Influence of solution treatment on microstructure and properties of in-situ Mg_2Si/AZ91D composites

The influence of solution treatment on the microstructure and properties of Mg2Si/AZ91D composites fabricated from Mg-SiO2 system via in-situ processing method was investigated.The results show that coarse Chinese script shape Mg2Si phases can be formed by adding SiO2 into AZ91D magnesium alloy with Si content up to 1.5% of the alloy melt.During solution treatment,the morphology and distribution of the coarse Chinese script shape Mg2Si phases are modified.Meanwhile,the β-Mg17Al12 phase is dissolved into the magnesium matrix.With increasing holding time,the coarse Mg2Si phases tend to dissolve,break and spheroidize.After solution treatment at 420 ℃ for 16 h,Mg2Si phases become the finest and relatively well-distributed phase.The tensile strength and elongation are increased by 14.9% and 38.9%,respectively.It is believed that the Mg2Si phases continuously dissolve and break,and finally the spheroidized Mg2Si particles are obtained due to the interface tension of Mg2Si/Mg interface.

Hot extrusion of SiC_p/AZ91 Mg matrix composites

SiC particles reinforced AZ91 Mg matrix composites （SiCp/AZ91） with SiC volume fractions of 5%, 10% and 15% were fabricated by stir casting. After T4 treatment, these composites were extruded at 350 °C with an extrusion ratio of 12：1. In the as-cast composite, particles segregated at a microscopic scale within the intergranular regions. Hot extrusion almost eliminated this particle aggregation and improved the particle distribution of the composites. In addition, extrusion refined the grains of matrix. The results show that hot extrusion significantly improves the mechanical properties of the composites. In the as-extruded composite, with the increase of SiCp contents, the grain size of the extruded composites decreases, the strength and elastic modulus increase but the elongation decreases.

Electrodeposition and corrosion resistance of Ni-P-TiN composite coating on AZ91D magnesium alloy

In order to improve the corrosion resistance and microhardness of AZ91D magnesium alloy, TiN nanoparticles were addedto fabricate Ni-P-TiN composite coating by electrodeposition. The surface, cross-section morphology and composition wereexamined using SEM, EDS and XRD, and the corrosion resistance was checked by electrochemical technology. The results indicatethat TiN nanoparticles were doped successfully in the Ni-P matrix after a series of complex pretreatments including activation, zincimmersion and pre-electroplating, which enhances the stability of magnesium alloy in electrolyte and the adhesion betweenmagnesium alloy and composite coating. The microhardness of the Ni-P coating increases dramatically by adding TiN nanoparticlesand subsequent heat treatment. The corrosion experimental results indicate that the corrosion resistance of Ni-P-TiN compositecoating is much higher than that of uncoated AZ91D magnesium alloy and similar with Ni-P coating in short immersion time.However, TiN nanoparticles play a significant role in long-term corrosion resistance of composite coatings.

Compressive Creep Behavior of TiC/AZ91D Magnesium-matrix Composites with Interpenetrating Networks

The 42.1 vol. pct TiC/AZ91D magnesium-matrix composites with interpenetrating networks were fabricated by in-situ reactive infiltration process. The compressive creep behavior of as-synthesized composites was investigated at temperature ranging from 673 to 723 K under loads of 95-108 MPa. For a comparative purpose,the creep behavior of the monolithic matrix alloy AZ91D was also conducted under loads of 15-55 MPa at 548-598 K. The creep mechanisms were theoretically analyzed based on the power-law relation. The results showed that the creep rates of both TiC/AZ91D composites and AZ91D alloy increase with increasing the temperature and load. The TiC/AZ91D composites possess superior creep resistance as compared with the AZ91D alloy. At deformation temperature below 573 K, the stress exponent n of AZ91D alloy approaches theoretical value of 5, which suggests that the creep process is controlled by dislocation climb. At 598 K, the stress exponentof AZ91D is close to 3, in which viscous non-basal slip deformation plays a key role in the process of creep deformation. However, the case differs from that of AZ91D alloy when the stress exponent n of TiC/AZ91D composites exceeds 9, which shows that there exists threshold stress in the creep process of the composites, similar to other types of composites. The average activation energies for the creep of the AZ91D alloy and TiC/AZ91D composites were calculated to be 144 and 152 k J/mol, respectively. The existence of threshold stress in the creep process of the composites leads to an increase in activation energy for creep.

