Particulate Metal Matrix Research Articles

Microstructural characterization and tensile properties of hot pressed Al–SiC composites prepared from pure Al and Cu powders

A conventional hot pressing method was used to produce Al–Cu–SiC particulate metal matrix composites. The matrix alloy was prepared from elemental powders of Al and 5 wt.% Cu. The composite comprises 13.3 or 27.2 vol.% SiC p with an average size of 10 μm. The powder mixtures were hot pressed uniaxially under nitrogen atmosphere. It is shown that Cu aids in the formation of a liquid phase. Practically pore free structures are obtained after pressing at 600°C. Elastic modulus, tensile strength and yield strength values were improved with incorporation of the reinforcement in composites prepared by fine Al powder. The ductility, on the other hand, decreased sharply with increasing amount of strengthening phase.

Particulate size effects in the particle-reinforced metal-matrix composites

The influences of particle size on the mechanical properties of the particulate metal matrix composite are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted.

Simulation and quantitative assessment of homogeneous and inhomogeneous particle distributions in particulate metal matrix composites

Reinforcement distributions play an important role in various aspects of the processing and final mechanical behaviour of particulate metal matrix composites (PMMCs). Methods for quantifying spatial distribution in such materials are, however, poorly developed, particularly in relation to the range of particle size, shape and orientation that may be present in any one system. The present work investigates via computer simulations the influences of particle morphology, homogeneity and inhomogeneity on spatial distribution measurements obtained by finite-body tessellation. Distribution inhomogeneity was simulated both by the segregation of particles away from specified regions within a microstructure and by generating point density peaks at random locations within a microstructure. Both isotropic and anisotropic inhomogeneous distributions were considered to simulate distribution patterns in PMMCs before and after mechanical working. It was found that the coefficient of variation of the mean near-neighbour distance (COV(dmean)), derived from particle interfaces using finite-body tessellation, was essentially independent of particle shape, size distribution, orientation and area fraction in homogeneous (random) distributions, but showed great sensitivity to inhomogeneity. Increased values of COV(dmean) were seen for both forms of inhomogeneous distributions considered here, with little influence of particle morphology. The COV(dmean) was also seen to be sensitive to anisotropic clustering, the presence of which was identified via nearest-neighbour angles and cell orientations. Although generally formulated for PMMCs, the present results may be generalized to other systems containing low aspect ratio finite bodies of low to moderate area fraction.

Cluster characterisation in a metal matrix composite

Inhomogeneities in the reinforcement of particulate metal matrix composites (PMMC) change their post elastic behaviour. Thus it is important to qualify and quantify particle clustering. Images of an Al–Si/SiC PMMC are analysed here in order to calculate interparticle distance, intercluster distance, and distance from isolated particles to clusters. This is done on six images at ×100 magnification, two images at ×200 magnification and one image at ×50 magnification. The analysis is done through increasing size dilatations. This is applied to the nine images but also to random images, allowing thus a comparison of the nine images and random ones. This gives four characteristic distances, and also the cluster size and the link between clustering and cracking. The particle and cluster alignment in the extrusion direction is also shown.

Measurement and modelling of some mechanical properties of electroless NiP/diamond particulate metal matrix composite thin films

Some mechanical and tribological properties of electroless Ni-P/ diamond composite films are measured, modelled by the finite element method and discussed. Preparation methods are briefly sketched. Hardness, Young's modulus and Milman's plasticity parameter are evaluated by diamond pyramid indentation methods; sliding wear behaviour is tested with the pin-on-disk technique. The application of the correlations for indentation testing is treated statistically in view of a quantitative use. The variations of mechanical properties induced by the dispersion of ceramic particles are quantitatively correlated with structural characteristics of the composite films.

