Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

高体积分数SiCp/A356复合材料的显微组织和电导率

铝基复合材料在电子封装领域存在着潜在的应用前景。为获得高体积分数的铝基复合材料,利用压力浸渗法制备了高体积分数SiC颗粒增强A356复合材料(SiC_p/A356),通过金相显微镜、XRD、SEM和EDS等分析手段对其物相、显微结构和电导率进行了表征。结果表明:用该方法制备的SiC_p/A356复合材料组织致密,颗粒分布均匀,界面结合性能较好;SiC增强颗粒与A356基体界面反应控制良好,仅有少量Al4C3脆性相生成。SiC粉体经颗粒表面氧化处理在其表面生成一层SiO_2薄膜,虽抑制了界面反应的发生,但也使复合材料的收缩减小,电阻率增大,导电性能变差。     

Microstructure and mechanical properties of SiC nanoparticles reinforced magnesium matrix composites fabricated by ultrasonic vibration

A particulate reinforced magnesium matrix nanocomposite was fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, grain size of matrix in the SiCp/AZ91 nanocomposite decreased and morphology of phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates. Although there were still some SiC microclusters in the nanocomposite, most of the SiC nanoparticles were dispersed well outside the microclusters. The ultimate tensile strength, yield strength and elongation to fracture of the SiCp/AZ91 nanocomposite were simultaneously enhanced compared with that of the AZ91 alloy. The study of interface between the SiC nanoparticles and the matrix in the nanocomposite suggested that SiC nanoparticles bonded well with the matrix without interfacial activity.     

Effect of high energy ball milling on solid state reactions in Al–25 at.-%Ni powders

Abstract

The effect of high energy ball milling on the solid state reactions between aluminium and nickel in Al–25 at.-%Ni powders has been investigated using scanning electron microscopy, thermal analysis techniques, and X-ray diffractometry. It has been observed that the microstructure of the powder particles evolves in three stages: stage I is the formation of entrapped nickel particles in the aluminium matrix structure; stage II is the formation of an Al–Ni multilayered structure; and stage III is the formation of Al₃Ni single phase. The temperature required to activate the reaction between aluminium and nickel during heating decreases by more than 200 K as the powder particle microstructure evolves from the entrapped particle structure to the multilayered structure, and then it decreases gradually with decreasing nickel layer thickness. The nucleation and lateral growth of Al₃Ni phase at the Al/Ni interfaces occurs at much lower temperatures than those required for the transverse growth of Al₃Ni. The fraction of Al₃Ni formed through nucleation and lateral growth at the interface is almost linearly proportional to the interfacial area. The activation energy for nucleation and lateral growth of Al₃Ni at the Al/Ni interfaces is independent of nickel layer thickness, but the activation energy for transverse growth of Al₃Ni decreases substantially with decreasing nickel layer thickness. The latter is attributed to the observation that the nickel layers are thinned by plastic deformation and thus contain an increasingly higher density of dislocations.     

Interfacial failure behavior of PyC-Cf/AZ91D composite fabricated by LSEVI

Carbon fiber reinforced AZ91D matrix composites with pyrolytic (PyC) coating deposited on fiber surface (PyC-C_f/AZ91D composites) have been fabricated by Liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI). Interfacial microstructure and failure behavior of the composites were investigated. Instead of interfacial reaction products, block-shaped interfacial precipitates Mg₁₇Al₁₂ were detected at the interface, which indicates that interfacial reaction was restrained by LSEVI and PyC coating. Nano-MgO was detected at the interface. Interfacial failure behavior of the PyC-C_f/AZ91D composites, which was the failure between PyC coating and AZ91D alloy due to the mismatch of thermal expansion and relatively poor bonding, was proposed. Fracture surface of the PyC-C_f/AZ91D composites was characterized by fibers pulling-out tests. PyC coating served not only as protection to the fibers, but also an adjustment of the interface of the composites.     

Microstructural characterisation and corrosion behaviour of laser cladded Al–12Si alloy onto magnesium AS41/carbon fibre composite

Abstract

The quality and properties of laser clad layers are dependent on the microstructure and properties of the interfaces with the substrate. The present paper reports, in details, on the characterisation of microstructure of the coating and interfacial layers evolved as a result of the CO₂ laser remelting of previously plasma sprayed Al–12Si alloy onto C short fibres reinforced AS41–Mg composite. Scanning electron microscopy (SEM), wavelength dispersive X-ray (WDX) analysis and X-ray diffraction (XRD) were employed to identify the phases arising in the interfacial layers. The latter are composed mainly of Mg₁₇Al₁₂ and Mg₂Si phases. XRD was conducted on the clad layers at different distances from the interface. At the same layers, the potentiodynamic polarisation in sodium chloride solution was measured and it was found that as the Mg content increases in the clad coating, the corrosion resistance decreases. However, the corrosion current of the clad coating is around two orders of magnitude lower than that of the C/Mg composite.     

