THE SIGNIFICANCE OF GRAIN-BOUNDARIES IN THE FLOW OF POLYCRYSTALLINE MATERIALS

The significance of grain boundaries in the mechanical properties of polycrystalline materials is dependent upon both the testing temperature and the grain size, At low temperatures, the boundaries act as obstacles to the easy movement of dislocations and thereby they provide strengthening through the Hall-Fetch relationship. However, the grain boundaries represent a source of weakness at high temperatures, both because of their ability to promote premature failure through the development of intercrystalline cracks and cavities and because they may contribute directly to the overall deformation through the occurrence of grain boundary sliding. This paper examines the influence of grain boundaries on the mechanical properties of metals and ceramics at high temperatures. It is demonstrated that it is convenient to codify the deformation behavior by defining four broad grain size ranges wherein the flow characteristics are reasonably similar: these ranges are termed macroscopic (>1000 mu m), mesoscopic (similar to 10-1000 mu m), microscopic (similar to 10 nm similar to 10 mu m) and nanoscopic (<10 nm), respectively. Although experiments tend to indicate that there are significant differences in the nature of grain boundary flow within the mesoscopic and microscopic ranges, it is suggested that there is a single deformation mechanism which occurs within both of these grain size ranges. A unified approach to grain boundary sliding is outlined for both the mesoscopic and microscopic ranges, with the rate of sliding controlled by the rate of accommodation through intragranular slip.