Influence of rare earth element Ce and La addition on corrosion behavior of AZ91 magnesium alloy

Two types of AZ91 magnesium alloys containing rare earth element Ce or La were fabricated. Hydrogen evolution and electrochemical tests were carried out to evaluate the corrosion behavior of new AZRE (RE = Ce or La) and AZ91 alloys in 3.5% NaCl solutions (pH 6.50). Various corrosion rate tests indicated that addition of RE obviously enhanced corrosion resistance of AZ91 magnesium alloy. The optimal content of RE was 0.92% for Ce and 0.66% for La. Scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and low‐angle X‐ray diffraction (XRD) were used to characterize the effect of RE addition on microstructure and corrosion product film of AZ91 magnesium alloy. The refined β phase and formation of γ phase in AZRE alloy were observed by SEM, which resulted in the improvement of corrosion resistance due to the depression of microgalvanic couples. Moreover, the enhanced protective effectiveness of corrosion products was another reason for the improved corrosion resistance.     

锶、钛对稀土改性压铸AZ91合金组织与性能的影响

研究了微量Sr、Ti对压铸AZ91合金微观组织与性能的影响。结果发现,与AZ91镁合金相比,添加稀土后,合金的强度提升、塑性下降。进一步添加Sr后,合金因组织细化而使塑性提升,但强度及耐腐蚀性降低。进一步添加Ti元素,则抑制了Sr元素的不利影响,并使合金保持良好的塑性。通过多元微合金化的方法使AZ91+RE+Sr+Ti合金达到最佳的综合性能。微观组织随组分添加而发生演变是材料性能变化的主要原因。     

NaCl溶液中AZ91D的腐蚀性能--RE与Sb及Si对AZ91D合金在NaCl溶液中腐蚀速率的影响

通过静态失重法研究了RE、Si和Sb元素加入后对AZ91D合金在NaCl溶液中腐蚀速率的影响.结果表明, RE加入可以提高AZ91D合金的腐蚀速率,而Si和Sb的加入使合金的腐蚀速率加大.     

应用稀土提高AZ91镁合金性能的研究进展

AZ91是目前应用最为广泛的一种镁合金,它具有良好的铸造性能和和高的屈服强度.但是在熔炼过程中极易燃烧、耐热性和耐蚀性较差等问题严重制约它的应用.稀土作为一种重要的合金化元素,可以显著改善AZ91镁合金的性能.本文综述稀土对AZ91镁合金的阻燃、力学和耐蚀等性能的影响,简述稀土用于改善AZ91镁合金性能的应用前景.     

The effect of rare earth elements on the kinetics of the isothermal coarsening of the globular solid phase in semisolid AZ91 alloy produced via SIMA process

In the present study, the effects of rare earth (RE) elements on the microstructure and coarsening kinetics of the solid globular particle in the semisolid slurry of AZ91 magnesium alloy have been studied at 570 °C and 580 °C. The results showed that the coarsening kinetics of the solid globular particles in semisolid slurry of AZ91 alloy satisfies the Ostwald ripening theory. It was shown that the coarsening rate of the solid particles decreases by adding RE elements into AZ91 alloy, specially at 580 °C, which results in the smaller particles size. It was attributed to the solid–liquid interfacial energy reduction due to the addition of RE elements.     

添加0.5%富铈混合稀土AZ91D镁合金半固态组织的形成

半固态浆料的固相颗粒尺寸、形态和分布主要取决于熔化过程中液相的形成与演化过程。采用添加0.5%富铈混合稀土来改善AZ91D镁合金的铸态组织,研究在半固态等温热处理中的组织演变以及非枝晶组织制备与控制的机理。结果表明:稀土合金化处理可促进初生相在等温热处理过程中由枝晶向粒状晶的转变,可获得更加细小、均匀的球状固相颗粒,并且其粗化速度较慢。半固态等温热处理过程中,整个系统处于熔化和结晶的动态平衡,铸态组织中枝晶根部高溶质浓度区或系统的温度、浓度起伏是固相颗粒内液相形成的内在和外在条件。     

