Toshihiro Tsuchiyama

Electrical resistivity measurement is a powerful method for evaluating the solute carbon concentration (Csol) in BCC iron. In this study, the effect of carbide precipitation on the accuracy of Csol quantification was examined by applying a model that predicts the electrical resistivity of composite microstructures containing dilute-dispersed spherical carbides. For Fe–C alloys with spheroidized cementite dispersed in ferrite, Csol in ferrite could be estimated with an uncertainty of only several tens of ppm. In Fe–0.2Mn–C alloys, high accuracy comparable to that of Fe–C alloys was achieved when Mn enrichment in ferrite accompanying cementite precipitation was properly taken into account. For Fe–2Mn–0.5Si–C martensitic alloys, Mn partitioning, dislocation annihilation, and grain coarsening do not occur during tempering at low-temperature below 573 K. Therefore, changes in electrical resistivity during tempering directly reflected the decrease in Csol, enabling accurate evaluation when the contribution of carbide precipitation was removed using the spherical dilute-dispersion model. In contrast, high-temperature tempering at 873 K induced pronounced Mn partitioning, and even slight uncertainties in the measured Mn concentration in ferrite led to errors of several hundred ppm in the estimated Csol. Although errors associated with volume fraction and electrical resistivity of carbides also affected the evaluation, their impact was much smaller than that of Mn partitioning. Consequently, except for the high-temperature-tempered Fe–2Mn–0.5Si–C alloy, the final accuracy of Csol measurement can be maintained within several tens of ppm when appropriate corrections are applied.

The intergranular crack growth in an Fe-25Cr-1.1N austenitic steel (in wt.%) was examined by in situ scanning electron microscopy and three-dimensional tomographic reconstruction based on Xe-focused-ion beam serial sectioning. The intergranular crack growth exhibited discontinuity, crack deflection/branching along {111}, and crack tip blunting. These features could be interpreted by considering the effects of planar dislocation slip that causes stress concentration at grain boundaries and Lomer-Cottrell sessile dislocations. The models explaining the intergranular cracking and associated crack deflection were proposed based on an assumption of intense planar slip and no cross slip until near-fracture, which was observed by in situ electron channeling contrast imaging under mechanical loading in the present study. In this context, because crack tip deformation is significantly constrained in the specimen interior (plain strain condition), the dislocation-driven intergranular crack growth occurred preferentially in the specimen interior, and subsequently, surface crack propagation occurs in a ligament portion. After blunting of the main crack tip, the coalescence of the main crack and planar-slip-induced brittle crack allows further crack growth.

To verify the effect of anisotropy of internal stress on the heterogeneous deformation behavior in martensitic steel, in-situ digital image correlation (DIC) method was conducted on as-quenched and pre-strained Fe–18%Ni–0.02%C alloys with lath martensitic structure. Uniaxial tensile loading and unloading were repeated in several steps to apply stepwise pre-strain, and it is considered that the pre-strain makes decreases in anisotropy of internal stress or alters the state of internal stress. In the 1st step of deformation, during the transition from elastic to plastic deformation, the heterogeneous deformation begins block-by-block. In the 2nd step (after pre-strain of the 1st step) and the later steps, a similar strain distribution to that in the 1st step was observed. If the effect of anisotropic internal stress generated during quenching appear in the initial stage of deformation and be reduced or altered by pre-strain, the strain distribution should gradually change with increasing pre-strain. The unchanged strain distribution seems to have demonstrated the effect of internal stress on the heterogeneous deformation is relatively small. Slip system analysis was conducted on almost all blocks in the DIC field of view. It was confirmed that in-lath plane slip systems and in-habit plane slip systems were activated in blocks with higher strain concentrations. Based on these results for the ultra-low-carbon martensitic steel, it is concluded that the preferential activation of in-lath plane slip systems could be a more dominant factor for the heterogeneous deformation of lath martensite, compared to the anisotropy of internal stress introduced during martensitic transformation.

