【作者】 胡中华；

【导师】 刘胜新；

【作者基本信息】 郑州大学 ， 材料加工工程， 2010， 硕士

【摘要】 AZ31镁合金是目前应用最广泛的变形镁合金之一,但由于镁合金具有典型的密排六方结构,塑性变性能力较差,生产效率普遍不高,加工成本高制约其广泛应用。稀土合金化是提高镁合金综合性能的有效方法,但含铝镁合金中稀土相的针状形态严重影响合金的塑韧性。如何控制稀土相的形态,提高变形镁合金的变形能力,是当前亟待解决的问题。本论文利用光学显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)及能谱仪(EDS)等分析测试手段,研究了Mg3Al系合金中添加RE和Sb的铸态合金显微组织；分析了不同温度液淬合金组织及其组成相形态的演变规律。采用固体与分子经验价电子(EET)理论研究针状稀土相和球化相在合金中的强化机制并利用相结构形成因子分析各合金相在熔体中的析出规律,为高性能镁合金的成分设计、制备及其进一步应用提供试验数据和理论依据。研究表明,Mg3Al系合金中加入1%Ce后,原先粗大的离异共晶Mg17Al12相数量减少,同时生成大量沿晶界分布的针状Al4Ce相；Mg3Al1Ce合金中加入1%Sb后,针状稀土相的数量基本消失,在晶界处生成了颗粒状CeSb相及少量呈不规则块状的β-Mg17Al12相。液淬实验研究表明,在Mg3Al合金中加入Mg-Ce中间合金及高温静置时,Al, Ce原子首先在熔体中自发扩散形成原子集团,随后开始形成细小而呈弥散分布的铝稀土相质点,在降温凝固过程中,Al、Ce原子向凝固界面前沿富集,发生Al-Ce原子集团向Al4Ce相的转变,逐渐长大成独立的长针状结构并沿晶界分布。随着温度的进一步降低,则发生共晶反应形成β-Mg17Al12相。Mg3Al合金中加入Ce, Sb元素后,首先在熔体中形成类似于针状的Al-Ce原子集团,随后Sb原子逐渐向Al-Ce原子集团中扩散,针状的铝稀土原子集团逐渐转化为颗粒状的CeSb原子集团。在连续冷却过程中,过冷液体中Ce-Sb原子团簇将开始形核并逐渐长大。当到达共晶反应温度时,发生共晶反应形成α-Mg+β-Mg17Al12共晶体。采用EET理论计算了纯镁和α-Mg固溶体的价电子结构,Zn、Mn元素固溶到α-Mg基体后最强键共价电子数增大,基体结构单元内原子间的成键能力变强,合金的强度增大,从而起到了固溶强化的作用。计算了Al4Ce相和CeSb相空间的价电子结构,两者最强键上共价电子数均远大于a-Mg中的最强键上的共价电子数,因此成键能力较基体a-Mg要强,在合金变形过程中对位错滑移和晶界迁移的阻碍作用比a-Mg基体更强。Al4Ce相中的最强Al-Ce键上共价电子电子数比其结构单元中的最强键要小得多,因此其为Al4Ce结构中的薄弱环节,在合金变形过程中容易引发应力集中,破坏其相结构,从而引起合金失效。CeSb相主要是由最近邻的Ce原子与Sb原子相互连接而成的均匀结构,该结构中各共价键上的价电子分布均匀,没有薄弱环节。因此CeSb相对Mg-Al系合金的强化效果比Al4Ce相的强化效果更好。用相结构形成因子S、最强键上共价电子数nA,相结构中原子间共价电子数na分析了合金相的凝固次序：在有Sb原子存在的情况下,Al4Ce相不可能析出,合金中的Ce元素仅以CeSb相的形式存在。综合考虑β-Mg17Al12相与α-Mg的结构形成因子S及最强键上的nA值认为,β-Mg17Al12相与α-Mg通过发生共晶反应形成(α-Mg+β-Mg17Al12)共晶体而析出。更多还原

