授凝固条件对Mg-Zn-Y合金显微组织及形核动力
学过程的影响
EFFECTS OF SOLIDIFYING CONDITION ON MICROSTRUCTURE AND NUCLEATION
DYNAMICS OF Mg-Zn-Y ALLOYS

摘  要

镁合金是实际应用中最轻的金属结构材料，具有高的比强度、比刚度和减震性，而且易于回收，在汽车、电子、航空等领域有广阔的应用前景。常规铸造镁合金显微组织及第二相比较粗大，高温易氧化，室温和高温强度都不理想，难以满足高性能结构材料的需求。快速凝固技术能显著细化晶粒、扩展固溶度及形成新的亚稳相，从而大幅度提高合金的强韧性和耐蚀性，因此快速凝固技术被广泛应用于制备高性能和新型镁合金。Mg-Zn系合金属于典型的高强镁合金，但该类合金的强度很难满足在更高条件下应用，尤其难以满足在较高温度范围内应用。Y在提高镁合金的强度和耐热性方面有很大的应用价值。
本文采用常规凝固技术制备出三种不同成分的Mg-Zn-Y合金，将合金在石英管中重熔后用单辊制带设备制备不同冷却速度的快速凝固Mg-Zn-Y合金条带。采用OM、SEM、EDS、XRD和TEM分析了常规凝固及快速凝固Mg-Zn-Y合金的显微组织和相组成，用DSC分析了合金升温过程中所发生的相变。运用与时间有关的非均质形核理论计算了在快速凝固Mg-Zn-Y合金中各竞争相的形核孕育期时间与温度的关系，研究了快速凝固合金的形核动力学过程。
研究表明，在常规凝固条件下，Mg-Zn-Y合金组织为树枝晶，随着Y的增加和Zr的添加，晶粒逐渐细化，等轴趋势明显加强。合金的晶界析出相主要以两种形态存在：一是在三角晶界形核，呈“鱼骨状”；一是几乎包围整个晶粒，呈连续网状。快速凝固Mg-Zn-Y合金条带的横截面组织分为三个区域：近辊面细晶区、内部柱状晶区、自由面等轴晶区；随着转速的增大，自由面等轴晶区的厚度越来越薄，并在某些地方消失。快速凝固Mg-Zn-Y合金条带贴辊面组织为等轴晶，晶粒内部有细小的颗粒状析出物，随着转速的增大，晶粒逐渐变细，颗粒状析出物越来越少。Mg7Zn2Y合金条带中析出物为Mg-Zn二元相，Mg7Zn3Y和Mg7Zn3Y0.55Zr合金条带中析出物为Mg-Zn-Y三元相。
合金成分的变化和凝固条件的不同引起相的变化。在常规凝固条件下， Mg7Zn2Y合金主要由α-Mg、Mg7Zn3、Mg12YZn相组成，Mg7Zn3Y和Mg7Zn3Y0.55Zr合金中主要由α-Mg、Mg3YZn6、Mg3Y2Zn3相组成。快速凝固条件下，Mg7Zn2Y条带主要由α-Mg和Mg2Zn11相组成，Mg7Zn3Y和Mg7Zn3Y0.55Zr合金条带中主要由α-Mg和Mg3Y2Zn3组成，快速凝固抑制了Mg3YZn6准晶相的形成。
快速凝固过程是一个非平衡凝固过程，用与时间有关的非均质形核理论分析了Mg-Zn-Y合金快速凝固过程中的形核特点，得出了形核孕育期与温度之间的关系：
 
做出了不同成分快速凝固Mg-Zn-Y合金的τ-T曲线。快速凝固Mg7Zn2Y合金的相选择顺序为：Mg2Zn11→α-Mg，快速凝固Mg7Zn3Y和Mg7Zn3Y0.55Zr合金的相选择顺序为：Mg3Y2Zn3→α-Mg，均为第二相优先形核。分析结果与试验结果吻合的很好。
合金的腐蚀试验表明，Mg7Zn2Y、Mg7Zn3Y、Mg7Zn3Y0.55Zr合金，转速为1800rpm的条带自腐蚀电位比铸态合金分别提高了0.03V、0.03V、0.027V，腐蚀电流密度分别降低到铸态合金的1/4、1/3、1/3，快速凝固条带的耐腐蚀性能比铸态合金有较大的提高。

