Discontinuously Reinforced Metal Matrix Composites Research Articles

Influence of micro-structural parameters on fatigue life of discontinuous reinforced metal matrix composites

The micro-structural parameters such as reinforcement shape, size, distribution, volume fraction, property mismatch, aging condition, bonding strength, and whisker orientation can influence the fatigue life of discontinuous reinforced metal matrix composites (DRMMCs). The strengthening effect plays a vital role in predicting the fatigue behaviour of DRMMCs. The modified shear lag (MSL) and enhance dislocation density (EDD) are two main factors that describes the strength of DRMMCs. In the present work, fatigue crack growth life model based MSL and EDD strengthening mechanism has been developed by integrating fatigue damage deformation at the crack-tip under the total strain-controlled conditions. The closed form expression predicts the dependency of particle size, reinforcement volume fraction and reinforcement constraint of the matrix on the fatigue crack growth life. The model fitting with experimental data affirms the appropriateness of proposed fatigue crack growth life prediction model for DRMMCs.

The energy‐based multistage fatigue crack growth life prediction model for DRMMCs

AbstractA complex closed form expression for the multistage fatigue crack growth life has been developed to predict the fatigue life of discontinuous reinforced metal matrix composites. The multistage fatigue crack growth model includes the microstructural features of composite such as aspect ratio, volume fraction of reinforcement, constraint of fibre in the matrix, and interfacial bonding strength. It characterizes the fatigue damage evolution in various stages of crack nucleation/initiation and the growth of crack culminating in failure. A fatigue crack initiation model is presented, using energy balance equation, considering the nucleation process of phase transformation. The short crack growth life is estimated using the dependency of cyclic crack‐tip opening displacement that is directly correlated with the fatigue crack growth rate whereas the long crack growth life is evaluated using Li and Ellyin model. The sensitivity analysis of crack initiation model is performed to provide in‐depth knowledge on the mechanistic aspect. The sensitivity analysis shows that the little variation in the parameters “cyclic strain hardening exponent” (n′), “cyclic strength coefficient” (K′), and short crack length (asc) significantly affects the total fatigue crack growth life. It is observed that reinforcement increases the cyclic plastic deformation level and cyclic plastic zone size near the crack tip, under the total strain‐controlled conditions. The effect of total fatigue crack growth life is found to be significant in low cycle fatigue applications.

In-situ space-confined synthesis of well-dispersed three-dimensional graphene/carbon nanotube hybrid reinforced copper nanocomposites with balanced strength and ductility

It is a tough issue to design and fabricate discontinuously reinforced metal matrix composites (DRMMCs) with desired mechanical and physical properties. Utilizing nanocarbon materials such as one-dimensional (1D) carbon nanotubes (CNTs), two-dimensional (2D) graphene or their hybrids as reinforcements for DRMMCs is now considered to be a good solution because of their outstanding intrinsic characterizations. In this work, we proposed a novel in-situ space-confined strategy to circumvent the problem of the controllable interconnection and bonding between CNTs and graphene and thus constructed a well-dispersed CNTs embedded in three-dimensional graphene network (3D GN) hybrid structure for fabricating reinforced Cu matrix nanocomposites. The as-obtained 3D GN/CNT hybrids reinforced copper bulk nanocomposites exhibited a significant strengthening efficiency and a more balanced strength vs. ductility relation compared with Cu matrix composites reinforced by single component (CNT or 3D GN) with the same volume fraction.

Influence of residual thermal stresses on fatigue crack growth life of discontinuous reinforcements in metal matrix composites

AbstractThe fatigue life prediction model based on crack propagation from micro‐structural features is derived and presented for planar and randomly oriented Discontinuous Reinforced Metal Matrix Composites (DRMMCs). The model contains the influence of micro‐structural properties such as aspect ratio, volume fraction of particle/fibre and constraint between particle and the matrix. The effect of residual thermal stresses generated within the matrix during development of composite is considered. The particle/fibre plays a dominant role in the development of the cyclic plastic zone size ahead of the crack tip; moreover, it enhances the cyclic plastic deformation characteristics of DRMMC. The theoretical model‐based evaluations for low cycle fatigue in DRMMCs are within the proximity of the experimental results.

