TTT and TTP diagrams for quench sensitivity of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy

摘要:

通过分级淬火技术得到Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc铝合金的时间-温度-转变（TTT）曲线和时间-温度-性能（TTP）曲线，采用透射电子显微镜（TEM）、差示扫描量热仪（DSC）和X射线衍射（XRD）进行相变分析。结果表明：在一定的温度下延长保温时间会导致试样的电导率增加，硬度降低。显微组织观察表明，随着保温时间的增加，许多大型杆状平衡相η（MgZn2）会在基体中析出并快速生长，导致淬火过程中溶质损失，削弱了随后的时效强化效果。η粒子沉淀析出的主要原因是溶质原子的快速扩散和强大的相变驱动力。淬火敏感温度范围为270～390℃。因此，在淬火敏感温度范围内，需适当提高冷却速度以获得较高的力学性能。其他温度范围内应考虑适当降低冷却速度以控制残余应力。

Abstract:

The time-temperature-transformation (TTT) curve and time-temperature-property (TTP) curve of Al-9.0Zn-2.5Mg-1.5Cu-0.15Zr-0.2Sc alloy were determined by interrupted quenching technique. Phase transformations have been investigated by TEM, DSC and XRD. The result shows that under certain temperature and prolonging the holding time, the electro-conductivity of the specimens tend to increase and the hardness will decreases. Microstructure observation indicates that with the time increased, a lot of large equilibrium rod-shaped η (MgZn2)phase precipitated in the matrix and grew rapidly, which results in loss of solutes in the aging process and weakened the strengthening effect. The main reasons for precipitation of η particles are that the rapid diffusion of solute atoms and the strong driving force for phase transformation. Quench sensitive temperature range is 270℃~390℃. Consequently, the cooling rate should?be consider?lowering appropriately from the solution temperature to 390℃ to control the residual stress. And then, in the quench sensitive range, it is necessary to increase the cooling rate properly to obtain high mechanical properties.