镁合金稀土镧化学转化工艺研究

镁合金稀土转化膜技术是近年来发展起来的一种环保型镁合金表面处理新技术。通过正交试验对压铸镁合金AZ91D稀土镧化学转化处理工艺进行了研究。利用扫描电子显微镜(SEM)、能量色散谱(EDS)和X射线光电子能谱(XPS)研究了膜层的表面形貌及其组成,并通过容量法对膜层在5%NaCl(pH=6.8~7.0)溶液中的耐腐蚀性能进行了研究。结果表明:La转化处理工艺能够在压铸镁合金AZ91D表面形成均匀、完整的转化膜;膜层主要由La2O3和MgO以及少量的Al2O3组成;La转化膜在浸泡初期的10 h内耐腐蚀性能与铬酸盐转化膜相当。     

微量Sr对AZ91D镁合金凝固过程的影响

利用XRD和DSC研究了微量Sr对AZ91D镁合金凝固过程的影响。结果表明：Sr的加入对合金的固相线温度影响不大，但能明显降低合金的液相线温度，从而减少结晶温度间隔。当Sr的添加量（质量分数，下同）分别为0.2%和0.5%时，合金中没有新相形成，合金中的Sr主要以固溶的形式存在。当Sr的添加量达到0.8%时，Sr与Al形成高熔点的Al4Sr相。     

AZ91D镁合金电偶腐蚀的研究

在自来水和3.5%NaCl溶液中测试了铸造AZ91D镁合金与铝合金、锌合金、Q235碳钢和Cu偶合后的电偶腐蚀行为,研究了腐蚀环境、偶接材料和阴阳极面积比(CAAR)对铸造AZ91D镁合金电偶腐蚀行为的影响。在电偶腐蚀过程中测量溶液的pH值以及电偶腐蚀电流;用失重法计算了铸造AZ91D镁合金的电偶腐蚀速率,利用SEM观察了AZ91D镁合金的腐蚀形貌,并对腐蚀产物进行XRD分析。结果表明,AZ91D镁合金在电偶腐蚀过程中会使溶液的pH值升高,并伴有以Mg(OH)₂为主的腐蚀产物的生成;溶液中Cl^-的存在会加速AZ91D镁合金的电偶腐蚀速率;低氢过电位金属Q235碳钢和Cu对AZ91D镁合金的电偶腐蚀加速效果明显高于中氢过电位金属铝合金和锌合金,偶接材料的极化性能对AZ91D镁合金的电偶腐蚀速率有较大影响。同时,大的阴阳极面积比会加速AZ91D镁合金的电偶腐蚀速率,且AZ91D镁合金的电偶腐蚀电流随阴阳极面积比的增大而呈线性增长趋势。     

Sr对AZ31B镁合金微观组织和腐蚀性能的影响

研究了Sr对AZ31B镁合金显微组织和腐蚀性能的影响。当AZ31B中加入的Sr含量大于0.3%时,合金组织中β相(Mg_(17)Al_(12))减少,分布更加弥散和均匀,并且在晶界上形成了断续网状分布的Al_4Sr相,合金组织明显细化。当AZ31B中加入的Sr含量达到0.9%时腐蚀速率下降为AZ31B的50%。但Sr含量达到1.2%时,晶界处出现共晶组织呈断续网状分布,合金晶粒再度增大。     

阻燃镁合金AZ91D-0．3％Be-Sr的研究

研究了镁合金AZ91D-0．3％Be-Sr的阻燃性及Sr对合金显微组织和力学性能的影响，结果表明，在镁合金AZ91D中加入0．3％Be。显著提高了阻燃性，使试验合金可在无覆盖和保护的条件下直接暴露在大气中熔炼。加入少量Sr可使镁合金AZ91D-0．3％Be-Sr的组织得到细化，力学性能显著提高。X射线衍射分析表明，阻燃机理是在合金液表面形成致密的MgO-BeO复合保护膜，阻止了镁合金的进一步氧化燃烧。     

