Al Alloy Composites Research Articles

Effects of volume fraction of alumina short fibers on high cycle fatigue properties of Al and Mg alloy composites

Rotating bending fatigue tests in high cycle fatigue regime were carried out on alumina short fiber reinforced aluminum alloy composites (Al-MMCs) and magnesium alloy composites (Mg-MMCs). Materials of an aluminum alloy A6061 and a magnesium alloy AZ91D, and three volume fractions of 10%, 18%, and 25% of alumina short fibers were prepared to investigate effects of fiber volume fraction on fatigue properties such as a fatigue strength, surface crack initiation, and propagation behaviors. As results, it was found that the fatigue strength increased with an increase of volume fraction of alumina fibers in both MMCs, and the fatigue strength in Al-MMCs at elevated temperature was improved by the alumina short fibers reinforcement. The crack initiation sites were larger size alumina fibers, cluster of short fibers and large size alumina particles in both MMCs.

Fabrication Process of Carbonaceous Fiber Reinforced Al and/or Mg Alloy(s) Composites by Squeeze Casting

Carbonaceous fiber has high strength, high modulus and high thermal conductivity. Preforms were fabricated by suction filtration method. Regardless of fiber type, the buckling strength and elastic modulus of preforms increased with quantity of aluminum borate (AlBO) whisker addition. The carbon nano tube (CNT) preforms were not buckled but only elastically compressed. Carbonaceous fiber reinforced Al, Mg composites were fabricated by squeeze casting. CF+AlBO hybrid reinforced composites were fabricated with 5% or less deformation. On the other hand, 60% deformation was found in CNT+50AlBO hybrid reinforced composites.

Recycling Mechanism of Discontinuously Reinforced Al Matrix Composites

In the present work, the effects of reinforcement type, reinforcement surface treatment, alloying additions, remelting time and remelting number upon the chemical and property degradations of discontinuously reinforced Al matrix composites, are investigated by means of Differential Scanning calorimeter (DSC), Transmission Electron Microscopy (TEM) and thermodynamic calculations. The results show that the SiCp/Pure Al and SiCp/wrought Al alloy composites could not be recycled via remelting, nevertheless SiCp/foundry Al alloy and Al2O3p/Al composites could be recycled via remelting. The remelt-recycling mechanism of discontinuously reinforced Al matrix composites is also discussed in detail in the present work.

Finite Element Simulation for Tensile Properties of δ-Al2O3f/Al Alloy Composites with Thermal Residual Stress Considered

First, the characteristics of thermal residual stress in randomly oriented short δ-Al2O3 fiber reinforced aluminum alloy composites were analyzed by an elasto-plastic finite element method regarding the effect of the variation in short fiber orientation. Then, the effective moduli and initial tensile stress-strain curves of the composites were simulated by finite element method with the thermal residual stress taken into account. It is concluded that the simulated results obtained with thermal residual stress contained agree with corresponding experimental results better than those obtained in thermal residual stress-free cases.

Lubricated sliding wear behaviour of aluminium alloy composites

Interest in aluminium alloy (Al-alloy) composites as wear resistant materials continues to grow. However, the use of the popular Al-alloy-SiC composite can be limited by the abrasive nature of the SiC, leading to increased counterface wear rates. This study reports new Al-alloy composites that offer high wear resistance, to a level similar to Al-alloy-SiC. Aluminium alloy (2124, 5056) matrix composites reinforced by nominally 15 vol.% of Cr 3Si, MoSi 2, Ni 3Al and SiC particles were prepared by a powder metallurgy route. The aluminium alloy matrix was produced by gas atomisation, and the Cr 3Si, MoSi 2 and Ni 3Al were prepared by self-propagating high temperature synthesis (SHS), while the SiC was from a standard commercial supply. Following blending, the particulates were consolidated by extrusion, producing a homogenous distribution of the reinforcement in the matrix. Wear testing was undertaken using a pin-on-ring configuration against an M2 steel counterface, with a commercial synthetic oil lubricant, at 0.94 m/s and a normal load of 630 N, corresponding to initial Hertzian contact pressures of ∼750–890 MPa (the exact value depending on the material properties). Specific wear rates at sliding distances exceeding 400 km were in the range 4.5–12.7 × 10 −10 mm 3/Nm. The monolithic alloys gave the highest specific wear rates, while the MoSi 2 and Cr 3Si reinforced alloys exhibited the lowest. The worn surface has been analysed in detail using focused ion beam (FIB) microscopy to determine the sub-surface structural evolution and by tomographic reconstruction of tilted scanning electron microscopy (SEM) images, to determine the local worn surface topography. Consequently, the wear mechanisms as a function of alloy composition and reinforcement type are discussed.

