铸态Mg-4Al-4Si-0.75Sb合金的显微组织与力学性能

采用重力铸造法制备Mg-4Al-4Si-0.75Sb(AS44-0.75Sb)(质量分数/%,下同)镁合金,研究铸态合金的显微组织和室温力学性能。结果表明:铸态AS44-0.75Sb合金主要由α-Mg基体、β-Mg17Al12相、Mg2Si相和Mg3Sb2相组成;加入0.75Sb后形成高熔点的Mg3Sb2相,显著改善了Mg2Si相的形貌,使粗大的骨骼状Mg2Si转变为相对细小的汉字状Mg2Si。铸态合金的硬度HV为65.9,屈服强度为136.4MPa,抗拉强度为172.3MPa,伸长率为3.3%;拉伸断裂形式为准解理脆性断裂。     

Si对AZ91D镁合金显微组织与力学性能的影响

利用光学金相显微镜OM和XRD分析了加入微量Si的AZ91D合金显微组织和相组成,测试了合金室温拉伸力学性能和硬度,利用SEM分析了合金拉伸断口形貌.结果表明,加入一定量Si后AZ91D合金组织中形成汉字状Mg2Si相,富集于固液界面前沿,阻碍α-Mg基体的自由长大,从而细化合金铸态组织;汉字状Mg2Si相的存在导致合金力学性能的降低;AZ91D合金室温拉伸断口是以解理断裂为主的脆性断裂,加入Si后,断裂常发生于α-Mg基体和汉字状Mg2Si相间的界面处.     

往复挤压Mg-4Al-2Si合金中Mg2Si颗粒形貌与分布

使用往复挤压细化Mg-4Al-2Si (AS42)合金组织,利用OM,SEM和TEM研究Mg2 Si颗粒形貌和分布特征.结果表明,铸态AS42合金中Mg2Si颗粒呈共晶汉字状和初生块状.共晶Mg2 Si经2道次往复挤压后全部破碎,且分布均匀.经6道次挤压后初生Mg2 Si颗粒全部破碎,细小的Mg2 Si颗粒已基本球化....     

Ca对AZ91显微组织及力学性能的影响

用气体保护法制备了含的合金,研究了对合金显微组织和力学性能的影响。结果表明,Ca能显著细化AZ91合金铸态和时效态的组织,改善了铸态组织形态。Ca的加入未形成新相或沉淀物,而是溶入β-Mg17Al12相中,并提高了β-Mg17Al12相的热稳定性。Ca的加入可以明显提高AZ91合金室温及高温下的拉伸性能。     

添加稀土元素Ce对AZ91D镁合金组织的影响

用金相显微镜、能量色散谱仪(EDS)和X射线衍射仪(XRD)研究了稀土元素Ce对AZ91D镁合金铸态组织的影响.结果表明,Ce对AZ91D镁合金具有明显的变质效果,加入0.4%Ce后,α-Mg树枝晶变化不明显,晶界上的β-Mg17Al12相呈断续网状分布;加入0.8%Ce后,合金晶界上的离异共晶β相基本上断裂成骨骼状,转变为颗粒状且分布比较均匀;加入1.2%稀土Ce后,枝晶变细,共晶β相完全变为颗粒相,弥散分布于晶界处.微结构分析发现,组织中出现了分布于晶界处的杆状Al10Ce2Mn7化合物.     

室温压缩AZ91镁合金显微组织及β-Mg17Al12相析出动力学

对常温压缩粗镁直接熔炼AZ91镁合金时效处理后的组织及β-Mg17Al12相析出动力学进行研究。结果表明:AZ91镁合金在常温压缩过程中出现大量的孪晶,为β-Mg17Al12相的析出提供了大量的形核基底;时效时β-Mg17Al12相优先在晶界、孪晶界析出,尤其易在孪晶与晶界、孪晶交接处析出并长大,且孪晶内析出的β-Mg17Al12相与α-Mg基体保持一定的位向关系;时效时间越长,析出的β-Mg17Al12相越多,温度越高,析出定量β-Mg17Al12相所需时间越短;结合实验数据,由JMAK方程计算得到AZ91镁合金析出β-Mg17Al12相激活能为23.8～37.9kJ/mol。     

Sr对AM50显微组织和力学性能的影响

研究了不同含量Sr对AM50合金微观组织和力学性能的影响.结果表明,添加Sr后,AM50合金铸体组织β-Mg17Al12变得细小,晶粒明显细化;Sr基本上与Al结合生成高熔点、高稳定的Al4Sr相,能够强化晶界并阻碍位错滑移,从而强化合金.适量的Sr明显提高了室温下合金的屈服强度和抗拉强度,而且不影响合金的延伸率,而过量的Sr会导致AM50延伸率和轻度的降低.含0.5%Sr的铸态合金可得到最高抗拉强度233MPa,屈服强度90MPa,延伸率16.3%的性能.     

