Effects of Cooling Rate on Solidified Microstructure of Mg-8Gd-1Er Alloy

A temperature acquisition system was used to measure average solidification cooling rate of GE81 alloys under different cooling conditions, including furnace-cooling in graphite mold, air-cooling in graphite mold, air-cooling in steel mold and air-cooling in copper mold. The relationship between cooling rate and grain density was studied based on classical nucleation theory. Optical microscope (OM) and scanning electron microscope (SEM) were used to observe the microstructure of as-cast GE81 alloys (Mg-8Gd-1Er, wt%) at various cooling rates in order to establish the relationship between solidification cooling rate, grain density, hardness and volume fraction of secondary phases. The results show that the solidification cooling rates of GE81 alloy in the four moulds are 0.23, 0.46, 2.17 and 3.88 K•s-1, respectively. The cooling rate is linear with the undercooling, and the formula is ∆T=13.5664v+6.9655. With the increasing cooling rate, the α-Mg grains are refined obviously. The relationship between the cooling rate and the grain density can be described as Nv=1.1135×1012exp(-46.8344/(13.5664v+6.9655)). Moreover, the secondary phase distribution is more homogeneous and their volume fraction decreases so that the hardness of the GE81 alloy increases significantly. The relationship between the hardness and the cooling rate can be expressed as HV=72.1772-12.6895/(1+exp(v-2.2570)).

Duke Scholars

Author Zhaohui Wang Pathology