On the origin of the two-dimensional electron gas at the AlGaN∕GaN heterostructure interface

Bare surface barrier heights (BSBHs) of AlGaN∕GaN heterostructures, with varying AlGaN layer thickness and ∼35% Al alloy composition, have been measured using UV laser induced transients. The BSBH has been observed to vary with AlGaN thickness, before it saturates beyond a critical thickness, and the variation found to be very similar to that of the two-dimensional electron gas (2DEG) density at the interface. Such a trend can be explained by considering the presence of surface donor states distributed in the band gap. The density of the surface donor states has been calculated from the variation of the surface barrier with the 2DEG density, and found to be almost constant at ∼1.6×1013cm−2eV−1 in the energy range of ∼1.0–1.8eV from the conduction band. After SiNx passivation of the surface, the BSBH reduces, and sheet charge density increases, indicating the presence of positive charges in the passivation layer.

<I>In-situ</I> Observations of Fracture Processes in 0.6 μm and 9.5 μm SiC<SUB>P</SUB>/6061Al Composites

ln-situ SEM observations of fracture processes in two 6061 Al alloy composites reinforced with coarse and line SiC particles, respectively. were carried out to clarify their fracture mechanisms. It was found that in the coarse particle reinforced composite, void were formed in the matrix around SiC particles ahead of the main crack tip. then coalesced with each other, and finally connected with the main crack tip. Causing propagation of the main crack. However. in the fine particle reinforced composite, multiple in micro-cracks were formed at the boundaries between SiC particle clusters and surrounding matrix or within the clusters. then connected with each other, and finally joined with the main crack tip. leading to crack branching and growth of the main crack, Crack branching, multiple cracking and crack deflection were proposed to contribute to the enhanced fracture toughness in the fine particle reinforced composite compared with the coarse particle reinforced composite.

Perturbation of charges in AlGaN∕GaN heterostructures by ultraviolet laser illumination

Perturbation of charges in AlGaN∕GaN heterostructures by ultraviolet (UV) laser illumination has been studied by Kelvin probe microscopy. It is observed that the charge dipole formed across the AlGaN barrier decreases under UV laser illumination, and typically takes a few days to a few weeks to revert back to the original equilibrium value. Bare surface barrier height of AlGaN∕GaN heterostructures has been calculated based on the observed recovery transients after the UV illumination is switched off, and found to increase with Al alloy composition as well as thickness of the AlGaN layer. After SiNx passivation, the surface barrier is reduced, and the charges cannot be perturbed to a significant extent. It is further observed that by UV illumination through a quartz mask, surface barrier patterns can be imprinted on AlGaN∕GaN heterostructures, which stay for several hours to several days. It is proposed that the imprinted patterns, with a spatial resolution ∼1–2μm, are caused by surface trapping of photogenerated holes.

