Grain coarsening in semi-solid state and tensile mechanical properties of thixoformed AZ91D-RE

For thixoforming to be possible,the microstructure of the starting material must be non-dendritic,which can be obtained by the strain induced melt activation(SIMA)route.Based on the SIMA route,as-cast AZ91D alloy with the addition of yttrium was deformed by cyclic closed-die forging(CCDF).Microstructure evolution of CCDF formed AZ91D-RE alloy during partial remelting were investigated.Furthermore,the mechanical properties of thixoformed AZ91D-RE magnesium alloy components were also studied.The results showed that prolonged holding time resulted in grain coarsening and the improvement in degree of spheroidization.The coarsening behaviour of solid grains in the semi-solid state obeyed Ostwald ripening mechanism.The coarsening rate constant of CCDF formed AZ91D-RE during partial remelting was 324 um3/s at 550℃.The value of yield strength,ultimate tensile strength and elongation to fracture of four-pass CCDF formed AZ91D-RE magnesium alloy were 214.9,290.5 MPa and 14%,respectively.Then the four-pass CCDF formed alloys were used for thixoforming.After holding at 550℃ for 5 min,the values of yield strength,ultimate tensile strength and elongation to fracture of thixoformed component were 189.6 MPa,274.6 MPa and 12%,respectively.However,prolonged holding time led to remarkable decrease in mechanical properties of thixoformed components.     

Microstructure and mechanical properties of friction stir welding of AZ31B magnesium alloy added with cerium

The AZ31B magnesium alloy sheet added with 0.5 wt.% Ce was welded with friction stir welding(FSW).The microstructures and mechanical properties of the welded joint were investigated.The results showed that the microstructures in the weld nugget zone were uniform and with small equiaxed grains.The grains in the heat-affected zone and the thermo-mechanical affected zone were coarser than those in the base metal zone and the weld nugget zone.The ultimate tensile strength of AZ31B magnesium alloy added with 0.5...     

Effects of RE on Microstructures and Mechanical Properties of Hot-Extruded AZ31 Magnesium Alloy

Effects of rare earth (RE) additions on microstructure and mechanical properties of the wrought AZ31 magnesium alloy were investigated. The results show that, by adding 0.3%, 0.6% and 1.0% RE elements, the as-cast microstructure can be refined, and the as-cast alloys‘ elongation and tensile strength can be improved. After extrusion, the alloy with 0.3 % and 0.6% RE additions obtain a finer microstructure and the best mechanical properties, but the alloy with 1.0% RE addition has the coarse A1-RE compound particles in grain boundaries which decreased elongation and tensile properties. Usually, Rare earth (RE) elements were used to improve the creep properties of aluminium-containing magnesium pressure die cast alloys at elevated temperatures. In this paper, it is also found that the high temperature strength of extruded materials can be increased by RE elements additions.     

铈镧混合稀土对AZ91D压铸镁合金显微组织和蠕变性能的影响

研究了不同含量Ce/La(x=0,0.1,0.5,1.0)对AZ91D镁合金显微组织及蠕变性能的影响。通过X射线衍射(XRD)、金相显微镜(OM)、扫描电镜(SEM)及能谱分析(EDX)观察与分析表明,压铸AZ91D镁合金中添加Ce/La后,除了α-Mg,β-Mg17Al12相之外,还生成了新的稀土化合物Al11RE3(RE=Ce/La)化合物,并且细化了合金显微组织、提高了合金室温和高温力学性能。生成的Al11RE3(RE=Ce/La)高温热稳定相使AZ91D+xCe/La(x=0,0.1,0.5,1.0)合金在150℃,50MPa下的蠕变抗力优于AZ91D镁合金,1%Ce/La的合金与AZ91D相比,蠕变延伸率低了0.2%,最小蠕变速率从2.30×10-8s-1降低到2.02×10-8s-1。蠕变试样的微观组织结构分析表明:AZ91D合金的蠕变机制主要以晶界滑移方式为主,Al11RE3(RE=Ce/La)热稳定相在晶界处延缓和阻碍了晶界断裂的过程。     

Sm对AZ61合金力学性能的影响

采用XRD、OM和SEM手段研究了Sm(0～2.0%,质量分数,下同)对AZ61合金显微组织和力学性能的影响。结果表明,Sm优先与A1形成高熔点Al2Sm弥散相,细化合金显微组织,提高时效态合金室温抗拉强度和屈服强度。在研究范围内,加入1.5%Sm的合金力学性能最优。Sm可以通过细晶强化、Ramakrishnan强化和Gypen固溶强化机制对AZ61合金强度产生影响。     

