Evolution of the Microstructure and Microsegregation in Subrapidly Solidified Mg–6Al–4Zn–1.2Sn Magnesium Alloy

The evolution of the microstructure and microsegregation in a new type of Mg–6Al–4Zn–1.2Sn (wt%) cast magnesium alloy solidified at cooling rates from 4.5 × 10¹ °C s⁻¹ to 2.3 × 10³ °C s⁻¹ is investigated. The results indicate that the secondary dendrite arm spacing and the average grain sizes decrease from 14.3 to 2.9 μm and from 187 to 78 μm, respectively, as the cooling rate increases from 4.5 × 10¹ °C s⁻¹ to 2.3 × 10³ °C s⁻¹. The relationship between the secondary dendrite arm spacing (λ₂) and cooling rate (R) is fitted as λ₂ = 69.7 × R^−0.42. When the alloy solidifies in the range of cooling rates, a new quasi‐crystalline I phase is formed due to faster solidification. The microsegregation ratios of Al, Zn, and Sn elements decrease significantly with the increase in cooling rate. Compared with the specimen solidified at 4.5 × 10¹ °C s⁻¹, the microsegregation ratios of Al, Zn, and Sn of that solidified at a cooling rate of 2.3 × 10³ °C s⁻¹ decrease by 37.4%, 45.7%, and 31.6%, respectively.