【作者】 张金山；

【导师】 梁伟；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2008， 博士

【摘要】 本作者以提高镁合金的强韧化和耐热性为目的,通过化学成分设计和工艺优化,用常规铸造制备出一种镁基球形准晶中间合金。以外加方式将其加入到镁合金熔体中,凝固结晶形成镁基准晶颗粒增强的镁合金。研究了镁基准晶的形成、结构演化和遗传效应对镁合金组织和性能的影响及其作用规律,探讨了镁基准晶颗粒增强镁合金的微观本质。在对Mg-Zn-Y系准晶制备研究的基础上,就Ca、Mn等对Mg-Zn-Y系准晶的形成过程和宏观硬度的影响进行了研究。结果表明:Ca、Mn的加入都会影响准晶的形成和生长形态。当Mg-Zn-Y系准晶合金中含有0.05%的Ca时,可获得球形准晶;当Mg-Zn-Y系准晶中间合金中随着锰添加量的提高,准晶的形貌由花瓣状变为球形状,并且球径逐渐变小,数量增多,当Mn含量超过2%时,球形准晶的数量逐渐减少,形态变坏。随着Ca、Mn元素加入量的提高,两种准晶中间合金的宏观硬度均下降。同时发现当加入Mn元素时还对结晶相α-Mg树枝状晶的生长有影响,随着锰含量的提高,使α-Mg树枝状晶变短,粗大的一次晶消失。常规铸造条件下可在大尺寸（φ30mm）范围内可获得镁基球形准晶中间合金。研究中发现,在Mg-Zn-Y花瓣状准晶和Mg-Zn-Y-Mn球形准晶的内部存在裂纹缺陷,这类准晶的形成温度即熔点相对较低,尽管合金组织细化和常温力学性能明显提高,但高温强度提高幅度有限。根据Nd可增强镁合金高温耐热性的作用,为了进一步提高镁合金的高温力学性能,我们又发现并在常规铸造条件下制备出一种新的Mg-Zn-Nd基球形准晶中间合金。这种新的球形准晶与Mg-Zn-Y-Mn球形准晶的最大不同之处,亦即可贵之处是在准晶内部没有裂纹,球形准晶相周围没有共生共晶相,但出现离异共晶和独立的层片状共晶相,准晶形成温度（熔点）较高,球化率和球化级别也好于Mg-Zn-Y-Mn球形准晶中间合金,这些特征均表明:把Mg-Zn-Nd基球形准晶作为耐热镁基合金的强化相,比Mg-Zn-Y-Mn球形准晶对镁合金耐热性显示出更高的增强作用、更好的工艺宽容性和更低的生产成本。论文利用准晶表面能低、热力学稳定的特点,以外加方式将自制的镁基准晶中间合金加入镁基合金熔体中,在常规铸造条件下凝固成形,获得一种准晶颗粒弥散均匀分布的由多相复合组成的镁基合金。由于准晶颗粒作用的结果,使铸态组织明显细化,其常温强韧度大幅度提高。研究发现:就准晶对镁合金的增强效果而言,Mg-Zn-Y-Mn球形准晶的强化效果大于Mg-Zn-Y花瓣状准晶,尤其韧度的提高效果更为明显。在AZ91合金中加入Mg-Zn-Y-Mn球形准晶中间合金后,在室温下该合金具有更高的抗拉强度和屈服强度,分别比加准晶前的AZ91合金提高38%和34%,伸长率提高了40%,冲击韧度提高60%以上,耐热性也有较大幅度的提高。在ZA85镁合金和高锌镁合金中加入Mg-Zn-Y-Mn球形准晶中间合金后,也同样具有很好的细化和强化效果。本文经过对准晶和出准晶增强的镁合金的制备研究,以及准晶结构演化规律研究,探明球形准晶在镁合金结晶凝固过程中的遗传效应与细化变质作用行为。研究表明:弥散均匀分布于α-Mg相中和晶界β-Mg₁₇Al₁₂相内部或附近的准晶颗粒可起到强化基体与晶界、降低镁合金基体中元素的扩散速率、阻碍位错运动和阻止晶界滑动的作用,使晶界β-Mg₁₇Al₁₂相断网和趋于球化,改变时效析出方式,从而有效提高镁合金的常温强韧性和高温耐热抗蠕变性。更多还原