铈对铸造镁合金AZ91D显微组织与力学性能的影响

利用光学金相显微镜OM和XRD分析了分别加入0．1％，0．3％，0．5％，0．7％和1．0％Ce的AZ91D合金显微组织和相组成，测试了室温力学性能和硬度。结果表明，加入一定量Ce后的AZ91D合金形成杆状化合物Al4Ce，被推移到生长界面，阻碍枝晶的自由生长，从而细化合金显微组织；Ce能提高AZ91D合金室温抗拉强度和硬度，而对其屈服强度和延伸率影响不大；加入0．7％Ce的AZ91D合金晶粒细化效果好，其综合力学性能比较理想。     

High Cycle Fatigue Properties of Die-Cast Magnesium Alloy AZ91D with Addition of Different Concentrations of Cerium

Recently , magnesiumalloy has become a very at-tractive material because of its lowdensity ,high spe-cific strength, and easy recyclability compared withother metals[1].As alight structural material ,recentlythere has been a significantincreaseinthe use of mag-nesiumalloy for automobiles ,aerospace components ,computers , mobile phones , and household equip-ments[2 ,3]. Mg-Al-Zn (AZ91D) alloyis the most wide-ly used die-cast magnesiumalloyin automotive applica-tions due toits excellent casta…     

Pb对AZ91镁合金铸态显微组织及力学性能的影响

采用扫描电镜观察、电子探针分析及拉伸性能测试等方法研究了Pb的添加对AZ91合金显微组织和力学性能的影响。结果表明,Pb能够细化AZ91镁合金中的d—Mg和β-Mgl7Al12的晶粒,抑制二次B的析出,且Pb可改善β-Mgl7Al12相形态和分布。Pb通过细晶强化增加了合金强度和硬度,使合金的断裂机制从脆性解理断裂转变为准解理断裂。     

Effect of rare-earth element additions on high-temperature mechanical properties of AZ91 magnesium alloy

The present article focuses on the high-temperature mechanical properties of the magnesium alloy AZ91. The addition of rare-earth (RE) elements up to 2 wt pct improves both yield and tensile strengths at 140 °C by replacing the Mg₁₇Al₁₂ phase with RE-containing intermetallic compounds. This intermetallic phase is thermally and metallurgically stable and is expected to boost the grain-boundary strengthening. It also increases the resistance of grain boundaries to flow at high temperatures. Further increases of RE additions reduce strength and ductility due to growth of the Al₁₁RE₃ brittle phase, which has sharp edges. Still, at a 3 wt pct RE addition, the strength of the alloy at high temperatures is more than that of AZ91.     

稀土元素铈对镁合金AZ91D显微组织和力学性能的影响

利用光学显微镜、X射线衍射和扫描电镜等分析研究含铈镁合金AZ91D（0．25％Ce、0．7％Ce、0．95％Ce）的显微组织,并对其力学性能进行了测试,同时与不含铈镁合金AZ91D进行了比较。结果表明,加入一定量Ce后的镁合金AZ91D形成杆状化合物Al4Ce,被推移到生长界面,阻碍枝晶的自由生长,从而细化合金显微组织;Ce能提高镁合金AZ91D抗拉强度和硬度,而对其屈服强度和延伸率影响不大;加入0．7％Ce的AZ91D镁合金晶粒细化效果和综合力学性能比较理想。     

Effect of Sb on the microstructure and mechanical properties of AZ91 magnesium alloy

Effects of Sb addition on the microstructure, mechanical properties, and fracture behaviors of AZ91 magnesium alloy, as well as the sensitivity to section thickness of the structure and mechanical properties, have been studied. The results show that when Sb is added into the AZ91 alloy, the grain is refined, the Mg₁₇Al₁₂ phase is refined and granulated, and a new Mg₃Sb₂ phase is formed and becomes coarse needle-shaped as Sb content increases. The room-temperature tensile strength, elongation, and impact toughness increase first, and then decrease with increasing Sb content. The study on sensitivity to section thickness shows that, when composition is constant, the room-temperature tensile strength and elongation increase with the reduction of section thickness; when section thickness is constant, the room-temperature tensile strength and elongation increase first, and then decrease with increasing Sb content. Additionally, the Sb addition improves the tensile strength of the AZ91 alloy at 100 °C and 150 °C. The room-temperature tensile and impact fractographs of the AZ91 alloy show intergranular fracture. With increasing Sb content, the tearing deformation zones on the both fractographs enlarge at first, and then diminish, which is consistent with the change of tensile strength, elongation, and impact toughness increasing first, and then reducing with increasing Sb content.     

