Microstructure,tensile properties and fracture behaviour of Al2O3 particulate-reinforced aluminium alloy metal matrix composites

The tensile deformation and fracture behaviour of aluminium alloy 2014 discontinuously-reinforced with particulates of Al₂O₃ was studied with the primary objective of understanding the influence of reinforcement content on composite microstructure, tensile properties and quasi-static fracture behaviour. Results reveal that elastic modulus and strength of the metal-matrix composite increased with reinforcement content in the metal matrix. With increase in test temperature the elastic modulus showed a marginal decrease while the ductility exhibited significant improvement. The improved strength of the Al-Al₂O₃ composite is ascribed to the concurrent and mutually interactive influences of residual stresses generated due to intrinsic differences in thermal expansion coefficients between constituents of the composite, constrained plastic flow and triaxiality in the soft and ductile aluminium alloy matrix due to the presence of hard and brittle particulate reinforcements. Fracture on a microscopic scale initiated by cracking of the individual or agglomerates of Al₂O₃ particulates in the metal matrix and decohesion at the matrix-particle interfaces. Failure through cracking and decohesion at the interfaces increased with reinforcement content in the matrix. The kinetics of the fracture process is discussed in terms of applied far-field stress and intrinsic composite microstructural effects.     

Machining of aluminium based metal matrix composites

Although metal matrix composites (MMCs) are generally regarded as extremely difficult to machine, it is also acknowledged that their machining behaviour is not fully understood. The work reviewed here confirms this widely held view but also suggests that the machinability of these materials can be improved by appropriate selection of the reinforcing phase, its volume fraction, size, and morphology as well as the composition and hardness of the matrix material. Cemented carbide tools can be used to machine some of the less abrasive materials at slow speeds but if higher production rates are required or the more abrasive materials are to be machined, polycrystalline diamond tooling is required.     

Reinforcement coatings and interfaces in aluminium metal matrix composites

The interface between the matrix and reinforcement plays a crucial role in determining the properties of metal matrix composites (MMC). Surface treatments and coating of the reinforcement are some of the important techniques by which the interfacial properties can be improved. This review reports the state of art knowledge available on the surface treatments and coating work carried out on reinforcements such as carbon/graphite, silicon carbide (SiC) and alumina (Al2O3) and their effects on the interface, structure and properties of aluminium alloy matrix composites.The metallic coatings improved the wettability of reinforcement but at the same time changed the matrix alloy composition by alloying with the matrix. Ceramic coatings reduce the interfacial reaction by acting as a diffusion barrier between the reinforcement and the matrix. Multilayer coatings have multifunctions, such as wetting agent, diffusion barrier and releaser of thermal residual stress. The roles of reinforcement coating as a means of in situ hybridising and in situ alloying are described.     

Failure behaviour of particulate-reinforced aluminium alloy composites under uniaxial tension

Tensile tests were carried out at room temperature on 6061-aluminium alloy reinforced with SiC and Al₂O₃ particulates. Although a significant increase in strength could be achieved by introducing ceramic reinforcements into the aluminium alloy matrix, it is associated with a substantial decrease in fracture strain. In order to understand the reason for the inferior ductility of such composites, analytical solutions were obtained using a simple composite model. SEM studies were carried out on the side surfaces of the fractured specimens to verify the proposed failure behaviour. Failure modes observed to operate in such composites under uniaxial tension are described.     

A review of processing techniques for graphene-reinforced metal matrix composites

In the quest to enhance reinforcement efficiency of graphene in metal matrices, various processing techniques have been devised over recent years. As the advancement in this field nowhere seems to slow down, the processing aspects of graphene-reinforced metal matrix composites are becoming more relevant than ever. In that premise, there lies an imminent need for a critical assessment of existing fabrication routes and their ability to extend a solution for the primary challenges of agglomeration, dispersion, interfacial interaction and structural integrity of graphene in metal matrix composites. This review presents a brief yet a meaningful insight to the processing techniques for graphene-reinforced metal matrix composites, while highlighting the key findings from individual studies, thereby expressing the primitive challenges and strengthens of these techniques. A critical evaluation of state of the art is presented alongside an inclusive review of improvement in mechanical, thermal, and electrical properties of composites fabricated by various processing routes. In the consideration of reviewed literature, it is established that a comprehensive processing strategy with a potential to simultaneously address all of the key processing challenges of graphene, is yet to nurture. Conclusively, future road map and a potential solution encompassing hybrid processing strategies, is opined.     

Miniature thermal cycling tests on aluminium alloy metal matrix composites

Abstract

A new miniaturised electrothermomechanical test system has been used to study the thermal cycling response of a number of aluminium alloy metal matrix composites reinforced with either Al₂0₃ or SiC particles. Tests were also performed on a monolithic 2618 aluminium alloy for comparison. The system showed good test discrimination between the different materials for both constant load-constant temperature (creep) tests and constant load-temperature cycling (50–200°C) tests. The system was also used to compare the yield behaviour at 200°C, and the thermal expansion and thermal diffusivity of several of the materials.     

