High Cycle Fatigue Behavior And Mechanism Of In-situ TiB₂/7050 Composite

As more stringent demands,such as specific strength,specific stiffness and high cycle fatigue property,have been put forward along with the rapidly developing aviation and aerospace fields,the conventional metal material can't meet those demands already.Therefore,new materials with high specific strength,specific stiffness and fatigue property are badly in need of.Particle reinforced aluminum matrix composites have great application prospect in aviation and aerospace fields due to their high specific strength,high specific stiffness and relatively good performance in high cycle fatigue property.The particle reinforced aluminum matrix composites synthesized by the ex-situ process show a limited improve in fatigue property attributed to the relative large scale and sharp corners of reinforced particles.For the in-situ particle reinforced aluminum matrix composites synthesized by the in-situ process,the round shaped in-situ particles are in submicron size scale and closely bonded with the matrix,hence a much superior high cycle fatigue property is desired for this material.A deep research on the fatigue crack initiation and fatigue crack propagation of in-situ particle reinforced aluminum matrix composites can provide theoretical basis for the futher improvement in fatigue property for this kind of materials.This thesis researched the high cycle fatigue properties of the in-situ TiB₂/7050 composite with different mass fractions,acquired the S-N curves of the composite,determined the fatigue crack initiation of the composite via observing the fatigue fractures,and analysed the mechanisms that the TiB₂ particles improve the crack initiation resistance ability of the composite.This thesis also studied the fatigue crack propagation behavior of the in-situ TiB₂/7050 composite and analized the effect mechanisms that the TiB₂ particles influence the fatigue crack propagation in intermediate?K region and near fractured?K region via the fatigue crack propagation rate VS stress intensity factor range curve.The research of the high cycle fatigue behavior shows that the fatigue limit of the in-situ TiB₂/7050 composite is much higher than the unreinforced alloy at T6 state,and the fatigue limit improves with the increase of the mass fraction for TiB₂ particles.Therefore,TiB₂ particles improve the fatigue crack initiation resistance ability of the in-situ TiB₂/7050 composite,and the increasement range increase with the increase of the mass fraction for the TiB₂ particles.The improvement above owes to the increasement in molule for the composite,the finer grain size caused by the TiB₂ particles and the inhibition ability to the dislocations for the TiB₂ particles.The study on the fatigue crack propagation shows that in intermediate?K region,the fatigue ceack propagation rate of the in-situ TiB₂/7050composite is similar to the unreinforced alloy.That is caused by the the finer grain size caused by the TiB₂ particles and the inhibition ability to the fatigue crack propagation for the TiB₂ particle bands.In near fractured?K region,the fatigue ceack propagation rate of the in-situ TiB₂/7050composite is higher than the unreinforced alloy,which is caused by the enhanced fatigue crack propagation rate as a result of the finer grain size caused by the the TiB₂ particles,and the fatigue crack propagation in the form of microvoid coalescence that caused by the debond of the TiB₂particles.