Study on the tensile deformation mechanism of high Mn austenite base steels using neutron diffraction experiments

DoctorThe micro-structural parameters, martensitic transformation and mechanical properties of high manganese austenite base steels were investigated by using neutron diffraction experiments. First, high energy time of flight (TOF) neutron source is utilized into the characterization of Fe-18Mn-0.6C-xAl wt% (x = 0, 1.4) twinning induced plasticity (TWIP) steels. It is observed that the Al addition in Fe-18Mn-0.6C wt% steel leads to lower tensile strength and higher elongation, reducing twin formation rate. In other words, Al addition weakens the tendency of twin and stacking faults evolution under applied strain, while the screw component of dislocations is increased. It is also confirmed that smaller austenite grain size in TWIP steels favors twin and stacking fault formation, which can be explained by larger accumulation of dislocation by Hall-Petch effect. In this study, double Voigt approximation is examined to the dislocation induced peak broadening up to fourth order of reflection and found that the approximation can explain well the peak broadening by structural defects. It is also proposed an effective method to separate between macro-strain and stacking fault induced peak shift. Secondly, in order to closely examine the tensile deformation behaviors of high manganese austenitic steels, in situ neutron diffraction experiments have been performed during tensile loading with the Fe-18Mn-0.6C-xAl wt% (x = 0, 1.5, 2.0, 3.0) steels. Significant variations of peak shift, broadening and asymmetry of the (hkl) neutron diffraction peak was observed in the plastic region with the measurement. Diffraction peak profile analysis was applied to determine microstructural parameters such as stacking/twin fault probabilities, dislocation density and stacking fault energy (SFE). These parameters are quantitatively correlated to the yield strength, serrated flow and stain hardening rate during tensile deformation. The main results show that twin/stacking fault probability and dislocation density decrease from 0.05 to 0.01 and from 1016 to 4 1015m-2 as a function of Al addition x, while the SFE increases from 21 to 44 mJ/m2 with relationship of SFE = 8.84 x +19 mJ/m2. Such microstructural parameters are also in good agreement with the results of the misorientation and pattern quality map obtained by electron backscatter method.Finally, Fe-17Mn-0.02C wt% steel was chosen to investigate transformation induced plasticity (TRIP) phenomena as a competitive mechanism to TWIP effects in high Mn austenite base steels. TOF in situ neutron diffraction experiments had been applied and examined the lattice strains, weight fractions of each constituent phase and stacking faults to explore this specific deformation mechanism. These results gave us an insight to resolve the complicated deformation mechanism, which involves multiple strain induced phase transformations such as austenite →e-martensite → a’-martensite, austenite → a’-martensite and e-martensite → austenite