ZA27合金的微观组织

Microstructure of casting ZA27 alloy

用SEM和TEM对金属型铸造并经 18个月自然时效的ZA2 7合金的微观组织进行了观察。结果表明 :ZA2 7合金凝固的初生相为富Al的树枝状α′相 ,随后在α′相周围形成一层富Zn的包晶 β相 ,最后剩余液体发生共晶反应 ,共晶 β优先依附于包晶 β相形核、长大 ,而共晶 η则在包晶 β相间形成一层薄膜 ,在有些区域则形成棒状共晶组织 ;在随后的冷却及时效过程中 ,β发生胞状分解形成规则的共析α +η层片组织或不规则的复杂形状组织 ,共析胞以共晶 η相为基形核 ,并向枝晶中心生长 ,使α′相也分解为层片α +η组织 ,当其生长被α′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 ;富Cu的ε相存在于所有

The microstructure of permanent mold casting ZA27 alloy was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) after natural aging for 18 months. The solidification begins with the formation of aluminum rich α′ dendrites, followed by the peritectic reaction whereby a zinc rich β phase forms around the edges of the primary phase. Finally, the solidification is completed by formation of eutectic β and η phases, the β phase joining the peritectic β phase and the η phase remaining as thin film in the interdendritic regions. In some regions, the eutectics form in the form of fibrous β within η phase. The morphologies of the eutectics are determined by the size of the eutectic pools. On cooling after casting and on aging, both of the β phases decompose into well formed eutectoid α+η lamellae or irregularly complex structures through cellular reaction. These cell colonies nucleate on the eutectic η phase and grow into the low aluminum α′ phase cores of the dendrites to make the α′ phases form lamella structures. 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 η phase in an aluminum matrix. The Cu rich ε phase particles form in all η phases.