日本金属学会誌

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Nanoindentation tests were performed on high purity aluminum and its alloys to clarify the effects of the types of additional elements and the effects of interactions between additional elements and grain boundaries or existing dislocations on local mechanical properties. The samples used were ultrahigh purity 5N-Al and high purity Al-Fe, Mn, Cu, Zr and Zn alloys processed by solution annealing or friction stir process (FSP). The results of nanoindentation hardness revealed that the amount of each element and the misfit had a significant effect on deformation resistance of plastic deformation in solution annealed samples. On the other hand, the results for the FSPed samples showed that the existing dislocations introduced by FSP have a significant effect on the plastic resistance. From the values of the nanoindentation hardness ratio of the grain boundary to the grain interior, it is concluded that the segregation effect of the additional elements at the grain boundary on the plastic resistance is small in solution annealed samples and FSPed samples. The results of critical load at pop-in revealed that elements with large misfit act as dislocation sources in solution annealed samples. For the FSPed samples showed that the density of existing dislocations introduced by FSP has a significant effect on the critical load for dislocation generation.

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The equilibrium crystal structure of LnMnO₃ (Ln: lanthanide) has been reported to be orthorhombic when relatively large ions from La³⁺ to Dy³⁺ are used as Ln³⁺ and hexagonal when relatively small ions from Ho³⁺ to Lu³⁺ are used. It has been reported that the hexagonal phase is formed when the tolerance factor, expressed as functions of radii of the constituent ions, is less than 0.840. In the present study, we attempted to induce oxygen deficiency in DyMnO₃ under the solidification at low oxygen partial pressure using aerodynamic levitator to reduce the tolerance factor through a decrease in the valence of manganese ions and the accompanying increase in the ionic radius. The results showed that the oxygen deficiency increases with decreasing oxygen partial pressure. Assuming that valence of manganese ions decreased due to the increase in oxygen deficiency, the corresponding tolerance factor evaluated from the average ionic radii of manganese and oxygen decreased, which promoted the formation of the hexagonal phase as is the case with the reduction of the ionic radius of Ln³⁺.

Fig. 3　Surface morphology of DyMnO₃ samples solidified under various P_O₂ observed by confocal laser scanning microscopy. Fullsize Image

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