Microstructure Evolution of Semi-Solid AZ91D Alloy During Isothermal Heat Treatment Process
The effect of isothermal heat treatment on the microstructure evolution of semi- solid AZ91 D magnesium alloy was investigated. The results show that the roundness of the primary α- Mg phase increases with the increasing temperature and time. The microstructure evolution of the semi-solid AZ91 D experiences three stages during isothermal heat treatment process, i.e. the structure separation,spheroidization and final coarsening. The liquid phase of semi- solid alloy has three types, namely, the molten pool, the"entrapped liquid"pool and the liner liquid film which separates two neighbor particles.The solid phase has two types, namely, the primary α-Mg particles and the α-Mg phase formed in the second solidification.
We present an asymptotic form of the Boussinesq equations in the limit of small P ´ eclet numbers i.e. when the time scale of motions is much larger than the time scale of ther- mal diffusion. We find that, in this limit, the effects of thermal diffusion and stable stratification combine in a single physical process. This process is an anisotropic dissipation (not effective for horizontal motions) which acts primarily on large scale mo- tions. The small-P´ eclet-number approximation presents also the great practical interest to avoid the numerical difficulty induced by the huge separation between the diffusive and dynamical time scales. The relevance of this approximation to study the flow dynamics within the stellar radiative zones is considered.