Effects of solidification rates on precipitation behavior of nano-particles and precipitation strengthening of ZA81M alloy

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.