Solid particle erosion of Al alloy and Al-alloy composites: Effect of heat treatment and angle of impingement

Erosive wear behavior of the as-cast and heat-treated Al alloy and the Al-alloy-SiC particle composite against Al2O3 erodent has been examined at different angles of impingement (15 to 90 deg). It has been noted that the cast Al alloy exhibited a higher erosion rate than the heat-treated alloy and composites irrespective of the angle of impingement. It is noted that the as-cast and heat-treated Al alloy exhibited a maximum wear rate at the 45 deg angle of impingement, whereas the composite, in as-cast as well as in heat-treated conditions, showed a maximum erosion rate at the 60 deg angle of impingement. Subsurface studies of the alloy and composite confirm that the material loss, during erosive wear, is primarily due to microcutting/microplowing (i.e., abrasive-type) and microfracture (i.e., impact-type) actions. At a low angle of impingement, the abrasive type is the dominating factor for material removal, and at a higher angle of impingement, both the impact-type and abrasive-type actions play critical roles. The impact type is mainly localized at the tip of the erosion profile, while the abrasive type takes place along the sidewalls of the profile. This is explained on the basis of the erosion mechanism using a schematic diagram.

Effect of reinforcement-particle-orientation anisotropy on the tensile and fatigue behavior of metal-matrix composites

The effect of extrusion-induced particle-orientation anisotropy on the mechanical behavior of metal-matrix composites (MMCs) was examined. In this study, we have shown that this anisotropy has a significant influence on the tensile and fatigue behavior SiC particle-reinforced Al alloy composites. The preferred orientation of SiC particles was observed parallel to the extrusion axis, with the extent of orientation being highest for the lowest-volume-fraction composites. The composites exhibited higher Young’s modulus and tensile strength along the longitudinal direction (parallel to the extrusion axis) than in the transverse direction. The extent of anisotropic behavior increased with increasing volume fraction, because of the increasing influence of the SiC reinforcement on the Young’s modulus and tensile properties. The preferred orientation also resulted in anisotropy in the fatigue behavior of the composite material. The trends mirrored those observed in tension, with higher overall fatigue strengths for both orientations and a higher anisotropy with increasing volume fraction of particles. The influence of particle-orientation anisotropy and the resulting tensile and fatigue damage mechanisms is discussed.

In‐Situ Processing of Al Alloy Composites

AbstractIn‐situ processing of discontinuously reinforced Al alloy composites (DRACs) by gas‐bubbling method was investigated in this paper. Fine SiC and AlN particles were synthesized by bubbling C‐bearing and N‐bearing gases respectively. These results show that processing of DRACs through in‐situ gas‐bubbling route is technically feasible. Compared with conventional processing techniques, this method has the potential to produce DRACs with better microstructure and properties while with lower cost.

Squeeze casting of SiCp/Al-alloy composites with various contents of reinforcements

Aluminum matrix composites reinforced with various contents and sizes of silicon carbide particles (SiCp) were fabricated by squeeze casting. A lower melting point AA A383 aluminum alloy (A383 Al) was squeezed into the higher melting point SiCp/pure aluminum (SiCp/pure Al) and SiCp/AA 6061 aluminum alloy (SiCp/6061 Al) preforms. The volume percents of the ceramic reinforcements were effectively lowered from traditional 50 to 8–25 vol% by the addition of pure Al and 6061 Al powders in the preforms. The SiC particles uniformly distributed within the matrices, and no pore was found in these composites. The growth of silicon precipitates in A383 Al alloys was limited by the addition of the aluminum alloy powders and SiC particles, and the tensile properties of the alloys were effectively enhanced by the refinement of the silicon precipitates. The tensile strengths and elongations of the SiCp/pure Al/A383 Al and SiCp/6061 Al/A383 Al composites were both better than those of the A383 Al alloy. The T6-treated 12-μm SiCp/6061 Al/A383 Al composite exhibited the highest tensile strength of 301 MPa.

Methodology to design the interfaces in SiC/Al composites

A methodology to control interfacial microstructures, while suppressing formation of Al4C3 in wrought Al alloy composites reinforced with SiC, was demonstrated. Thermodynamic calculations were carried out to elucidate how one can select process parameters in terms of alloy composition and fabrication temperature to obtain intended interfaces. Experimental verifications were conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to validate calculated results. The reaction mechanisms for forming various interfaces were identified both theoretically and experimentally. Evaluations of the interfacial bonding strengths and interfacial stability at elevated temperatures were also carried out for various interface types.