Analytical, numerical and experimental approach for design and development of optimal die profile for the cold extrusion of B4C DRMM Al 6061 composite billet into hexagonal section

Aluminium based matrix composites with boron carbide as particle reinforcement called Discontinuous reinforced metal matrix composites (DRMMs) possess high specific strength, high elastic modulus, good wear resistance, damping capacity and thermal stability. But during the development of DRMM composites, compression process like extrusion is an advisable secondary process for homogenous structure. This research work investigates the metal flow behavior of Al-B4C based DRMM composite through six different die profiles namely third order polynomial, fourth order polynomial, cosine, elliptical, hyperbolic and conical geometry. Extrusion load, stress and strain distribution, and metal flow for above said die profiles are predicted by using analytical approach upper bound technique and compared with finite element method. Cosine and third order polynomial profiles are found to be most optimal in terms of homogenous and minimal extrusion load requirement. To validate the results, specially made Al-B4C composite through stir casting route was extruded from round to hexagon through an exclusively fabricated cosine die. Results observed from the experiment have good agreement with both analytical and numerical.

Design and analysis of optimal die profile for the extrusion of round SiC DRMM Al 6061 composite billet into hexagonal section

Aluminium based matrix composites with silicon carbide as particle reinforcement called discontinuous reinforced metal matrix composites (DRMMs) have excellent mechanical properties like high yield strength, resistance to wear and vibration resistance. But during the development of DRMM composites, compression process like extrusion is an advisable secondary process for homogenous structure. This research work investigates the metal flow behaviour of Al–SiC based DRMM composite through six different die profiles namely third-order polynomial, fourth-order polynomial, cosine, elliptical, hyperbolic and conical geometry. Extrusion load, stress and strain distribution, and metal flow for above said die profiles are predicted using analytical approach upper bound technique and numerical technique finite element method. Cosine and third order polynomial profiles are found to be most optimal in terms of homogenous and less extrusion load requirement. To validate the results, specially made Al–SiC composite through stir casting route was extruded from round to hexagon through an exclusively fabricated cosine die. Results observed from the experiment have good agreement with both analytical and numerical methods.

The Effects of Porosity on Mechanical Properties of Cast Discontinuous Reinforced Metal–Matrix Composite

This paper presents the findings of porosity effects on the mechanical properties of cast discontinuous reinforced metal–matrix composites (DRMMCs). Aluminum–silicon alloy composite specimens are produced by the stir casting method with varied content of reinforcing silicon carbide (SiC) particles for metallographic study, porosity measurement, and tensile and fatigue testing. The factors of porosity formation are assessed from three different levels of stirring speeds, 100, 200, and 500 rpm, varied SiC content, and modification of the stir casting method. The evaluated porosity data exhibits increased SiC particle content and stirring speed has increased the porosity formation in the cast DRMMC whereas the modification of the stir casting method significantly has decreased the porosity content. The least content of porosity evaluated is at 0.09% in the modified stir cast DRMMC, while the highest is at 12.45% in the conventionally stir cast DRMMC. In monotonic tensile testing, increasing porosity content has decreased the ductility, tensile strength, and yield strength of cast DRMMC. Though, based on fully reversed (R 1/4 1) fatigue test, porosity formation has increased the cast DRMMC fatigue strength at 1 10 7cycles. The fatigue strengths of cast DRMMCs at 5, 10, and 15% reinforcing SiC particle are 129.7, 141.5, and 157.3 MPa, respectively. These are due to either isolated porosity formation or porosity presence among particle clusters as observed in the metallographic studies.

Bounds of the Thermal Conductivity in Discontinuously Reinforced Metal-Matrix Composites

A simple model based on the network simulation method is proposed to estimate numerically the thermal conductivity of particulate reinforced metal-matrix composites. The estimation is carried out running the model in the standard Pspice code, the computing time being negligible. The 3-D solid is discretized in 1000 cubic volume elements which represent an acceptable approximation of the shape of the particles. For each reinforcement percentage and each combination of matrix and reinforcement more than 200 tests were carried out, so that the results may be considered close to the exact values. The limit values are scarcely influenced by the effect of the 3-D geometry and basically depend on the amount of the reinforcement. Applications to aluminum and titanium matrix composites reinforced with different types of particles are presented covering a wide range of reinforcement percentages.

