The effect of one-pass strain, |±Δε|, on grain refinement was systematically investigated by cyclic - HPT straining with a repetitive deformation process in which positive and negative shear strain are introduced. The steady-state grain size, d_ss, depended on |±Δε| rather than the given total strain, Σ |±Δε|. The unstable dislocation cell walls formed by positive strain, +Δε, was discomposed by negative strain, −Δε. The stability of dislocation cell wall increased as the number of dislocations introduced by applying |±Δε| in a grain, n, increased. The decrease in n was caused by decreasing +Δε and grain size. It was found that n affected the stability of dislocation cell walls and was an important factor in determining d_ss.

In tensile tests, α-Mg/C14-Mg₂Ca eutectic alloy with a lamellar structure is plastically deformed above 473 K but ruptures before yielding at temperatures below 423 K. This study investigates the effect of the α/C14 interface on the creep strength of α-Mg/C14-Mg₂Ca eutectic alloy at 473 K under 40 MPa stress. The creep curves of the alloy exhibited three stages: a normal transient creep stage, minimum creep-rate stage, and accelerating stage. The minimum creep rate was proportional to the lamellar spacing, indicating that the α/C14 lamellar interface plays a creep-strengthening role. In high-resolution transmission electron microscope observations of the specimens after the creep test, a dislocations appeared within the α-Mg lamellae and were randomly distributed on the α/C14 interface. It was deduced that the α/C14 interface presents a barrier to dislocation glide and does not annihilate and/or rearrange its dislocation caused by the creep test.

A prediction method for grain boundary segregation using a nano-polycrystalline grain boundary model is applied to the grain boundary segregation of Cr and Mn in bcc-Fe polycrystals for which experimental results exist, and the validity of the prediction method is verified. In this prediction method, focusing on the fact that the atomic structure of grain boundaries is almost independent of grain size, grain boundaries of polycrystals with grain sizes of the order of micrometers are modeled as grain boundaries of nano-polycrystals, for which structural relaxation calculations by molecular dynamics calculations are possible. For this grain boundary model, the grain boundary segregation energy of each site is calculated exhaustively using the interatomic potential. In addition, the average amount of grain boundary segregation in the polycrystal is calculated from the obtained grain boundary segregation energy. With this prediction method, the average amounts of grain boundary segregation and segregation energies of Cr and Mn in bcc-Fe polycrystals can be calculated and compared with the experimental results. Calculated results for both elements reproduced the experimental results well, suggesting that this prediction method is also effective in predicting the grain boundary segregation of other solute elements.

A plasticity resistance by a grain boundary was evaluated by nanoindentation measurement in the vicinity of the grain boundary for austenitic stainless steels. Converting a load (P) - displacement (h) relation to a P/h - h curve, the plasticity resistance can be evaluated in the parameter α with hardness dimension obtained as the slope on the loading curve. The slope α is constant until the plastic zone underneath the indenter reaches to the grain boundary at a certain penetration depth (stage I), and then turns into a higher value in the deeper penetration depth range (stage II). The lower α in the stage I (α_I) means the deformation resistance in the grain interior, and that in the stage II (α_II) includes the deformation resistances of both the grain interior and the grain boundary. Therefore, Δα = α_II − α_I could correspond to the plasticity resistance by the grain boundary. It has been found that the average of Δα is higher for the nitrogen-added sample than that for the nitrogen-free one, suggesting a nitrogen effect including segregation on the plasticity resistance by a grain boundary.

The importance of controlling grain boundary (GB) segregation is increasing with the strengthening of steel. In this study, a theoretical prediction method for the amount of GB segregation for a solute element in polycrystals, is established. In this prediction method, a nano-polycrystalline GB model for simulating GBs in polycrystals is developed, and the segregation energy of a solute element is calculated comprehensively for all atomic sites constituting the GB model by using an interatomic potential. From the obtained segregation energies, the segregation amount of the solute element at each atomic site is determined. Subsequently, each atomic site is classified for based on its distance from the GB center, and averaged to calculate the segregation profile of the solute element for that distance from the GB center. By applying this method to the GB segregation of P in bcc-Fe and comparing its results with experimental findings, it is determined that this prediction method can attain a good prediction accuracy.