On the fibre/matrix interface in boron/aluminium metal matrix composites

The fibre/matrix interface of B; AI metal matrix composites (MMCs) has been examined by transmission and scanning electron microscopy (TEM and SEW. As-fabricated samples show no fibre/matrix reaction whereas isothermal exposure for increasing periods of time leads to the formation of at least four distinct borides. The extent and location of the fibre/matrix reaction is strongly influenced by the presence of an oxide layer which is present at all the interfaces. The effect of these reaction products upon mechanical properties is considered.     

Processing of Ti-SiC metal matrix composites by tape casting

Abstract

Titanium-silicon carbide (Ti-SiC) continuous fibre composites are very attractive for aerospace applications. Although development of various components is under way, a cost effective method to manufacture the material still has to be identified. Here, a tape casting technique is investigated as a viable method of producing the composites. It involves relatively large inexpensive titanium powder and simple apparatus. Furthermore, the powder particles ensure good fibre distribution, reduced consolidation time, and little damage to the reinforcement. It is shown that uniform powder tapes with good packing density can be readily produced using appropriate casting parameters. Both thermogravimetry and mass spectrometry are used to analyse the burnout process of a fugitive binder system used to produce the tapes. Removal of the organics is found to take place in two stages, separated by over 100 K. composite materials processed by the tape casting route exhibit good fibre distribution and no signs of fibre damage.     

Fracture characteristics of a particulate-reinforced metal matrix composite

The effect of particulates on the failure mechanism of an Al-Mg-Si alloy 6061 with 20% angular alumina particles was studied. Fracture toughness tests were conducted on compact tension peak-aged specimens. The interaction of the reinforcement phase with the crack was investigated by optical microscopy and scanning electron microscopy, both on the surface and in the mid-thickness of the fractured specimen. It is shown that the fractured particles ahead of the crack tip, in particular the larger particles, play an important role in the void-initiation phase of the fracture process. Particle size and aspect ratio determine the likelihood of fracture. Some differences in the failure mechanisms have been observed between the mid-thickness and the surface of the specimen because of the difference between plane strain and plane stress fractures.     

Use of spray techniques to synthesize particulate-reinforced metal-matrix composites

In an attempt to optimize the structure and properties of particulate-reinforced metal-matrix composites, a variety of novel synthesis techniques have evolved over the last few years. Among these, the technique of spray processing offers a unique opportunity to synergize the benefits associated with fine particulate technology, namely microstructural refinement and compositional modifications, coupled within situ processing, and in some cases, near-net shape manufacturing. Spray technology has resurrected much interest during the last decade and there now exists a variety of spray-based methods. These include spray atomization and deposition processing, low-pressure plasma deposition, modified gas welding techniques and high velocity oxyfuel thermal spraying. Spray processing involves the mixing of reinforcements with the matrix material under non-equilibrium conditions. As a result, these processes offer an opportunity of modifying and enhancing the properties of existing alloy Systems, and also developing novel alloy compositions. In principle, such an approach will inherently avoid the extreme thermal excursions, and the concomitant macrosegregation associated with conventional casting processes. Furthermore, the spray processing technique also eliminates the need to handle fine reactive particulates associated with powder metallurgical processes. In this paper, recent developments in the area of spray synthesis or processing of discontinuously reinforced metal-matrix composites are presented and discussed with particular emphasis on the synergism between processing, microstructure and mechanical properties.     

Fibre and fibre-surface treatment effects in carbon/aluminium metal matrix composites

A systematic study of the relationship between the microstructure of the interface in C/Al composites and its dependence on variations in squeeze-casting parameters has been undertaken. This research has shown that the amount of Al₄C₃ reaction product at the interface is dependent on the surface structure of the reinforcing fibre and the surface treatment of the fibre. Additionally, the interface shear strength increases with an increase in the amount of reaction product at the interface. An increase in interface shear strength leads to a decrease in composite longitudinal strength. High-resolution electron microscopy and X-ray photoelectron spectroscopy analyses indicate that carbide formation is a conventional two-step process of nucleation and growth. Nucleation occurs preferentially at graphite edge planes on the carbon fibre surface, and growth is restricted along certain matrix planes and directions.     

