Dimitrios Myriounis

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.

Experimental analysis of constant-amplitude fatigue properties in 6156 (Al-Mg-Si) sheet aluminum alloy

Fatigue mechanical behavior of wrought aluminum alloy (Al-Mg-Si) 6156 at T4 temper is experimentally investigated. Constant-amplitude fatigue tests, at fixed stress ratio R = 0.1, were carried out, and the respective stress–life diagram was constructed and compared against the competitive 6xxx aluminum alloys, for example, 6082 and 6061. Fatigue endurance limit of AA6156 was found to be approximately 155 ± 5 MPa, that is, almost 30% below yield stress Rp of the material. AA6156 presents almost 50% higher fatigue life in the high-cycle fatigue area and approximately 20% higher fatigue endurance limit, when compared with other 6xxx series aluminum alloys. Significant work hardening was induced due to fatigue and was experimentally validated by the measurements of residual stiffness of fatigue loops as well as of absorbed energy per fatigue loop. Work-hardening exponent was essentially decreased by almost 25% from the first fatigue cycles and up to 10% of fatigue life. Fracture surfaces of specimens loaded at applied stresses close to fatigue endurance limit exhibited signs of coarse voids due to the formed precipitates at the matrix. The fracture mechanism was a mixture of transgranunal and intergranular fracture for the fatigue specimens tested at higher applied fatigue loadings.

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.