The high ultimate tensile strength

The high ultimate tensile strength (up to 800 MPa) and excellent formability (up to 95% ductility) of Twinning Induced Plasticity (TWIP) steels have been attracted great interest over the past few years 1–4. The superior tensile properties of TWIP steels result from continuous formation of mechanical twins during plastic deformation, which enhances the work hardening capacity in the steel. Plastic deformation by TWIP mechanism is observed in alloys which have a medium stacking fault energy (SFE), typically in the range of 18–45 mJ.m-2 5 and is characterized by the formation of discrete sheared grain sub regions containing a mirror plane at the interface, i.e. nanometer thick deformation twins. The excellent strength-ductility combination makes the TWIP steels most suitable material for high strain rate energy absorption demands such as military vehicle armor plates and automotive crash safety 6. The remarkable strain hardening of TWIP steels is mainly related to dynamic Hall–Petch effect 7, 8. As twins nucleated during plastic deformation of TWIP steels, they can act as barriers for dislocation glide and resulting in a continuously dynamic grain refinement. Therefore, dislocation mean free path reduces as a consequence of twin boundaries formation which leads to high strain-hardening rate 9. Various mechanisms have been suggested for the deformation twinning formation via gliding of dislocations for example; pole mechanism and twin nucleation through the stacking faults 10. However, the relatively low yield strength of TWIP steels (200-450 MPa) in comparison to other advanced high strength steels (AHSS) limits the extensive use of these alloys in industrial applications (specially automotive) 11. The yield strength of materials can be modified using different strengthening mechanisms such as grain size reduction, solid solution and precipitation hardening 12. Although yield and ultimate tensile strengths increase by solid solution and precipitation hardening process but elongation and toughness decrease 13,14. The grain refinement is the most effective method for improving both strength and ductility simultaneously without any change in the chemical composition of the alloy 15.