Estimation of creep deformation behavior in Mg-Al alloys by using .THETA. projection method.

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Abstract
The creep deformation behavior of magnesium and Mg-Al alloys was analyzed by using the θ projection concept based on the following equation: ε=θ1{1-exp(-θ2t)}+θ3{exp(θ4t)-1} where ε is creep strain, t time and θ1-θ4 the parameters determined by curve fitting. The equation was able to reproduce the creep curves even in the tertiary creep stage. In solid solution hardened alloys containing up to 6 mass% Al parameters θ1 and θ3 were independent of temperature, but temperature dependent in the precipitation hardened alloy containing 9 mass%Al. The activation energy for θ1 and θ3 in Mg-9 mass%Al alloy was 30kJ/mol, and this value was equal to that of discontinuous grain boundary precipitation of Mg17Al12 during aging. The activation energy for θ2 related to strain hardening rate was 100kJ/mol, and that for θ4 related to recovery or weakening rate was 134kJ/mol. The activation energy of θ4 was almost same as that of self-diffusion in pure Mg, indicating that the recovery or weakening rate is controlled by climbing of dislocations.

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