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

• The present investigation systematically examined the effects of cooling rate during solidification on the morphological characteristics and crystallographic structure of nano-particle precipitates, coarse second phases and microsegregation of Zn element in as-cast microstructures through XRD, EDS, EPMA, TEM analysis combined with SAED and HRTEM techniques. • The experimental findings conclusively demonstrate that increased cooling rates significantly inhibit both nucleation kinetics and growth of precipitates, thereby reducing the precipitate population density. This decreased quantity of precipitation directly correlates with attenuated precipitation strengthening effects, consequently leading to a measurable reduction in the microhardness of the alloy matrix. • Two distinct types of precipitates exhibiting multiple orientation relationships with the Mg substrate were identified, including basal and pyramidal precipitates. The synergistic effects of crystallographic orientations and morphological characteristics on precipitation strengthening mechanisms were discussed. • 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, while the tensile mechanical properties are mainly affected by fine-grain strengthening. 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 Mg 4 Zn 7 structure (β 1 ′) and plate-like nano-particles with an MgZn 2 structure (β 2 ′). 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 β 1 ′ and β 2 ′ significantly decreased with an increase in cooling rate from 0.70 °C·s −1 to 33.98 °C·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 β 1 ′ and β 2 ′ and the Mg substrate: basal and pyramidal precipitates, respectively. Specifically, the β 1 ′ 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 β 2 ′ 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.