Effects of Zn/Sn mass ratio on microstructure and mechanical properties of Mg–Zn–Sn–Al–Ca alloys

Abstract

The effects of the Zn/Sn mass ratio on microstructure and mechanical properties of the as cast and the as rolled Mg–xZn–ySn–2Al–0·2Ca (x+y = 9 wt-) alloys have been investigated by optical microscopy, X-ray diffraction, SEM and mechanical testing to explore the feasibility of developing a heat treatable high strength wrought Mg alloy by water cooling copper mould casting under pressure plus plastic forming. The as cast alloys are characteristic of divorced eutectics and are composed of four phases, i.e. α-Mg, Mg₃₂(Al, Zn)₄₉, Mg₅₁Zn₂₀ and Mg₂Sn. The alloy with the lower Zn/Sn mass ratio contains a higher amount of the Mg₂Sn phase and a lower fraction of the Zn containing phases. The as cast Mg–4·5Zn–4·5Sn–2Al–0·2Ca alloy is characteristic of the finer grain size and the fewer amount of the intermetallic compounds at grain boundaries, showing a fairly good roll forming ability and the optimal combined mechanical properties, with the ultimate tensile strength (UTS), yield tensile strength (YTS) and elongation to rupture (El) of 243 MPa, 170 MPa and 13·9 respectively. Solution treatment can improve the roll forming ability of the Mg–Zn–Sn–Al–Ca alloys to a large extent. The as rolled Mg–4·5Zn–4·5Sn–2Al–0·2Ca alloy exhibits the well balanced mechanical properties among the three alloys. The alloy prepared by direct rolling exhibits UTS, YTS and El of 351 MPa, 245 MPa and 7·1 respectively, while that prepared by solution rolling exhibits UTS, YTS and El of 406 MPa, 285 MPa and 6·6 respectively. Solution rolling has a more obvious strengthening effect on the Mg–Zn–Sn–Al alloys than direct rolling. The as investigated Mg–4·5Zn–4·5Sn–2Al–0·2Ca alloy is a promising candidate for future commercial applications in the Mg–Zn–Sn system.