Effect of Sb on the microstructure and mechanical properties of AZ91 magnesium alloy

Effects of Sb addition on the microstructure, mechanical properties, and fracture behaviors of AZ91 magnesium alloy, as well as the sensitivity to section thickness of the structure and mechanical properties, have been studied. The results show that when Sb is added into the AZ91 alloy, the grain is refined, the Mg₁₇Al₁₂ phase is refined and granulated, and a new Mg₃Sb₂ phase is formed and becomes coarse needle-shaped as Sb content increases. The room-temperature tensile strength, elongation, and impact toughness increase first, and then decrease with increasing Sb content. The study on sensitivity to section thickness shows that, when composition is constant, the room-temperature tensile strength and elongation increase with the reduction of section thickness; when section thickness is constant, the room-temperature tensile strength and elongation increase first, and then decrease with increasing Sb content. Additionally, the Sb addition improves the tensile strength of the AZ91 alloy at 100°C and 150°C. The room-temperature tensile and impact fractographs of the AZ91 alloy show intergranular fracture. With increasing Sb content, the tearing deformation zones on the both fractographs enlarge at first, and then diminish, which is consistent with the change of tensile strength, elongation, and impact toughness increasing first, and then reducing with increasing Sb content.     

稀土Nd对AZ 31镁合金铸态组织和力学性能的影响

利用气氛电阻炉制备了AZ 31-xNd合金(x=0.05%,0.1%,0.2%,0.4%,0.6%),采用光学显微镜(OM)、扫描电子显微镜(SEM)和电子能谱分析仪(EDS)对不同Nd含量的实验合金进行了显微组织观察和分析,结果发现,Nd在AZ 31-xNd合金中形成了Al_3Nd和Mg_(12)Nd相,这些含Nd相导致AZ 31镁合金在凝固过程中的晶粒细化,从而提高了AZ 31镁合金的铸态室温力学性能,随着Nd含量的增加,合金的铸态室温抗拉强度极限和延伸率均先升高后降低.     

Modification of AZ91?Mg Alloys for High Temperature Applications

The good specific strength and specific modulus of magnesium alloys had drawn the attention of the automotive manufacturers for use in fuel efficient vehicles. Among the cast magnesium alloys, AZ91 (Mg?C9Al?C1Zn) is the most sought alloy because of its good casting properties. However, this alloy loses its strength and creep resistance properties above 120?°C due to softening of the ?? phase (Mg₁₇Al₁₂). Hence, this alloy cannot be used for making heavier engine components (power train), which require the thermal stability up to about 250?°C. The paper discusses the approach of modifying the AZ91 alloy by minor alloying additions to improve the high temperature withstanding capability without significantly affecting its casting properties. Additions of Ca to AZ91 alloy to the levels of about 0.4?wt% increased the ambient and high temperature strength of the base alloy. Additions of other minor alloying elements such as Sb, Pb, rare earths etc. can also increase the high temperature capability of the AZ91 by further modifying the ?? phase structure. The paper overviews the work carried out by the authors on the role of different alloying additions on the microstructure and mechanical properties of AZ91 magnesium alloys.     

Correlation of abrasive wear with microstructure and mechanical properties of pressure die-cast aluminum hard-particle composite

Aluminum hard particle composites were synthesized by the solidification processing technique and the composite melt was solidified using gravity and pressure die castings. An aluminum-silicon alloy (A 332.1) has been used as the matrix and silicon carbide particles (quantity: 10 wt pct, and size: 50 to 80 μm) have been used as reinforcement for synthesis of the composite. The microstructure of the pressure die cast composite is found to be finer than those of the gravity cast ones. Additionally, the distribution of SiC particles in the Al alloy matrix is found to be more uniform in the pressure die-cast composites compared to the gravity die-cast ones. The mechanical properties such as ultimate tensile strength, hardness, and ductility are observed to be superior in the case of pressure die-cast composites compared to the gravity-cast one. The two-body abrasive wear resistance of the Al-composite is also noted to be greater in the pressure die-cast composite than in the gravity-cast one. The effects of injection pressure on the mechanical properties and wear resistance of the pressure die-cast composites are examined. It is observed that the wear resistance (inverse of wear rate), hardness, and strength of the Al-SiC composites increase with the increase in injection pressure during pressure die casting. This may be due to the finer microstructure, the absence of casting defects, and the stronger interfacial bonding between the matrix and hard dispersoid in pressure die-cast composites. The wear rate of the alloys and composites is studied as a function of their hardness, strength, and Young’s modulus. It is noted that the wear rate is primarily controlled by hardness even though other mechanical properties influence the wear behavior of the materials to some extent. An attempt is made to establish an empirical relation to correlate the wear rate of material with the mechanical properties such as hardness, ultimate tensile strength, and elongation.     

