Microstructure and deformation mechanism of TWIP steels

Microstructure before and after deformation of two Fe-Mn-Si-Al TWIP steels containg different amount of manganese (15wt% and 25wt%) were investigated by optical microscope, X-ray diffraction, SEM and TEM. The results show that γfcc &rarr Εhcp and γfcc &rarr Εhcp &rarr αbcc phase transformation induces plasticity in the Fe-15Mn-3Si-3Al TWIP steel and there is no detected deformation twins. However, lots of deformation twins appeares in Fe-25Mn-3Si-3Al TWIP steel after tensile test, which greatly increases its plasticity. TEM results show that mechanical twinning and dislocation gliding interacts in grains, two or more twin systems are activated in most grains, twin and grain boundaries are strong obstacles for twins. Stacking fault energy (SFE) played an important role in twinning mechanism, with the increase of Mn content, SFE increases, and twinning mechanism is then favored.