Effect of deep cryogenic treatment on microstructure, creep and wear behaviors of AZ91 magnesium alloy

This paper focuses on the effect of deep cryogenic treatment (−196 °C) on microstructure and mechanical properties of AZ91 magnesium alloy. The execution of deep cryogenic treatment on samples changed the distribution of β precipitates. The tiny laminar β particles almost dissolved in the microstructure and the coarse divorced eutectic β phase penetrated into the matrix. This microstructural modification resulted in a significant improvement on mechanical properties of the alloy. The steady state creep rates were measured and it was found that the creep behavior of the alloy, which is dependent on the stability of the near grain boundary microstructure, was improved by the deep cryogenic treatment. For the AZ91 alloy, the results indicate a mixed mode of creep behavior, with some grain boundary effects contributing to the overall behavior. However for the deep cryogenic samples dislocation climb controlled creep is the dominant deformation mechanism. After the deep cryogenic treatment the sliding of grain boundaries was greatly suppressed due to morphological changes. As a result, the grain boundaries are less susceptible for grain boundary sliding at high temperatures. Dry sliding wear tests were also applied and the wear resistance of the alloy improved remarkably after deep cryogenic treatment.