Role of Carbon atoms on the Deformation Induced Phase Transformation of Austenitic Fe-17 Mn Steels

DoctorThe Fe-17 wt.% manganese (Mn) steels usually exhibit deformation induced phase transformation from γ-austenite to ε-martensite by an elastic strain energy on cooling and by plastic strain under external deformation. In addition, these steels offer superior combination of tensile strength and elongation to provide a good application potential for automobile parts and damping alloys. Meanwhile, various mechanical properties and damping capability depend significantly on the amount of carbon atoms. Indeed, their bulk properties of interest depend significantly on the composition of carbon atoms due to the effects of nanoscale microstructural phenomena, such as solute clustering, solute partitioning and segregation, and nanoscale compositional gradients as well as highly controlled nucleation and growth of nanoscale precipitates. These nanoscale phenomena are, therefore, emerging as the key factors in microstructural design. The role of carbon atoms on the deformation-induced phase transformation and mechanical properties has, thus, been investigated in the above regards by using a high resolution transmission electron microscopy (HR-TEM) and electron backscatter diffraction (EBSD) combined with step-wise straining. In addition, carbon segregation behavior has also been successfully investigated both in atomic scale by using an atom probe tomography (APT) and in microscale by using a nano-beam secondary ion mass spectroscopy (Nano-SIMS). In the present study, volume fraction of ε-martensite was observed to decrease by the addition of carbon atoms to a 17Mn steel in as quenched state with increased hardness and tensile strength. This suggests that the hindrance of thermally activated phase transformation of γ → ε-martensite on cooling might be due to the segregation and partition behavior of carbon atoms. The APT results showed that C atoms were definitely segregated to prior austenite grain boundaries (G.B) as well as to crystal imperfections, such as stacking faults and phase boundaries. This segregation to GB area was caused both by the decrease in nucleation sites of ε-martensite and by the local strain induced by the difference of lattice plane spacing between the parallel (111) plane of γ-austenite and the (0002) planes of ε-martensite. The lattice parameter of austenite phase was found to increase with the increase in C atoms through a high resolution electron microscope (HREM) analysis. An increase of carbon content was found to hinder the phase transformation of ε-martensite to α’-martensite during deformation, which could be due to the difference in the number of slip systems available in austenite (γ) and ε-martensite. Interestingly, the fraction of retained γ after straining decreased in a higher carbon contents steel after tensile deformation, despite the higher carbon initial fraction of γ. This implies that the transformation rate of γ either to α’-martensite or to ε-martensite becomes faster with the increase in the amount of carbon atoms, caused by an enhanced stain-induced stability of γ due to a higher carbon content.In addition, a serrated flow behavior was observed to appear gradually in the strain-stress curves by higher carbon content, originating from strong interactions between interstitial carbon atoms and the various defects such as vacancies and dislocations, and initiating from carbon-carbon interactions (carbide formation). The origin of serration in strain-stress curves of Fe-17Mn steel containing 0.18% C during tensile tests at 150℃ was due to the formation of clustering introduced by the interaction between moving dislocations and point-defect complexes, which was clearly observed by an APT and the procedure of sample treatment proposed first in this study. Based on what was reported in the various literatures, this first direct observation of C-clustering tends to support the theory of interactions between mobile dislocations and carbon-vacancy-carbon pairs as a mechanism of the dynamic strain aging (DSA) observed in the present 17Mn steels with high carbon atoms. Indeed, tensile tests performed at room temperature and at different strain rates have been combined with a digital image correlation (DIC) technique to monitor the strain localization, which was clearly caused by Portevin-Le Chatelier bands, during deformation of the steels