| SiC participates reinforced Al matrix composite is one of advanced metal matrix composites (MMCs), having widespread prospect of applications. Since its excellent strength at high temperatures, good wear resistance, high specific modulus and easy processing, the composite has been used as structure material in aerospace, automobile and other industries. Recently, it was found that the composite with high volume fraction of SiC particulate could be used in semiconductor and electronic industry due to its high thermal conductivity and low thermal expansion coefficient. If the reinforcements are graded or gradually changed along thickness direction, the composite will be more satisfied with the requirements of applications. This is the concept of functionally graded materials. In this thesis, SiC particulates reinforced Al matrix composites have been chosen, and the influence of graded SiC particulates on fatigue crack propagation and subcritical crack propagation has been investigated.The thesis consists of six chapters, hi the first chapter, research and development of SiC particulates reinforced metal matrix composite and functionally graded materials were reviewed. The problems to be solved on development and application of functionally graded materials were pointed out. Then, the objectives and outline of the present thesis were described.In the second chapter, the graded SiC particulates reinforced Al matrix composites (graded composite) were designed and fabricated by stacking powder metallurgy processing, in which volume percentage of SiC particulates is from 5% to 30%. For comparison, homogeneous SiC particulates reinforced Al matrix composites (homogeneous composite) and bi-layered graded composites (the volume percentage of SiC was 5% and 15%, respectively) were also fabricated in similar processiong conditions. Since the Al matrix of the graded composites was well resolved together, there was no interface between two layers with different volume fractions of SiC particulates. The density of the homogeneous composites increased with an increasing in volume fraction of SiC particulates. It reached the top value when the volume fraction of SiC was 15%, after that, the density slightly decreased. It was found that the hardness increased with an increase in volume fraction of SiC particulates with a parabolic relationship.In the third chapter, the tensile strength and resistance curve (R-curve) of crack growth in the graded composites were investigated. The Young's modulus increased with increasing SiC particulates with an exponential relationship, while the yield strength and the ultimate tensile strength increased with increasing SiC particulates with a linear relationship. The fracture behavior of the graded composites was inhomogeneous and unique. The fracture originated from the highest volume fraction of SiC layer and propagated along the direction of decreasing of SiC contents. At the high volume fraction layer, limited dissipation of energy by restrained plastic deformation of the matrix at the crack tip produced low fracture toughness values. Thegraded composite exhibited an increasing R-curve behavior. The fractography showed brittle fracture in high SiC layer and dimple fracture in the matrix of low SiC layer.In the fourth chapter, fatigue crack propagation behavior of bi-layered graded composite has been investigated. The fatigue crack propagation rates of bi-layered graded composite were lower than those of homogeneous composites when the fatigue crack growth from high volume fraction of SiC to low volume fraction of SiC in bi-layered composite. The residual stress in front of crack tip was calculated by finite element analysis, but tensile residual stress should caused crack growth acceleration. Fatigue crack closure effects were measured and analysed. It could interpret retardation of crack growth caused by roughness induced crack closure hi the high volume fraction of SiC particulates. Furthermore, one of the reasons for the retardation of crack growth was attributed to the crack...
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