Nucleation mechanism of the cubic σ phase in squeeze-cast aluminium matrix composites

An Al-4.3 wt% Cu-2.0 wt% Mg alloy reinforced with 20 vol% reinforcing fibres was examined after a T7 heat treatment. The expected precipitate phase was equilibrium S′ (Al₂CuMg), which was confirmed to form in the monolithic alloy. However when this Al-Cu-Mg alloy was squeeze-cast into a fibre preform and given an identical T7 heat treatment a number of other phases also nucleated; these included θ′ (Al₂Cu), β′ (Mg₂Si) and the cubic σ phase (Al₅Cu₆Mg₂). These additional phases were determined to nucleate and grow rapidly during the water-quench following solution treatment. The existence of excess Si (approximately 0.5 wt%) in the matrix was determined to be responsible for nucleation of these additional phases. This extra Si entered the composite matrix during squeeze-casting through breakdown of an SiO₂ layer which existed at the fibre interfaces. During quenching Si clusters rapidly form and provide nucleation sites for the σ and θ′ phases. The Si clusters apparently created a compressive strain in the matrix which attracted a high concentration of small Cu atoms to their interface. The σ phase nucleated in this high-Cu region since, on a localized scale, σ became the equilibrium phase. This type of nucleation process may also explain the enhanced precipitate nucleation which occasionally takes place in other alloy systems when trace amounts of certain elements are added.     

Interfacial characteristics of electron beam welding joints of SiCp/Al composites

Abstract

The electron beam welding of SiCp/101Al composites has been carried out. The influence of welding parameters on weldability and mechanical properties of the welded joints was discussed. The welding parameters were therefore optimised under the current experimental circumstance. Results show that only weak interfacial reaction between SiC particle and liquid aluminium occurred. Minute quantity brittle Al₄C₃ compounds and single phase Si were generated in the welded joint. The interfacial reaction between SiC particles and Al matrix could be greatly suppressed by adopting appropriate technique measures such as high welding speed and low heat input. The content of Al₄C₃ can be therefore greatly decreased in the welded joint. Moreover, modification welding and electron beam scanning could further improve the appearance of weld, and the welded joint with better quality could be obtained.     

Investigation of interfacial structure of Mg/Al vacuum diffusion-bonded joint

The interfacial structure of the Mg/Al diffusion-bonded joint was studied by means of SEM, EPMA and TEM. The test results indicated that the interface zone of Mg/Al diffusion-bonded joint included the transition region on Mg side, the mid-diffusion region and the transition region on Al side. The concentration distribution of Mg and Al atoms in the diffusion zone includes three different regions. The three regions are intermetallic compounds Mg₂Al₃, Mg₃Al₂ and MgAl, respectively. The location of Mg₂Al₃, Mg₃Al₂ and MgAl phases in the diffusion zone can be determined. The Mg₃Al₂ phases were observed in the transition region on Mg side by means of TEM.     

The influence of Si and Mg rich phases on the mechanical properties of 6061 Al-matrix composites reinforced with Al2O3

Besides altering the kinetics of precipitation, the reinforcements with alumina particles appear to alter the relative proportions of the various phases formed in the matrix alloy during ageing. Alumina also seems to reduce the volume fractions of hardening phase in these materials, especially at higher contents. One reason for this effect could be the dislocation relief of the matrix strain associated with the early precipitates. Additionally, the diffusion of Mg and its subsequent incorporation into the reinforcement at the Al₂O₃/Al interface could also result in Mg depletion from the matrix, accounting for the reduction of the (Mg₂Si) particle sizes. Magnesium incorporation into interfacial alumina to form MgAl₂O₄ has been observed in these materials. This paper shows that the nature and morphology of the second phases depend the heat treatment conditions on.     

Interfacial reactions in the liquid diffusion couples of Mg/Ni,Al/Ni and Al/(Ni)-Al2O3 systems

The interfacial reactions between various molten metals and solid plates were investigated in this diffusion couple study. The molten metals were pure magnesium, pure aluminium, aluminium-rich Al-Mg alloy, and aluminium-rich Al-Cu alloys, and the solid plates were pure nickel plate, alumina plate, and nickel-plated alumina plate. The interfacial reactions in the diffusion couples were determined by using optical microscopy, scanning electron microscopy and electron probe microanalysis in regard to the formation of intermetallic phases, the dissolution rates of the nickel plates, and the morphology of the interfaces. Mg₂Ni phase was found in the pure Mg/Ni plate diffusion couples, and the Al₃Ni and Al₃Ni₂ phases were observed in the pure Al/Ni plate and Al-alloys/Ni plate diffusion couples. In the Al-Cu alloy/Ni-plated alumina plate diffusion couple, Al₂O₃ formed at the interface, while spinel particles were found in the diffusion couples of Al-7.4wt% Mg alloy/Ni-plated alumina plate. Experimental difficulty was encountered in preparing the diffusion couples with alumina plate, and a gap existing at the interface prohibited reactions between the molten metal with alumina plate.     

