Particle reinforced metal matrix composites are now being produced commerically, and in this paper the current status of these materials is reviewed. The different types of reinforcement being used, together with the alternative processing methods, are discussed. Depending on the initial processing method, different factors have to be taken into consideration to produce a high quality billet. With powder metallurgy processing, the composition of the matrix and the type of reinforcement are independent of one another. However, in molten metal processing they are intimately linked in terms of the different reactivities which occur between reinforcement and matrix in the molten state. The factors controlling the distribution of reinforcement are also dependent on the initial processing method. Secondary fabrication methods, such as extrusion and rolling, are essential in processing composites produced by powder metallurgy, since they are required to consolidate the composite fully. Other methods, suc...

Particle reinforced aluminium and magnesium matrix composites

Tensile properties of nanometric Al2O3 particulate-reinforced aluminum matrix composites

Review on the physicochemical treatments of rice husk for production of advanced materials

A composite powder with a fine homogeneous distribution of the reinforcement phase in the whole particle can be obtained by mechanical alloying. Aluminium PM6061 unreinforced, and matrix composite reinforced with Si 3 N 4 and AlN powder, are milled in a high-energy attritor mill and the powder properties are compared with those of the same composite composition mixed in a horizontal low-energy ball mill. The correlation observed between the apparent densities and the milling time, explained by the morphological and microstructural evolution of the powder particles during the high-energy milling process, is used to determine the steady state of the process. At short milling times, the apparent density decreases as the milling time is extended, due to the deformation dominant at this stage; at longer milling times, it starts to increase with increasing milling time due to the piling up of the flattened particles and fracture of the welded particles. When mechanical alloying reaches the steady state, the apparent density is stabilized. A simple model is proposed to illustrate the mechanical alloying of a ductile–brittle component system. The particle size distribution and the microhardness of the mechanically alloyed particles are determined.

Effect of mechanical alloying on the morphology, microstructure and properties of aluminium matrix composite powders

In the present investigation, the influence of B 4 C on the mechanical and Tribological behavior of Al 7075 composites is identified. Al 7075 particle reinforced composites were produced through casting, K 2 TiF 6 added as the flux, to overcome the wetting problem between B 4 C and liquid aluminium metal. The aluminium B 4 C composites thus produced were subsequently subjected to T6 heat treatment. The samples of Al 7075 composites were tested for hardness, tensile, compression, flexural strengths and wear behavior. The test results showed increasing hardness of composites compared with the base alloy because of the presence of the increased ceramic phase. The wear resistance of the composites increased with increasing content of B 4 C particles, and the wear rate was significantly less for the composite material compared to the matrix alloy. A mechanically mixed layer containing oxygen and iron was observed on the surface, and this acted as an effective insulation layer preventing metal to metal contact. The coefficient of friction decreased with increased B 4 C content and reached its minimum at 10 vol% B 4 C.

Influence of B4C on the tribological and mechanical properties of Al 7075–B4C composites

The roles of volume fraction and size of reinfrecement on the steady state creep behaviour of pure aluminium matrix-silicon carbide particulate composites have been studied in the temperature range 623–723 K. The observed apparent stress exponents are higher than 15 and apparent activation energy is 249 kJ mol−1. By considering the existence of a threshold stress, the data for 1.7 μm particulate reinforced composites with different volume fraction can be rationalized according to the substructure invariant model. The effective stress-strain rate behaviour of composites with 10 vol.% of coarser particulates (14.5 and 45.9 μm), however, agree with the stress dependent substructure model. The present analysis is validated by constructing a new type of “dislocation creep mechanism map”. The observed threshold stress varies with the volume fraction of reinforcement and is independent of particulate sizes and test temperatures. It is suggested that a model based on applied stress independent load transfer is required to explain the origin of such a threshold stress.

Steady state creep behaviour of silicon carbide particulate reinforced aluminium composites

Microstructure-property correlation in (XD) composites

Monolithic MoSi 2 and MoSi 2 20vol.%SiC composites have been prepared by reaction hot-pressing Mo and Si or Mo, Si and SiC powder mixtures in the range 1450–1500°C. As Si atoms diffuse into the Mo lattice under external pressure, a complete reaction takes place between Mo and Si leading to the formation of MoSi 2 phase. The initial Mo particle size affects the grain size of the MoSi 2 that is formed. Grain boundaries and interfaces of MoSi 2 with SiC were abrupt on the atomic scale, except for some MoSi 2 /SiC boundaries, where a 5–8 nm thick amorphous phase, believed to be SiO 2 , could be seen. MoSi 2 was also processed by vacuum hot-pressing MoSi 2 powder at 1700°C for comparison. The composites show a moderate increase in flexural strength and hardness over monolithic hot-pressed polycrystalline MoSi 2 . There is, however, a significant increase in the high-temperature compressive yield strength due to addition of SiC reinforcements. Grain sizes of the MoSi 2 matrix strongly influence the fracture toughness and related micromechanisms.

Processing-microstructure-property relationships in reaction hot-pressed MoSi2 and MoSi2/SiCp composites

Creep behaviour of an aluminium-silicon carbide particulate composite

Two Al20vol.%TiB 2 composites produced by different processing routes, namely conventional powder metallurgy (PM) and XD TM (XD TM is a proprietary process of Martin Marietta Laboratories), were creep tested under compression in the temperature range 573–673 K to evaluate the steady state creep mechanisms. The steady state creep rate data of the conventional composite, covering alomost five orders of magnitude in creep rate, were rationalized in terms of the substructure invariant model, which predicts a stress exponent of eight and lattice diffusion control, together with the existence of a threshold stress. The higher values of the apparent stress exponent (8–14) and the apparent activation energy (339 kJ mol −1 ), observed for the XD composite, do not agree with the existing models for dislocation creep even after considering the threshold stress. This is possibly because of the anomalous creep behaviour of the XD composite observed at higher temperatures. A comparison of the steady sreep data for PM AlTiB 2 and AlSiC composites shows that the SiC particulate provides better creep resistance than the TiB 2 particulate. The XD AlTiB 2 composite has better creep strength than the conventional AlTiB 2 composite; however, its creep strength is inferior to the AlSiC composite. It has been suggested that an applied stress-independent load transfer mechanism is required to explain the origin of the threshold stress for steady state creep in discontinuously reinforced Al matrix composites.

Y. R. Mahajan

Papers

Steady state creep behaviour of silicon carbide particulate reinforced aluminium composites

Microstructure-property correlation in (XD) composites

Processing-microstructure-property relationships in reaction hot-pressed MoSi2 and MoSi2/SiCp composites

Creep behaviour of an aluminium-silicon carbide particulate composite

High-temperature creep of AlTiB2 particulate composites