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Yield stresses of ferritic iron pre-deformed at 298 K and 213 K were compared under an identical dislocation density. The specimens pre-deformed at 213 K introduced uniformly distributed dislocations, while those at 298 K formed dislocation cell structure. After the pre-deformation, tensile tests and stress-relaxation tests were performed at room temperature. The specimens with uniformly distributed dislocations exhibited significantly lower yield stress and larger stress-relaxation than those with cell-structured dislocations even under almost same dislocation density. This result suggests that the uniformly distributed dislocations contain a large amount of mobile dislocations which can easily move at a low stress compared with the cell-structured dislocations.

The critical shear stress for dislocation emission at grain boundary was directly measured by nanoindentation technique. The change in the grain boundary strength with low-temperature aging was then discussed. It was found that the critical grain boundary shear stress increased, and the increment was correlated to the amount of carbon segregated at the grain boundary. From the relationship between the lower yield point and the amount of segregated carbon, it was concluded that the Hall–Petch coefficient (ky) increased with grain boundary segregation. The validity of the pile-up theory explaining ky, which depends on the grain boundary strength, was demonstrated.

An austenitic stainless steel (SUS316L) was prepared with and without addition of solute nitrogen. The effect of cold-working and nitrogen addition on hydrogen solubility and hydrogen diffusion were investigated. High-pressure hydrogen gas and thermal desorption techniques were used. Increasing dislocation densities were related to a higher hydrogen content and higher nitrogen content related to a lower hydrogen content. Both dislocations and nitrogen had a negligible effect on hydrogen diffusion. The different hydrogen contents in the dislocations and the metal lattice, as well as trapping and diffusion activation energies explained the lack of effect of dislocations on hydrogen solubility.

Plastic lattice strain anisotropy in polycrystalline aggregates involves complicated integration of single crystal elastic anisotropy, grain-grain interactions and orientation-dependent slip activations. It is essential to experimentally determine plastic lattice strain anisotropy for in-depth understanding elasto-plastic deformation mechanisms and rationalizing advanced models. Unlike the widely applied method for quantifying elastic lattice strain anisotropy utilizing diffraction peak shift, accurately evaluating plastic lattice strain anisotropy is still enormously challenging over the decades. In this work, we developed a new diffraction line profile breadth analysis approach to reliably assessing plastic lattice strain anisotropy based on the simple linear dependence in quasi elasto-plastic model. The approach is confirmed to be effective and adaptive in practical application by linearization of the experimental dependences for line profile broadening in nickel under various cold-working strain paths. Then the orientation-dependent plastic lattice strain values could be reliably estimated up to high strain levels, which were identical to the magnitude of published elastic lattice strain. The strain levels and strain paths dependent plastic anisotropy magnitude was also inferred and further compared with the results from classical elastic models, i.e., Reuss, Voigt, Reuss-Voigt average and Eshelby-Kroner models. Simultaneously, by further carrying out microstructural characterization and dislocation model based line profile analysis, correlative texture and dislocation (arrangement and edge/screw constituent) developments were found to be strongly depended on strain paths, as well as verified to be the primary contributions to strain anisotropy. In addition to the proposed explanations in two-phase composite model, the strain hardening was demonstrated to be impacted by strain anisotropy behaviors as well. The fundamental mechanisms and significance of above interrelated effects under various strain paths were also well discussed.

The effects of carbon and nitrogen addition on the athermal α′-martensitic transformation in metastable 18%Cr-8%Ni austenite are investigated. The thermal stability of austenite in nitrogen-added steel is higher than that in carbon-added steel, and the difference between the effects of carbon and nitrogen addition becomes remarkable as the amount of both elements increases. As the two-step transformation (γ → e → α′) always occurs in metastable 18%Cr-8%Ni austenite, the suppression of e-martensite results in the thermal stabilization of austenite. Nitrogen is more effective than carbon for increasing the stacking fault energy, leading to a higher thermal stability of the nitrogen-bearing 18%Cr-8%Ni steels.

Aged Fe-2%Cu alloys containing precipitated Cu particles with different sizes (peak-aged and over-aged) were cold-worked by wire drawing to investigate the dissolution behavior of the Cu particles. Electrical resistivity measurements quantitatively revealed the increment of solute Cu concentration in the ferrite matrix during working. The peak-aged steel containing finer Cu particles experienced a more rapid dissolution compared with the over-aged steel. Work softening of the peak-aged steel was also found at strains between 3 and 5, because of the reduction in the particle dispersion-strengthening component from the total strength. Finally, further working resulted in the near complete solution of Cu.

To understand the microstructural factors dominantly affecting the mechanical properties of metals with duplex microstructures on a mesoscale, the change in the 0.2% proof stress as a function of the hard austenite fraction of an Fe–Ni austenitic alloy having a duplex microstructure composed of soft and hard austenite was investigated in terms of the microstructural connectivity. The 0.2% proof stress continuously increased with increasing fraction of the hard austenite. However, the strengthening behavior changed when the hard austenite fraction exceeded a critical value; the 0.2% proof stress shifted from the lower limit stress to the upper limit one calculated by secant method at the critical fraction. This demonstrates that the matrix of duplex microstructure was switched from soft austenite to hard austenite at the critical fraction. On the other hand, it was also confirmed that the non-monotonic strengthening behavior has a good correlation with the microstructural connectivity of the hard austenite, that is, the percolation phenomenon. From these result, it was concluded that the strength of metals with duplex microstructure can be wholly predicted based on secant method while taking into account the matrix switching between soft and hard phases, and the percolation index has a possibility to be a useful parameter to distinguish between soft and hard phases which is the matrix of the duplex microstructure, although further investigation is needed on a degree of the hard phase connection.