A model for the low cycle fatigue life prediction of discontinuously reinforced MMCs

An analytical model for predicting the crack initiation life of low cycle fatigue (LCF) of discontinuously reinforced metal matrix composites (DR-MMCs) has been proposed on the basis of Gibbs free energy law. The formation of a fatigue crack was considered to be associated with the reduction of the internal energy that could be expressed as a part of the area of the saturated hysteresis loop. The effects of the volume fraction V f, cyclic strain hardening exponent n′ and cyclic strength coefficient K′ on the LCF fatigue crack initiation of DR-MMCs were analyzed at different levels of the plastic strain amplitude. While both the lower level of the plastic strain amplitude and the lower volume fraction of the reinforcement particles were found to increase the LCF crack initiation resistance, the effects of n′ and K′ were more complicated. Although a high n′ value or a low K′ value would lead to a longer LCF crack initiation life, their influence was interrelated. The theoretical predictions based on the proposed crack initiation model, in conjunction with Ding et al.’s [1] crack propagation model, were found to be in good agreement with the experimental data reported in the literature. Since both processes of the crack initiation and propagation have been considered, the accuracy of predicting the total LCF life of DR-MMCs was improved.

Relaxation of the residual stress in 6061Al-15 vol.% SiCw composites by isothermal annealing

Abstract The residual stress (RS) in extruded 6061Al-15 vol.% SiC w composites manufactured by a powder metallurgical route has been studied by neutron diffraction. Several precipitation states were investigated, going from a fully hardened state, T6, to an over-aged (OA) condition. For comparison, the unreinforced powder metallurgy 6061Al alloy was also studied. The end precipitation stages T6 and OA were obtained after appropriate heat treatments. The T6 condition involves a quenching step, which provokes a macro-RS state. Instead, the OA treatment (at 300 °C) does create relaxation at intermediate times and leads to total relaxation of the axial deviatoric component and to partial relaxation of the hydrostatic term of the stress in the composite. Total relaxation of both deviatoric and hydrostatic terms occurs in the unreinforced alloy. Furthermore, it is found that relaxation of the macro-RS occurs more rapidly in the unreinforced alloy than in the composite. These results are in agreement with the idea that discontinuously reinforced metal matrix composites (DRMMCs) have a more stable microstructure than the corresponding unreinforced alloys. On the other hand, no decrease of the deviatoric micro-RS developed in the composite and a small reduction of the hydrostatic one occurs when treating from the T6 to the OA condition. This implies that the geometry of the reinforcing particles plays an important role and the geometrically necessary dislocations do not disappear with heat treatment.

Multiscale modelling of anisotropic damage of discontinuous reinforced metal matrix composites

Composite materials are used in many industries. Depending of the limit conditions of application it is possible to choose the type of reinforcement, its volume fraction, its orientation, its size, the type of matrix and its mechanical properties. It has been experimentally observed that damage often initiates very early during the loading. It is diffused in all the material on the microcracks form. It contribute to the strain of the composite under stress. The mechanical properties as stiffness, yield stress, hardening are influenced by damage. So, it is necessary to take into account the damage mechanisms as source of deformation coupled with the classical deformation mechanisms as elasticity, plasticity, viscosity. This paper deals with the way of taking into account damage in the behaviour law based on a micro-macro relationship. We will focus only on discontinuous reinforced composite. A Mori and Tanaka model has been used with damage laws written at the microscale. The macroscopic mechanical properties are obtained by homogenisation. The type of reinforcement could be particulates, short fibres, plates, which are randomly oriented in the matrix. The matrix could be in polymer, in metal.

High temperature fatigue of discontinuously-reinforced metal–matrix composites

This paper reviews the current knowledge on the fatigue behavior of discontinuously-reinforced metal–matrix composites at high temperature. The effect of cyclic loading at high temperature on the micromechanims of deformation, crack nucleation, and crack propagation are dealt with. The overall performance of these composites under isothermal and thermo-mechanical fatigue loading have been examined. A brief account of the current industrial applications of discontinuously-reinforced metal–matrix composites in components subjected to fatigue at high temperature is provided

Study on fracture strain of discontinuously reinforced metal matrix composites

The fracture strain of discontinuously reinforced metal matrix composites (DRMMCs) is studied according to a damage mechanics model proposed by McClintock, with a localized crack propagation hypothesis and particle-related crack initiation mechanism. An estimation equation is proposed. The estimated results are verified with the data available in the literature and those measured on some types of DRMMCs obtained by powder metallurgical techniques. The experimental results show good agreement with the estimation model.

