Research On Gradient Nanostructure Formation Mechanism And Mechanical Properties Of HCP Alloys Subjected To Laser Shock Peening

With the increasingly serious global environmental pollution,it is urgent to reduce the energy consumption of automobiles,aviation,and other fields.Lightweight is one of the effective ways to reduce energy consumption and environmental pollution,and then realize the sustainable development of mankind.With the development of engineering structures towards high strength and light weight,traditional homogeneous materials(coarse-grained materials and nanocrystalline materials)have been unable to meet the performance requirements of engineering structural parts in extreme service environments.Gradient nanostructures emerged,which can effectively overcome the defects of low plasticity of nanostructures while improving the strength of materials.Among the three light alloys,magnesium(Mg)alloys and titanium(Ti)alloys have hexagonal close-packed(HCP)structure,but their melting point,lamination fault energy,critical shear stress,and lattice are different.Therefore,the characteristic microstructure and microstructure evolution laws are different under the same plastic deformation condition.In this thesis,gradient nanostructures were prepared on the surface of two HCP light alloys by laser shock peening(LSP)strengthening technology.Theoretical analysis and experimental study were used to investigate the grain refinement mechanisms and the dominant deformation mechanisms at different plastic deformation stages.Most current studies only take LSP as a post-treatment process,ignoring the hidden hazards caused by the formation of micro-pits on the LSP treated surface.In this thesis,I also investigated that LSP was taken as a pre-treatment process and supersonic fine particles bombarding(SFPB)surface nanocrystallization technology was taken as a post-treatment process.The effects of LSP as different treatment processes(pre-treatment/post-treatment)on surface integrity,mechanical properties,and fatigue crack propagation behavior of AM50 magnesium alloy were comparatively studied.The research contents and conclusions of this paper are as follows:(1)The microstructure characterization,grain refinement model,and microhardness enhancement mechanism of TA2 commercial pure titanium(CPTi)subjected to LSP were deeply studied.The effects of LSP on the surface and depth microhardness of TA2 CP-Ti was discussed by measuring the microhardness of TA2 CP-Ti subjected to LSP.At the same time,the microstructure of TA2 CP-Ti subjected to LSP was characterized in detail,and four types of typical microstructure induced by plastic deformation were summarized:layered slip zone in tensile deformation region,martensite reverse phase transformation,microtwin gate,and microtwin collision in compressive deformation region.The evolution of microstructure caused by the severe plastic deformation induced by LSP was summarized.According to the experimental data and the theory of materials science,the grain refinement mechanism induced by severe plastic deformation after multi-layer LSP impacts was proposed.After multi-layer LSP,the grain refinement mechanism on the surface of TA2 CP-Ti includes two types of simultaneous refinement models,namely multi-directional(sub-)microntwin intersections subdivision mode and nanoscaled double twins-dislocation wall subdivision mode.The results of microhardness test showed that an～650 μm-thick hardened layer could be induced in the surface layer of LSP treated TA2 CP-Ti,and the surface microhardness of the LSP treated specimens with 1 and 3 impact layers increased by 16.37%and 41.42%,respectively.(2)The microstructure and evolution mechanism of depth gradient nanostructures of AM50 Mg alloy induced by LSP were systematically studied.Gradient nanostructures were prepared in the surface layer of AM50 Mg alloy by LSP.The internal strain rate of metal materials gradually decreases with the increase of the depth from the LSP treated surface.Therefore,the microstructure of AM50 Mg alloy strengthened by LSP at different strain rates could be characterized at the different depths.The plastic deformation-induced dislocation structures and three types of twinning structures found in the plastic deformation layer of AM50 Mg alloy induced by LSP were summarized,including the matrix/twinning structure,the secondary twinning within the primary twinning structure,and the multi-direction twin-twin intersection structure(bidirectional twin-twin intersection structure and three-direction twin-twin intersection structure).Based on the experimental data and the theory of materials science,the microscopic evolution model of gradient nanostructures was summarized,and the dominant deformation mechanism of the gradient nanostructures on the surface of AM50 Mg alloy induced by LSP in three different plastic deformation stages is revealed.The {1012}<1011>deformation twinning dominates at the initial plastic deformation stage.Both deformation twinning and dislocation movement dominate at the severe plastic deformation stage.The discontinuous dynamic recrystallization process leads to nanoscale grain refinement at the surface nanocrystalline stage.(3)The effects of gradient nanostructures induced by LSP on surface integrity and mechanical properties of AM50 Mg alloy were comprehensively studied.Aiming to improve the strength-ductility inverse relationship,gradient nanostructures induced by LSP can effectively control the sharp reduction of ductility while improving the strength of materials.The effects of LSP on surface roughness,in-depth residual stress distribution,and in-depth microhardness distribution of AM50 Mg alloy were analyzed,and the effects of LSP induced in-depth gradient nanostructures on tensile,properties fatigue properties,and fatigue crack growth behavior of AM50 Mg alloy were studied.The enhancement mechanism of LSP on the service life of AM50 Mg alloy was explored,and the strengthening mechanism of in-depth gradient nanostructures induced by LSP on AM50 Mg alloy was revealed.The results showed that LSP induced gradient residual compressive stress and gradient microhardness in the surface layer of AM50 Mg alloy,and the maximum values appeared on the treated surface.At the same time,LSP could significantly improve the fatigue properties and effectively extend the fatigue life of AM50 Mg alloy.(4)The effects of gradient nanostructures induced by LSP and SFPB composite process on the fatigue properties of AM50 Mg alloy was explored.The AM50 Mg alloy specimens treated by LSP were further strengthened by SFPB.The scale of the gradient nanostructures(the surface grain size and the in-depth gradient distribution)was regulated by the combined process to regulate the strength-ductility matching model.The surface integrity and related mechanical properties of the specimens treated by the composite process were tested,and the surface roughness,in-depthe residual stress distribution,in-depth microhardness distribution,tensile properties,and fatigue properties of AM50 Mg alloy were analyzed.The results showed that the SFPB further increased the residual compressive stress in the near top surface layer and microhardness enhancement layer,and effectively reduces the surface roughness of the LSP treated specimens,thereby improving the mechanical properties of AM50 Mg alloy.