关键词： Mg-Zn-Y镁合金，快速凝固，显微组织，相组成，形核动力学
Abstract

As the lightest metal structure material, magnesium alloys are found using widely in automotive, electronic and aeronautical industries because of a number of desirable features, including high strength/weight, high hardness/weight, damping characteristic and recycled easily. The microstructures、secondary phase of as-cast alloys are coarse and easily oxidized in high temperature, the room temperature strength and elevated temperature strength is undesirable, which is difficult to meet the needs of high performance structure materials. The rapid solidification appears under this condition, the materials made by rapid solidification have many advantages, such as high strength、high toughness、wonderful corrosion resistance because of fine grains、large solid solubility、new metastable phase. New magnesium alloys with high performance can be developed by rapid solidification. Mg-Zn based alloy is typical high strength magnesium alloy. But its strength is difficult to apply in higher condition. Its strength and heat resistance can be improved by the addition of Y.
The rapidly solidified samples of Mg-Zn-Y alloys were prepared using single-roller equipment. In this thesis, the microstructures, phase composition were systemically investigated by using measurements of OM, XRD, SEM, EDS, TEM. The phase transformation was studied by DSC. An expression adapts to the heterogeneous nucleation rate was induced, utilizing time depended nucleation theory.
Microstructures of as-cast Mg-Zn-Y alloys are dendritic morphology. As the addition of Y and Zr, the grain size is refined. The grain boundary appears two kinds of morphology: one is “fish bone”, which nucleates in triangle grain; the other is continuous graticule, which enclose the whole grain. Microstructures of the cross section of rapidly solidified Mg-Zn-Y alloy ribbons are composed of three regions: fine equiaxed region near the copper roll side、the inner columnar grain region and the outer equiaxed region near the free surface. The microstructure of the region near the copper roll side is fine equiaxed morphology, as the increasing of the rotation speed, the thickness of the outer equiaxed region near the free surface becomes thinner, even disappears in some region. Microstructures of the region near the copper roll side is equiaxed morphology, there are fine precipitates in grain. As the increasing of the rotation speed, the grain is refined. The precipitates are fewer. In the Mg7Zn2Y alloy ribbons, the precipitates are Mg-Zn binary phase，In Mg7Zn3Y、Mg7Zn3Y0.55Zr alloy ribbons, the precipitates are Mg-Zn-Y ternary phase.
The phase composition changes as the variation of alloying component and solidified condition. There are α-Mg、Mg7Zn3、Mg12YZn phase in as-cast Mg7Zn2Y alloy, there are α-Mg，Mg3YZn6、Mg3Y2Zn3 phase in as-cast Mg7Zn3Y、Mg7Zn3Y0.55Zr alloys. While rapidly solidified Mg7Zn2Y alloy ribbons consist of α-Mg、Mg2Zn11 phase, rapidly solidified Mg7Zn3Y、Mg7Zn3Y0.55Zr alloys ribbons consist of α-Mg、Mg3Y2Zn3 phase, rejecting the precipitation of Mg3YZn6 phase.
Aiming at non-equilibrium characteristics of the rapidly solidified process, an expression adapts to the heterogeneous nucleation was deduced, utilizing time dependent nucleation theory. We induce the connection between incubation period and temperature:
 
Get the curve of τ-T. The results show the sequence of phase selection of rapidly solidified Mg7Zn2Y alloy is Mg2Zn11→α-Mg, the sequence of phase selection of rapidly solidified Mg7Zn3Y、Mg7Zn3Y0.55Zr alloy is Mg3Y2Zn3→α-Mg. In rapidly solidified Mg-Zn-Y alloy the second-phase nucleate firstly. The results are concord with results of experiments.
Corrosion experimental results show the resistance to corrosion is improved obviously by rapid solidification. The corrosion potential of Mg7Zn2Y、Mg7Zn3Y、Mg7Zn3Y0.55Zr alloys was separately 0.03V、0.03V、0.027V higher than as-cast alloys, the corrosion current is1/4、1/3、1/3 of as-cast alloys.

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