Optimal drilling of particulate metal matrix composites based on experimental and numerical procedures

An experimental and numerical study of the evolution of cutting forces, tool wear and surface finish, measured when drilling the particulate metal matrix composite A356/20/SiCp-T6 is presented. The experimental work was developed through the continuous measurement of the cutting forces with an appropriate piezoelectric dynamometer. The wear type was identified and its evolution with cutting time was measured. Drills with polycrystalline diamond were tested. The surface finish of the holes was evaluated with a profilometer. Using the experimental results, a numerical search of optimal drilling conditions was performed. Since there are contradictory objectives, such as maximisation of tool life and minimisation of tool wear, the concept of the Pareto optimum solution is considered in the optimisation procedure. An evolution strategy is adopted to obtain the optimal solution for cutting speed, feed rate and tool life prediction with industrial interest.

Production of titanium particulate metal matrix composite by mechanical milling

Mechanical milling is an established production method for aluminium particulate metal matrix composites (MMCs). There are examples of its use for high performance automotive applications and within the aerospace industry. The production of a titanium particulate MMC is still in the developmental stage. However, compared to conventional titanium alloys such materials offer improvements in stiffness, strength, fatigue and creep properties, high temperature capability, and wear resistance. This paper describes the use of mechanical milling for the production of titanium particulate MMCs with the addition of 10 vol.-%TiB. Gas atomised titanium powders with additions of either boron or TiB2 were milled in a high purity argon atmosphere to avoid contamination of the powders by oxygen or nitrogen. The distribution of the boron or TiB2 with increasing milling time is discussed along with the effect of the alloy composition. Gas atomised, hydride dehydride, and sponge fine powder blends are also compared. The powders were subsequently hot isostatically pressed at 500°C for 2 h at 150 MPa followed by 900°C for 2 h at 150 MPa. During this consolidation process TiB was formed by an in situ reaction between either the TiB2 or boron and the titanium matrix.

Simulation and quantitative assessment of finite­size particle distributions in metal matrix composites

A series of finite­size particle distributions were simulated to investigate the effects of particle size, shape, orientation, and area fraction on the quantification of homogeneity in structural particulate metal matrix composites (MMCs). It is found that, for nominally random distributions, the values of conventional centre-to-centre nearest-neighbour spacing parameters are influenced by particle morphology, and, as such, are unsuitable for characterising distributions of finite-size particles. However, the coefficient of variation of the mean near-neighbour distance COV(dmean), derived from particle interfaces using finite-body tessellation, appears independent of particle shape, size distribution, orientation, and area fraction, while showing great sensitivity to particle clustering. In the range of particle morphological characteristics studied, the random distributions were found to exhibit a consistent value of COV(dmean) equal to 0.36±0.02. The degree of inhomogeneity of any given distribution may then be evaluated by simply comparing the measured COV(dmean) with this value.

Materiales compuestos de matriz metálica. II parte. Métodos de procesado y consolidación de MMCs reforzados con partículas

This work is a review of the different forming and consolidation methods for particulate metal matrix composites. In the first part we focus in the forming methods from raw materials while the second is centred in the consolidation from preforms in order to get the final part.

Extrusion Modelling of Aluminum Particulate Composites

Particulate Al matrix composites inexpensively fabricated into large billets by liquid metal mixing receive final fabrication into components by mechanical forming, notably at high temperatures to minimize cracking and particle-matrix degradation. A hot torsion project on alloys 6061, A356, 7075 and 2618 with 10 to 20% of Al 2 O 3 or SiC determined hot strength and ductility, constitutive constants and microstructural evolution. The substructure is very heterogeneous exhibiting regions of subgrains, non-cellular masses of dislocations and small highly misoriented cells. The modeling of extrusion of the various particulate metal matrix composites was conducted by finite element analysis to develop strain rate, mean stress and temperature distributions. The relative load-stroke curves and extrudability were compared.