Planar slips have been frequently recognized in nitrogen-added austenitic stainless steels, and often discussed in terms of stacking-fault energy (SFE). On the other hand, nitrogen addition promotes the formation of N-Cr short-range order (SRO), which has been proposed to cause the planar slips. In this study, a chromium-free Fe-N binary austenite with 2.4 mass% of fully solid-solutioned N was fabricated to simplify the relationship among deformation structures, SFE and N-Cr SRO. The Fe-N alloy was found to exhibit wavy slips and possess almost the same SFE as a nitrogen-added austenitic stainless steel with planar slips. TEM results indicate the formation of modulated structures along in the nitrogen-added austenitic stainless steels, which should be attributed to the presence of N-Cr SRO, as reported previously in the literature. These results indicate that the planar slips are primarily caused by N-Cr SRO rather than by SFE.

The present study investigates the strengthening mechanism of ferritic steels containing mono-layered VC (V-C clusters) and B1-structured VC-precipitates formed by interphase precipitation. Vickers hardness and TEM analysis were performed on isothermally transformed steels at 973 K and 923 K. Finely aligned particles of a few nanometers in diameter were identified, and Orowan loops were observed around these particles in the cold-rolled specimen after isothermal transformation at 923 K. While atomic-scale scanning-TEM revealed the formation of fully coherent mono-layered V-C clusters containing atomic shift corresponding to the Burgers vector in the cold-rolled specimen, suggesting that V-C clusters were sheared by dislocations. The particle strengthening estimated from the Orowan model overestimated the yield strength derived from hardness measurements as transformation temperatures decreased. These results demonstrate that the evolution from V-C clusters to VC precipitates correspond to the shearable to non-shearable transition for the dislocation interaction mechanism.

This study systematically examined the abnormal grain growth (AGG) behavior of ferrite in pure iron subjected to cold torsion followed by annealing. Continuous shear strain in the radial direction of the cylindrical test specimen was introduced by cold torsion. Two torsion angles 1300 ° and 3700 ° were applied, and the resulting microstructural change was characterized through hardness distribution, optical microscopy, electron backscatter diffraction (EBSD), and inclusion particle analysis. Annealing at 1123 K revealed that AGG consistently occurred at regions with a shear strain of approximately 1.5, independent of the torsion angle or radial position. The abnormal grains formed in a circumferential chain, reaching diameters of 6 mm and 2 mm for 1300 ° and 3700 ° torsion, respectively. The recovery and recrystallization behavior were influenced by the shear strain. The emergence of AGG was successfully interpreted using the defect-model proposed by Hillert, which defines AGG conditions based on the balance between the driving force of grain growth and the pinning force exerted by dispersed particles.

The addition of nitrogen has been widely recognized to significantly enhance the elevated-temperature strength of austenitic stainless steels. However, the effect of solute interstitial nitrogen remains unclear in many cases owing to nitride precipitation. This study investigated the influence of solute interstitial nitrogen on the strength of austenitic stainless steels at elevated temperatures under conditions where nitrides do not precipitate. The 0.2% proof stress and tensile strength at 1173 K increased with increasing solute nitrogen concentration. For the Fe-18%Cr-8%Ni-N alloy with up to 0.1% nitrogen, the strength increased linearly with the square root of the solute nitrogen concentration. However, at higher nitrogen concentrations, the strength deviated from this relationship and exhibited an even greater increase. Observations of the dislocation microstructures revealed distinct subgrains in the 0.1N steel, whereas subgrain formation was less pronounced in the 0.2N steel, suggesting different strengthening mechanisms for the 0.1N and 0.2N steels at elevated temperatures. A theoretical investigation of the evolution of deformation at elevated temperature using creep and stress-change tests suggested that the 0.1N steel exhibits recovery-controlled deformation, in which the climbing motion of edge dislocations dominates the deformation, whereas the 0.2N steel exhibits glide-controlled deformation, in which the dislocations are dragged by the solute atmosphere. This may stem from the sufficient segregation of the nitrogen and Cr-N atmospheres into the dislocations and stacking faults at higher nitrogen concentrations, which reduces the mobility of the dislocations.