INVESTIGATIONS ON TEMPERATURE DEPENDENCE OF MECHANICAL PROPERTIES AND THE DEFORMATION MECHANISM OF A TWIP STEEL

The TWIP(twinning induced plasticity) steel is a new developed super toughness steel.In the TWIP steel,deformation twinning is the dominate mechanism controlled by stacking fault energy(SFE) in austenitic phase during plastic deformation.Since SFE depends on temperature,it has a major influence on mechanical properties of alloys.The evolution of deformation mode in Fe-Mn-C austenitic steels with temperature and SFE has been extensively reported in literatures.However,in Fe-Mn-Al-Si austenitic steels,the literatures only focused attention on the deformation structure and mechanical properties of Fe-28Mn-1Al-0.5Si and Fe-24Mn-3.5Al-0.4Si steels in compression under different temperatures.The relationship between deformation structure and temperature for Fe-Mn-Al -Si TWIP steel under tensile test has not yet been established.More importantly,a thorough investigation on dependence of deformation mechanism on deformation temperature and SFE is still lacking,which is one of the key factors in alloy design and new processing exploitation. In this paper,the mechanical properties of Fe-25Mn-3Si-3Al TWIP steel and the microstructure evolution with temperature have been investigated through tensile testing at 298,373,473 and 673 K. It was found that the strength and elongation decrease with deformation temperatures increasing. The SFE of the TWIP steel,(?),at different temperatures have been calculated.It was pointed out that when 21 mJ/m~2≤(?)≤34 mJ/m~2 in 298 K≤T≤373 K,the deformation twinning is a main deformation mechanism,while the slipping is a predominant deformation mode when(?)≥76 mJ/m~2 in T≥673 K.The SFE value was found to decrease with temperature decreasing,and lower values of SFE would promote deformation twin production and inhibit slip.Deformation twins formed in plastic deformation act as obstacles to dislocations,resulting in high strain hardening effect so that both high elongation and ultimate tensile strength can be obtained at relatively low temperatures.