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The combined effect of high-intensity ultrasonic treatment (HIUST) and 0.3 wt.%Ca addition on modification of primary Mg 2 Si and wear resistance in the hypereutectic Mg–5 wt.%Si alloy has been investigated. The results show that without treatment, the dendrites of primary Mg 2 Si are coarse and non-uniform in size. With HIUST or 0.3 wt.%Ca addition, nearly fine uniform and polyhedral shape of primary Mg 2 Si are achieved. Interestingly, modification and refinement of primary Mg 2 Si arise from the combined effect of HIUST and 0.3 wt.%Ca addition. The sample due to combined effect of HIUST and 0.3 wt.%Ca addition has the best wear resistance due to the lowest weight loss among all the other samples for various loads (10, 30 and 50 N) at constant sliding speed (0.3 m/s). Modification mechanisms resulting in the development of microstructures are also investigated. [ABSTRACT FROM AUTHOR]

The effect of Ca addition on modification of primary Mg 2 Si, hardness and wear behavior in Mg–5 wt.%Si hypereutectic alloy has been investigated. The results showed clearly that without Ca addition, most of primary Mg 2 Si appeared as coarse dendritic morphology with average size of about 215 μm. With the addition of 0.1 wt.%Ca, the average size of primary Mg 2 Si decreased to about 98 μm, but their morphologies did not significantly changed. As the addition level of Ca increased to 0.3 wt.%, the average size of primary Mg 2 Si decreased significantly to about 50 μm and their morphologies changed to polyhedral shape. However, with further increasing Ca addition to 0.6 wt.% and 1 wt.%, some needle-like and blocky CaMgSi particles formed and the average size of primary Mg 2 Si increased slightly, which could described as over-modification. The present work showed that the optimal modification effect could be obtained when the Ca content in the investigated alloy reached 0.3 wt.%. The modification mechanism may be referred mainly due to poisoning effect resulting from the segregation of Ca atoms at the growth front of the Mg 2 Si and the adsorption effect of some Ca atoms in the Mg 2 Si crystal growth plane. The 0.3 wt.%Ca-added alloy has the highest hardness value and the best wear resistance among all other alloys. An excessive Ca addition resulted in the formation of some needle-like and blocky CaMgSi particles, which was detrimental to hardness and wear behavior of the 0.6 wt.% and 1 wt.%Ca-added alloys. The wear mechanism of investigated alloys is a mild abrasive oxidative wear with little adhesion. [ABSTRACT FROM AUTHOR]

Highlights: [•] The high intensity ultrasonic treatment (HIUST) can modify and refine the primary Mg2Si. [•] The optimal pouring temperature of HIUST is 800°C. [•] The optimal vibration time of HIUST is 90s. [•] The mechanisms for HIUST influence on solidification have been discussed. [ABSTRACT FROM AUTHOR]