Syed Hasan

Microdeformation behaviour of Al–SiC metal matrix composites

The satisfactory performance of metal matrix composites depends critically on their integrity, the heart of which is the quality of the matrix-reinforcement interface. The nature of the interface depends in turn on the processing of the MMC component. At the micro-level, the development of local concentration gradients around the reinforcement can be very different according to the nominal conditions. These concentration gradients are due to the metal matrix attempting to deform during processing. This plays a crucial role in the micro-structural events of segregation and precipitation at the matrix-reinforcement interface. Equilibrium segregation occurs as a result of impurity atoms relaxing in disordered sites found at interfaces, such as grain boundaries, whereas non-equilibrium segregation arises because of imbalances in point defect concentrations set up around interfaces during non-equilibrium heat treatment processing. The amount and width of segregation depend very much on (a) the heat treatment temperature and the cooling rate, (b) the concentration of solute atoms and (c) the binding energy between solute atoms and vacancies. An aluminium–silicon–magnesium alloy matrix reinforced with varying amounts of silicon carbide particles was used in this study. A method of calculation has been applied to predict the interfacial fracture strength of aluminium, in the presence of magnesium segregation at metal matrix interface. Preliminary results show that the model succeeds in predicting the trends in relation to segregation and intergranular fracture strength behaviour in these materials. Microhardness profiles of reinforced and un-reinforced aluminium alloys are reported. The presence of precipitates at alloy-reinforcement interface identified by Nano-SEM.

Heat treatment and interface effects on the mechanical behavior of SiC-particle reinforced aluminium matrix composites

The interface plays a vital role in composites. Strengthening behavior of SiC-particle reinforced aluminium matrix composites relies on load transfer behavior across the interface, whereas toughness is influenced by crack deflection at the boundary between matrix and reinforcement and ductility is affected by relaxation of peak stresses near the interface. In general, metal matrix composites often behave asymmetrically in tension and in compression and have higher ultimate tensile strength, yet lower proportional limits, than monolithic alloys. Such behavior of composites lies with the factors governing matrix plasticity, which can be divided into two areas: those affecting the stress rate of the matrix, and those which alter the flow properties of the matrix through changes in microstructure induced by inclusion of the reinforcement. This work focuses on the characterization of the mechanical response of the interface to stresses arising from an applied load in SiC-particle reinforced aluminium matrix composites. The composites have been studied in the as-received (T1) and in the T6 and modified T6 (HT1) conditions. In the nonequilibrium heat treatment processing of the composites, nonequilibrium segregation arises due to imbalances in point defect concentrations set up around interfaces. Mechanical properties, including microhardness and stress-strain behavior, of aluminum matrix composites containing various percentages of SiC particulate reinforcement have been investigated. The elastic modulus, the yield/tensile strengths, and ductility of the composites were controlled primarily by the volume percentage of SiC reinforcement, the temper condition, and the precipitation hardening.

Predicting Interfacial Strengthening Behaviour of Particulate-Reinforced MMC — A Micro-mechanistic Approach

The fracture properties of particulate-reinforced metal matrix composites (MMCs) are influenced by several factors, such as particle size, inter-particle spacing and volume fraction of the reinforcement. In addition, complex microstructural mechanisms, such as precipitation hardening induced by heat treatment processing, affect the fracture toughness of MMCs. Precipitates that are formed at the particle/matrix interface region, lead to improvement of the interfacial strength, and hence enhancement of the macroscopic strength properties of the composite material. In this paper, a micro-mechanics model, based on thermodynamics principles, is proposed to determine the fracture strength of the interface at a segregated state in MMCs. This model uses energy considerations to express the fracture toughness of the interface in terms of interfacial critical strain energy release rate and elastic modulus. The interfacial fracture toughness is further expressed as a function of the macroscopic fracture toughness and mechanical properties of the composite, using a toughening mechanism model based on crack deflection and interface cracking. Mechanical testing is also performed to obtain macroscopic data, such as the fracture strength, elastic modulus and fracture toughness of the composite, which are used as input to the model. Based on the experimental data and the analysis, the interfacial strength is determined for SiC particle-reinforced aluminium matrix composites subjected to different heat treatment processing conditions.

Predicting micro-mechanics damage behaviour at a metal-ceramic interface in a reinforced alloy

The performance of metal matrix composites (MMCs) depends critically on the quality of the matrix-reinforcement interface. The nature of the interface in turn depends on the processing of the MMCs. At the micro-level, local concentration gradients around the reinforcement are being developed during processing and due to the metal matrix attempting to deform during deformation which can be very different to the nominal conditions. This plays a crucial role in the development of micro-structural events such as segregation and precipitation at the matrix-reinforcement interface. Micro-deformation characteristics of matrix reinforcement interface are modelled using commercial FE software and compared with analytical and experimental data. A method of calculation has been applied to predict the interfacial fracture strength of aluminium silicon carbide (Al-SiC) with 20% Vol fraction. Preliminary results show that the model succeeds in predicting the trends in relation to segregation and intergranular fracture strength behaviour in these materials. The proposed hypothesis will help the design engineers to select and use the materials in structural/load bearing applications. Interfacial strengthening characteristics will in turn give more accurate life predictions of such smart composite systems.

