In this work, the solidification behavior and precipitation strengthening of Mg–8Zn–1Al–0.5Cu–0.5Mn (wt%, ZA81M) alloy solidified with different cooling rates of solidification were investigated. Selected area electron diffraction (SAED) analysis revealed the presence of rod-like nano-particles with an Mg₄Zn₇ structure (β₁^′) and plate-like nano-particles with an MgZn₂ structure (β₂^′). X-ray diffraction(XRD) and Energy dispersive spectrometer (EDS) indicate that the coarse second phases in the solidification structure are MgZn eutectic phase, MgZnCu separated eutectic phase, and AlMn phase. The effect of precipitation strengthening was caused by the precipitated phases in the solidified microstructure. The quantities of both β₁^′ and β₂^′ significantly decreased with an increase in cooling rate from 0.70 ℃·s⁻¹ to 33.98 ℃·s⁻¹, resulting in a reduction of Vickers hardness from 87.70 to 75.53 HV. The higher cooling rate of solidification effectively inhibited the formation of precipitates, thereby diminished the precipitation-strengthening effect. Two orientation relationships (ORs) were identified between the nano-particles β₁^′ and β₂^′ and the Mg substrate: basal and pyramidal precipitates, respectively. Specifically, the β₁′ phase exhibited the following ORs: (3 1 2) _β1^′//(0 0 0 2)_Mg, (−9 1 2) _β1^′//(1 0 −1 1)_Mg, (⁻10 0 3)_β1^′//(1 0 −1 1)_Mg, and (1 1 0) _β1^′//(1 0 −1 1)_Mg. Moreover, the β₂′ phase exhibited ORs with basal and pyramidal planes, presented as (1 1 −2 0)_β2^′//(0 0 0 2)_Mg and (1 0 −1 3)_β2^′//(1 0 −1 1)_Mg. In addition, the variation of coarse second phase and nano-particle precipitated phase shows a competitive relationship, which is attributed to the microsegregation of Zn element. Precipitation strengthening mainly improves microhardness, and tensile mechanical properties are mainly affected by fine-grain strengthening.

Poor formability is a key problem that limits the application of flame-retardant Mg-Al-Ca based alloys at room temperature. In this study, we present a new Mg-6Al-3Ca-0.4Mn-2Zn (wt%) alloy which exhibits excellent flame-retardant performance and excellent formability. Due to the high Ca content, the Mg-6Al-3Ca-0.4Mn-2Zn (wt%) alloy does not burn at 1065 ℃. The formability of the alloys is measured using a three-point bending test, and the Mg-6Al-3Ca-0.4Mn-2Zn (wt%) alloy shows excellent formability, with a significant increase in bending displacement from 7.1 mm to 23.8 mm compared to the Mg-6Al-3Ca-0.4Mn (wt%) alloy. The combined effect of the weakened basal texture, the reduction of twins and the plastically deformable Al₂Ca phase particles ensures good formability of the Mg-6Al-3Ca-0.4Mn-2Zn (wt%) alloy. The dynamic recrystallization mechanisms of the alloys have been analyzed, and the promotion of dynamic recrystallization by the PSN mechanism is responsible for the weakened basal texture and the reduction of twins in the Mg-6Al-3Ca-0.4Mn-2Zn (wt%) alloy. The new Mg alloy is attractive for industrial applications due to its excellent flame-retardant performance and formability.

Magnesium alloys with homogeneous degradation and controlled degradation rate are desirable for biodegradable materials. In the present work, Mg-3 wt.%Zn-0.2 wt.%Ca alloys with different columnar structures were fabricated and the degradation in 0.9 wt.% NaCl were investigated. With the increase of the growth rate for the directional solidification, the microstructure of the directionally solidified (DSed) alloy evolved from cellular to dendritic coupled with the change of the spacing of the primary trunks (λ₁) and the volume fraction (f_v) of Ca₂Mg₆Zn₃ phase. The results of the corrosion test suggested that the alloy with cellular structure experienced homogeneous corrosion and exhibited the lowest corrosion rate. The good corrosion resistance of the alloy with cellular structure was attributed to the protective corrosion products film (CPF), which was closely related to the f_v of Ca₂Mg₆Zn₃ phase and λ₁. To evaluate the corrosion rates (CR) of the DSed Mg-Zn-Ca alloys with different microstructures, a parameter α was proposed in this work, which was calculated by λ₁ and the f_v of Ca₂Mg₆Zn₃ phase. The fitting result showed that there was a linear relationship between CR and α, which was CR = 4.1899 + 0.00432α. This means that the CR of the DSed Mg-Zn-Ca alloy can be evaluated if the microstructure had been characterized.