Two-body abrasion of a cast Al–Cu (2014 Al) alloy–Al 2O 3 particle composite: influence of heat treatment and abrasion test parameters

An Al–Cu (2014 Al) alloy reinforced with 10 vol.% Al 2O 3 particles (size: 75–150 μm), prepared by liquid metallurgy route, has been investigated under two-body (high stress) abrasive wear condition. The influence of varying load, abrasive size and sliding distance on the abrasive wear behaviour of the specimens was also studied. The base alloy prepared under similar condition has also been studied under identical test conditions in order to understand the influence of the dispersoid phase on the abrasive wear characteristics of the (base) alloy. In order to assess the role of matrix microstructure, the composite as well as the base alloy was subjected to abrasion in heat treated as well as in cast conditions. The results indicate that the Al 2O 3 particle reinforced cast Al alloy composite was more wear resistant (less wear rate) than the (unreinforced) matrix alloy when tested against 20, 35 and 60 μm size abrasive particles over the entire range of loads and sliding distances due to protection provided by the Al 2O 3 particles to the alloy matrix. On the other hand, a reverse trend was observed when 100 μm size abrasive particle was used as the abrasive medium above 1 N load due to greater microcracking tendency followed by fragmentation of dispersoid/abrasive particles. Unlike cast composite, heat treated composite exhibited better wear resistance (lesser wear rate) than the matrix alloy even when the tests were conducted against 100 μm size abrasive particles over the entire range of loads and sliding distances. Matrix strengthening and the morphological changes of the precipitate particles caused lower wear rate of the heat treated base alloy and composite over the cast ones. Abrasive wear rates were observed to decrease with sliding distance because of the increased extent of clogging, capping, shelling and attrition of the abrasive particles as well as subsurface hardening of the matrix. The wear behaviour of the specimens has been explained in terms of wear-induced subsurface work hardening, protection provided by the reinforced particles (in the case of composite) and degradation of the abrasive. Material removal mechanisms have also been studied through the examination of wear surfaces, subsurfaces, debris and abrasive particles.

Effect of SiC volume fraction on the age-hardening behavior in SiC particulate-reinforced 6061 aluminum alloy composites

The age-hardening behavior in the SiC particulate-reinforced 6061 aluminum alloy composites (SiCp/6061 Al alloy composites) was investigated using hardness measurement, calorimetric technique and transmission electron microscopy. The aging time for the peak hardness in 10%SiCp/6061 Al and 20%SiCp/6061 Al alloy composites was shorter than that of the unreinforced 6061 Al alloy, and the age-hardening was most accelerated in the 20%SiCp/6061 Al alloy composite. It is induced from the high density of dislocations in SiCp/6061 Al alloy composites which act as heterogeneous nucleation sites of precipitates and as a high diffusivity path of alloy elements. The precipitation sequence and the aspect of precipitates in the 6061 Al alloy cannot be affected considerably by the presence of SiC particulate and the volume fraction of SiC particulate. The activation energies for the formation of the intermediate β' phase in the unreinforced 6061 Al alloy, 10%SiCp/6061 Al and 20%SiCp/6061 Al alloy composite were 142.6 kJ/mol, 127.3 kJ/mol, 106.1 kJ/mol, respectively.

Interfacial reaction and its influence on the mechanical properties of the ASZ short fibre-reinforced Al alloy composite.

The effect of interfacial reaction on the mechanical properties of the AC8A Al alloy reinforced with ASZ short fibres (ASZ/AC8A composite) was studied. In the ASZ/AC8A composite, the interfacial reaction was observed to proceed between the SiO2 binder layer and Mg of the matrix to form MgAl2O4 at the interface. Formation of MgAl2O4 was believed to enhance the interfacial bonding strength, resulting in the improved composite strength. However, the interfacial reaction in the ASZ/AC8A composite always took place at the expense of Mg in the matrix, resulting in the composite devoid of the Mg bearing precipitates such as Al2CuMg and Mg2Si. Interfacial reaction mechanisms were investigated for composites containing various Mg contents. The resultant mechanical properties of the composite were measured to determine the adequate amount of Mg within the composite. Microstructural changes of the composite were observed using transmission electron microscopy and differential scanning calorimetry to provide qualitative analyses on the experimental observations.

