Theoretical Studies On Failure Model,Steady State And Stability Of Metallic Material Subjected To Plastic Deformation

As structural or functional materials,metals play a very important role in modern industrial applications,and are generally treated with plastic deformation processing techniques prior to use.Plastic deformation technique is the thermomechanical treatment of metallic material under the given load and boundary conditions in order to obtain the designed shape and the specific properties.Conventional plastic deformation techniques including rolling,extrusion and forging can only achieve relatively small deformation,while severe plastic deformation(SPD)techniques enable the metallic material to withstand large deformation without failure by modulating the stress state.Based on this principle,integral plastic forming of complex-shaped surface can be realized.During SPD,the microstructure of metallic material can be refined to micron or even nanometer scale,thus leading to excellent mechanical properties.However,since the microstructure cannot be further refined with the increase of plastic deformation under the given deformation conditions,the steady state represents the limit of microstructural features and mechanical properties for a specific SPD technique,and the relationship between the steady state and the initial state determines constitutive behaviors of metallic materials.The stability of SPD processed metals will be significantly reduced due to the microscale or nanoscale microstructure,and the strain burst phenomenon is widely observed during compression of polycrystalline or single crystal micropillars,which limit their applications.As a consequence,the deformation stability of metallic materials with sizes ranging from tens of nanometers to tens of microns has aroused extensive research interest.By combing the molecular dynamics and crystal plasticity method,three scientific problems during plastic deformation of metallic materials are theoretically studied in this thesis,which are of great significance to industrial applications.1.Failure problem during plastic deformation under complex stress state.For this purpose,the Extended Unified Strength Theory(EUST)fracture locus is developed which can be used to investigate the effect of stress state on ductile fracture.Many other fracture loci are special cases of the EUST and the relations of the parameters between these models are established.The testing data on 2024-T351 aluminum alloy and TRIP 780 steel sheets are used to calibrate and verify the EUST model and several typical extended models are selected for comparison,and then a detailed parametric study is performed to understand how and why the transformed fracture locus evolves with the parameters.2.Steady state and constitutive behaviors during plastic deformation.The dominant strengthening mechanism for SPD processed metals is analyzed,and then a general method based on the microstructure evolution model is proposed to study the steady-state dislocation density and mechanical properties.Within the developed crystal plasticity framework,a consistent physical interpretation is provided for the various experimental observations such as near-perfect elastoplasticity,strain hardening,and strain softening.3.Stability analysis for microscale or nanoscale structure subjected to plastic deformation.The strain burst phenomenon is interpreted in terms of the instability of loading system.Thus,after analyzing the detailed physical process of the loading system and the pillar under the stress or strain control during the period of strain burst,the theoretical model with the loading system that is assumed to be linearly elastic and the pillar connected in series is established,while the constitutive relations of micropillars are obtained by molecular dynamics(MD).Based on the model,the critical condition of the instability of loading system is derived,and the size of strain burst is also predicted under the assumption.