Sn、Ca合金化Mg-Al系合金制备及组织性能研究

发布时间：2018-01-28 08:48

  本文关键词： Mg-Al合金 合金化 热处理 力学性能 第一性原理计算　出处：《沈阳工业大学》2016年硕士论文　论文类型：学位论文

【摘要】：Mg-Al系合金是目前应用最为广泛的压铸镁合金,但其有限的强韧性和较差的高温力学性能在一定程度上限制了其应用范围。本文以Mg-7Al合金为研究对象,通过Sn、Ca合金化和热处理方法来提高合金的强韧性。通过第一性原理计算方法研究了Mg-Al-Ca-Sn系合金中主要二元相的合金化能力及其晶体结构稳定性。利用OM、XRD、DSC、SEM、TEM及力学性能试验、失重试验及电化学试验等手段分析了Sn、Ca合金化Mg-7Al合金组织、力学性能及腐蚀性能的变化规律,探讨了热处理对合金组织及力学性能的影响,并揭示了合金的强韧性机理与腐蚀机理。Mg-Al-Ca-Sn合金中二元相Mg17Al12、Al2Ca、Mg2Sn和Mg2Ca的合金形成热及结合能计算结果表明,Al2Ca与Mg2Sn具有较强的合金形成能力及结构稳定性。通过计算各相的态密度、Mulliken电子占据数、金属性和差分电荷密度,分析了相结构稳定性的机制。各相弹性常数计算结果表明,Al2Ca具有最强的抵抗变形能力,Mg2Sn的塑性最好。热力学性质计算结果表明,Al2Ca和Mg2Sn的Gibbs自由能低于Mg17Al12,即Al2Ca和Mg2Sn的晶体结构稳定性优于Mg17Al12相。因此,理论上通过添加Ca和Sn元素可以提高Mg-Al系合金的热力学稳定性。向Mg-7Al合金中添加2wt.%Sn元素可以细化晶粒,抑制Mg17Al12相的生长,并在组织中形成新相Mg2Sn,其以颗粒状弥散分布于基体中。由于合金组织细化、第二相数量的增加、Mg17Al12相形貌改善以及具有良好热力学性质的Mg2Sn相析出的综合作用,使得AT72合金表现出比Mg-7Al合金更优的室温及高温拉伸力学性能。AT72合金经T4固溶处理(400℃?20h)后,晶粒尺寸变化较小,Al和Sn元素固溶到基体中产生固溶强化;同时,由于Mg17Al12相分解,使得高熔点的Mg2Sn颗粒相的析出强化效果更加明显。T4处理后的AT72合金的抗拉强度、屈服强度和伸长率分别达到276MPa、202.6MPa和10.6%,其比压铸态AT72合金分别提高了7.0%,18.2%和24.7%。T6(400℃?20h+200℃?15h)处理后,由于脆性相Mg17Al12的非连续析出以及Mg2Sn相的粗化,造成AT72-T6合金强度和塑性的下降。不同Sn含量对Mg-7Al-1Ca合金的组织、力学性能和腐蚀性能有着明显的影响。Sn元素的加入可以有效地细化合金晶粒,改变第二相的数量和形貌。添加Sn元素可以抑制(Mg,Al)2Ca(C36)的形成,导致了CaMgSn和Mg2Sn相的形成。随着Sn元素的添加Mg-7Al-1Ca合金的力学性能和耐蚀性能得到明显地改善。Mg-7Al-1Ca-0.5Sn合金表现出最优的室温拉伸性能和冲击韧性,其值分别为σb=258.36MPa、σ0.2=188.08MPa、δ=10.21%和Ak=25J。Mg-7Al-1Ca-1.0Sn合金表现出最优的高温拉伸性能和硬度,其值分别为σb=175.47MPa、σ0.2=132.65MPa、δ=15.39%和61.9HB。失重试验和电化学试验结果表明,少量的Sn能够提高合金的腐蚀电位,降低腐蚀电流密度,明显地提高合金的耐蚀性,但随着Sn含量的增加,大量非连续的金属间化合物会恶化高Sn含量合金的耐蚀性。热处理后Mg-7Al-1Ca-xSn合金晶粒粗化,使得合金力学性能的改善效果较小。
[Abstract]:Mg-Al alloy is the most widely used die casting magnesium alloy. However, its limited strength and toughness and poor mechanical properties at high temperature limit its application to a certain extent. In this paper, Mg-7Al alloy as a research object, through Sn. CA alloying and heat treatment were used to improve the strength and toughness of the alloy. The alloying ability and crystal structure stability of the main binary phases in Mg-Al-Ca-Sn alloy were studied by first-principle calculation method. Using OM. The microstructure of Mg-7Al alloy alloyed with Sno Ca was analyzed by means of TEM, mechanical properties test, weightlessness test and electrochemical test. The influence of heat treatment on the microstructure and mechanical properties of the alloy was discussed. The mechanism of strength and toughness and the corrosion mechanism of the alloy. The binary phase Mg17Al12Al2Ca in Mg-Al-Ca-Sn alloy was also revealed. The results of heat of formation and binding energy of Mg2Sn and Mg2Ca show that Al _ 2CA and Mg2Sn have strong forming ability and structural stability. The density of states of each phase is calculated. The mechanism of phase structure stability is analyzed by Mulliken electron occupation number, gold property and differential charge density. The results of calculation of the elastic constants of each phase show that Al _ 2Ca has the strongest resistance to deformation. The calculated thermodynamic properties of Mg2Sn show that the Gibbs free energy of Al2Ca and Mg2Sn is lower than that of Mg17Al12. That is, the crystal structure stability of Al2Ca and Mg2Sn is better than that of Mg17Al12 phase. In theory, the thermodynamic stability of Mg-Al alloy can be improved by adding Ca and Sn elements, and the grain size can be refined by adding 2wt. Sn element to Mg-7Al alloy. The growth of Mg17Al12 phase was inhibited and a new phase mg _ 2SnN was formed in the microstructure, which was dispersed in the matrix as granular. The number of the second phase increased due to the refinement of the alloy structure. The morphology of Mg17Al12 phase was improved and the precipitation of Mg2Sn phase with good thermodynamic properties was synthesized. The tensile mechanical properties of AT72 alloy at room and high temperature are better than that of Mg-7Al alloy. After 20h, the change of grain size was smaller, and the solution of Al and Sn elements in the matrix resulted in solution strengthening. At the same time, due to the decomposition of Mg17Al12 phase, the precipitation strengthening effect of Mg2Sn particle phase with high melting point is more obvious. The tensile strength of AT72 alloy treated with T4 is more obvious. The yield strength and elongation reached 276MPA 202.6 MPA and 10.6 MPA, respectively, which were 7.0% higher than those of die-cast AT72 alloy. 18.2% and 24.7. T6? 20 h 200 鈩，

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