Effect of Different Heat Treatments with and without Deep Cryogenic Treatment on the Morphology of Mg₁₇Al₁₂Intermetallic Phase and Hardness of AZ91 Magnesium Alloy

Abstract

In this study, dissolution of the Mg17Al12 phase carried out at 413 °C within 24 h and process followed by cooling the samples in different media with various cooling rates. Some samples were directly aged and others were held in liquid nitrogen for 24 h prior to aging. In this study for investigation of microstructure, the specimens were studied using a Meiji optical microscope and Scanning Electron Microscopy and Energy Dispersive Spectroscopy analysis was used to characterize the chemical composition of the phases. Results showed that at low cooling rates, a large amount of discontinuous precipitations were formed and by increasing the cooling rate, their amount reduced and their number and sizes decreased. Application of the deep cryogenic treatment with aging causes the dimensions of the secondary phases to be slightly larger than that of the non-deep cryogenic treatment state. Deep cryogenic treatment especially in samples with high cooling rate causes reducing the amount of Al, compared to usual stoichiometric value by formation of shrinkage stress in two stages (cooling after solutioning and deep cryogenic treatment). Deep cryogenic treatment in all of the samples increased the hardness, compared to the case without deep cryogenic treatment, by increasing the amount of Al out of the structure and providing conditions for the formation of more continuous and discontinuous phases.