Influence of matrix alloying elements on reactive synthesis of 2124 aluminium alloy metal matrix composites

Abstract

In situ TiB₂ particle reinforced Al alloys are produced by reactive synthesis from elemental and prealloyed powders. The influence of 2124 alloying elements on the reactive synthesis is evaluated with a comparison of elemental AI, elemental AI-Cu mixture, and 2124 Al prealloyed powders as matrix materials. Experimental investigations by differential scanning calorimetry and dilatometry showed that the presence of Cu leads to an increase in the reaction rate during the formation of intermediate reaction products in comparison with the elemental Al matrix. X-ray diffraction of the reaction products showed a more complete conversion of the intermediate Al₃ Ti as a result of Cu addition. The Cu has no influence on the TiB₂ particle size, but the TiB₂ morphology changed from pure hexagonal to a more rounded morphology.     

Corrosion behaviour of aluminium matrix composites

Samples of aluminium alloy 2014 reinforced with 20–40 vol % of alumina or silicon carbide particles were tested by the potentiodynamic polarization technique. The chosen medium was 0.1m lithium perchlorate which tends to cause localized corrosion. The measurements revealed no impairment of the corrosion performance of the matrix alloy as a result of the presence of the reinforcement phase.     

Cast metal matrix composites

Abstract

Metal matrix composites have been available in certain forms for at least two decades, e.g. boron fibre reinforced aluminium and various dispersed phase alloys and cermets. Recently, a range of alumina and silicon carbide fibres, whiskers, and particles with diameters <20 μm have become available. The possibilities of incorporating these materials into metals to improve stiffness, wear resistance, and elevated temperature strength without incurring weight penalties have attracted the attention of design engineers in the aerospace and automobile industries. The aim of the present paper is to outline the manufacturing processes for such composites, in particular those based upon liquid metal technology, e.g. squeeze casting and spray forming. Some of the mechanical and physical properties which have been determined for these materials are described. An analysis of how matrix alloy selection may influence tensile and fracture behaviour of short fibre and particle reinforced composites is attempted.

MST/770     

Effect of particle size on fatigue behaviour in SiC particulate-reinforced aluminium alloy composites

The effect of particle size on rotary bending fatigue behaviour was studied for powder metallurgy 2024 aluminium alloy composites reinforced with 10 wt% silicon carbide particles (SiC_p ). Average particle sizes of 5, 20 and 60 μm were evaluated. Particle size had a significant influence on fatigue strength, indicating an increased fatigue strength with decreasing particle size. The composite with 5 μm SiC particles showed higher fatigue strength than the unreinforced alloy. The incorporation of 20 μm SiC particles led to an increase in fatigue strength at a high stress level, but the improvement diminished with decreasing stress level, and a slightly decreased fatigue strength was observed at low stress level, as compared with the unreinforced alloy. The composite with 60 μm SiC particles exhibited a considerable decrease in fatigue strength. Fatigue cracks initiated at several different microstructural features, e.g. surface defects, inclusions and particle–matrix interfaces, and crack initiation was considerably affected by particle size. Fatigue strength was found to depend strongly on the resistance to crack initiation, because there was no discernible difference in small crack growth between the unreinforced alloy and the composites, particularly at a low maximum stress intensity factor.     

Particle size dependence of fatigue crack propagation in SiC particulate-reinforced aluminium alloy composites

Fatigue crack propagation (FCP) and fracture mechanisms have been studied for two orientations in powder metallurgy 2024 aluminium alloy matrix composites reinforced with three different sizes of silicon carbide particles. Particular attention has been paid to make a better understanding for the mechanistic role of particle size. The FCP rates of the composites decreased with increasing particle size regardless of orientation and were slightly faster in the FCP direction parallel to the extrusion direction. After allowing for crack closure, the differences in FCP rate among the composites and between two orientations were significantly diminished, but the composites showed lower FCP rates than the corresponding unreinforced alloy. Fracture surface roughness was found to be more remarkable with increasing particle size and in the FCP direction perpendicular to the extrusion direction. Taking into account the difference in the modulus of elasticity in addition to crack closure, the differences in FCP rate between the unreinforced alloy and the composites were almost eliminated.     

Opportunities for new graphitic aluminium metal matrix composite

Abstract

Nickel coated graphite particles have been incorporated into aluminium with a second particulate phase to produce graphitic aluminium metal matrix composites (Gr A-Ni) with improved processing, wear, and scuffing resistance. Excellent wear behaviour is provided by a combination of solid lubrication by graphite as well as high temperature strengthening of the matrix alloy by nickel present as Al₃ Ni intermetallics. Applications being developed include cylinder liners, pistons, connecting rods, various types of brakes, air diffusers and bushings. Neutral buoyancy of two particles, one of which is lighter and the other heavier than the aluminium matrix alloy, makes this a readily sand and die castable material. The presence of graphite and Al₃Ni intermetallics reduces the amount of ceramic particulate required to achieve the desired wear properties, with resulting improved machinability. The composition of the material can be tailored to the application. All these factors influence the finished part cost.