Fatigue Crack Coalescence in Discontinuously Reinforced Metal Matrix Composites: Implications for Reliability Prediction

This paper examines aspects of the distribution of the longest possible microcracks in specimens of a fatigued aluminum-matrix silicon carbide whisker composite and the probability of coalescence in general. The length of the longest microcracks in this material is determined largely by coalescence and making reasonable assumptions is bounded and an upper bound can be calculated. This upper bound is used to fit sample data to an asymptotic extreme value distribution from which reliabilities based on an arbitrarily chosen critical length are calculated. It is shown that for reliabilities corresponding to failure probabilities less than even 10−10 the mere presence of an upper bound on microcrack length can determine the extreme value distribution independent of the actual value of that bound provided that the experimental data supporting the extreme value distribution are entirely characteristic of actual service and that a loading criterion is met.

Interface characterization of the discontinuously reinforced metal matrix composites based on wavelet analysis of acoustic emission behaviour

Interface characterization of the discontinuously reinforced metal matrix composites based on wavelet analysis of acoustic emission behaviour

Interfacial microchemistry and microstructure of Al-Mg-Si alloy matrix composites reinforced with Al 2O 3 particulates

In discontinuously reinforced metal matrix composites produced by ingot metallurgy, the ceramic particles are introduced in the molten alloy which provides adequate opportunity for reactions to quickly occur between the particles and the molten alloy. Similar reactions, although less rapid, can occur in composites produced by powder metallurgy routes. Using transmission electron microscopy (TEM) and X-ray spectroscopy, matrix and interfacial microchemistry can be analyzed in the metal matrix composites and correlated with the observed characteristics. In this study, the microstructure and the microchemistry of Al-Mg-Si alloy matrix composite reinforced with 10 vol.% Al{sub 2}O{sub 3} were characterized in order to examine the effect of the reinforcements on the distribution of alloying elements.

Microstructure evolution during solidification of DRMMCs (Discontinuously reinforced metal matrix composites): State of art

Distribution of particle reinforcements in cast composites is determined by the morphology of the solidification front. Interestingly, during solidification, the morphology of the interface is intrinsically affected by the presence of dispersed reinforcements. Thus the dispersoid distribution and length scale of matrix microstructure is a result of the interplay between these two. A proper combination of material and process parameters can be used to obtain composites with tailored microstructures. This requires the generation of a broad data base and optimization of the complete solidification process. The length scale of soldification microstructure has a large influence on the mechanical properties of the composites. This presentation addresses the concept of a particle distribution map which can help in predicting particle distribution under different solidification conditions. Future research directions have also been indicated.

Microstructural Optimization of Discontinuously-Reinforced Metal-Matrix Composites

Microstructural Optimization of Discontinuously-Reinforced Metal-Matrix Composites

A quantitative evaluation and experimental study on the elastic limit for DRMMCs

The evaluation of the properties of discontinuously reinforced metal matrix composites (DRMMCs) has been the focus in research of composite fields, though the quantitative evaluation of the properties of composites is difficult. Until now in spite of some property evaluation models were proposed, because of their omission of the coordinated effects of the continuous matrix and discontinuous reinforcements, or over simplification of the stress/strain fields in practical DRMMCs, both diverging the real conditions in DRMMCs, the evaluation of strength properties of DRMMCs are empirical, except for elastic constants, and there is few reports about the quantitative evaluation of elastic limit of DRMMCs. In present work, the effective axial divided stress in arbitrary section and the strain constraint in matrix are introduced into analysis, to establish an expression of predictive elastic limits of DRMMCs, and to evaluate the engineering elastic limit of this type of composites.

Interfacial Effects on Fatigue and Fracture in Discontinuously Reinforced Metal Matrix Composites

Interfaces in conventional monolithic alloys exert an important influence on fatigue and fracture behavior. In discontinuously reinforced metal matrix composites (MMCs), the role of interface is even more dominant. The interfacial is higher in MMCs and the interfaces are generally of high energy and chemically unstable. This paper reviews the factors which can affect interfacial strength in discontinuously reinforced MMCs, and the ways in which interfacial strength can be controlled. The effects of interfacial strength on fatigue crack propagation and fracture behavior are then illustrated.