【Abstract】 AZ31 is one of the most widely used wrought magnesium alloys, however, its typical close-packed hexagonal structure, poor plastic deformation character, generally low productivity efficiency and high processing costs have restricted its wider application. Alloying with addition of RE is an effective means to improve the combined properties of magnesium alloy, but the acicular phases in the alloy lead to a deterioration to alloy’s plastic property. So how to control the shape of RE phase and to increase the plastic deformation ability of magnesium alloy are the problems which need to be solved urgently.The influences of RE and Sb on Mg3Al magnesium alloy’s microstructure and liquid quenching alloys’morphology at different temperatures were investigated by using optical microstructure(OM), X-ray diffraction(XRD) and scanning electric microscopy(SEM). The strengthening mechanisms of acicular rare earth phase and sphere-like phase in the alloys and its precipitation process in the melt were studied by empirical electron theory of solids and molecules (EET) with phase structure formation factor. The obtained results could provide experimental data and theory references for composition design, preparation and further application of high performance deformation magnesium alloy.The results showed that the amount of coarsen divorced eutectic P-Mg17Al12 phases decreased by adding 1%Ce into Mg3Al alloy, with a great deal of acicular Al4Ce phases distributing along the grain boundary. After adding 1% Sb, the number of acicular shaped rare earth phase disappeared, however, a small amount of granular CeSb phase and irregular bulkβ-Mg17Al12 phase generated in the grain boundary.Liquid quenching experiments showed that, in the process of adding Mg-Ce master alloy in the Mg3Al alloy and high temperature depositing, Al and Ce atoms first formed atomic clusters in the melt spontaneously, and then began to form small, dispersed Al-Ce particles. During cooling solidification, Al and Ce atoms enriched in the front of liquid/solid interface and transition of atomic clusters to the Al4Ce phase occurred, which gradually grew into an independent and long acicular-like structure and distributed along the grain boundaries. As the temperature further reduced, the eutectic reaction occurred and formedβ-Mg17Al12 phase. After adding Ce and Sb to Mg3Al alloy, the acicular-like Al-Ce atomic clusters formed firstly in the melt. Then Sb atoms gradually diffused towards Al-Ce atom group. The acicular-shaped Al-Ce atom group changed into granulated Ce-Sb atom group gradually. In the continuous cooling process, Ce-Sb clusters in the super-cooled liquid would form the nuclear and grow up gradually. When it reached the temperature of Eutectic reaction, Eutectic reaction occurred andα-Mg+β-Mg17Al12 eutectic was formed.The empirical electron theory of solids and molecules (EET) was used to calculate the valence electron structure of pure magnesium and a-Mg solid solution. After Zn, Mn elements dissolved into the a-Mg matrix, the number of the valence electron in the strongest bond increased and the bonding capacity of atoms in structural unit became stronger. The strength of the alloy increased, thus they played a solid solution strengthening role in matrix.Space valence electron structures of Al4Ce and CeSb phase are calculated. The number of valence electron on their strongest bonds is much larger than that on the strongest bonds of a-Mg, thus the bonding capacity is much stronger. The role in hindering the dislocation slip and grain boundary migration during the deformation process is much stronger than a-Mg matrix.The structure stability of Al4Ce and CeSb phase is analyzed. The number of valence electron on the strongest Al-Ce bond in Al4Ce phase is much smaller than that of the strongest bond in its structural unit, so this is the weak point of the Al4Ce structure, which would be inclined to bring about stress concentration in the process of deformation and destroy the phase structure, then lead to failure. CeSb phase was a uniform structural unit which was mainly interconnected by the nearest neighbor Ce and Sb atom, and the valence electron on the covalent bond distributed evenly, so there is no weak point. Therefore we can draw a conclusion that the strengthening effect of CeSb is better than Al4Ce in Mg-Al base alloys.The solidification sequence is analyzed by the phase structure factor S, the number of valence electron on the strongest bond nA, the number of valence electron between the atoms na. Al4Ce phase is impossible to precipitate, under the condition that Sb atoms exist, and Ce element can only exist in the form of Cesb phase in this alloy. Considering the phase structure factor S and the nA value, the eutectic reaction betweenβ-Mg17Al12 and a-Mg phase occurred and the Eutectic (α-Mg+β-Mg17Ali2) phase precipitated.更多还原