Quasicrystalline icosahedral and amorphous phases prepared as thin films or bulk samples in the systems Al–Pd–Re, Al–Cu–Fe and Ti–Zr–Ni, have been investigated with respect to their temperature ...dependence of conductance and thermopower, in the range between 10 and 300 K. Various conduction mechanisms yield different exponent values in the power‐law temperature dependence of conductance, Tζ. A semi‐metal behaviour ζ ≌ 1.5) is found in the amorphous phase and in the disordered icosahedral Al–Pd–Re film and Ti–Zr–Ni ribbons, as well as in icosahedral Al–Cu–Fe films. Lower ζ values found in the quasi‐perfect icosahedral Al–Pd–Re bulk sample and in two of the icosahedral Al–Cu–Fe films suggest that electron localization in a hierarchy of clusters is the main conduction mechanism in these samples. The thermopower data support the existence of a pseudo‐gap at the Fermi level and suggest a similar short‐range order in both amorphous and icosahedral phases. The special sensitivity of the thermopower to the DOS details at EF is responsible for the strong variation of its magnitude with the degree of icosahedral order.
Metastable phase in the Ag-Ge rapidly quenched alloys Manaila, R.; Zavaliche, F.; Popescu, R. ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
06/1997, Letnik:
226
Journal Article
Recenzirano
Metastable phases can appear in metallic alloys if their surface response function (liquidus temperature versus cooling rate) is less steep than that of the stable ones. In this case, rapid ...solidification procedures can result in their formation. On the other hand, metastable alloys are expected to yield special microstructures when the equilibrium phase composition is restored by their transformation or decomposition. These microstructures depend on the thermal history of the sample (e.g., the heating rate and temperature). Metastable phase was obtained by melt-quenching in the Ag
1−
x
Ge
x
(x = 0.2, 0.35). X-ray diffraction revealed in Ag-Ge samples a metastable h.c.p. The h.c.p. Ag-Ge phase is the Hume-Rothery type. Besides, an α-Ag-Ge solid solution was present. Amounts of non-reacted Ge were also found, with a lattice constant close to the ideal value. Differential scanning calorimetry (DSC) runs revealed stability of the Ag-Ge metastable phase up to temperatures of 521 K (x = 2.0) and 626 K (x = 0.35), where exothermic peaks appeared with relatively high heat release (46.6 and 48.8 J g
−1, respectively). Work is in progress on the inhomogeneous lattice distortions in the metastable phases, which could be induced in the rapid solidification process and are expected to affect their stability.
Models for Al-TM (TM is transition metal) quasicrystals consisting of networks of icosahedral units have the merit to be specific about the transition metal surrounding. Experimental arguments are ...brought for the importance of local stable Al12TM clusters in the formation of disorder-free icosahedral phases.
Ag-substituted AlCuFe icosahedral phases Manaila, R.; Macovei, D.; Jianu, A. ...
Physica status solidi. B. Basic research,
1 February 1996, Letnik:
193, Številka:
2
Journal Article
Recenzirano
The gradual substitution of Cu by Ag in the Al62Cu25.5−xAgxFe12.5 icosahedral phase (x ≤ 10) is investigated by X‐ray diffraction from the viewpoint of the induced structural changes. The phonon and ...random phason disorders, estimated from diffraction linewidths, increase with x, the latter showing, however, an induction range of Ag concentrations. EXAFS results at the CuK and FeK edges show, for low Ag contents, and enhanced disorder in the first coordination sphere, partly relaxed for higher x values. The 6D hypercubic lattice parameter is found to be independent of the Ag content, despite the large diameter of monovalent Ag. The latter facts suggest a d‐sp hybridization of the Ag bonds to the surrounding Al, taking place above a certain x value.
Le remplacement graduel du Cu par Ag dans la phase icosaédrale Al62Cu25.5−xAgxFe12.5 (x ≤ 10) est étudié par diffraction des rayons X, sous l'aspect des changements de structure. Le désordre de type phononique et phasonique (aléatoire), évalué en utilisant les largeurs des raies de diffraction, s'accroit avec x, le dernier ayant un domaine d'induction. Les résultats EXAFS pour les limites d'absorption CuK et FeK montrent un excès de désordre dans la première sphère de coordination, pour les concentrations basses en Ag, désordre qui est partiellement relaxée pour des plus larges x. Le paramètre du réseau 6D est indépendant du contenu en Ag, malgré le rayon atomique large de celuici. Les résultats suggèrent une hybridisation d‐sp pour les liens Ag‐Al, au‐dessus d'un certain x.
