Deformation and fracture behaviour of twinning-induced plasticity steels

Abstract

The present thesis is aimed to study the deformation and fracture behaviour of twinning-induced plasticity (TWIP) steels.It has been reported that deformation twins play an essential role in the work-hardening behaviour of TWIP steels. However, the present study found that the work-hardening rate and dislocation density in a TWIP steel deformed at 373 K and 473 K were comparable to that deformed at 298 K, but deformation twins were considerably prohibited at these temperatures. Furthermore, it was also found that the twin fractions in 30Mn0.6C and 30Mn3Al3Si steels were comparable. But the 30Mn0.6C steel possessed much higher dislocation density and work-hardening rate than that of 30Mn3Al3Si steel. Therefore, we proposed that deformation twins are not important in TWIP steels because (i) they offer minor strengthening effect to the flow stress and (ii) they are insignificant for the accumulation of dislocations. The fracture mechanisms of TWIP steels have received less attention compared to their deformation behaviour. It was showed that the dominant fracture mechanism of the TWIP steel could either be the formation of void sheets or the propagation of quasi-cleavage cracks, which depends on the stress triaxiality. We suggested that such fracture mechanisms of TWIP steels contribute to the limited post elongation, high notch sensitivity and low fracture toughness. However, the fracture toughness of TWIP steels can be improved by grain refinement as well as the addition of Al. Furthermore, it was found that the cracking process of TWIP steel at 298 K was stable, even though cleavage patches were observed on the fracture surface. A physical model based on the dislocation behaviour at the crack tip was proposed to explain qualitatively the mechanism responsible for the stable cleavage cracking in TWIP steels. TWIP steels are candidate material for cryogenic applications. The low-temperature tensile and fracture properties were also studied in the present thesis. The results showed that the carbon-free TWIP steel exhibited an excellent combination of strength-toughness at cryogenic temperatures. On the contrary, ductile-to-brittle transition (DBT) was observed in the carbon-alloyed TWIP steels with decreasing temperature. No martensitic transformation occurs during the tensile and impact tests as demonstrated by electron backscatter diffraction (EBSD) and X-ray diffraction. We proposed that the occurrence of DBT in TWIP steels was due to the combined effect of (i) the high temperature sensitivity of flow stress due to the interstitial carbon, and (ii) the reduced grain boundary cohesive energy as a result of the intergranular carbide and carbon segregation. The last part of the present thesis investigated the delayed fracture (DF) behaviour of TWIP steel in the neutral environment by using deep-drawn cup specimens. The results revealed that the DF of the TWIP steel was caused by a hydrogen-assisted transgranular stress corrosion cracking (tSCC) mechanism. Besides the residual stress, we demonstrated that the plastic strain plays a crucial role on the DF of TWIP steels by accelerating the crack growth rate due to the increased density of dislocation and deformation twins.