Study On Microstructure And Properties Of Steel With Superior Strength And Ductility Designed By A Novel SFE-dependent Strategy

The strength-ductility trade-off is a long-standing dilemma in steel.A novel stacking fault energy(SFE)dependent strategy is designed and carried out on austenitic steel,metastable austenitic steel,medium carbon bainitic steel and martensitic steel,and the corresponding microstructure and mechanical properties are tested and analyzed.The influence of the coupling strengthening and plastic mechanisms on the mechanical properties of the steel is explored to circumvent the strength-ductility trade-off.The deformation mechanisms of FCC metals are strongly related to their SFE,thereby leading to a competition among grain boundary/dislocation gliding,twinning and martensite transformation.Accordingly,we proposed a SFE-dependent strategy to improve the mechanical properties.The SFE was calculated and the heat treatment process was established on the relationship between SFE and deformation mechanism.Then deformations were conducted in the SFE ranges dominated by grain boundary migration,dislocation slip and twinning deformation,which developed steels with high density dislocations and/or nanotwins.The microstructure evolution and consequently the mechanical properties of the studied steels were tested and analyzed.In austenitic steel,the deformation effect,conducted in the SFE ranges dominated by dislocation slip or twinning deformation,on dislocation density and twin spacing was studied,revealing that dislocation density and twin spacing increase and decrease with the deformation amount,respectively.Meanwhile,high density dislocation and higher order mechanical twins were obtained via coupling the two deformation processes,which simultaneously improved its yield strength,tensile strength and total elongation.In metastable austenitic steel,two successive cycles of deformation were conducted in the SFE ranges dominated by dislocation slip and twinning deformation,which developed steel with high density dislocations and 3-order nanotwins.Dislocation strengthening results in 1.2 GPa yield strength and martensitic transformation within nanotwins provides continuous hardening capacity and consequently 1599 MPa tensile strength and 12.5% elongation.Compared with the water quenching sample,the yield strength,ultimate tensile strength and total elongation increase by 3.8,1.7 and 0.6 times respectively.In bainitic steel,the SFE-dependent strategy developed bainite steel with high density dislocation and nanotwins.High density dislocation could retard and even prevent the diffusion of the supersaturated carbon atom from BF into adjacent residual austenite,which significantly reduced the carbon content and consequently the stability of the residual austenite.The accelerated martensitic transformation within metastable retained austenite results in an excellent work hardening capacity and consequently high strength.Meanwhile,the interaction between nanotwins with dislocations and mobile dislocation slip provide superior ductility.The SFE-dependent strategy developed the bainitic steel with 2.4 GPa high strength and 15.7% elongation.In martensitic steel,the SFE-dependent strategy developed martensite steel with 2.9GPa ultimate tensile strength and 3.5% uniform elongation via the coupling dislocation strengthening and precipitation strengthening.Meanwhile,the uniform elongation is increased to beyond 11.8% by coupling TRIP effect,TWIP effect and high-density mobile dislocation slip of retained austenite through regulating the carbon content and consequently the deformation mechanism via adjusting tempering temperature and tempering time.The improved SFE-dependent strategy developed martensite steel with yield strength of 1730 MPa and 2156 MPa,tensile strength of 2150 MPa and 2782 MPa and uniform elongation of 11.8% and 18.9%.In summary,the simultaneous improvement of strength and ductility in steels indicates the SFE-dependent strategy is a potential pathway for the development of highstrength,high-ductility materials.