纳米SiCp增强AZ91D复合材料的摩擦磨损性能

采用超声波分散法制备了纳米SiCp增强AZ91D复合材料,研究了基体及复合材料在干滑动摩擦情况下的摩擦磨损性能。结果表明,与基体镁合金相比,复合材料的硬度、耐磨性都有显著提高。随着SiCp含量的增加,复合材料的耐磨性也逐渐增强。用激光扫描显微镜对试样的磨痕进行了分析,发现AZ91D的磨损机制以严重的粘着磨损即擦伤磨损为主,而复合材料的磨损机制以磨粒磨损为主,纳米SiCp的加入改变了材料的磨损机制。

Mechanical and wear behaviour of AZ91D magnesium matrix hybrid composite reinforced with boron carbide and graphite

In this experimental study,magnesium(AZ91D)based boron carbide(B4C)and graphite(Gr)particle reinforced hybrid composite materials were manufactured by stir casting.The tribological and mechanical properties of these composite materials were investigated.The results of the tests revealed that the graphite reinforced hybrid composites exhibited a lower wear loss compared to the unreinforced AZ91D alloy and AZ91D–B4C composites.It was found that with an increase in the B4C content,the wear resistance increased monotonically with hardness and ultimate tensile strength decreased.This study revealed that the addition of both a hard reinforcement(e.g.,B4C)and soft reinforcement(e.g.,graphite)significantly improves the wear resistance of magnesium composites.These entire results designate that the hybrid magnesium composites can be considered as an excellent material where high strength,ultimate tensile strength and wear-resistant components are of major importance,primarily in the aerospace and automotive engineering sectors.

Microstructure of Cu-based Amorphous Composite Coatings on AZ91D Magnesium Alloy by Laser Cladding

To improve the sliding wear resistance of AZ91D magnesium alloy, Cu-based amorphous composite coatings made of CuaTTi34Zr11Nis and Cu47Ti34Zr11Ni8＋20 wt pct SiC powders were fabricated on AZ91D magnesium alloy by laser cladding, respectively. SEM （scanning electron microscopy）, EDS （energy dispersive X-ray spectroscopy）, XRD （X-ray diffraction） and TEM （transmission electron microscopy） techniques were employed to study the phases of the coatings. The results show that the coatings mainly consist of amorphous phase and different intermetallic compounds. The reason of formation of amorphous phase and the function of SiC particles were explained in details.

Influence of chemical composition on corrosion resistance of AZ91D magnesium alloy ingots

The influence of chemical composition on corrosion resistance of AZ91D magnesium alloys ingots has been investigated. Mass loss method was applied to evaluate the corrosion resistance of AZ91D alloys and the data were analyzed by multiple regression. The results show that the corrosion resistance of this alloy can be improved by increasing Al, Zn and Mn in a certain degree, and will drop with increasing Si and heavy metals (Fe, Cu, Ni). It is found that ingots received from company F should be listed into unusable materials in terms of the corrosion resistance, while among the five suppliers, the only local company E supplied excellent AZ91D magnesium alloy ingots with the best corrosion resistance.

Effects of in-situ ZrB_2 particle on grain refinement of ZrB_2/AZ91D magnesium matrix composite

In-situ ZrB2/AZ91D magnesium matrix composite was successfully synthesized with AI/K2ZrF6＋NH4BF4 by means of Direct Melt Reaction. The fabricated ZrB2/AZ91D magnesium matrix composite through direct melt mixing method was investigated. Results from X-ray diffraction （XRD） and energy dispersive X-ray spectroscopy （EDS） confirmed the existence of ZrB2 particles in the AZ91D alloy, and most ZrB2 particles were in the size range of just several microns, some even to 100 nm. The cast specimens were studied through corrosion testing and heat treatment. The average grain size of AZ91D decreased markedly from about 250 pm to 50 IJm. In addition, the shape and size of the ,β-MglTAI12 phase as well as the morphologies of primary a-Mg in the magnesium matrix composite were greatly changed. The network structure of the β-MglTAI12 phase was broken into small blocks and the size of a-Mg decreased significantly.

Friction Stir Processing of Thixoformed AZ91D Magnesium Alloy and Fabrication of Surface Composite Reinforced by SiC_ps

The microstructural evolution characteristics of the thermomechanically affected zone （TMAZ） alloy during friction stir processing （FSP） of thixoformed （TF） AZ91D alloy were investigated. Simultaneously, a surface composite layer reinforced by SiC particles （SiCps） was prepared on the alloy by FSP and the corresponding tribological properties were examined. The experimental results indicate that dynamic recrystallization and mechanical separation （including splitting and fracture of the primary grains） are the main mechanisms of grain refinement for the TMAZ. A composite surface reinforced by uniformly distributed SiCps was prepared on the alloy. Compared with the corresponding permanent mould casting alloy and the TF alloy without composite surface, the TF alloy with composite surface has the highest wear resistance and lowest friction coefficient.