Machining of Al/SiC particulate metal matrix composites part III: comprehensive tool wear models

A robust 3-D finite element model of a cutting tool is presented in this paper. The model takes into consideration the thermal and mechanical interactions at the tool/chip and tool/workpiece interfaces. Temperature-dependant material properties are incorporated in the model. The model was applied to the special case of the turning of Al/SiC particulate metal matrix composites, where high tool wear rates represent a challenge to the industry. The temperatures and stresses predicted by the model are in agreement with experimental measurements and tool wear observations. The wear patterns predicted by the model include crater wear, tool pitting and tool chipping. Thus, the model presented could be utilized in the selection of the tool material, geometry and cutting parameters that would result in the least tool wear, and hence helps to reduce machining costs and tool change down-time. Controlling tool wear is expected to enhance the surface integrity of the workpiece.

Microstructure and tensile properties of mechanically alloyed Ti–6A1–4V with boron additions

Particulate titanium metal matrix composites (MMCs) offer advantages for a wide range of structural applications with increases in stiffness and possibly wear resistance. A powder metallurgy/mechanical alloy route was used successfully to give a fine, uniform distribution of boron in titanium alloy powder. This powder was hot isostatically pressed and during the process TiB reinforcement was formed by an in-situ reaction. Significant tensile ductility in MA titanium alloy (without boron), equivalent to wrought material was achieved, along with good tensile strength. In materials with high TiB volume fractions it is thought that the fine boride size and high strength matrix caused embrittlement. This suggests that the use of a lower strength matrix, such as commercial purity titanium, and shorter milling times would provide a better balance of strength, stiffness and ductility for this type of composite.

Microstructural characterization of cemented carbide samples by image analysis techniques

The microstructure of various cemented carbide samples have been characterized by an automatic image analyzer (Imagist II, Princeton Gamma Technologies) connected to both light optical and scanning electron microscopes (SEM). The volume fraction, size and shape of the carbide grains were measured by image analyzer and the result have been co-related with bulk properties such as hardness, and toughness. It was concluded that the variation of volume fraction, grain size and morphology of carbide particles in metal matrix corresponds closely to the final properties of the cemented carbide samples.

A Study on the Improvement of Fatigue Strength in Particulate Reinforced Metal Matrix Composites at Elevated Temperatures

Fatigue strength of NiAl and NiAl particulate reinforced aluminum alloy composites fabricated by the diecasting method was examined at room and elevated temperatures. The results were compared wit h that of SiC particulate reinforced one. The particulate reinforced composites showed some improvement in the static and fatigue strength at elevated temperatures when compared with that of Al alloy. The composites reinforced by intermetallic compound particles showed good fatigue strengths at elevated temperatures especially alloy composite showed good fatigue limit up to high temperature of 30. Adopting intermetallic compound particle as a reinforcement phase, it will be possible to develop MMC representing better fatigue property at elevated temperature.

Low cycle fatigue behaviour of particulate reinforced metal matrix composites

The low cycle fatigue (LCF) resistance of two different 6061 Al/20 vol% alumina particulate metal matrix composites (MMCs) in a peaked-aged condition has been evaluated under fully reversed strain control testing. Test results were combined with scanning electron and optical microscopy investigations to determine the effects of reinforcement particles and strain amplitude on the LCF behaviour of these MMCs. Both materials show three stages of response to LCF: initial fast hardening or softening in the first few cycles; gradual softening for most of the fatigue life; and a rapid drop in the stress carrying capability prior to failure. Both MMCs exhibit short LCF life which follows a Coffin-Manson relationship. All tested specimens demonstrate ductile fracture morphology at final failure. The experimental results are discussed in respect of strain amplitude, matrix composition and reinforcement shape and crack initiation.

