时效5年的触变成形ZA27合金微观形貌观察

Microstructure Characteristic of Thixoformed ZA27 Alloy After Aging for 5 Years

通过OM、SEM和XRD，对触变成形并经5年自然时效的ZA27合金的微观组织进行了观察。结果表明：ZA27合金是由富Al枝晶和枝晶间的共晶组织组成。经触变成形和自然时效后的ZA27合金由球状初生粒子α′相、晶间的二次凝固组织及半固态成形时没有凝固完全的小液池组成。共晶方式主要为离异共晶及其中“蜂窝”状棒状共晶，部分区域出现不规则层片共晶组织。ZA27合金通过随后的冷却和自然时效过程，经过了一系列的相变，β相发生胞状分解，形成规则的共析（α＋η）层片组织或是不规则的复杂形状组织，从一些层片区域逐渐向低铝的α′相区域中心生长为粗大的层片组织，当其生长被α′相中心的连续分解所阻挡时，致使其芯部形成细小的（α＋η）颗粒组织。

The microstructure of thixoformed ZA27 was examined by optical microscope, scanning electron microscope and X-ray diffraction after aging for 5 years at ambient temperatures. The results indicat that the microstruture is composed of aluminum-rich α′ dendrites and eutectic microstructure in the interdendritic regions. After thixoforming and natural aging, the microstructure is composed of nearly globular primary α′ phase, secondary solidification structure inner or outer primary grains and the liquid not freezed completely during semi-solid forming. Eutectic modes dominate in divorced eutectic and rod-like eutectic in shape of honeycomb, and the irregular lamellar eutectic appeared in some regions. On cooling after casting and on natural aging, both of the β phases decompose eutectoidally into wellformed lamellae a and η or irregularly complex structures, and some lamellar colonies spread into the low-aluminium α′ phase cores of the microstructure to form coarse lamellar products. But the growth of these lamellae is always retarded by the continuous decomposition of the α′ phase in the dendritic cores and they decompose into mixture of fine zinc-rich rich ase in an aluminum matrix.