Si-RE复合合金化对AZ31镁合金显微组织及力学性能的影响

采用Si-RE元素对AZ31镁合金进行复合微合金化,利用光学显微镜(OM)、扫描电镜(SEM)、X射线衍射仪(XRD)及拉伸实验仪等手段研究Si-RE元素对AZ31镁合金的微观组织和力学性能的影响规律。结果表明,Si-RE复合加入可以明显细化AZ31镁合金的铸态组织,β-Mg17Al12相也由网状分布变为颗粒状,同时生成了短棒状和针状的Mg2Si相及Al11RE3相;合金的综合力学性能显著改善,AZ31+0.4%Si+0.5%RE的强度和延伸率比原始AZ31镁合金分别提高了42MPa和3.9%,试样室温拉伸断口虽然是以解理为主的脆性断裂,但断口中出现了少量的韧窝,解理面也较小。     

宽幅AZ31B镁合金铸轧板显微组织和性能研究

通过金相观测、断口扫描和力学性能测试等实验,研究了宽幅AZ31B镁合金铸轧板的组织分布,及其对板带边裂和力学性能的影响。结果表明:AZ31B镁合金铸轧板的组织主要由α-Mg基体、析出相β-Mg17Al12相及α+β离异共晶组成,呈树枝晶形貌;β-Mg17Al12相在AZ31B镁合金铸轧板坯的表层及边部主要分布于晶界处且密度较大、构成网状,而在板坯的心部和中部呈球状弥散于α-Mg基体中,构成层片状;在板坯边部枝晶间低熔点的共晶相及晶界上的β-Mg17Al12相易成为裂纹源,并沿晶向外扩展,是铸轧过程中产生裂纹的主要原因;试样拉伸断口呈脆性解理断裂的特征,其力学性能呈明显的各向异性。     

形变热处理对Mg-4Al-1Si-1Gd合金组织及性能的影响

为改善Mg-Al-Si系(AS系)变形镁合金因第二相粗大、分布不均而性能较低的现状,在200℃下对挤压态Mg-4Al-1Si-1Gd合金分别进行不同时间(5 h、10 h、15 h)的等温时效和等通道挤压(ECAP)处理,并利用光学显微镜、扫描显微镜和拉伸实验分析其组织及拉伸性能.结果表明:随着等温时效时间的延长,晶粒尺寸增大,晶界处析出少量聚集的大尺寸Mg17 Al12相,时效10 h后合金的室温拉伸性能较优,但与挤压态相比强塑性明显降低.而形变时效提供的热变形能和应变累积量促进了动态再结晶的充分进行,晶粒尺寸由挤压态的10.68μm减小至2.20μm,Mg2 Si相和Si3 Gd5相碎化完全且分布更均匀,晶界处析出了大量颗粒状Mg17 Al12相,基面织构明显弱化,形成了新的非基面织构组分,抗拉强度、屈服强度和延伸率与挤压态相比分别提高了11.7％、33.7％和19.9％.经计算,细晶强化对屈服强度的贡献值为42.8 MPa,Orowan强化的贡献值为4.25 MPa.     

Microstructure and mechanical behaviour of semi-solid die-casting AZ91D magnesium alloy

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with semi-solid die-casting (SSDC) were characterized in as-cast conditions. The SSDC alloy exhibits a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg₁₇Al₁₂ intermetallic. High ultimate tensile strength and elongation have been achieved before fracture. Observations on the vertical-section microstructure of the fractured sample by scanning electron microscopy (SEM) show that the crack mainly originated from the brittle fracture of the eutectic phase causes the interface decohesion of the ductile Mg phase, making fracture a rather critical event. Before that, the deformation of ductile α-Mg phase in the matrix as well as the “pulling out” of primary α-Mg phase combines to provide the SSDC alloy a certain strain.     