Analytical solution of isothermal fatigue crack growth in solid cylinder

Abstract Nowadays many industries deal with components which are subjected to high loads at elevated temperatures than before due to the increasing requirements regarding weight and performance. The simplest process to check the behaviour of the material at high temperature is the isothermal fatigue (IF), by designing a fatigue cycle at constant and uniform temperature to estimate stress-strain required to predict fatigue life of the material. Generally it is assumed that the maximum temperature in the loading cycle represents the most damaging condition likely to be experienced during service life of the component. An empirical IF model for solid cylinder subjected to constant temperature superimposed with sinusoidal mechanical load applied at different stress levels is being proposed. Linear equations are developed to describe the severity of the temperature gradient, thermal stresses, and stress and strain intensity factors through the solid cylinder wall as function of time. Results show the effect of temperature can be explained as increase in von Mises thermal stress increase as a function of increasing temperature. The highest stress at 400 °C recorded is due to inherent hardness increase of the material indicated by high modulus of elasticity. The mechanical stress is more effective than thermal loading and results show that the stress intensity factor decreases with temperature, except at 400 °C (due to hardness increase).

An empirical method of calculating interfacial strength in a second phase reinforced alloy

Abstract Innovations in MMCs are beginning to pay off with new military and commercial developments underway. Engineered solutions, capitalising on the advantages of lightweight and effective thermal performance, are proving the superiority of MMCs over traditional approaches and materials. As a technology-driven 21st century dawns, demand for better performance, productivity and/or efficiency in transportation, aerospace and industrial processes/products will increasingly require the use of these remarkable composite materials. The understanding of the interfacial strengthening mechanisms, therefore, is the key factor for optimising the properties of these remarkable new advanced materials. A method of calculation has been applied in order to predict the interfacial fracture strength of aluminium, in the presence of silicon segregation. The interface fracture toughness was determined as a function of the macroscopic experimental measurements (mechanical properties of the composite) and the microscopic modification parameters (tailoring of interface properties). The model shows success in making prediction possible of trends in relation to segregation and interfacial fracture strength behaviour in SiC particle-reinforced aluminium matrix composites. The model developed here can be used to predict possible trends in relation to segregation and the interfacial fracture strength behaviour in metal matrix composites. The results obtained from this work conclude that the role of precipitation and segregation on the mechanical properties of Al/SiC composites is crucial, affecting overall mechanical behaviour.

A Method of Analysis to Estimate Thermal Down-Shock Stress Profiles in Hollow Cylinders when Subjected to Transient Heat/Cooling Cycle

An empirical solution for the thermal shock stresses in cylindrical shell presented when cylinder is subjected to heating or re-heating case and down-shock cooling by forced air case. Linear equations are developed to describe the severity of thermal shock loading. When thermal gradient and time period are in consideration, it is shown the equations displays good approximation for major characteristics of the thermal shock stress profiles.

Deformation Characteristics of Aluminium Composites for Structural Applications

Silicon carbide (SiC) particulate-reinforced aluminium matrix composites (AMC) are attractive engineering materials for a variety of structural applications, due to their superior strength, stiffness, low cycle fatigue and corrosion fatigue behaviour, creep and wear resistance, compared to the aluminium monolithic alloys. An important feature of the microstructure in the Al/SiC composite system is the increased amount of thermal residual stresses, compared to unreinforced alloys, which are developed due to mismatch in thermal expansion coefficients of matrix and reinforcement phases. The introduction of the reinforcement plays a key role in both the mechanical and thermal ageing behaviour of the composite material. Micro-compositional changes which occur during the thermomechanical forming process of these materials can cause substantial changes in mechanical properties, such as ductility, fracture toughness and stress corrosion resistance. The satisfactory performance of aluminium matrix composites depends critically on their integrity, the heart of which is the quality of the matrix/particle reinforcement interface. The nature of the interface depends in turn on the processing of the AMC component. At the micro-level, the development of local concentration gradients around the reinforcement can be very different to the nominal conditions. The latter is due to the aluminium alloy matrix attempt to deform during processing. This plays a crucial role in the micro-structural events of segregation and precipitation at the matrix-reinforcement interface. The strength of particulate-reinforced composites also depends on the size of the particles, interparticle spacing, and the volume fraction of the reinforcement [1]. The microstructure and mechanical properties of these materials can be altered by thermo-mechanical treatment as well as by varying the reinforcement volume fraction. The strengthening of monolithic metallic material is carried out by alloying and supersaturating, to an extent, that on suitable heat treatment the excess alloying additions precipitates out (ageing). To study the deformation behaviour of precipitate hardened alloy or particulate reinforced metal matrix composites the interaction of dislocation with the reinforcing particles is much more dependent on the particle size, spacing and density than on the composition [2]. Furthermore, when a particle is introduced in a matrix, an additional barrier to the movement of dislocation is created and the dislocation must behave either by cutting through the particles or by taking a path around the obstacles [3].