Mechanical Behavior of TWIP steels

Introduction

Twinning Induced Plasticity (TWIP) steels are class of austenitic steels with mechanically induced twin deformations.TWIP steels have highly desirable properties for automotive applications exhibiting both high strength and large ductility.The composition of TWIP steels is mainly a high amount of Mn, from 20-35 wt.% with alloying additions that may include <3% Al, <3% Si, <1.5% C and some other microalloying elements. TWIP steels are face centered cubic (fcc) with a low stacking fault energy (SFE) that promotes deformation twinning. The twinning increases the strength via additional hardening and extends ductility. The strength and ductility of these high Mn TWIP steels are considerably greater than all other classes of steels for automotive applications as illustrated in Fig. 1.

Fig. 1. Strength and ductility of steels for automotive applications

Stress-strain plots for TWIP steels

Fig. 2. True stress true strain curve (a) and normalized strain hardening rate curve as a function of the true strain (b) of the Fe-22Mn-0.6C fine-grained TWIP steel

Fig. 3. Engineering stress-strain response of different advanced high strength sheet steels with 980 TWIP being Fe-18Mn-1.5Al-0.6C

Fig. 4. Effect of grain size on tensile response of Fe-22Mn-0.6C TWIP steels< ref name=ref1/

Fig. 5. Engineering stress-engineering strain curves for TWIP steel samples with different grain sizes

Fig. 6. The representative engineering stress-strain curves of Fe-28Mn-9Al-0.8C with three different grain sizes tested at 25 C with the initial strain rate of 10^-3 s^-1 (a), and the true stress-strain curves and the corresponding strain hardening rate as a function of true strain (b)

Fig. 7. Engineering stress-strain curves of Fe-23Mn-2Al-0.2 steel at different deformation temperatures

Fig. 8. Mechanical properties of the Fe-23Mn-2Al-0.2C TWIP steel deformed at different strain rates

Fig. 9. Typical ranges for the mechanical properties of TWIP steel

Fig. 10. Stress-strain curves Fe-18Mn-0.6C TWIP steel with increasing Al alloying additions. The additions delay the onset of the serrations, and at 2.3% Al no serrations are observed.

Fig. 11. Influence of N alloying additions on the stress-strain curve of Fe-22Mn-0.6C TWIP steel

Fig. 12. True stress-strain curves of single hit compression testing of TWIP steel under strain rate of 0.1 s^-1.

Fatigue Behavior of TWIP steels

Fig. 15. Stress amplitude–fatigue life data for the investigated steels. Data for annealed Type 301LN and 316L austenitic stainless steels are included for comparison

Fig. 16. Cyclic behavior of the investigated TWIP steels at three initial stress amplitudes.

Fig. 17. Stress amplitude-fatigue life data and plots for 301 LN and 6C22 Mn TWIP steels with (a) ultra-fine grain structures and (b) coarse grain structures