High-Temperature Fatigue Characteristic and Life Extension Mechanism of IN718 Nickel Base Alloy Strengthened by Laser Peening
Supervisor:zhou jian zhong
Laser Peening(LP),as a novel surface modification technology,makes use of the mechanical effect of high-pressure shock wave induced by high-energy short-pulse laser to produce plastic deformation on the surface of metal materials.The fatigue properties of LP treated metal materials are improved by generating high-amplitude compressive residual stress and refined grain structure.In this study,IN718nickel-based alloy was selected as the research object,and the model for high-temperature stress relaxation was established first,then the formula for estimating high-temperature fatigue life and the model for fatigue crack growth under high-temperature oxidation environment were deduced.After that,the influence of microstructure evolution induced by LP on the high-temperature fatigue crack growth performance was systematically studied by macro and micro-fracture.In addition,the formation mechanism of high-temperature oxidation film and its effects on improving fatigue properties were also revealed.The main contents are as follows:(1)The interaction mechanism of creep-fatigue-oxidation damage during high-temperature fatigue of IN718 nickel-base alloy was studied,and the macro and micro processes of high-temperature fatigue fracture of typical specimens were elaborated.The stress strengthening and structural strengthening effects induced by LP were analyzed,and the improvement mechanism on high-temperature fatigue under interaction between temperature and ultra-high strain rate was discussed in combination with dislocation proliferation and high-temperature precipitation induced by LP.Based on the activation entropy,grain size,dislocation density and precipitated phase,the macro and micro relaxation mechanism of LP-induced compressive residual stress at high temperature was explored,and the formula for estimating compressive residual stress during high-temperature fatigue was established.The crack initiation,propagation and full life of LP treated single-central-hole sample were calculated according to the continuous damage mechanics.The effects of LP on the oxidation kinetics of IN718 nickel-base alloy at high temperature were discussed,and the coupling model of stress and oxidation behavior under high-temperature environment and the model for high-temperature oxidation fatigue crack growth were further established.(2)LP experiments were carried out at different laser power densities,and the effects of different laser power densities(6.05 GW/cm~2,6.58 GW/cm~2 and 7.37GW/cm~2)and service temperatures(600℃,700℃and 800℃)on the surface microhardness and surface morphology of the samples were analyzed.The compressive residual stresses in the surface and depth directions of IN718nickel-based alloy specimens after peening were studied.The thermal exposure test of LPed specimens at different service temperatures was carried out to study the relaxation law of compressive residual stress induced by LP during high-temperature maintenance,and the high-temperature stress relaxation model was obtained.The results show that LP can significantly enhance the microhardness of IN718 nickel-based alloy in the near surface layer.However,there exists a critical depth of hardening layer,and the depth of plastic deformation layer can be increased by increasing laser power density.LP induced compressive residual stress is gradient distributed in depth direction,and dislocation motion caused by temperature rise is the main reason for the relaxation of compressive residual stress.(3)The typical LP treated single-central-hole tensile specimens of IN718nickel-based alloy were utilized for high-temperature fatigue to study the effects of different laser power densities and service temperatures on the fatigue life of the specimens.The morphologies of fatigue initiation zone,propagation zone and final fracture zone,combined with fatigue strip,precipitate phase,dimples structure were used to study the effect of different laser power densities and service temperatures on the fracture characteristics of IN718 nickel-based alloy from macro and micro-fracture morphologies,respectively.Finally,and the essential law of high-temperature fatigue fracture of samples before and after LP was revealed.The results show that LP can inhibit or eliminate the fatigue crack initiation tendency on the surface of IN718 nickel-based alloy,significantly improve its fatigue life at room and high temperature.Laser power density and service temperature are two important factors that affecting the final fatigue life of the material.(4)TheoxidationbehaviorofIN718nickel-basealloyduring high-temperature fatigue was analyzed,and the different physical and chemical reaction stages of oxide film formation under high-temperature cyclic loading were explored to reveal the influence of oxide film on fatigue crack growth characteristics.The microstructural strengthening of IN718 nickel-base alloy under ultra-high strain rate produced by LP was studied,such as grain refinement and dislocation proliferation induced by intense plastic deformation on the surface of material.Further more,the equilibrium law of the internal structure energy state in the micro-scale,aswell as the relationshipamong dislocation substructure transition,service temperature and laser power density were exhibited.The evolution of grain structure,dislocation and strengthening phase in the strengthening zone near the fracture surface during high-temperature fatigue,as well as their effects on the damage mode and crack growth rate in the plastic zone near the fracture tip were investigated.Microstructure strengthening mechanism induced by LP on improving fatigue properties of high-temperature service parts was also revealed.The results show that the increase of dislocation density induced by LP,as well as the formation of refined grain structure,are helpful to improve the local mechanical properties of materials.In addition,new dislocation structures,such as dislocation walls and dislocation cells,form a unique"dislocation-precipitated phase"entanglement with theγ’phase,which results in pinning effect on moving dislocations and ultimately improve the high temperature fatigue resistance of materials.Further more,the oxide film during the high temperature process inhibits the initiation and propagation of cracks at the initial stage of crack propagation.