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In the present study, a Mg-Y-RE-Zr alloy was isothermally heat treated in semi-solid mushy region. The experimental alloy was deliberately composed of a higher amount of Zr (1.9 wt%) to take advantages of grain refinement capabilities of zirconium in both dissolved and undissolved condition. Accordingly, the experimental material possessed a fine equiaxed non-dendritic as-cast structure. The effects of holding time and temperature in the semi-solid region were evaluated on the microstructural evolution through applying a predetermined isothermal heat treatment scheme. The results indicate that increasing the isothermal temperature and holding time would increase the liquid fraction, shape factor, and particle size. It is discussed that the particle spheroidization as well as Ostwald ripening mechanism are triggered at higher temperature. The incremental trends of particle size and shape factor are mitigated after mid-term holding time at 600 and 610°C. This observation is attributed to isolating more discrete particles. A lower coarsening rate of solid particles in the present work is related to the dragging effects of redistributed Zr particles in hindering the boundary motion. Furthermore, the results suggest that the microstructure with optimum characteristics would be achieved by holding the material for 30 min at 620°C. 14395, Iran

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