Breaks in Hall-Petch Relationship in Magnesium

Magnesium and its alloys display a non-usual relationship between flow stress and grain size at room temperature. Breaks in the Hall-Petch relationship have been reported in the literature. Inverse Hall-Petch behavior in which flow stress reduces with grain size decreasing has also been reported in pure magnesium and magnesium alloys with ultrafine and nanocrystalline structures. The present overview discusses these effects in terms of controlling deformation mechanisms. The distinct strength observed in pure magnesium and magnesium alloys with ultrafine grained structure is also discussed. It is shown that experimental data for fine and ultrafine grained magnesium alloys agree with a model suggested recently based on the mechanism of grain boundary sliding. It is also exhibited that the stability of the grain structure might control the strength of ultrafine grained samples.