Aging hardening and precipitate behavior of a solution-treated Mg-6Zn-4Sn-1Mn (wt.%) wrought Mg alloy

Mg-Zn-Sn-Mn alloy is a new type of age hardening wrought Mg alloy. However, the precipitation behavior of the alloy is still not well understood. A solution-treated Mg-6Zn-4Sn-1Mn alloy is selected as the experimental alloy in this paper. The hardening behavior, precipitate microstructure and precipitation sequence of the solution-treated alloy during single aging and double aging at 180 °C are systematically studied by means of XRD, OM, SEM, TEM, EDS and microhardness. The results show that four types of precipitated phases are successively precipitated in the experimental alloy during the aging treatment, namely, G.P. zone parallel to the (0001)_α base plane, rod-like β₁^′(MgZn₂) phase perpendicular to the (0001)_α base plane, disc-like β₂^′ (MgZn₂) phase parallel to the (0001)_α base plane, and irregular β' (Mg₂Sn or Mg₃Sn) phase. The habitual plane of most β' phases is the (0001)_α basal plane, and a small part of the β' phase forms a certain angle with the (0001)_α basal plane. A large number of finely dispersed G.P. zones can provide the heterogeneous nucleation sites for the β₁^′ phase during the pre-aging stage, which promotes the nucleation of the precipitates during the second aging, so the peak aging hardness (87HV) of the double-aged alloy is much higher than that of the single-aged alloy (74HV), and the peak aging time (6 h) during second aging of the double aging is shorter than that (12 h) of single aging. The possible precipitation sequence of Mg-6Zn-4Sn-1Mn alloy during the isothermal aging treatment can be obtained, that is, α-Mn→β₁^′ (MgZn₂)→β (MgZn) in the Mn-rich zone, and SSSS→G.P. zones→β₁^′ (MgZn₂)→β₂^′(MgZn₂)→β' (Mg₂Sn or Mg₃Sn)→(β (MgZn) and β (Mg₂Sn)) in Zn-rich and Sn-rich zone.