本文關(guān)鍵詞： Mg-Al合金 合金化 熱處理 力學(xué)性能 第一性原理計(jì)算　出處：《沈陽工業(yè)大學(xué)》2016年碩士論文　論文類型：學(xué)位論文

【摘要】：Mg-Al系合金是目前應(yīng)用最為廣泛的壓鑄鎂合金,但其有限的強(qiáng)韌性和較差的高溫力學(xué)性能在一定程度上限制了其應(yīng)用范圍。本文以Mg-7Al合金為研究對象,通過Sn、Ca合金化和熱處理方法來提高合金的強(qiáng)韌性。通過第一性原理計(jì)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體結(jié)構(gòu)穩(wěn)定性。利用OM、XRD、DSC、SEM、TEM及力學(xué)性能試驗(yàn)、失重試驗(yàn)及電化學(xué)試驗(yàn)等手段分析了Sn、Ca合金化Mg-7Al合金組織、力學(xué)性能及腐蝕性能的變化規(guī)律,探討了熱處理對合金組織及力學(xué)性能的影響,并揭示了合金的強(qiáng)韌性機(jī)理與腐蝕機(jī)理。Mg-Al-Ca-Sn合金中二元相Mg17Al12、Al2Ca、Mg2Sn和Mg2Ca的合金形成熱及結(jié)合能計(jì)算結(jié)果表明,Al2Ca與Mg2Sn具有較強(qiáng)的合金形成能力及結(jié)構(gòu)穩(wěn)定性。通過計(jì)算各相的態(tài)密度、Mulliken電子占據(jù)數(shù)、金屬性和差分電荷密度,分析了相結(jié)構(gòu)穩(wěn)定性的機(jī)制。各相彈性常數(shù)計(jì)算結(jié)果表明,Al2Ca具有最強(qiáng)的抵抗變形能力,Mg2Sn的塑性最好。熱力學(xué)性質(zhì)計(jì)算結(jié)果表明,Al2Ca和Mg2Sn的Gibbs自由能低于Mg17Al12,即Al2Ca和Mg2Sn的晶體結(jié)構(gòu)穩(wěn)定性優(yōu)于Mg17Al12相。因此,理論上通過添加Ca和Sn元素可以提高M(jìn)g-Al系合金的熱力學(xué)穩(wěn)定性。向Mg-7Al合金中添加2wt.%Sn元素可以細(xì)化晶粒,抑制Mg17Al12相的生長,并在組織中形成新相Mg2Sn,其以顆粒狀彌散分布于基體中。由于合金組織細(xì)化、第二相數(shù)量的增加、Mg17Al12相形貌改善以及具有良好熱力學(xué)性質(zhì)的Mg2Sn相析出的綜合作用,使得AT72合金表現(xiàn)出比Mg-7Al合金更優(yōu)的室溫及高溫拉伸力學(xué)性能。AT72合金經(jīng)T4固溶處理(400℃?20h)后,晶粒尺寸變化較小,Al和Sn元素固溶到基體中產(chǎn)生固溶強(qiáng)化;同時,由于Mg17Al12相分解,使得高熔點(diǎn)的Mg2Sn顆粒相的析出強(qiáng)化效果更加明顯。T4處理后的AT72合金的抗拉強(qiáng)度、屈服強(qiáng)度和伸長率分別達(dá)到276MPa、202.6MPa和10.6%,其比壓鑄態(tài)AT72合金分別提高了7.0%,18.2%和24.7%。T6(400℃?20h+200℃?15h)處理后,由于脆性相Mg17Al12的非連續(xù)析出以及Mg2Sn相的粗化,造成AT72-T6合金強(qiáng)度和塑性的下降。不同Sn含量對Mg-7Al-1Ca合金的組織、力學(xué)性能和腐蝕性能有著明顯的影響。Sn元素的加入可以有效地細(xì)化合金晶粒,改變第二相的數(shù)量和形貌。添加Sn元素可以抑制(Mg,Al)2Ca(C36)的形成,導(dǎo)致了CaMgSn和Mg2Sn相的形成。隨著Sn元素的添加Mg-7Al-1Ca合金的力學(xué)性能和耐蝕性能得到明顯地改善。Mg-7Al-1Ca-0.5Sn合金表現(xiàn)出最優(yōu)的室溫拉伸性能和沖擊韌性,其值分別為σb=258.36MPa、σ0.2=188.08MPa、δ=10.21%和Ak=25J。Mg-7Al-1Ca-1.0Sn合金表現(xiàn)出最優(yōu)的高溫拉伸性能和硬度,其值分別為σb=175.47MPa、σ0.2=132.65MPa、δ=15.39%和61.9HB。失重試驗(yàn)和電化學(xué)試驗(yàn)結(jié)果表明,少量的Sn能夠提高合金的腐蝕電位,降低腐蝕電流密度,明顯地提高合金的耐蝕性,但隨著Sn含量的增加,大量非連續(xù)的金屬間化合物會惡化高Sn含量合金的耐蝕性。熱處理后Mg-7Al-1Ca-xSn合金晶粒粗化,使得合金力學(xué)性能的改善效果較小。
[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，

本文編號：1470300