Influence of matrix characteristics on fracture toughness of high volume fraction Al2O3/Al–AlN composites

The role of matrix microstructure on the fracture of Al-alloy composites with 60 vol% alumina particulates was studied. The matrix composition and microstructure were systematically varied by changing the infiltration temperature and heat treatment. Characterization was carried out by a combination of metallography, hardness measurements, and fracture studies conducted on compact tension specimens to study the fracture toughness and crack growth in the composites. The composites showed a rise in crack resistance with crack extension (R curves) due to bridges of intact matrix ligaments formed in the crack wake. The steady-state or plateau toughness reached upon stable crack growth was observed to be more sensitive to the process temperature rather than to the heat treatment. Fracture in the composites was predominantly by particle fracture, extensive deformation, and void nucleation in the matrix. Void nucleation occurred in the matrix in the as-solutionized and peak-aged conditions and preferentially near the interface in the underaged and overaged conditions. Micromechanical models based on crack bridging by intact ductile ligaments were modified by a plastic constraint factor from estimates of the plastic zone formed under indentations, and are shown to be adequate in predicting the steady-state toughness of the composite.

Interfacial reaction and its effect on strength of Al18 B4O33 /Al composites

Aluminium alloy 6061, AC8A, Al–1Mg, Al–9Cu and pure aluminium composites reinforced with aluminium borate whiskers were fabricated by a squeeze casting process. The interfacial reaction in the composites and its effect on the bending strength are discussed, together with the results from SEM, TEM, and X-ray diffraction. A slight interfacial reaction is favourable for composite strength as it has the effect of anchoring the whiskers. A T6 treatment can enhance the strength of an Al–9Cu matrix composite, but is not efficient for magnesium containing 6061 and AC8A matrix composites. Furthermore, if heated at temperatures higher than 793 K for a long time, the composite strength drops rapidly owing to whisker damage and shortening during the interfacial reaction. It is suggested that the interface in an Al18 B4O33 /Al alloy composite is stable below 623 K which is the temperature requirement for automobile engine components.

Relationship Between High-Strain-Rate Superplasticity and Interface Microstructure in Aluminum Alloy Composites

Abstract The Al alloy composites reinforced with Si3N4 or SiC have been reported to exhibit superplasticity at high strain rate of faster than 1x10−2 s−1. It has been shown in many aluminum alloy composites that the optimum superplastic temperature coincides with an incipient melting temperature. The coincidence suggests a contribution of the liquid phase to the superplasticity mechanism. This paper shows a direct evidence of partial melting along matrix grain boundaries and matrix-reinforcement interfaces. Based on the obtained results, the role of the liquid phase in the high-strain-rate superplasticity is discussed. The sample was Al-Mg (5052) alloy reinforced with 20vol% Si3,N4 particles, fabricated by a powder metallurgy process. The sample showed an excellent superplasticity under the conditions given in Table 1. Partial melting was confirmed to occur at 821 K by differentail scanning calorimetry. The microstructural changes during heating were observed in situ by TEM using a heating stage. The structure of interfaces and grain boundaries was observed by HREM. Chemical analysis was performed with EDS attached to VG-STEM.

High temperature creep deformation of Al 18B 4O 33 whisker-reinforced 8009 Al composite

The compression creep deformation of an Al 18B 4O 33 whisker-reinforced Al–Fe–V–Si (8009 Al alloy) composite has been investigated in the temperatures range from 573 to 723 K. The addition of 15 vol.% Al 18B 4O 33 whiskers to the 8009 alloy decreases the creep rate by two orders of magnitude compared to the 8009 alloy. The stress and temperature dependence of the creep rate of the composite is similar to that of the 8009 alloy. The threshold stress for creep was used to analyze the experimental data for the composite. The creep rates of the composite are shown to be controlled by lattice diffusion of aluminum. The contribution of the whiskers on creep rate of the composite is interpreted by a modified load-transfer model.