis. Some defects are essentially point-like, others volumetric, others planar or linear. Grain boundaries will be discussed as representative of planar defects. The interface between two polymer blends represents another planar defect. Defects can be dangerous, they can be advantageous, or they can be benign. The tiny copper clusters found in pressure vessels may certainly be dangerous, but the clusters found in many aged hardened alloys are beneficial. Temperature plays a key role for many defects. The atomic defects which give rise to superionicity appear with increasing temperature as the thermal energy becomes comparable with their energy of formation. Many volumetric defects, such as alloy clusters, break up at high temperatures, but represent a condensed state of matter which becomes stable at intermediate temperatures. Macroscopic defects may be the hardest to characterize. A region of high residual strain in a structure can be more dangerous than many more easily located defects. Figure 1 illustrates these axes for some types of defect which will be discussed here.
Mat. Res. Soc. Symp. Proc. Vol. 209. @1991 Materials Research Society
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Planarl Figure1 Some of the variables used to characterize defects. In italics are some of the defects considered in this review.
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POINT DEFECTS: SUPERIONICITY IN UO2+x, ZrO2 and SrCI2. The detailed understanding of the high temperature properties of uranium di
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