Since most of early generation of steels shows somewhat inadequate ductility at high strength level, there is a growing need for high strength steels with exceptional ductility. So these are the low strength steels, the first generation. But as you can see from here, although you can obtain quite a range of tensile strength, the ductility becomes much lower if you increase the strength, so there is a strong need for the steel which shows high strength with exceptional ductility. TWIP steel is the one which can answer this issue. This TWIP steel actually shows excellent combination of strength and ductility, mainly because of twinning. The key point is if the twinning activity occurs very rapidly, you are not going to have good combination. If the twinning activity becomes very slow, at the same time, you are not going to have a good combination of strength and ductility. So there should be a gradual formation of twins during deformation. So how can you make TWIP steels? Now everybody knows, nowadays, deformation behavior of steels depends on the stacking fault energy. If the stacking fault energy is low, then material will show TRIP phenomena. If the stacking fault energy is high, the material will deform by dislocation glide. So at the intermediate level, the stacking fault energy for TRIP and dislocation glide, then you will have a formation of twinning. This is the TWIP steel. Like I said before, you should have gradual formation of twins during the deformation. So if you look at this figure, now, this is the EBSD map of TWIP steels and the deformation and you can see there are lots of twins formed inside the grain. If you look at this one at the higher magnification by TEM, you will see there is a formation of very fine twins inside the microstructure. Even though some twins look thicker but actually this is composed of very, very fine twins. What's really important about this issue is, because of high combination of high strength and high ductility the material can find many applications, which require very high strength and good formability such as A and B pillars, floor side and cross members, door impact beam in the automobile. The stacking fault energy is a function of composition. That's why you should have some alloying element which would increase the stacking fault energy to the right level. It is usually done by the manganese so TWIP steels usually contain about 17 to 24 percent of manganese. So how does the twinning effect the mechanical properties? As I just showed before, twinning actually partitions the grains into several sub-units, so originally one grain will be divided into several sub-units. Actually twinning is like a mirror image. That means when once the twin forms, it'll have mirror image of your slip planes, so one grain will be divided into several grains having different orientations. So it all depends on the number of twins. So that��s the part, so that's why materials will always show so-called dynamic Hall-Petch relationship. Why it's called dynamic? Because the grain size keeps on changing during the deformation. Because twinning partitions the grain into finer grains, that simply means not just orientation but also it means it will decrease the mean free paths for the subsequent dislocation glide. That is why you will have high instantaneous work hardening rate as compared to others. Usually n value, work hardening exponent, of TRIP steels is around 0.4. On the other hand, conventional steel will show n value about between 0.2 and 0.3. So as compared to stainless steel, for example standard steel, which deforms mostly by dislocation glide, TWIP steel will show higher work hardening rate. This is the example, TEM micrograph of deformed stainless steel which shows the dislocated glide, and this is the case for the TWIP steel which shows that the deformation is done by twinning. So because of continuous decrease in mean free paths like I said before, the material will always show very, very good strength when you increase the stress. At the same time, not just for strength and ductility, TWIP steel can also show very good low temperature toughness. Why? Because TWIP steel is mainly based on the austenite structures, austenite means FCC which has many sleep systems. So material will always show good ductility at a low temperatures, so nowadays this kind of TWIP steel finds many application subjected to low temperatures and if you look at carefully the deformation behavior or tensile behavior of TWIP steels, we found that TWIP steel will always show largest elongation at intermediate temperatures between room temperature and very low temperature such as liquid nitrogen temperature, -196 degrees celsius. Why? The key point like I mentioned, you should have gradual formation of twins, not too fast not too slow. At the room temperature, twinning activity is slow, at the very low temperature twinning activity too high, so you are not going to have largest elongation at either room temperature or at very low temperatures, but somewhere in between room temperature and very low temperatures, steel will show gradual formation of twinning. That is why you will have largest elongation at the intermediate temperatures.
