【作者】 宋佩维；

【导师】 井晓天； 郭学锋；

【作者基本信息】 西安理工大学 ， 材料学， 2007， 博士

【摘要】 本文采用普通凝固法制备了Mg-4Al-2Si（AS42）、Mg-4Al-4Si（AS44）和Mg-6Al-6Si（AS66）三种合金。深入研究了普通凝固、往复挤压（RE）、热处理及Sb微合金化对三种合金微观组织、力学性能的影响规律，讨论了基体组织和Mg₂Si相颗粒的细化、强化机制。研究结果表明：通过在镁合金熔体中加入Al-Si中间合金制备的AS42、AS44和AS66合金，由α-Mg基体、汉字状及块状Mg₂Si相和β-Mg₁₇Al₁₂相组成。往复挤压可显著细化AS42合金的组织。挤压过程中，AS42合金基体发生了动态再结晶。随着挤压道次的增加，基体晶粒与Mg₂Si相颗粒不断细化。往复挤压6道次时，通过再结晶，基体形成了尺寸约3μm的等轴晶，Mg₂Si相颗粒尺寸减小到约1.4μm；挤压8道次时，基体组织和Mg₂Si相颗粒尺寸进一步减小，此时合金的抗拉强度和屈服强度均达到最大值，分别为283.5MPa和269.1MPa；挤压道次介于6～8时，晶粒和强化相尺寸基本不再变化，即合金组织的细化趋于稳定，达到了细化极限。但是，如果继续增加往复挤压道次，晶粒和强化相则会出现长大。如，当挤压道次为11时，由于挤压温度过高，导致基体晶粒和Mg₂Si相颗粒均发生长大。往复挤压也可细化AS44和AS66合金组织。挤压过程中基体发生了动态再结晶而得到细化；汉字状Mg₂Si相颗粒得到了显著细化，粗大的骨骼状Mg₂Si相在挤压破碎后呈较大的块状分布于基体中。往复挤压可大幅提高铸态AS42合金的力学性能。铸态和往复挤压态AS42合金中，Mg₂Si颗粒尺寸遵从Weibull分布；在挤压过程中，汉字状Mg₂Si颗粒依弯曲机制而破碎成块状或条状，随挤压道次的增加，条状颗粒依短纤维加载机制而破碎，块状颗粒依剪切机制而破碎；颗粒开裂的概率与颗粒相对密度成正比，与基体晶粒尺寸平方成正比，与外加真应力的5次方成正比。对挤压态AS42合金进行T4与T6处理后，Mg₂Si颗粒的形貌与尺寸只发生微小变化，说明Mg₂Si相具有较高的热稳定性；而基体晶粒粗化，使合金的力学性能降低。挤压态AS42合金在150℃高温下的短时拉伸试验表明，挤压道次小于8时，挤压道次对合金抗拉强度影响不大；当挤压道次为8时，合金的抗拉强度开始迅速增加；当挤压道次为11时，合金的抗拉强度最高，为256MPa，与铸态高温抗拉强度相比，增长163.9％；挤压态AS42合金的高温抗拉强度均高于相同道次挤压态AS44和AS66合金的。微量Sb的加入使铸态AS44和AS66合金中的Mg₂Si相由粗大枝晶状转变为细小枝晶状，随着组织的改善，合金的抗拉强度和屈服强度均得到不同程度的提高。研究认为，合金中加入Sb后形成高熔点Mg₃Sb₂相，弥散分布的Mg₃Sb₂相与Mg₂Si相存在界面共格对应，成为Mg₂Si相非均质形核的核心，阻碍其进一步长大。更多还原

【Abstract】 In this paper Mg-4Al-2Si （AS42）, Mg-4Al-4Si （AS44） and Mg-6Al-6Si （AS66） alloys were prepared by traditional solidification. The effects of solidification, reciprocating extrusion （RE）, heat treatment and alloying with Sb on microstructures and mechanical properties were investigated. The grain and strengthening phase refinement and strengthening mechanisms were analyzed.The results show that AS42, AS44 and AS66 alloys can be prepared by adding Al-Si intermediate alloy into magnesium-aluminum melt system. The microstructures of these alloys are composed ofα-Mg matrix,β-Mg₁₇Al₁₂ and Mg₂Si which in Chinese script or/and massive shapes.RE is an effective method to be as grain and strengthening particles refining to AS42 alloy. The grain refinement of AS42 is mostly due to dynamic recrystallization RE. The dimension of theα-Mg matrix and Mg₂Si particle decreases with increasing passes of the RE. As fine as 3μm and 1.4μm of equaxedα-Mg and Mg₂Si particle, respectively, can be achieved after REed 6 passes. And 8 pass of RE also brought about finer matrix grain and Mg₂Si particle and maximum ultimate tensile strength and yield strength of 283.5MPa and 269.1Mpa, respectively. The results also showed that 6-8 pass of RE resulted in limitaion of finememt of grain The strain-hardening and dynamic recrystallization are equilibrium, which means that the rate of multiplication and annihilation of dislocations are almost the same. The grain size and particle size begin to coarse due to high temperature from 11 passes RE.RE can refine microstructures of AS44 and AS66 alloys apparently. The dynamic recrystallization of AS44 and AS66 matrix has happened; the ’Chinese’ script Mg₂Si particle has been refined; and the large skeleton Mg₂Si has broken into large block in matrix by extrusion.RE can improve mechanical properties of as-cast AS42 alloy. The tensile properties at room temperature, such as yielding strength, ultimate strength and hardness of the alloys increase with increasing the pass of extrusion.The results of cracking Mg₂Si particles of AS42 alloy showed that Mg₂Si particles size distribution fits ’Weibull’ rule. The ’Chinese’ script Mg₂Si particles could break into massive or strip particles by the bending mechanisms, the plate particles could crack by the short-fiber loading mechanisms, the massive particles, crack by the shear mechanism during RE process. The crack probability of Mg₂Si particles are proportional to relative density of Mg₂Si particles, and also proportional to square of matrix grain size and 5 power of applied true stress.AS42 alloy were heat treated after extrusion. The results show that the morphology and size of Mg₂Si particles were changed slightly by T4 and T6 treatment. It indicated that Mg₂Si phase has a high thermal stability. However, the matrix grain coursed obviously and resulted in decreased mechanical properties.The tensile results of AS42 alloy after RE at 150℃showed that: RE pass have little effects on the ultimate tensile strength when it is less than 8 passes; and the ultimate tensile strength improved rapidly when the pass is more than 8 passes. RE-11 passes resulted in the maximum ultimate tensile strength, 256Mpa. The ultimate tensile strength increased by 163.9% compared with as-cast AS42 at 150℃. Moreover, the ultimate tensile strength of RE AS42 was higher than that of RE AS44 and RE AS66 at 150℃.The morphology of Mg₂Si particle in as-cast AS44 and AS66 changed from coarse dendritic-like to fine ’Chinese’ script-like by alloying element Sb. The ultimate tensile strength and yield strength are also improved by refine microstructures. It is considered that the high melting-point Mg₃Sb₂ phase is dispersed in the matrix due to addition of Sb. Mg₃Sb₂ phase is conherent to Mg₂Si phase and become the inhomgenious nuclei of Mg₂Si and Mg₃Sb₂ phase gathered in the front of primary Mg₂Si phase and hinder its growth.更多还原