冷却速度对Mg-8Gd-1Er合金凝固组织的影响

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

摘要:

本文采用温度采集装置测定了Mg-8Gd-1Er(GE81)合金在石墨型炉冷、石墨型空冷、铁型空冷、铜型空冷四种不同冷却方式下的平均冷却速度，基于经典形核理论分析了晶粒密度与冷却速度的关系；利用金相显微镜和扫描电镜观察了不同冷却速度下合金的铸态显微组织，分析了晶粒密度、第二相体积分数及硬度与冷却速度的关系。研究结果表明：合金在不同冷却方式下的平均冷却速度分别为0.23，0.46，2.17，3.88 K·s-1，冷却速度与过冷度为线性关系：?T=13.5664v+6.9655；随冷却速度增加，晶粒明显细化，晶粒密度与冷却速度的关系为：Nv=1.1135×1012exp(-46.8344/(13.5664v+6.9655))；此外，第二相体积分数减小，分布更加细小均匀，合金硬度明显增大，硬度与冷却速度的关系为：HV=72.1772-12.6895/(1+exp(v-2.2570))。

Abstract:

In the study, a temperature acquisition system was used to measure average solidification cooling rate of GE81 alloys under different cooling conditions, such as furnace-cooled in graphite mould, air-cooled in graphite mould, air-cooled in steel mould and air-cooled in copper mould. The connection between cooling rate and grain density was studied on the basis of classical nucleation theory. The optical microscope (OM) and scanning electron microscope (SEM) were used to observe the microstructure of as-cast GE81 alloys (Mg-8%Gd-1%Er, mass fraction) under various cooling rate in order to establish the relationship between solidification cooling rate, grain density, hardness and volume fraction of secondary phases. The results showed that the solidification cooling rate of GE81 alloy in the four moulds was 0.23, 0.46 2.17 and 3.88 K·s-1, respectively. The cooling rate is linear with the undercooling and the formula was ?T=13.5664v+6.9655. With the increasing cooling rate, the α-Mg grains were refined obviously. The relationship between the cooling rate and the grain density could be described as Nv=1.1135×1012exp(-46.8344/(13.5664v+6.9655)). Moreover, the secondary phases distributed more homogeneously and their volume fraction decreased so that the hardness of GE81 alloy increased significantly. The relationship between the hardness and the cooling rate could be expressed as HV=72.1772-12.6895/(1+exp(v-2.2570)).