No abstract

No abstract

The quantitative description of highly nonequilibrium processes for the preparation of metastable and unstable phases requires the determination of the thermodynamic functions of the system under investigation. However, in systems such as Cu–Cr which are immiscible in the equilibrium states, the determination of the thermodynamic functions over the entire concentration range is often difficult if not impossible because reliable experimental data are not available for the metastable or unstable regime. The present paper demonstrates that such data can be obtained by a combination of thin film deposition techniques and differential scanning calorimetry. It is concluded that the phase formation in such thin films can be described in terms of the thermodynamics of the system, even when the heats of mixing are highly positive. The results indicate that models of the regular solution type still provide a reasonable description of the thermodynamic functions of such alloys.

Amorphous metallic alloys, frequently observed to occur in systems with large negative heats of mixing, are much less common in systems which are immiscible in the equilibrium solid state, such as Nb-Cu. However, amorphous Nb-Cu alloys can be produced over a wide composition range by sputtering. Using isothermal and nonisothermal differential scanning calorimetry, both the kinetics and the thermodynamics of these amorphous Nb-Cu alloys were characterized quantitatively. It was found that the formation enthalpies of the amorphous alloys amounted to only 4.5-7.6 kJ/g atom. These data were combined with a modeling of the thermodynamic functions of the system. The unexpected low enthalpies and Gibbs energies of the amorphous phase demonstrate the thermodynamic stabilization of the liquid phase which develops with undercooling. This is connected with a change of sign in the heat of mixing of the liquid phase, which is positive at high temperatures and negative at low temperatures.

An unstable fcc-Fe50Cu50 alloy has been prepared by milling of elemental powder blends. The structure and the decomposition behavior of the alloy were studied by x-ray diffraction and Mössbauer spectroscopy. A broad distribution of different local environments of the iron atoms was observed in the fcc-FeCu phase. This indicates that Fe and Cu are mixed on an atomic level. In the initial state of decomposition, iron atoms precipitated coherently in the fcc-FeCu matrix as fcc-Fe particles. At higher annealing temperatures the particle size increased during the thermal treatment, and the fcc-Fe precipitates transformed into the bcc-Fe structure.

A fcc-Fe50Cu50 solid solution was prepared by mechanical alloying of elemental Fe and Cu powder blends. The alloying process was studied by using x-ray diffraction and Mössbauer spectroscopy. Initially, the milling process reduced the crystallite sizes of both elemental powders. After 20 h milling, some Fe particles transformed into the fcc structure. Due to the structural similarity of the fcc-Fe and fcc-Cu phases, composites consisting of coherent Cu and Fe regions were formed. The increasing density of interfaces during further milling resulted in an interdiffusion of Cu and Fe. The alloying process was monitored by Mössbauer investigations which showed an increasing Fe concentration in fcc Cu. After 50 h of milling, the Mössbauer spectra consisted of a broadened sextet caused by a hyperfine field distribution, which demonstrates that the Fe and Cu were alloyed on an atomic level. These observations are in agreement with a model proposed by C. Gente, M. Oehring, and R. Bormann [Phys. Rev. B 48, 13244 (1993)] describing the formation of unstable alloys by mechanical alloying.

The temperature dependence of saturation magnetization has been observed for an unstable FCC Fe50Cu50 solid-solution sample prepared by mechanical alloying. The unstable Fee Fe50Cu50 solid solution has a Curie temperature of 505+or-5 K. At low temperatures the saturation magnetization is well approximated by the relationship M(T)=M(0)(1-BT32/), where M(0)=108.7 emu g-1 and B=4.21*10-5 K-32/. This holds over a large temperature range (T/Tc<0.4) indicating the presence of long-wavelength spin-wave excitations and disordered atomic structure. On annealing the as-milled sample at annealing temperatures above about 500 K, decomposition of the metastable FCC Fe50Cu50 solid solution was observed. The magnetic moment of Fe atoms in the FCC Fe100-xCux Phases remains at about 2.35 mu B for 50

No abstract