【Abstract】 To improve the strength-toughness and heat-resistance, spherical quasicrystal contained Mg-based （SQCM） master alloy has been fabricated under conventional casting conditions by designing chemical compositions and optimizing processing parameters. Mg-based quasicrystal-particle-reinforced magnesium alloys were obtained by introducing SQCM master alloy into the magnesium alloy melt. The formation mechanism, microstructure evolution and heredity effects of Mg-based quasicrystal was investigated in this paper. Besides, its effects on magnesium alloys and the microscopic essence of strengthening behavior of Mg-based quasicrystal were also discussed.Based on the investigation of Mg-Zn-Y quasicrystal alloy, effects of alloying elements such as Ca and Mn on formation and macro-hardness of Mg-Zn-Y quasicrystal were investigated. Experiment results showed that the formation mechanism and macro-morphology of quasicrystal have changed with the addition of Ca or Mn elements. Spherical quasicrystal can be obtained by adding 0.3% Ca into Mg-Zn-Y quasicrystal alloy. The morphology of quasicrystal changes from petal-like to sphericity as the content of Mn increases, with its diameter decreasing and its quantity increasing. However, with the addition of Mn surpassing 2.0%, the quantity of spherical quasicrystal decreases and its morphology varies from sphericity to polyhedron or petal-like. Both the macro-hardness of these two kinds of quasicrystal master alloys gradually decreases with the increase of Mn or Ca addition. It was also found that the addition of Mn influences growing process of dendriteα-Mg phase. As the content of Mn increases, dendrite arm space ofα-Mg phase tends to be smaller and coarse primary dendrite disappears. Spherical quasicrystal contained Mg-based master alloy can be achieved under conventional casting condition with mould diameter less than 30mm.It was found that cracks exist in petal-like Mg-Zn-Y quasicrystal and spherical Mg-Zn-Y-Mn quasicrystal. Although adding these kinds of relatively low melting-point quasicrystal leads to obvious grain refinement and enhances mechanical properties both at room and elevated temperature, the improvement of elevated-temperature mechanical properties is limited. Since Nd can improve the elevated-temperature heat-resistance, a new spherical quasicrystal contained Mg-Zn-Nd master alloy was fabricated under conventional casting condition to improve the mechanical properties of magnesium alloys at elevated-temperature. Compared to spherical Mg-Zn-Y-Mn quasicrystal, no cracks in the spherical Mg-Zn-Nd quasicrystal and no eutectic phase surrounds the quasicrystal while divorced eutectic and individual lamellar eutectic phase appears, besides its melting point, spherical ratio and spherical rating become much higher. All these features indicate that superior heat resistance, better processing tolerance and lower production cost can be achieved than that of spherical Mg-Zn-Y-Mn quasicrystal when adding Mg-Zn-Nd quasicrystal into master alloy with the spherical Mg-Zn-Nd quasicrystal acting as reinforced phase of heat resistant Mg-based alloy.Because of its low surface energy and high thermal stability, a multi-phase Mg-based alloy with homogeneously distributed quasicrystal particles was obtained under conventional casting condition after adding SQCM master alloy into Mg-based alloy melt. The as-cast microstructure was obviously refined and room-temperature strength-toughness was dramatically improved due to the strengthening effect of quasicrystal particles. It was found that spherical Mg-Zn-Y-Mn quasicrystal exhibits superior strengthening effect than petal-like Mg-Zn-Y quasicrystal. After the addition of spherical quasicrystal containing Mg-Zn-Y-Mn master alloy into AZ91 alloy, room-temperature tensile strength, yield strength, percentage elongation, impact toughness was improved by 38%, 34%, 40% and 60%, respectively; heat resistance was also improved greatly. Moreover, excellent grain-refining and strengthening effects can be achieved after adding spherical quasicrystal containing Mg-Zn-Y-Mn master alloy into ZA85 alloy or high Zinc magnesium alloy. In this paper, both quasicrystal master alloy and quasicrystal reinforced magnesium alloys were fabricated, microstructure evolution rule of quasicrystal was discussed, and heredity effect and grain-refining effect of spherical quasicrystal during the solidification of magnesium alloy were investigated. The results showed that dispersive phase which distributes in the interior ofα-Mg matrix and in the vicinity ofβ-Mg₁₂Al₁₇ phase can strengthen the matrix and the grain boundaries, slow down the diffusion of elements inα-Mg matrix, impede the dislocation movement in the matrix, and prevent grain boundary sliding. Besides, morphology ofβ-Mg₁₂Al₁₇ phase changes from continuous nets to discontinuous nets or even sphere, and its aging precipitation changes too. Consequently, the strength and toughness at room temperature and the heat-resistance at elevated temperature can be improved efficiently.更多还原