SiC_p/AZ91D复合材料真空压力浸渗制备工艺及微观组织的研究

采用真空压力浸渗装置制备SiCp/AZ91D复合材料。对制备工艺进行了改进,提出了以SiC颗粒覆盖法保护镁合金熔体的措施,可以有效解决熔剂覆盖法易造成的熔剂夹杂问题;在压力0.4MPa、浸渗温度700℃、保压5min的条件下,制备SiC单一颗粒尺寸为5μm、体积分数为44.7%的SiCp/Mg复合材料;并且成功制备32μmSiC单一颗粒体积分数为56.4%的SiCp/AZ91D复合材料。经过光镜、扫描电镜和X射线衍射仪分析表明,采用SiC颗粒覆盖法制备SiCp/AZ91D复合材料组织致密、无明显孔洞及夹杂等铸造缺陷,有新相Mg2Si生成。

热处理对SiC_p/AZ91D复合材料组织和力学性能的影响

在无溶剂熔炼的条件下,采用挤压铸造法制备了SiCp/AZ91D复合材料,并研究了热处理工艺对SiCp/AZ91D复合材料组织和力学性能的影响。结果表明:经热处理后,SiCp/AZ91D复合材料的晶粒明显细化,平均尺寸在100μm以下;显微组织致密,无明显缩孔及夹杂等铸造缺陷,拉伸断口表现为韧脆混合断裂特征,主要由解理台阶、撕裂棱和小尺寸的韧窝组成。此外,其力学性能较未热处理的SiCp/AZ91D复合材料和AZ91D合金均有大幅提高,抗拉强度和伸长率分别提高了46.7%、147.8%和61.2%、88.1%。

超声辅助机械搅拌法制备SiC_P/AZ91D复合材料的研究

采用超声辅助机械搅拌的方法制备了Si CP/AZ91D复合材料,研究了超声输出功率和处理时间对其凝固组织及拉伸力学性能的影响。结果表明,通过控制超声参数可以有效改善Si C颗粒在基体中的分布,并提高其力学性能。随着超声输出功率和处理时间的增加,Si C颗粒在基体中呈现先分散均匀后又团聚的趋势,同时力学性能先增加后降低,在700 W和30 s时复合材料的组织和力学性能均达到最佳。施加超声处理使复合材料的断口由脆性断裂为主转化为部分韧性断裂的混合断裂。

纳米SiC对AZ91D镁合金组织和性能的影响

为了研究不同SiC_(p)含量对AZ91D镁合金的强化机理,采用扫描电子显微镜测定了各相的微观结构组织形貌和分布特征。同时,结合X射线衍射分析结果,探究其强化机制。结果表明,纳米SiC_(p)加入使基体组织变为柱状晶和树枝晶。纳米SiC_(p)的加入可以显著改善基体结构。在本次实验中,纳米SiC_(p)的含量与晶粒尺寸成反比。当纳米SiC_(p)质量分数为0.5%时,AZ91D镁合金的晶粒细化效果特别显著,此时晶粒尺寸达到103.05μm。适量的纳米SiC_(p)可以改善AZ91D镁合金的力学性能,但抗拉强度随着增强体含量先增加后减少。当增强体含量为0.5%时,拉伸强度达到最大值。

Effects of SiC_(p) on microstructures of semi-solid extruded AZ91D magnesium alloys in recrystallization process

AZ91 D and 8.5 vol.%SiC_(p)/AZ91 D magnesium matrix composites were fabricated by a semi-solid extruded processing method,and treated with solution and aging heat treatment.The effects of SiC_(p)on the microstructures of the semi-solid extruded AZ91 D magnesium alloy during recrystallization were studied by observing and analyzing the microstructure evolution during extrusions and heat treatments.The results show that the addition of SiC_(p)inhibits the dynamic recrystallization of AZ91 D during the semi-solid extrusion with only 26% of the volume fraction of recrystallization.Furthermore,the addition of SiC_(p)refines the sizes of grains and second phases,and upgrades the volume fraction of second phase.After solution and aging treatment,the recrystallization continues,and the addition of SiC_(p)promotes the recrystallization and the recrystallized microstructure is much more stable.Meanwhile,the sizes of grains and second phases continue to be refined,and the volume fraction of second phases continues to increase.

Preparation and characterization of the micro-arc oxidation composite coatings on magnesium alloys

The magnesium alloys attract the light-weight manufacture due to its high strength to weight ratio,however the poor corrosion resistance limits the application in automobile industry.The Micro-arc Composite Ceramic(MCC)coatings on AZ91D magnesium alloys were prepared by Micro-arc Oxidation(MAO)and electrophoresis technologies.The microstructure,corrosion resistance,abrasion resistance,stone impact resistance and adhesion of MCC coatings were studied respectively.The cross section morphologies showed that the outer organic coating was filled into the hole on surface of MAO coating,and it acted as a shelter against corrosive products.The copper-accelerated acetic acid salt spray Test,abrasion resistance test,stone impact resistance test,thermal shock resistance test and adhesion test were used to evaluate the protective characterization by the third testing organization which approved by GM.The test results showed the composite coatings meet all the requirements.The MCC coating on Mg presents excellent properties,and it is a promising surface treatment technology on magnesium alloys for production vehicles.

