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.

To investigate the reason why low-carbon steels with carbon-clusters shows the maximum strength during low-temperature aging, interactions between an edge dislocation and carbon clusters are performed through molecular dynamics (MD) simulations. Carbon clusters are modeled based on atom probe tomography (APT) observations. To express a transition process of carbon configurations from solid solution state to carbon cluster state to precipitation state during aging process, we reduce a carbon presence area with a fixed number of carbon atoms, i.e., the carbon concentration can be continuously increased. The MD simulations can represent the age hardening/softening tendency observed in the experiment and the carbon cluster state shows the maximum strength where the dislocation passes through the carbon cluster not by the Orowan but by the cutting mechanism. The MD analysis found that partial clusters in the carbon cluster act as the main resistance to dislocation passage; the biased distribution of carbon atoms is also confirmed in the actual observed carbon clusters by APT. A new interaction mechanism between dislocation and carbon clusters is developed based on the phenomena in the MD simulations and the availability is discussed.

To evaluate the stability of the α-Mg/C14–Mg₂Ca lamellar microstructure, the aging treatment was carried out for the Mg–14.8 mass％Ca near-eutectic alloy at 573-723 K for 1-150 h. The spacing of the lamellar structure obtained by the eutectic reaction L → α-Mg＋C14–Mg₂Ca after casting is approximately 250 nm. It was found by the HRTEM observation that the α/C14 interface is composed of terraces and steps, and the terrace is parallel to the (1101)α pyramidal plane of the α-Mg lamellae. The lamellar microstructure is stable below 573 K. By contrast, the lamellar spacing (λ) continuously increases with the increasing aging time (t) above 573 K, and the increase of λ can be described as λ²−λ₀² = k_T t, where λ₀ is the lamellar spacing for the as cast specimen and k_T is a constant depending on aging temperature. The activation energy for the coarsening of the α/C14 lamellar microstructure was evaluated as 112 kJ/mol, which is close to the activation energy for the inter-diffusion of Ca in Mg. The hardness of the lamellar region decreases with the increasing λ, which is indicative that the α/C14 interface acts as an obstacle to the basal slip of dislocations in the α-Mg lamellae.

The surface morphological change of electro-deposited Pt–75 at％ Cu (Pt–75Cu) nanoparticles under potential cycling were investigated by identical-location scanning electron microscopy. Electro-deposited nanoparticles consisted of numerous nuclei (～3 nm in diameter) that agglomerated to form larger secondary particles (30-50 nm). In the initial immersion in the sulfuric acid, Pt-shell formed on the surface due to the selective dissolution of Cu from Pt-75Cu. Although around 40％ of Cu dissolved, noticeable surface morphological change does not appear. Then Pt-75Cu was subjected to potential cycling between 0.05-1.0 V, surface smoothing of the nanoparticles slightly progressed due to surface diffusion of Pt. On the other hand, when Pt-75Cu was potential-cycled between 0.05-1.4 V, a particle diameter of the nanoparticles drastically decreased, and the nucleus on the surface completely disappeared owing to Pt dissolution and re-deposition. The heat-treated nanoparticles formed numerous pores on the surface during the immersion and the initial stage of the potential cycle. However, the final morphology was similar to the non-heated one. The formation of pores can be explained by the coarsening of nuclei by heat treatment.

The formation of texture through thermomechanical treatment was investigated in Mg-18.8 at％ Sc shape memory alloy to enhance its superelasticity at room temperature. The samples were cold rolled in an α phase or in a β phase and then finally heat treated at 690℃ followed by water quenching to obtain a β phase. In the case of cold rolling in the α phase, a basal-plane texture was formed, while no preferential texture was observed along in-plane direction. After the final heat treatment, {011}<uvw>_β transformation texture was obtained, according to Burgers relationship, indicating no improvement of the superelasticity along in-plane direction. In the case of the cold rolling in the β phase, a weak {111}<011>_β recrystallization texture was obtained. The sample showed about 0.65％ superelastic tensile strain along rolling direction, while that along transverse direction (//~<113>β) showed only about 0.43％. This trend is in good agreement with the orientation dependence on the transformation strain, but, the obtained superelastic strain was much lower than the expected value, which is due to the weak texture and suggests the existence of a strong grain constraint in the Mg-Sc shape memory alloy.