| The harmonic structure is a special bimodal structure,which can obtain high strength and maintain good ductility.It provides an important method for the development of metal materials with high strength,high toughness and good fatigue resistance.Studying the fatigue properties of harmonic structured materials and revealing the fatigue failure mechanism are important for design of high performance structural materials having heterogeneous multi-scale nanograined structure.In this thesis,the fatigue properties of harmonic structured 316 L austenitic stainless steels prepared by mechanical milling and subsequent hot isostatic pressing were studied.The effect of harmonic structure on the fatigue limit and microstructure stability were studied,and the fatigue fracture mechanism was analyzed.Moreover,the effects of stress ratio and grain size on the fatigue crack growth performance of harmonic structured materials were investigated.The effect of coarse grains(core)and ultrafine grains(shell)on the fatigue crack propagation rate and path were revealed.In addition,the effect of grain size,mean stress and peak-stress holding time on the ratcheting strain of harmonic structured materials,and the fatigue failure mechanism were analyzed.Based on these analyses,the following conclusions can be drawn:(1)Compared to the conventional CG austenitic stainless steels,the increased fatigue limit can be achieved in the harmonic structured austenitic stainless steels,which also demonstrates good cyclic stability.(2)The stress ratio had little effect on the fatigue crack growth rate.For harmonic structured materials,the continuously network shell(UFG structure)provides an increase of tensile strength,which enhance the fatigue crack nucleation resistance.Meanwhile,the fatigue crack growth path is impeded by embedded coarse grains,which results in fatigue crack deflection and secondary cracks.Therefore,the harmonic structure design is proposed to be an effective method to enhance fatigue performance.(3)The increase of mean stress and peak stress holding time leads to an increase in the ratcheting strain of the harmonic structured austenitic stainless steels,which reduces the ratcheting fatigue life.However,the reduction of grain size reduces ratcheting strain of harmonic structured materials.(4)The increase of mean stress increases the tensile deformation of material,which causes internal secondary crack formation easily,reducing the fatigue life. |
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