Hideki Tamaki

Concentration profiles and the rafting mechanism of Ni base superalloys in the initial stage of high temperature creep tests

The rafting mechanism is one of the most important and interesting phenomena which appears in Ni base superalloys during high temperature creep tests. The precipitated {gamma}{prime} particles, which are arrayed periodically in the {gamma} matrix, coalesce and form rafting structures in the direction perpendicular to tensile stresses. To understand this phenomenon, element profiles and diffusion process have been investigated in the vicinity of the {gamma}/{gamma}{prime} interfaces. In this paper, the authors analyzed accurate element profiles, especially Al and Cr which are main alloying elements, in the most vicinity of {gamma}/{gamma}{prime} interface regions. The rafting mechanism is correlated with the diffusion process on the basis of Al concentration profiles in a few nano meter distance from the {gamma}/{gamma}{prime} interfaces.

Molecular orbital approach to the chemical bonding at grain boundary in γ′-Ni3Al

The nature of chemical bonding at grain boundary in {gamma}{prime}-Ni{sub 3}Al has been analyzed by extended Hueckel molecular orbital method in order to investigate the intrinsic brittleness of grain boundary and the effect of doping element for enhancing ductility of polycrystalline {gamma}{prime}-Ni{sub 3}Al. For doping elements, B, C, P and S positioned at the grain boundary have been examined and following results are concluded. (1) Brittleness of {gamma}{prime}-Ni{sub 3}Al grain boundary attributes to the coulombic repulsive force between same atoms that arrange adjacently at the grain boundary. In particular, Al atom repels strongly against each other. (2) Boron can bind with both Ni and Al without decreasing bonding energy between constituent atoms, which would lead to improve the strength of grain boundary.

Chemical bonding state of Ni-based single crystal alloy surface with oxygen and sulfur molecules

Abstract The chemical bonding state of an Ni-based single crystal alloy surface with oxygen and sulfur molecules has been investigated by replacing foreign atoms with the alloy surface atoms. For foreign atoms, transition metals such as Cr, Re and Nb are adopted and the effect of these elements on the chemical bonding state has been examined by the molecular orbital method. The results are summarized as follows: (1) The energy level of the Cr electrons is demonstrated to be higher than that of Ni. Cr was found positively charged on the alloy surface. This can help Cr to bond more tightly with the oxygen molecule than Ni when oxygen is approaching the alloy surface. (2) Electrons of Nb and Re are located at lower energy levels than those of Ni and hold a negative charge. These elements can act as electron donors and can bond more strongly with sulfur than Ni when sulfur molecules are approaching the surface.