Study On The Fatigue And Fracture Of In-situ TiB₂ /Al Composites

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.