Plastically Induced Anisotropy on Metal Matrix Composites

The influence of the plastically induced anisotropy on the formability of particulate metal matrix composites (MMCs) is studied with the aid of the strain ratio, R, and the concept of the forming limit diagram (FLD). Series of simple tension experiments were carried out for the determination of R both for the matrix material and the respective composite one. For the theoretical construction of the FLDs a modified version of the T-criterion of failure is employed, according to which materials fail by either yielding or fracture when their volume or shape change capacity is exhausted. The results are compared both with the theoretical predictions obtained by the same criterion, but ignoring the plastic anisotropy, and also with the experimental results obtained from a series of in-plane and out-of-plane sheet forming experiments. It is concluded that neglecting the plastic anisotropy may lead to fallacious conclusions.

Imaging damage evolution in a small particle metal matrix composite

It is difficult to study effectively microstructural damage in metal matrix composites (MMCs) due to artefacts arising from traditional metallographic sample preparation techniques. The sectioning and imaging capabilities of the focused ion beam (FIB) microscope provide an excellent method for studying damage accumulation in MMCs. The capabilities of the FIB system have been used to carry out a study of damage evolution in a powder-processed/hot-extruded Al2080/SiCp MMC. Microvoid damage is found to be preserved accurately during FIB sectioning, allowing measurements of the fraction of decohered particles and the void area fraction. These microscopic damage measurements are correlated with the macroscopic damage parameter, D, as determined by density measurements. Using transmission electron microscopy, the evolution of dislocation structures at the SiC-matrix interfaces has been examined. A previously unreported decohesion mechanism has been observed.

Multiscale Modeling of Highly Heterogeneous MMCs

AbstractA hierarchical modeling concept incorporating different material descriptions at appropriate length scales is presented and applied to High Speed Steel (HSS) in order to quantify the overall thermomechanical behavior. Such materials can be viewed as particulate metal matrix composites (MMCs) with a strongly clustered arrangement of linear‐elastic carbidic particles in an elastic‐plastic martensitic‐austenitic matrix. On the mesoscale mean field methods are used to describe the effective material behavior of the mesophases depending on instantaneous constituent properties and local fields. Plastic strains in the matrix show a strongly inhomogeneous distribution which hardly can be predicted by standard micromechanics based methods neglecting the mesostructure. On the microscale the influence of the geometrical paramters describing the stress distribution inside the inclusions is studied by means of quasi‐random structure unit cells. Statistical methods are employed in order to quantify the relationship between particle size and their probability of fracture. The role of residual stresses due to quenching of the material prior to mechanical loading and their beneficial effect on the particles is emphasized.

Effect of hot isostatic pressing on cast A357 aluminium alloy with and without SiC particle reinforcement

The effect of Hot Isostatic Pressing (HIPping) treatments on porosity in the aluminium casting alloy A357 and stir-cast A357/15 vol % SiC particulate Metal Matrix Composite has been investigated. Densitometry and image analysis have been used to determine the apparent percentage porosity. Four HIPping temperature profiles have been investigated (103 MPa pressure) and mechanical testing has been carried out by single notch four point bending. The bend strength was increased after HIPping relative to as-received. The optimum HIPping cycle involved HIPping for a short time in the semi-solid region followed by a sustain below the solidus.

Multiscale modeling of highly heterogeneous particulate MMCs

A continuum mechanics based multiscale modeling concept incorporating different material descriptions at appropriate length scales is presented and applied to High Speed Steels (HSSs) in order to study their overall and local thermomechanical behavior. Such materials can be viewed as particulate Metal Matrix Composites (MMCs) with a strongly clustered arrangement of thermoelastic carbidic particles embedded in a thermoelastoplastic steel matrix. In the contribution a Mesophase Cell Hierarchical Model is presented, which combines a unit cell approach for handling the clustered topology at the mesoscale with mean field based constitutive material laws (a mean field based version of the Transformation Field Analysis and an incremental Mori-Tanaka approach) for describing the matrix -inclusion-type composite at the microscale. As an altemative approach, a Finite Element based 2/D micromechanical method is used, in which the HSS is modeled as a material with a graded microstructure. Results are discussed in terms of the overall thermoelastoplastic behavior and of microscale parameters relevant for local damage initiation and evolution.