Nitride Nanoparticle Addition to Beneficially Reinforce Hybrid Magnesium Alloys

This study is aimed at understanding the function of two nitride nanoparticles regarding altering the mechanical properties of hybrid magnesium alloys in relation to nanoparticle-matrix reactivity. Nitride nanoparticles were selected for reinforcement purposes due to the affinity between magnesium and nitrogen (in parallel with the well-known magnesium-oxygen affinity). AZ91/ZK60A and AZ31/AZ91 hybrid magnesium alloys were reinforced with AlN and Si₃N₄ nanoparticles (respectively) using solidification processing followed by hot extrusion. Each nitride nanocomposite exhibited higher tensile strength than the corresponding monolithic hybrid alloy. However, AZ91/ZK60A/AlN exhibited slightly lower tensile ductility than AZ91/ZK60A, while AZ31/AZ91/Si₃N₄ exhibited higher tensile ductility than AZ31/AZ91. The formation of high strain zones (HSZs) (from particle surfaces inclusive) during tensile deformation as a significant mechanism supporting ductility enhancement was addressed. AZ91/ZK60A/AlN exhibited lower and higher compressive strength and ductility (respectively) compared to AZ91/ZK60A, while AZ31/AZ91/Si₃N₄ exhibited higher and unchanged compressive strength and ductility (respectively) compared to AZ31/AZ91. Nanograin formation (recrystallization) during room temperature compressive deformation (as a toughening mechanism) in relation to nanoparticle-stimulated nucleation (NSN) ability was also discussed. The beneficial (as well as comparative) effects of the respective nitride nanoparticle on each hybrid alloy are studied in this article.     

Effects of precipitates on grain size and mechanical properties of AZ31-x% Nd magnesium alloy

The effects of precipitates on grain size and mechanical properties of as-cast AZ3 1-x%Nd magnesium alloy were investi- gated, and the affecting mechanism was also discussed. The results indicated that Al2Nd phase, AlllNd3 phase and a few AI-Mn-Nd-Fe phase were furmed when adding 0.38 wt.%-1.46 wt.% Nd into AZ31 melt, coarse AI2Nd transformed into Al11Nd3 gradually with the increasing of Nd content. Due to structure and size transformation and content increasing of AI-Nd phase, the grain size of AZ31-x% Nd alloy increased firstly, and then decreased with the increment of Nd content. After reaching a minimum value, once again it rose up, provided that Nd content was further increased. The tensile property reached its optimal value when the adding amount of Nd content was 1.05 wl.%, however, adding excessive amount of Nd deteriorated both ultimate strength and elongation ofAZ31 alloy.     

细晶Mg-Al-Ti-C中间合金的制备及其对AZ91D组织和性能的影响

采用两步法+铜模喷铸的方式制备具有颗粒增强和细晶强化作用的Mg-Al-Ti-C中间合金,并研究了其对AZ91D组织和性能的影响。通过OM、SEM、XRD、硬度及拉伸试验对中间合金和AZ91D的组织形貌和力学性能进行分析。结果表明,两步法制备得到了Mg-Al-Ti-C中间合金,经过铜模喷铸后,TiC颗粒尺寸大幅度减小,且弥散分布于Mg-Al-Ti-C中间合金中;用该细晶Mg-Al-Ti-C中间合金处理AZ91D,后者的抗拉强度提高了19.8%,硬度提高了64.9%;TiC颗粒既有颗粒增强作用,也能作为AZ91D凝固时的异质形核核心,起到细晶强化作用。     

Corrosion and mechanical properties of AM50 magnesium alloy after being modified by 1 wt.% rare earth element gadolinium

In order to improve the corrosion and mechanical properties of AM50 magnesium alloy, 1 wt.% Gd was used to modify the AM50 magnesium alloy. The microstructure, corrosion and mechanical properties were evaluated by X-ray diffraction(XRD), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), electrochemical and mechanical stretch methods. The results indicated that β-Mg17Al12 phase decreased and Al2Gd3 and Al0.4GdMn1.6 phase existed after Gd addition. Because of the Gd addition, the grain of AM50 magnesium alloy was refined significantly, which improved the tensile strength of AM50 magnesium alloy. The decreasing of β phase improved the corrosion resistance of the magnesium alloy. The fracture mechanism of the Gd modified AM50 magnesium alloy was quasi-cleavage fracture. The corrosion residual strength(CRS) of AM50 magnesium alloy was improved after 1 wt.% Gd addition.     

热处理对含Y元素AZ31镁合金板材组织和性能的影响

对含Y元素AZ31镁合金板材进行退火处理后的组织和性能进行了研究.结果表明:随着退火温度的升高,镁合金晶粒尺寸逐渐增大,力学性能略有提高然后降低;退火时间对镁合金晶粒尺寸影响不大;在300℃下退火1 h后板材性能达到最佳,抗拉强度为255 MPa,屈服强度为170 MPa,延伸率为24%;经过热处理后镁合金断裂方式为准解理断裂和韧性断裂的复合形式.