Preparation and performance of coating on rare-earth compounds-immersed magnesium alloy by micro-arc oxidation

A composite ceramic coating containing Y2O3-ZrO2-MgO（YSZ-MgO） was prepared on AZ91D magnesium alloy,which was immersed in Y（NO3）3 aqueous solution as pretreatment,by micro-arc oxidation（MAO） process.The morphology,elemental and phase compositions,corrosion behavior and thermal stability of the coatings were studied by SEM,EDX,XRD,electrochemical corrosion test,high temperature oxidation and thermal shock test.The results show that the coating mainly consists of ZrO2,Y2O3,MgO,Mg2SiO4,and MgF2.Among these compounds,Y2O3 accounts for 26.7% of（Y2O3 ＋ ZrO2）.The thickness of YSZ-MgO coating is smaller than that of ZrO2-MgO coating,but its compactness and surface roughness are better than those of ZrO2-MgO coating.YSZ-MgO coating has a good corrosion resistance,and its corrosion rate in 5% NaCl aqueous solution is lower than that of ZrO2-MgO and only about 8.5% of that of AZ91D magnesium alloy.After oxidation at 410 °C,the mass gain of AZ91D magnesium alloy presents a linear increase with the oxidation time.The YSZ-MgO coating and ZrO2-MgO coating can remarkably decrease the oxidation mass gain.The oxidation mass gain of YSZ-MgO coating is lower than that of ZrO2-MgO coating,especially during a long oxidation period.The thermal shock resistance of YSZ-MgO coating is superior to ZrO2-MgO coating.

外场对SiCp/AZ91D镁基复合材料组织和性能的影响

在常规条件、电磁模拟微重力场、脉冲电流场和"电磁模拟微重力+脉冲电流"复合场四种外场条件下制备出SiCp含量5wt%的SiCp/AZ91D镁基复合材料,分析测试结果表明:在常规条件下,样品晶粒粗大,β-Mg_(2)Al;相呈现出无规律的网格状结构,新生相Mg_(2)Si以颗粒状的初生相和树枝状的共晶相存在;在电磁模拟微重力场条件下,β-Mg_(2)Al;相细化为零散分布的短棒状和颗粒状,Mg_(2)Si全部转化为花纹状的共晶相;在脉冲电流场条件下,β-Mg_(2)Al;相呈现出规律分布的矩形网格状,Mg_(2)Si以方向性明显的树枝状共晶相存在;"电磁模拟微重力+脉冲电流"复合场则兼具前述两种外场的优点,样品中的晶粒明显细化,β-Mg_(2)Al;相呈现为颗粒状、短棒状和矩形网格状,Mg_(2)Si转变为花纹状和树枝状的共晶组织贯穿组织晶粒,相比常规条件下的样品,其显微硬度提高了25.1%,摩擦性能提高了31%。

Physical,Mechanical,and Tribological Attributes of Stir-Cast AZ91/SiC_p Composite

In the present investigation, composites with silicon carbide particle （SiCp） as reinforcement and AZ91 magnesium alloy as matrix have been synthesized using liquid metal stir-casting technique with optimized processing conditions. The composites with good particle distribution in the matrix, and better grain refinement and good interfacial bonding between the matrix and reinforcement have been obtained. The effect of SiCp content on the physical, mechanical, and tribological properties of Mg-based metal matrix composite （MMC） is studied with respect to particle distribution, grain refinement, and particle/matrix interfacial reactions. The electrical conductivity, coefficient of thermal expansion, microas well as macro-hardness, tensile and compressive properties, and the fracture behavior of the composites along with dry sliding wear of the composites have been evaluated and compared with the base alloy.

固溶处理对SiC_p增强AZ91D镁合金组织及硬度的影响

采用不同固溶温度及保温时间,研究了SiC p增强AZ91D镁合金组织演变及硬度变化行为。结果表明,随固溶温度提高,铸态组织中连续网状β相逐渐消失,汉字状Mg2Si相发生熔断,球化速度加快,400℃时晶界清晰。400℃×16h固溶时,不稳定β相几乎完全固溶至基体中,汉字状Mg2Si球化效果最佳,Al8Mn5相未发生明显变化。400℃×20h固溶后,Mg2Si球化趋势开始减弱,晶粒尺寸出现显著增加。结果表明,β相的消失和Mg2Si相的球化导致合金硬度发生连续下降。400℃×16h后由于固溶强化效果的增加,合金硬度有所提高。