A Study On The Deformation Mechanism And Work Hardening Behavior Of Gradient Structure TWIP Steel Using Surface Mechanical Strengthening

The fracture of engineering materials was usually initiate from the surface.Surface mechanical strengthening technology can effectively restrain the initiation of cracks from the surface.With the help of surface mechanical strengthening treatment,the performance of the materials in service is much improved,which was highly recommended in field like aerospace engineering,automobile industry,railway system,etc.It is found that the gradient structure obtained by surface mechanical strengthening of polycrystalline copper,copper zinc alloy,stainless steel and other materials has excellent comprehensive mechanical properties.Comparing to traditional strengthening methods,the properties of the strengthen zone and matrix could be coordinated effectively by gradient structure.However,the microscopic mechanism of the influence of gradient structure on the material is still unclear.In this work,twining-induced plastic deformation(TWIP)steel of single-phase austenite structures with low dislocation energy was studied.Gradient structure of a certain thickness was formed on the surface of TWIP steel through mechanical rolling manufacturing.Surface feature,microstructure and fracture morphology were investigated using laser scanning confocal microscope,optical microscope and scanning electron microscope respectively.And Vickers hardness tester and MTS universal testing system were employed to evaluate the hardness gradient and tensile property.A highresolution camera was used to track the tensile full-field strain throughout the whole process,and the positions of structures and feature points at different depths in different stages of tensile deformation were measured.The structural evolution process of tensile deformation of gradient structures was restored,and the structure evolution and property evolution were closely linked with the measured position information,so as to conduct in-depth analysis of the strengthening and toughening mechanism.The main results are as follows:(1)The thickness of TWIP gradient nanostructure layer prepared by SMRT is about 800μm.The uniaxial tensile experiments show that,compared with coarse-grained sample,the yield strength of the gradient structured TWIP steels increased from 270 MPa to 740 MPa,and the uniform elongation decreased from 73% to 20.6%.Through the hardness test of the gradient structure TWIP steel section,the surface layer of TWIP steel after SMRT treatment formed a hardness gradient,and the maximum surface hardness reached 647.5 HV,and gradually decreased from the surface to the coarse-crystal hardness of 230 HV.(2)The surface gradient structure layer of TWIP steel treated by mechanical rolling maintains a good deformation relationship with the matrix coarse crystals,and no spalling or premature cracking occurs in the process of tensile deformation.At the initial stage of tensile,the gradient structure of the surface layer is still in elastic deformation when the core coarse crystals undergo plastic deformation,so the gradient structure provides high strength and the coarse crystals provide high plasticity.The grain with gradient structure is coarsened during the tensile process and multistage deformation twins and dislocation interaction occur in the grain.The fracture of tensile test rod in the original and rolling state is typical equiaxed dimple structure.The gradient layer has low dimple density and poor plasticity.The source of the original tensile crack is generated from the surface and the source of the rolling tensile crack is generated from the subsurface.(3)The work hardening rate of TWIP steel in rolling state is significantly higher than that in original state at the early stage of tensile stretch,and the work hardening rate in original state is better than that in rolling state at the middle and later stage of tensile stretch.With the increase of deformation,the hardness of gradient structure layer first decreases and then increases,and finally the whole structure tends to be consistent.The production of multistage deformation twins greatly increases the work hardening rate of CG-TWIP steel in the middle and late stages of deformation.The back stress analysis shows that the back stress strengthening and effective stress strengthening of TWIP steel with gradient structure are obviously higher than those in the original state.The high yield strength of TWIP steel with gradient structure is the synergistic effect of back stress strengthening and effective stress strengthening.