| Twinning-induced plasticity(TWIP)steels possess excellent tensile strength,ductility and formability,making them satisfy the stringent demands of high strength and high plasticity for the automotive steel.The deformation mechanisms of TWIP steels are dominated by deformation twinning and dislocation glide.It is urgent to understand the interaction between different mechanisms and their respective contributions to strength and plasticity.In addition,the yield strength of TWIP steels is very low,which limits their wider industrial applications in the future.In this dissertation,Fe-Mn-C(-Al)TWIP steels were used.Through experimental methods,the evolution behaviors of dislocations and deformation twins under different temperature and strain conditions were quantitatively studied,and their respective contributions to the mechanical behaviors at room temperature were clarified.Microstructure evolution and mechanical properties of gradient-substructured(GS)TWIP steels were also studied.The main conclusions were listed below:(1)Through tensile experiments and microstructural characterization,the storage of geometrically necessary dislocation(GND)and statistically stored dislocation(SSD)densities,occurrence of deformation twinning,as well as their respective roles in strengthening and strain hardening were quantitatively investigated in a Fe-22Mn-0.6C TWIP steel during the true strain level from 0 to 0.34 at room temperature.The study found that GNDs are non-uniformly distributed,and mainly concentrate near/at the deformation twins and grain boundaries.The GND density increases non-linearly with the increase of strain.The SSD density is heavily dependent on the degree of deformation with a faster rate of multiplication than GND density as a function plastic strain.At the early strain stage(before the true strain of 0.14),the total dislocation density is mainly composed of the GNDs,so it controls the contribution of dislocation hardening to strength.At the large deformation stage(after 0.14 true strain),the SSD density outnumbers the GND density,so its contribution to dislocation hardening is more.The contribution of twin hardening to the strength is less than 100 MPa.Depending on dislocation types,dislocation multiplication governs the strengthening and strain hardening behaviors of TWIP steels during deformation.(2)By combining EBSD and TEM measurements,the evolution behaviors of deformation microstructures of the Fe-22Mn-0.6C TWIP steel at different temperatures were quantitatively studied.The study found that the deformation mechanism of the TWIP steel at 423 K is dominated by dislocation slip,accompanied by a very small amount of deformation twinning;at 293 K,it is dominated by deformation twinning and dislocation slip;at 77K,it is dominated by strain-induced hexagonal martensite transformation and dislocation slip,accompanied by a small amount of deformation twinning.As a result,the twin volume fraction is the most at room temperature.It is found that at the same temperature,the average twin thickness is independent on the strain level,and the average spacing between adjacent twins in a twin bundle decreases with the increase of strain;at the same strain,the average twin thickness and the average spacing between adjacent twins in the twin bundle both decrease with decreasing temperature.In addition,it is found that as the temperature decreases,the tendency of dislocation plane slip increases,and the GNDs mainly concentrate near/at the grain boundaries,deformation twins and phase interfaces.(3)Based on the law of stress partitioning,hetero-deformation induced(HDI)stress was quantitatively studied in Fe-22Mn-0.6C and Fe-22Mn-0.6C-3Al TWIP steels through the loading-unloading-reloading tensile tests.The study found that the HDI stress in two steels increases with the increase of true strain,and the contribution of deformation twins to HDI stress is limited.Further microstructural observations also confirmed that the deformation twins are less effective for storage of GNDs than the grain boundaries,so the heterogeneous deformation around deformation twins is smaller.(4)A simple torsional-treatment was used to prepare the GS TWIP steels with high strength and high ductility.The study found that there exists the heterogeneous deformation at both grain and sample scales in the GS TWIP steels,which leads to a higher HDI stress;in addition,the heterogenous deformation at the sample scale enhances the multiplication of dislocations and deformation twins,which induces a higher short-range effective stress.Therefore,excellent tensile properties of the GS TWIP steels derive from synergistic effect of the two stresses. |
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