A TEM study of the interfaces and matrices of SiC-coated carbon fibre/aluminium composites made by the K2ZrF6 process

Carbon fibre-reinforced aluminium composites were pressurelessly cast by using K₂ZrF₆ as the wetting promotion agent. Transmission electron microscopy (TEM) and energy dispersed analysis of X-rays, (EDAX) were used. The results showed that interfacial reactions were very active after K₂ZrF₆ treatment. This was caused by the diffusion and reaction of zirconium in the surface of carbon fibres or in the SiC coating. Silicon alloying of aluminium could suppress the interfacial reactions by decreasing the activity of zirconium and changing intermetallic Al₃Zr to Zr₃Al₄Si₅, and building up the phase equilibrium between SiC, aluminium and silicon. The requested silicon content was higher than the equilibrium content of Al-Si-SiC system to suppress the SiC/Al interfacial reaction. A perfect interface was achieved in SiC-coated carbon fibre Al-12 wt% Si composite.     

Crystallography of the second phase/SiC particles interface,nucleation of the second phase at β-SiC and its effect on interfacial bonding,elastic properties and ductility of magnesium matrix composites

The nucleation and crystallographic orientation of both the magnesium matrix and the eutectic {Mg(ZnCu)₂} at the -SiC particle surface were investigated. The eutectic nucleates at the particle surfaces with an identical crystallographic orientation and covers the particle surfaces in ZC63 and ZC71 magnesium matrix composites, but no distinct crystallographic orientation of the magnesium matrix/SiC particle interface has been resolved. At the interface no extensive chemical reaction takes place and no reaction product forms as a layer at the particle surfaces, but in some cases, during composite processing Mg₂Si particles, and during heat treatment Mg₂Si precipitates, nucleated at the particles and in the matrix close to the interface. In addition, other phases having the composition of CuFeCrSi and CuMoMgSi, the formation of which was not known in ZC63, and manganese- and iron-rich phases in ZC71 alloy, were also detected at the particle surfaces. There is some indication that the coarse and brittle manganese- and iron-containing particles may be involved in the fracture, but no direct effect of the other phases on tensile properties and/or fracture toughness was observed. Nucleation of the eutectic resulting in a good interfacial bonding, however, increases both the Young's modulus and the ductility of the composites, modifying the brittle nature of the matrix/particle interface. As well as -SiC, -SiC particles are intended for use in the fabrication of magnesium matrix composites. Therefore, the possibility for similar nucleation and orientation relationships between the eutectic and -SiC particles has been examined.     

Effect of re-melting on particle distribution and interface formation in SiC reinforced 2124Al matrix composite

The interface between metal matrix and ceramic reinforcement particles plays an important role in improving properties of the metal matrix composites. Hence, it is important to find out the interface structure of composite after re-melting. In the present investigation, the 2124Al matrix with 10 wt.% SiC particle reinforced composite was re-melted at 800 °C and 900 °C for 10 min followed by pouring into a permanent mould. The microstructures reveal that the SiC particles are distributed throughout the Al-matrix. The volume fraction of SiC particles varies from top to bottom of the composite plate and the difference increases with the decrease of re-melting temperature. The interfacial structure of re-melted 2124Al–10 wt.%SiC composite was investigated using scanning electron microscopy, an electron probe micro-analyzer, a scanning transmission electron detector fitted with scanning electron microscopy and an X-ray energy dispersive spectrometer. It is found that a thick layer of reaction product is formed at the interface of composite after re-melting. The experimental results show that the reaction products at the interface are associated with high concentration of Cu, Mg, Si and C. At re-melting temperature, liquid Al reacts with SiC to form Al₄C₃ and Al–Si eutectic phase or elemental Si at the interface. High concentration of Si at the interface indicates that SiC is dissociated during re-melting. The X-ray energy dispersive spectrometer analyses confirm that Mg- and Cu-enrich phases are formed at the interface region. The Mg is segregated at the interface region and formed MgAl₂O₄ in the presence of oxygen. The several elements identified at the interface region indicate that different types of interfaces are formed in between Al matrix and SiC particles. The Al–Si eutectic phase is formed around SiC particles during re-melting which restricts the SiC dissolution.     

