Structure and Mechanical Properties of Ultrafine-Grained Chromium Produced by Severe Plastic Deformation Processing

Abstract

Due to their elevated temperature properties, such as high strength, melting temperature and oxidation resistance, the chromium-based alloys are potentially very attractive for variety of structural applications, including their use as low activation materials for fusion reactor components [1]. To date, however, the potential of this class of alloys has not been translated into practice, because, though they are strong and hard, they are inherently brittle. In fact, they exhibit little or no ductility at low temperatures together with an unacceptably high ductile-to-brittle transition (DTBT) temperature. The sixties and early seventies, marks the period when mechanical properties of these alloys were actively studied and their inherently brittle behavior was amply demonstrated. However, the mechanical results from some of the studies, published during this period, appear to suggest that the low temperature ductility and fracture toughness properties of these alloys could be markedly improved by grain refinement. Among the results reported during this period, are those of Wilcox [2], Gilbert [3], and Wain [4] and their respective co-workers, all of which point to the critical role of grain size on the resulting mechanical properties. For example, Wilcox and co-workers showed that decreasing the grain size of chromium from 37 to 2.4 µm by the addition of a few percent of thoria particles resulted in a significant decrease in DTBT temperature (from 140 to 15°C) together with significant improvements both in the ductility and strength [2].