Martensitic transformation is a pressing issue in austenitic steels in the context of hydrogen embrittlement; strain-induced martensite formation may increase the level of hydrogen embrittlement in metastable austenite. Therefore, the effect of hydrogen on phase stability was investigated. An austenitic stainless steel with several grain sizes was charged with various hydrogen contents, and the mechanical stability of austenite under cold-rolling was studied. Grain size did not affect the amount of martensite formed in the material for any condition. Increasing hydrogen contents decreased the amount of strain-induced martensite at each tested strain. The results were confirmed with magnetization saturation, X-ray diffraction, and electron backscattered diffraction measurements. Hydrogen appears to both change the deformation mechanism and the martensite forming rate. The stabilization is caused by a lower population of martensite nucleation locations.

To evaluate heterogeneous strain distribution developed by pre-deformations in dual phase (DP) steel accurately, a combinational technique of Electron Backscatter Diffraction (EBSD) and Digital Image Correlation (DIC) methods was newly introduced in this study. A good correlation is established between kernel average misorientation calculated by EBSD and local equivalent strain measured by DIC in ferrite matrix of DP steels regardless of the difference in deformation process, which means that an EBSD orientation map can be easily converted into an applicative strain map by employing the individual correlation formula. This new technique reveals that very large strain region is locally formed within dozens of micrometer from the punched edge in a punched DP steel. On the other hand, hard martensite grains dispersed in DP steel remarkably promote the heterogeneity of strain distribution in ferrite matrix. As a result, the large strain region is also developed in the form of bands in a cold-rolled DP steel by only 60% thickness reduction at least, as if it is affected by the distribution and morphology of martensite grains. In addition, the local strain mapping demonstrates that the equivalent strain of the large strain band in cold-rolled material is comparable to that of the heavily deformed edge in punched one. The very large strain band in ferrite matrix is characterized by ultrafine grained structure, which leads to the possibility for the losing ductility in ferrite matrix and the martensite cracking.

An Fe-5Mn-0.1C alloy was subjected to interrupted quenching, which involves quenching to a temperature between the martensite start temperature (Ms) and finish temperature (Mf), and subsequent intercritical annealing. This resulted in the formation of a unique multi-phase structure consisting of tempered martensite, two types of retained austenite, namely granular and film-like, and fresh martensite. The film-like austenite was observed in the area swept by the untransformed austenite/tempered martensite interface, which is formed during intercritical annealing. Tensile testing revealed that the fresh martensite was effective for increasing the strength, whilst adequate ductility was achieved owing to the transformation-induced plasticity (TRIP) effect.

Deformation and dissolution behaviors are important characteristics of soft dispersion particles in steel to improve the deformability of high strength steel sheets by controlling the work hardening/softening and fracture processes associated with particle dispersion strengthening. In this study, the deformation and dissolution behaviors of e-Cu precipitate particles in a ferrite matrix were investigated to understand how relatively soft dispersion particles influence mechanical responses of the steel compared to hard dispersion particles such as alloy carbides. 35 nm diameter nearly spherical e-Cu particles were initially elongated along the rolling direction by cold rolling, and then these were partly dissolved into the ferrite matrix as the equivalent strain increased. The dissolution of the Cu particles was suggested to be caused by a dynamic partitioning of Cu atoms from the precipitates into the matrix by dislocation shearing at the particle tip sharpened by severe cold working. Simultaneously, the contribution of the e-Cu particles to the dispersion strengthening appeared to be decreased with increasing dislocation density in the matrix during cold rolling. The dislocation density and other defects in the e-Cu particles also increased and the particle/matrix interfaces were serrated or became indistinct as deformation progressed. The morphology, internal defects, and interface structure changes of fine dispersion particles and their correlation with mechanical responses of the alloy are discussed in reference to an identically processed, hard particles dispersed Fe-V binary alloy.

For improving the understanding of austenite stability in steel, hydrostatic pressure in untransformed austenite that is generated via martensitic transformation was evaluated from macro- and micro-viewpoints, and its effect on austenite stability was investigated in a Fe-27%Ni austenitic alloy. X-ray diffractometry revealed that the lattice parameter of untransformed austenite is continuously decreased via martensitic transformation only when martensite becomes the dominant phase in the microstructure. This suggests that the untransformed austenite is isotropically compressed by the surrounding martensite grains, i.e., hydrostatic pressure is generated in untransformed austenite dynamically at a later stage of martensitic transformation. On the other hand, microscopic strain mapping using the electron backscatter diffraction technique indicated that a finer untransformed austenite grain has a higher hydrostatic pressure, while a high density of dislocations is also introduced in untransformed austenite near the austenite/martensite interface because of lattice-invariant shear characterized by non-thermoelastic martensitic transformation. Furthermore, it was experimentally demonstrated that the hydrostatic pressure stabilizes the untransformed austenite; however, the austenite stabilization effect alone is not large enough to fully explain a large gap between martensite start and finish temperatures in steel.