The Effect Of Carbon Content On The Stacking Fault Energy And Twinning-induced Plasticity Behavior Of Fe-Mn-Cu-C TWIP Steel

Three kinds of carbon content Fe-20Mn-3Cu-XC (X=0.23、0.83、1.41; wt%) twinning induced plasticity (TWIP) steels are fabricated in this paper. The effect of carbon content on stacking fault energy, microstructures, mechanical properties and strain hardening behavior of TWIP steels were studied by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). And the deformation mechanism and the twinning-induced plasticity mechanism of TWIP steels were systematically investigated. The main conclusions are described as follows:(1) The stacking fault energy of TWIP steels with different carbon contents was investigated by means of X-ray diffraction Peak-shift method and thermodynamic modeling, and the phase identification of prior to and after deformed TWIP steels was carried using X-ray diffraction. The results shows that, the stacking fault energy of TWIP steel increased by carbon addition and that suppressed the occurrence of γ→ε martensite transformation, promoted the generation of deformation twins and TWIP effect. Fe-20Mn-3Cu-XC TWIP steel specimens were fully composed of austenite without phase transition.(2) The microstructures of TWIP steels during tensile deformation were observed by OM. The results shows that, the number of deformation twins in austenite increased with strain, and the addition of carbon delayed the occurrence of the initial deformation twins, whereas the formation rate of deformation twins increased with the carbon content as the strain increased.(3) The results of mechanical properties of TWIP steels with different carbon content showed that, the yield strength, tensile strength and elongation of TWIP steel were significantly improved with carbon content. The Fe-20Mn-3Cu-1.41C TWIP steel has an extraordinary mechanical property, the yield strength, tensile strength and elongation were 501.62MPa,1178.4MPa and 90.4%, respectively. The excellent strain hardening ability of TWIP steel was attributable to the interaction of dislocation glide and deformation twins strengthen. At low strain stage, the dislocation strengthening plays the dominant role, the twins strengthening plays the leading role when strain increases to a certain extent. The strain hardening exponent n of TWIP steel increased with the carbon content, the value of n increased from 0.602 to 0.782 with the carbon content increases from 0.23wt.% to 1.41wt.%.(4) The microstructures of TWIP steels after tensile deformation were observed by SEM. It was found that the fracture of Fe-20Mn-3Cu-XC TWIP steel is typical ductile fracture pattern with small and uniform equiaxed dimples. The carbon addition was beneficial to the improvement of TWIP steel’s fracture toughness.(5) The microstructure of Fe-20Mn-3Cu-1.41C TWIP steel during tensile deformation and the mechanism of twinning-induced plasticity were investigated by TEM. The results show that dislocation glide and multiplication was the primary mechanism of plastic deformation at low strain, whereas as the deformation twins and secondary twins were formed with the strain increasing, the twins strengthening played the dominant role. The generations of a large number of dislocations provide potential nucleation sites to deformation twins, and the increase of strength and plasticity was attributable to large quantity of dislocation pile-up, the generation of deformation twins and the interaction of each other.