Effect of Nd content on microstructure and mechanical properties of Grf/Al composite

M40 graphite fibre reinforced Al-17Mg matrix composites with different neodymium (Nd) content (Al-17Mg, Al-17Mg-0.2Nd, Al-17Mg-0.5Nd and Al-17Mg-2Nd) were fabricated by pressure infiltration method. Microstructure of Gr_f/Al composites was investigated by XRD, SEM, TEM and HRTEM. Effect of Nd on microstructure and mechanical properties of Gr_f/Al composites were deeply discussed. Al₃Mg₂ and Al₁₁Nd₃ phases followed by segregation of Nd and Mg at carbon-aluminum interface were detected in composites containing Nd. The size and amount of Al₄C₃ phase were increased with Nd content. Bending strength of Gr_f/Al composites were decreased sharply from 1463 MPa (Gr_f/Al-17Mg) to 791 MPa (Gr_f/Al-17Mg-2Nd) after the addition of Nd. The increased Nd content decreased the pull-out of single fibre and bundles, which was due to the high interfacial bonding strength with formation of Al₄C₃, Al₃Mg₂, Al₁₁Nd₃ phases and the transition layer.     

Microscopic examination of the interface region in AC8A/Al18B4O33w composites reinforced with as-received and artificial nitrided Al18B4O33 whiskers

Aimed to control the interfacial reaction in Al₁₈B₄O₃₃/Al composites, the artificial nitridation process is proposed based on thermodynamic calculations, which leads to a 50-nm thick artificial nitrided coating surrounding a Al₁₈B₄O₃₃ whisker. Aluminum (AC8A) matrix composites reinforced with different Al₁₈B₄O₃₃ whiskers were processed and Al₁₈B₄O₃₃ whiskers were used in their as-received form and after artificial nitridation. The interface region between the aluminum matrix and Al₁₈B₄O₃₃ was charactered using transmission electron microscopy. Results show that extensive reaction takes place during the incorporation process between the as-received Al₁₈B₄O₃₃ whiskers and the aluminum matrix and Mg in the base alloy forms MgAl₂O₄ spinel phases. Such interfacial reaction is enhanced after T6 heat treatment. In the case of artificially nitrided Al₁₈B₄O₃₃ whiskers, even though the artificial nitrided coating surrounds the Al₁₈B₄O₃₃ whisker, it can react with the AC8A alloy to produce MgAl₂O₄, but the degree of the interfacial reaction is reduced. The interfacial reaction only consumes the artificial nitrided coating instead of the reinforcement whiskers and the integrity of the whisker is preserved.     

Interfacial microstructure and its effect on thermal conductivity of SiCp/Cu composites

Thermal conductivity of SiCp/Cu composites was usually far below the expectation, which is usually attributed to the low real thermal conductivity of matrix. In the present work, highly pure Cu matrix composites reinforced with acid washed SiC particles were prepared by the pressure infiltration method. The interfacial microstructure of SiCp/Cu composites was characterized by layered interfacial products, including un-reacted SiC particles, a Cu–Si layer, a polycrystalline C layer and Cu–Si matrix. However, no Cu₃Si was found in the present work, which is evidence for the hypothesis that the formation of Cu₃Si phase in SiC/Cu system might be related to the alloying elements in Cu matrix and residual Si in SiC particles. The thermal conductivity of SiCp/Cu composites was slightly increased with the particle size from 69.9 to 78.6 W/(m K). Due to high density defects, the real thermal conductivity of Cu matrix calculated by H–J model was only about 70 W/(m K). The significant decrease in thermal conductivity of Cu matrix is an important factor for the low thermal conductivity of SiCp/Cu composites. However, even considered the significant decrease of thermal conductivity of Cu matrix, theoretical values of SiCp/Cu composites calculated by H–J model were still higher than the experimental results. Therefore, an ideal particle was introduced in the present work to evaluate the effect of interfacial thermal resistance. The reverse-deduced effective thermal conductivities of ideal particles according to H–J model was about 80 W/(m K). Therefore, severe interfacial reaction in SiCp/Cu composites also leads to the low thermal conductivity of SiCp/Cu composites.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Abstract

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Effect of Sc and Zr alloying on microstructure and precipitation evolution of as cast Al–B4C metal matrix composites

Abstract

Additions of Sc and Zr were introduced into Al–15 vol.-%B₄C composites, and eight experimental composites with different Sc and Zr levels were prepared via a conventional cast process. Optical microscopy, SEM and TEM were applied for observing the as cast microstructures, including the interfaces between the Al matrix and the B₄C as well as the evolution of the precipitates. It was found that Sc involved the interfacial reactions with B₄C that partially consumed the Sc. On the other hand, no major Zr reaction products were found in the interfaces, and the major part of Zr remained in the matrix for precipitation strengthening. The Sc addition yielded considerable precipitation strengthening in the as cast and peak aged conditions. The combination of Sc and Zr significantly enhanced the precipitation strengthening. Nanoscale precipitates Al₃Sc and Al₃(Sc,Zr) were found in the as cast microstructure and contributed to the significant increase of matrix hardness.