表面纳米化对AZ91D镁合金渗铝行为的影响

镁合金表面渗铝是提高耐蚀性的一种有效方法。本研究将表面纳米化作为渗铝的预处理过程。采用高能喷丸对AZ91D镁合金进行表面纳米化处理,然后进行真空铝扩散得到渗铝层。用透射电镜(TEM)和扫描电镜(SEM)观察了渗铝层的形貌。结果表明,在对AZ91D镁合金表面高能喷丸后获得了100 nm的晶粒尺寸。在高能喷丸之后,渗铝层的深度比未高能喷丸的渗铝层厚。在440℃下扩散12 h后,渗铝层的深度增加到70μm。采用电化学方法对AZ91D镁合金的耐腐蚀性能进行了表征。结果表明,渗铝层明显降低了AZ91D镁合金的腐蚀速率。因此,高能喷丸强化有利于镁合金表面渗铝,提高镁合金的耐蚀性。     

Effect of cerium addition on microstructure and corrosion resistance of die cast AZ91 magnesium alloy

A novel AZ91 Ce containing magnesium alloy characterized by excellent corrosion resistance is fabricated by adding rare earth Ce (cerium) in the form of a Mg‐Ce master alloy. The metallographic investigation shows that Ce added to AZ91 can obviously decrease the size of β‐Mg₁₇Al₁₂ and forms Al₁₁Ce₃ intermetallic compounds in the shape of fine needles. The corrosion tests and electrochemical measurements indicate that the corrosion resistance of AZ91 Ce containing magnesium alloy is obviously higher than that of AZ91. Furthermore, increasing the content of Ce in the magnesium alloy can further enhance the corrosion resistance. X‐ray photoelectron spectroscopy (XPS) reveals that Ce can be incorporated into corrosion products in the form of CeO₂ in the course of corrosion. Based on the preliminary analysis, the addition of Ce can improve the corrosion resistance of AZ91 by decreasing the size of β‐Mg₁₇Al₁₂ and enhancing the protective effectiveness of corrosion products.     

Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings

The corrosion performance of WE43-T6 and AZ91D magnesium alloys with and without treatment by plasma electrolytic oxidation (PEO) was investigated by electrochemical measurements in 3.5 wt.% NaCl solution. For untreated WE43-T6 alloy, formation of a uniform corrosion layer (Mg(OH)₂) was accompanied by initial pits around magnesium-rare earth intermetallic compounds. The AZ91D alloy disclosed increased corrosion susceptibility, with localized corrosion around the β-phase, though the β-phase network phase acted as a barrier for corrosion progression. PEO treatment in alkaline phosphate electrolyte improved the corrosion resistance of WE43-T6 alloy only at the initial stages of immersion in the test solution. However, PEO-treated AZ91D alloy revealed a relatively high corrosion resistance for much increased immersion times, contrary to the relative corrosion resistances of the untreated alloys. The improved performance of the PEO-treated AZ91D alloy appears to be related to the formation of a more compact coating.     

Corrosion behaviour of AZ91D and AM50 magnesium alloys with Nd and Gd additions in humid environments

AM50 and AZ91D alloys modified with rare earths (RE) were evaluated under atmospheric conditions. Nd and Gd additions resulted in formation of Al₂RE and Al–Mn–RE compounds and reduction of the fraction of β-phase. According to surface potential maps, RE-containing intermetallics were more noble than the β-phase, but less than Al–Mn inclusions. As a result, the action of micro-galvanic couples depended on the added amount of RE and the initial alloy microstructure. Nd or Gd additions improved the corrosion resistance of the AM50 alloy by up to 43%, but had no significant effect on the corrosion resistance of the AZ91D alloy.     

