Analysis of Dislocation-Point Defect Interactions in Ferrous Alloys

Abstract

The dislocation-point defect interactions in various ferrous alloys, including bake hardenable steel, Quenched Fe-N-C steel and Fe-18Mn-0.6C-xAl TWIP steel, were analyzed by the impulse internal friction technique. Internal friction technique allows for the analysis of the thermally activated motion of various defects such as interstitial C and N atoms, Mn-C complexes and kinks on dislocation segments in the presence of C atmosphere. The simultaneous presence of interstitial solutes and dislocations in an ultra low carbon bake-hardenable steel gives rise to two characteristic peaks in the internal friction spectrum: the dislocation-enhanced Snoek peak and the Snoek-Kê-Köster peak. These internal friction peaks were used to study the dislocation structure developed by pre-straining and the static strain aging effect of C during the bake-hardening process. A Ti-stabilized interstitial-free steel was used to ascertain the absence of a -peak in the internal friction spectrum of the deformed ultra low carbon steel. The analysis of the internal friction data shows clearly that the bake hardening effect in ultra low carbon steel is entirely due to atmosphere formation, with the dislocation segment length being the main parameter affecting the internal friction Snoek-Kê-Köster peak amplitude. Recovery annealing experiments showed that the rearrangement of the dislocation structure lead to the elimination of the C atmospheres.The yielding behavior of a quenched and aged Fe-114ppmN-30ppmC steel was also studied. The material was aged at 473K for different times. Vacancy loops were observed by Transmission Electron Microscopy after water quenching. The temperature-dependent Internal Friction technique was used to determine the amount of interstitial N and C in octahedral interstitial sites by analyzing the Snoek peak. No clear yield point elongation was observed after water quenching. Aging caused the appearance of a yield point elongation and a reduction of the N and C in the octahedral interstitial sites. The results suggest that the yield point elongation, i.e. Lüder’s elongation is closely related to the pinning of dislocation and vacancy loops by solute N and C atoms.The temperature dependent internal friction and elastic modulus of Fe-18%Mn-0.6%C-xAl TWIP steel with different Al contents were investigated. The modulus effect was reduced and the Néel temperature was lowered by the Al addition. The activation energy of the Finkelshtein-Rosin peak arsing from rearrange of Mn-C complex increased with Al addition. The deformation twinning behavior of the Fe18Mn0.6C and Fe18Mn0.6C-2.5Al twinning-induced plasticity steels were compared by in-situ electron backscattering diffraction. Al suppressed deformation-induced twinning. A constitutive model considering the effect of Al on the twin formation kinetics was used to show that the work hardening and the ultimate tensile strength were lowered by the suppression of deformation twinning related to dynamic hall-patch effect.The tensile properties of Fe-18Mn-0.6C with Al-alloying addition of 0%, 1.5% and 2.5wt% were investigated in the temperature range of 213K (-60oC) to 413K (140oC). The addition of Al resulted in an increase of the yield strength due to solid solution hardening and a decrease of the work hardening due to the suppression of deformation twinning. Both the decrease of the deformation temperature and the addition of Al suppressed the dynamic strain aging, clearly indicating an interaction between the stacking fault region of the mobile dislocation and Mn-C point defect complexes. It is clear that the suppression of dynamic strain aging by Al addition in TWIP steel is due to the higher activation energy of C motion which was verified by the internal friction technique. A constitutive model for the temperature dependence of the flow stress taking into account the thermally activated dislocation glide, Al solid solution hardening and the dynamic Hall-Petch effect caused by deformation twinning, was developed.