Microstructure Characteristics of the Eutectics of Die Cast AM60B Magnesium Alloy

Under the cold-chamber high pressure die casting (HPDC) process, samples were produced with AM60B magnesium alloy to investigate the microstructure characteristics of the eutectics, especially focusing on the constitution, morphology and distribution of the eutectics over cross section of the castings. Attentions were also paid to study the effect of heat treatment on the eutectics in the die castings. Based on experimental analysis using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), it was determined that fully divorced eutectics consisting of α-Mg and β-Mg17Al12 appeared at the grain boundary of the primary α-Mg in the as-cast microstructure. Islands and networks of β-Mg17Al12 phase were observed in the central region of the castings, while the β-Mg17Al12 phase revealed a more dispersed and granular morphology on the surface layer. The two phases ratio β/α in the central region of the castings was approximately 10%, which was higher than that on the surface layer. Besides, the defect bands contained a higher percentage of the eutectics than the adjacent regions. After aging treatment (T6), only α-Mg phase was detected by XRD in the AM60B magnesium alloy, though a small amount of precipitated β-Mg17Al12 phase was observed at the grain boundary. In contrast to the microstructure of die cast AZ91D magnesium alloy under the same T6 heat treatment, no discontinuous precipitation of the β-Mg17Al12 phase was observed in AM60B magnesium alloy die castings.     

ECAP变形对Mg2Si增强耐热镁合金组织及性能的影响

为了提高镁合金的耐热性能,在Mg-Zn合金中加入Si,形成Mg-Zn-Si镁合金.采用ECAP工艺在变形温度为573 K和挤压路径为Bc条件下对Mg-Zn-Si镁合金进行不同道次的变形.运用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等手段对变形后的Mg-Zn-Si镁合金进行了组织表征,对变形后的合金进行了室温拉伸和高温蠕变等力学性能测试.结果表明:随着挤压道次增加,α-Mg基体、Mg Zn相及Mg2Si相均得到细化且分布趋于均匀.1道次挤压后部分基体α-Mg细化,4道次挤压后α-Mg的尺寸减小为5~10μm,且晶粒大小趋于均匀;2道次挤压后Mg2Si相枝晶在原位置破碎为颗粒状,6、8道次挤压后Mg_2Si相呈弥散分布.4道次挤压后合金的屈服强度和抗拉强度均提高120%,伸长率提高353%;8道次挤压后合金的抗拉强度和伸长率与4道次相比变化不大,但屈服强度进一步提高了19%.随着挤压道次增加,高温抗蠕变性能提高,8道次后高温稳态蠕变速率降低5倍.Mg2Si相细化机理为受剪切而机械碎断.     

电磁搅拌对富Ce混合稀土合金化AZ91D组织结构的影响

用光学显微镜、扫描电镜、电子耗散能谱和X射线衍射仪研究了电磁搅拌对富Ce混合稀土(MM)合金化AZ91D镁合金组织和结构的影响。结果表明,随富Ce混合稀土加入量的增加,初生相细化,β相(Mg17Al12)的相对量降低,尺寸减小,由连续网状向分离的球粒状转变。形成的Al11MM3化合物数量增多,由主要以短棒状在晶内和晶界分布向大的针状在晶界分布过渡,甚至在晶界成簇聚集。强电磁搅拌作用下,随等温搅拌温度降低,初生相尺寸变小,其形貌由球粒状加少量的玫瑰状向类球状和多边形演变;Al11MM3以类球状和细小的针状分布于晶粒和液相中。富Ce混合稀土可显著细化晶粒,改善β相的形貌和分布,降低体积分数;改变富Ce混合稀土含量可控制Al11MM3化合物的形貌和分布。电磁搅拌可同时使初生相和第二相Al11MM3细化和球粒化。     

Comparative studies on the microstructure evolution and fracture behavior between hot-rolled and as-cast Mg96ZnY3 alloys

