Effect of Sr on microstructure, tensile properties and wear behavior of as-cast Mg–6Zn–4Si alloy

• Sr can modify the morphology and size of primary and eutectic Mg2Si in Mg–6Zn–4Si alloys.
• Sr improves tensile strength, elongation and wear behavior of the Mg–6Zn–4Si alloys.
• The change in morphology and size of Mg2Si phase leads to the increase of tensile properties.
• The needle-like Sr–Mg–Si phase leads to a reduction of mechanical property and an increase of the wear rates.

The microstructure, tensile properties and wear behavior of as-cast Mg–6Zn–4Si alloy with strontium additions at ambient and elevated temperature were investigated by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), standard high temperature tensile testing and a pin-on-disc type apparatus. The results indicated that the grain size of the primary Mg2Si decreased initially and then gradually increased with increasing Sr amount. Meanwhile, the morphology of the primary Mg2Si in the alloys changed from large dendritic to polygonal or fine block, and that of the eutectic Mg2Si phase turned to fine fibre with increasing Sr content. Tensile testing results showed that Sr addition improved the ultimate tensile and elongation of the Mg–6Zn–4Si alloys at both ambient temperature and 150 °C. Dry sliding wear tests indicated that the change trend of wear rate was basically coincident with that about the average size of the primary Mg2Si phases. Optimal mechanical properties and wear behavior could be achieved by a Sr addition of 0.5%. An excessive Sr addition resulted in the formation of the needle-like SrMgSi compound, which was detrimental to the tensile properties and wear behavior of the alloys.