Gd对AZ91D合金铸态组织及性能的影响

研究了稀土元素Gd对铸态AZ91D镁合金组织与性能的影响.试验结果表明,Gd元素对合金的组织有细化作用,加入了Gd元素后,合金中会形成Al2Gd相,适量添加Gd元素会提升合金的性能,当Gd的质量分数为1.0％时,合金的性能最好.     

AZ91D镁合金在北京地区的大气腐蚀行为研究

利用室外大气暴露试验，研究了AZ91D镁合金在北京地区的平均大气腐蚀速率，并与相同条件下A3钢的大气腐蚀情况进行了对比．采用扫描电镜(SEM)、X-射线衍射(XRD)、X射线能谱(EDS)以及红外光谱(IR)等多种现代表面分析技术对腐蚀产物的形貌、组成、结构等进行分析，与A3钢相比，AZ91D镁合金具有良好的耐大气腐蚀性能，具有局部腐蚀的特征。     

AZ91D镁合金电偶腐蚀的扫描Kelvin探针研究

利用扫描Kelvin探针技术(SKP)研究AZ91D镁合金与316L不锈钢偶接件在盐雾试验中电偶腐蚀规律。通过在中性盐雾试验不同周期的表面腐蚀形貌的观察和伏打电位分布图的测量结果分析表明,AZ91D镁合金电偶腐蚀效应与偶接阴阳极的伏打电位差密切相关,AZ91D镁合金与316L不锈钢偶接件存在较大的电位差(约为–1.28V),其电偶腐蚀效应非常显著。在盐雾试验初始阶段,腐蚀主要发生在偶接界面AZ91D镁合金一侧,该腐蚀区域的伏打电位增加幅度较大,而316L不锈钢受到保护没有发生明显腐蚀。随着盐雾试验时间的延长,AZ91D镁合金腐蚀加快,腐蚀产物覆盖区域不断扩大,24h盐雾试验后,偶接件的平均伏打单位差由原始的–1.29V增加到–1.53V,AZ91D镁合金的电偶腐蚀效应加大。由于AZ91D镁合金在盐雾中生成的腐蚀产物对基体具有一定的保护作用,当腐蚀产物不断增加并覆盖表面,偶接件的电位差减小导致AZ91D镁合金的电偶腐蚀效应降低。     

Gd对镁合金AZ81耐腐蚀性能的影响

为了改善镁合金的耐腐蚀性能,进一步拓宽镁合金的应用范围,利用静态腐蚀试验,辅以腐蚀速率计算和腐蚀形貌观察,研究了稀土元素Gd对镁合金AZ81在0.5%NaCl溶液中耐腐蚀性能的影响作用。结果表明,随着Gd含量的增加,镁合金AZ81的腐蚀速率总体上表现出先增加后降低的变化趋势;加入适量的Gd,可改善镁合金AZ81的耐腐蚀性能。     

Microstructure and mechanical properties of AZ81 magnesium alloy with Y and Nd elements

The effects of rare earth（RE） elements Y and Nd （w（Y）/w（Nd）=3：2） with total content of 1%-4% on microstructures and elevated temperature mechanical properties of AZ81 magnesium alloy were investigated. The results show that, proper content of rare earth elements makes the microstructures of AZ81 magnesium alloy refine obviously and the quantity offl-MglTAll2 phases reduce, and Al2Y and Al2Nd form. After solid solution treatment, with increasing content of rare earth elements, the tensile strength and elongation of the alloys （at room temperature, 150 ℃ and 250 ℃） increase first, then decrease. When the content of rare earth elements is up to 2%, the values of tensile strength at room temperature and 150 ℃ are up to their maxima simultaneously, 282 MPa and 212 MPa, respectively. Meanwhile, the values of elongation at room temperature and at elevated temperature are also up to their maxima, 13% and 15%, respectively.