Microstructures and microsegregation of directionally solidified Mg-1.5Gd magnesium alloy with different growth rates

Directional solidification of Mg-1.5Gd (wt%) magnesium alloy was carried out to investigate the effects of the growth rate on the microstructures under controlled solidification conditions. A Bridgman-type directional solidification furnace with a liquid metal cooling (LMC) technique was used to solidify the specimens, which could provide steady state conditions with a constant temperature gradient (40 K/mm) at a wide range of growth rate (10~200 μm/s). Results show that the microstructures are cellular, and the relationship between cellular spacing (λ) and growth rate (V) is established in the form: λ=130.2827V^-0.2228 by a linear regression analysis, which is in good agreement with the calculated values by Trivedi model. The thermodynamics solidification path calculations by Scheil model and experimental observations confirm that the solidification microstructure in the alloy consists of primary α(Mg) phase and binary eutectic α(Mg)+Mg₅Gd phase. Meanwhile, the microsegregation of the alloying element predicted by the Scheil model agrees reasonably with the electron probe microanalysis (EPMA) measurements.