Critical Analysis of Threshold Stresses in the Creep Mechanisms of a Powder Metallurgy Magnesium Alloy AZ31

In this work, tensile tests were conducted on two powder metallurgy materials based on the AZ31 magnesium alloy. The first material, named AZ31-PM, was processed using atomized powder, followed by isostatic cold compaction at 340 MPa, plus hot extrusion at 350 °C. The second material, named AZ31-PMCM, used metal powder that was cryomilled in liquid nitrogen environment and processed as for the AZ31-PM material. Creep mechanisms were analyzed in the range 300 °C to 400 °C based on the creep laws determined previously for ingot metallurgy AZ31. Two threshold stresses, one for grain boundary sliding and the other for slip creep, both independent of temperature, were introduced, providing a consistent description of creep for the different AZ31-based materials. The variations observed in threshold stresses in the AZ31-PM material were attributed to an inhomogeneous distribution of dispersoids and satisfactorily explained by a composite type model. This method of analyzing the data is compared with the traditional one in the literature, demonstrating that the dependence of the threshold stresses with the temperature, reported in those articles, could actually be derived from the method used rather than from a real physical effect.The authors acknowledge the financial support of CICYT, Spain, under Programs MAT2012-38962 and MAT2015-68919 (MINECO/FEDER)