Microstructure evolution and fracture mechanism of as-cast and hot-rolled Mg₉₆ZnY₃ alloys have been investigated in the present paper. Microstructure of the as-cast Mg₉₆ZnY₃ alloy consists of α-Mg dendrite and lamellar Mg₁₂YZn phase distributed mainly in interdendritic region. After a initial thickness reduction of ～ 62% through a series of rolling passes at 420 °C, the hot-rolled Mg₉₆ZnY₃ alloys were prepared by a final rolling reduction of 15% at 350 °C and 400 °C, respectively. The long period stacking order phase having a special orientation with α-Mg matrix in hot-rolled alloy was identified as 18R structure phase. Though highly dense dislocations were formed in the α-Mg matrix of the hot-rolled alloys, the parallel dislocations and sub-grain boundary were easily formed in the alloy during the recovery due to the relatively high working temperature of 400 °C. In the alloy rolled at 350 °C, the long period stacking order phase was traversed in the grain by the twins, which were induced by the relatively low working temperature. The morphology of dislocation and the influence of twin boundary may be the main reason for higher strength of the alloy rolled at 350 °C. While the tensile elongations for both as-cast and hot-rolled Mg₉₆ZnY₃ alloys are similar, their respective fracture mechanisms are different. The fracture along the phase interface of lamellar Mg₁₂YZn phase at grain boundary and cleavage fracture are considered as main fracture mechanisms for the as-cast alloy. The fracture caused by stress concentration at the phase interface between the matrix and Mg₁₂YZn phase in grain is the main fracture mechanism for the hot-rolled alloy.     

Microstructure and Mechanical Properties of Mg-Al-Ca-Nd Alloys Fabricated by Gravity Casting

The aims of this study are to investigate the effects of Nd addition in the Mg-Al-Ca alloys on microstructure and mechanical properties.Microstructure of as-cast Mg-5Al-3Ca alloy containing Nd consists ofα-Mg matrix, eutectic phase and Al-Nd rich intermetallic compound.As Nd addition was increased,α-Mg matrix morphology was changed from dendritic to equiaxed grains and average value of grain size was decreased.Nd addition to Mg-5Al-3Ca based alloys resulted in the formation of Al-Nd rich intermetallic compounds at grain boundary andα-Mg matrix grains.And these Al-Nd rich intermetallic compounds were dispersed homogeneously.In these alloys,two kinds of eutectic phases were observed,i.e.coarse irregular-shape structure at grain boundary and fine needle-shape structure in theα-Mg matrix grain.It is found that the ultimate strength showed the maximum value of 271 MPa at Mg-5Al-3Ca-2Nd alloy and elongation was decreased as the addition of Nd was increased.     

Effect of homogenization on the microstructure and mechanical properties of the repetitive-upsetting processed AZ91D alloy

The current research investigates the effect of homogenization on the microstructure and mechanical properties of the AZ91 D alloy processed by repetitive upsetting(RU). Results show that during RU processing, the initial large Mg_(17)Al_(12) particles in the as-cast specimen accelerate the dynamic recrystallization(DRX) due to the particle stimulating nucleation(PSN) mechanism. With the progress of RU,the inherent large strain breaks the large second phases into small fragments, which indicates the PSN gradually disappears, while the pinning effect becomes obvious. As for the homogenized specimen, a pre-heat treatment leads to the absence of Mg_(17)Al_(12) particles but a uniform distribution of Al atoms in the Mg alloy. Though the subsequent RU promotes the precipitation of Mg_(17)Al_(12) particles, the relatively small particle size and the uniform distribution are more favorable to act as obstacles for grain growth than contributors to PSN. Finally, a more homogeneous and refined microstructure is obtained in the specimen with prior homogenization than the as-cast one.     

Ageing Behavior of Mg-xAl-Zn with Variation of Al Content

The ageing behavior of Mg-xAl-Zn alloys with a variation of AI contents （x=3,5,7,9 and 11 wt pct） were investigated.Solid solution treatment and artificial ageing were carried out at 693 and 493 K,respectively. After the solid solution treatment,β-Mg-（17）Al-（12） was resolved to theα-Mg matrix in the specimens which contained Al up to 9 wt pct and partially resolved in the specimen of 11 wt pct Al.During ageing,theβ- Mg-（17）Al-（12） phase was precipitated at the grain boundary in the form of discontinuous precipitation and grown into the matrix.The volume fraction ofβ-phase was increased with the Al contents.Tensile strength increased with the volume fraction of precipitatedβ-phase,which was believed to strengthen the matrix.