Chen Jihua

The effect of thermal exposure on the microstructure and hardness of as-cast Mg–Zn–Al alloys with Sn addition

Abstract Microstructural evolution and hardness changes of as-cast Mg-6Zn-2Al alloys with Sn addition during prolonged thermal exposure to 150 °C and 200 °C were studied using optical microscopy, X-ray diffraction, scanning electron microscopy and hardness tests. Neither obvious grain growth nor new phases were detected in these alloys during thermal exposure. There were initial precipitation hardening peaks followed by long-term hardness stability during thermal exposure to 150 °C, indicating that all the alloys exhibited excellent microstructural stability. There were two peaks in the hardness vs. annealing time curves of these alloys during thermal exposure to 200 °C, which corresponded to the precipitation of MgZn and Mg2Sn. The overall hardness of these alloys decreased with prolonged time, resulting from the aggregation and coarsening of Mg2Sn.

Microstructure, Tensile Properties and Creep Resistance of Sub-rapidly Solidified Mg-Zn-Sn-Al-Ca Alloys

Abstract Microstructures, tensile properties and creep resistances of seven kinds of sub-rapidly solidified Mg-Zn-Sn-Al-Ca alloys were studied. Results show that as for the Mg- x Zn- y Sn-2Al-0.2Ca ( x + y =9, x / y =2, 1 and 0.5, in mass%) alloys, the highest UTS and YTS at RT are obtained with the Zn/Sn mass ratio of 1. The YTS at 150 °C increases but that at 200 °C decreases with the increase of Zn/Sn mass ratio. It is possibly ascribed to the elevated-temperature strengthening of Mg 2 Sn particles since the alloy with the lower Zn/Sn mass ratio contains a higher amount of Mg 2 Sn and a lower fraction of Zn-containing phases. As for the Mg-4.5Zn-4.5Sn-2Al- z Ca ( z =0, 0.2, 0.4 and 0.6) alloys, UTS and YTS increase at first and then decrease with the increase of Ca content at both RT and 200 °C. The strength peaks occur at 0.2% Ca and 0.4% Ca, respectively. Both the initial strain and the steady creep rate decrease with the increase of Ca content below 0.4% Ca at 200 °C under the compression stress of 55 MPa. Minor Ca addition can improve the RT and elevated temperature strength and enhance the compressive creep resistance of the Mg-4.5Zn-4.5Sn-2Al alloy, but reduce the elevated-temperature plasticity. In addition, it also has an obvious influence on the tensile fracture mode, which changes from cleavage to quasi-cleavage at RT and from ductile to quasi-cleavage at 200 °C with the increasing of Ca addition.