The changes in the γ’ solvus temperature and the volume fraction of Co-Al-W based alloys with fcc / L1 2 two-phase microstructures upon alloying with quaternary elements have been investigated. All investigated quaternary elements, except for Fe and Re, increase the γ’ solvus temperatures of Co-Al-W based alloys with varying efficiencies depending on quaternary element. On the other hand, the variation of the γ’ volume fraction with alloying depends on the alloying element. Of the investigated quaternary elements, Ta is found to be the most effective in increasing the γ’ solvus temperature of Co-Al-W based alloys. The lattice mismatch significantly increase upon alloying with Ta of 4at.%, which destroys the coherent cuboidal structure.
The changes in microstructure and defect structure of two different semiconducting transition-metal silicides, ReSi1.75 and Ru2Si3 with ternary alloying of substitutional elements with a valence electron number different from that of the constituent metal have been investigated in order to see if the crystal and defect structures of these silicides and thereby their physical properties can be controlled through defect engineering according to the valence electron counting rule. The Si vacancy concentration and its arrangement can be successfully controlled in ReSi1.75 while the relative magnitude of the metal and silicon subcell dimensions in the chimney-ladder structures can be successfully controlled in Ru2Si3.
Cr-segregation to a lamellar interface in NbSi2/MoSi2 duplex silicide has been examined by a newly developed phase-field model. The model can take into account the segregation energy evaluated by a first principles calculation to reflect the chemical interaction between solute atoms and the interface in addition to the elastic interaction. Cr segregation occurs at the interface in the case with segregation energy whereas no segregation occurs in the case with only elastic interaction. However, the segregation is much smaller than that observed in the experiment when the segregation energy was evaluated by the first principles calculation without lattice vibration (i.e. for 0 K). Another simulations with the segregation energy with lattice vibration results in segregation comparable to that in the experiment. Thus, it has been revealed that the solute-interface chemical interaction and its temperature dependence is responsible for the interfacial segregation of Cr.
The thermoelectric properties of type-III clathrate compounds in the Ba–Al–Ge system have been investigated as a function of Al content. The solid solubility of Al in the type-III clathrate compounds is determined to be slightly less than X=12 when expressed with the formula of Ba24AlXGe100−X. As the Al content increases, values of electrical resistivity and Seebeck coefficient increase, while that of lattice thermal conductivity decreases. The changes in electrical resistivity and Seebeck coefficient with the Al content are explained in terms of the change in the number of excess electrons upon alloying with Al, which can be described with the simple Zintl concept. The changes in lattice thermal conductivity with the Al content are explained in terms of the very low vibration frequency for the rattling motion of the Ba guest atom, which is caused by the increased size of the encapsulating cage of open dodecahedron upon alloying with Al.
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