Al-Zn-Mg-Sc合金连续冷却转变曲线的测定

测定了Al-Zn-Mg-Sc合金固溶处理后的连续冷却转变（CCT）曲线,通过动态电阻法测得冷却过程的电阻-温度曲线,根据曲线斜率的变化规律确定相变开始点、结束点以及临界冷却速度范围,绘制出该合金的CCT图,通过扫描电镜和透射电镜分析观察连续冷却过程的组织转变.结果表明,动态电阻法测得的CCT图是可信的;在470℃,保温1 h固溶处理后,抑制相变发生的临界冷速在2 168.0℃/min以下,但高于716.8℃/min,相变主要集中在150~420℃的温度区间发生;当冷却速度较慢时,平衡相η在晶内和晶界大量析出并逐渐长大和粗化,当冷却速度较快时,合金保持了较高的过饱和度,冷却到70℃以下仍有相变发生.

Measurment of continuous cooling transformation curves of Al-Zn-Mg-Sc aluminum alloy

CCT curves of an Al-Zn-Mg-Sc alloy were tested in the paper.Electrical resistance-temperature curves during continuous cooling processes were obtained by in-situ resistance measurement.Beginning and ending temperatures of phase transformation and critical cooling rate range were established by analyzing slope change of those curves and CCT curves were drawn.Microstructure evolution during cooling was examined by SEM and TEM observation,EDS and EDP analysis.The results show that the CCT curves obtained by in-situ resistance measurement are credible.Critical cooling rate to prevent phase transformation during solution treatment in 470 ℃ for 1 h is above 716.8 ℃/min but below 2 168.0 ℃/min,phase transformation beginning and ending temperatures happened mainly in 150～420 ℃.Low speed cooling results in growth and coarsening of η equilibrium phase in grains and on grain boundaries.However,high speed cooling leads to high concentration of solute,which make precipitates happen when temperature is below 70 ℃.