等温热处理过程中铸态AZ91镁合金的微观组织演化

铸态AZ91镁合金加热至液固相温度区间内等温热处理后演变成半固态金属,对淬火组织形态分析的试验表明:在高固相体积分数条件下,网状分布的共晶组织首先熔化,二次枝晶臂消失或熔断脱落并重新沉淀在枝晶主干上,在Gibbs-Thomson效应下,发生Ostwald熟化,固相颗粒由枝晶逐渐演变成球形颗粒,并随着热处理时间的延长发生合并现象,而且热处理温度越高晶粒间合并现象越严重。同时在界面能降低的驱使下,晶粒内部包裹的液滴以两种方式形成:晶粒内富Al、Zn的质点经过熔化、扩散、合并、或者脱落的二次枝晶臂沉淀后搭桥生长形成。

Microstructure Evolution of As-Cast AZ91 Magnesium Alloy During Isothermal Heat Treatment

Microstructure evolution of AZ91 magnesium alloy, which is isothermally treated in solid-liquid region at a high volume fraction of solid from as-cast microstructure then transforms to semisolid state is investigated. The results show that eutectic phase with net-work shape is first to melt, and second dendritic arms disappear or melt off and then re-precipitated on the dendritic trunk. Solid particles go through Ostwald ripening due to Gibbs-Thomson effect resulting from difference of curvatures, and transform into globular particles, and coalesce together with the time. Moreover, higher the isothermally treating temperature, more severe the coalescence. At the same time, the entrapped liquid drops form intra-grain by two mechanisms owing to decreasing interfacial energy:one is that inclusions including rich solute Al and Zn melt, diffuse and then coalesce together into larger liquid drops, the other is that second dendrite arms, which melted off and reprecipitated on the trunk, enclose to form liquid drops by bridge-growth.