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Study on the Fatigue and Fracture of In-situ TiB2 /Al Composites

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Tutor: SuYueZeng
School: Shanghai Jiaotong University
Course: Aircraft design
Keywords: in-situ,particles reinforced,tensile perproties,fatigue life,fatigue crack initi
CLC: TB331
Type: Master's thesis
Year:  2012
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advantages of ceramic particles and aluminum matrix alloy and break through the strength limit of homogenous material. Their own advantages make them appealing materials in areas of automotive and aerospace, such as high specific strength, modulus, stiffness, etc.In this thesis, an investigation is given on in-situ TiB2/Al composites, in which the reinforcement is in-situ TiB2 particles and the matrix is the aluminum alloy A356. The major target is to research the tensile strength, fatigue life and fracture toughness of 3.0 vol.%, 5.6 vol.% and 7.8 vol.% TiB2/Al composites in order to provide the scientific basis for the wider industrial application of TiB2/Al composites.First of all, test the tensile strength of TiB2/Al composites. Through the test results we find that the elastic module, yield strength and ultimate tensile strength of TiB2/Al composites have improved accordingly as the TiB2 particles volume fraction increased in compared with unreinforced matrix alloy A356, however, the elongation decreases slightly. In the same time, simplified elastic module model and the strengthen mechanism of TiB2/Al composites have also discussed.Meanwhile, study the organic structure by SEM and observe that the size of in-situ TiB2 particles is in the range of 80-500nm and present circle, hexagon and square shapes with no apparent sharp edges. And uniform distribution of in-situ TiB2 particles prevents large-scale reunion in TiB2/Al composites. All above advantages of in-situ TiB2 particles are the guarantee of good performances of TiB2/Al composites. Secondly, study the fatigue properties £¨S-N curve£© of TiB2/Al composites and unreinforced matrix alloy A356 under pull-pull cycle loading. Results show that the fatigue life of 7.8 vol.% TiB2/Al composites is higher than matrix alloy A356 and the fatigue limit has improved about 40%, mainly because the in-situ TiB2 particles have undertook the part load. In this test, we also study the fatigue crack initiation mechanism and the influence factors of fatigue properties, mainly including the volume fraction of TiB2 particles, particles diameter and uniform distribution, etc.Finally, carry on the plane fracture strain test of TiB2/Al composites and study their fracture toughness and fracture mechanism. Generally speaking, the fracture toughness of PMMCs have dropped remarkly in compared with unreinforced matrix, however, there is slightly lower for the TiB2/Al composites than matrix alloy A356. Observe the fracture surfaces of TiB2/Al composites samples, and research their fracture mechanism, mainly including three stages of holes initiation, crack growth and final fracture failure. Through the results, we also find that there are different roles for micron-sized Si particles and submicron-sized TiB2 particles in ductile fracture process in which the Si particles decide the position of holes initiation and the in-situ TiB2 particles are the main factors in affecting last two phases. There are many factors in affecting the fracture toughness of TiB2/Al composites, such as Si particles, the size, shape and distribution of TiB2 particles, the TiB2/matrix interfaces and the volume fraction of TiB2 particles. Among these factors, the volume fraction of TiB2 particles is the major factor.
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