Amorphous alloys of the types Zr1-xNix, Zr1-xCox, Zr1-xFex and Zr1-xCux were prepared by melt-spinning and studied by means of differential scanning calorimetry and X-ray diffraction. The crystallisation temperatures (Tx) and the activation energies for crystallisation ( Delta E) were determined. The former values were found to vary only slightly with concentration while the latter show a marked concentration dependence, which is particularly pronounced in Zr1-xNix. The occurrence of compositional short-range ordering (CSRO) in amorphous systems is discussed with respect to its consequences for the hole model proposed earlier. It is shown that the predictions of the hole model are only slightly affected by the occurrence of CSRO, owing to a gross cancellation of the effects of a decreasing hole formation enthalpy and decreasing configuration entropy. The relatively strong temperature dependence of the configurational entropy expected for alloy compositions with strong CSRO leads to an increase in the experimental values of the apparent activation energies. From the concentration dependence of the experimental activation energies indications can be obtained about the concentration dependence of CSRO. It appears that the regions with a maximum CSRO do not, in general, correspond to the concentrations at which intermetallic compounds occur, nor do they correspond in all cases to regions located around the deepest eutectic composition.

https://iopscience.iop.org/article/10.1088/0305-4608/14/3/005/pdf

Short-range order and thermal stability in amorphous alloys

The function fBA-which defines the degree by which an A atom is surrounded by neighbours of B atoms in Miedema's semi-empirical model and is used for the calculation of the formation enthalpy of crystalline alloys-is modified for amorphous alloys. For the five alloy systems considered, the fBA function turns out to be well described by fBA=cBs(1+5(cAscBs)2). This fBA also reflects the existence of chemical short-range order. In addition, Buschow's prediction for the crystallisation temperature of amorphous alloys is modified using this fBA, resulting in the relation Tx=5 Delta Hhs+275, where Delta Hhs represents the formation enthalpy of a hole of the same size as the smaller atom.

https://iopscience.iop.org/article/10.1088/0305-4608/17/4/008/pdf

Application of the Miedema model to formation enthalpies and crystallisation temperatures of amorphous alloys

Standard enthalpies of formation of some 3d transition metal silicides by high temperature direct synthesis calorimetry

New critical evaluations and optimizations of the Al–Ce, Al–Y, Al–Sc and Mg–Sc systems are presented. The Modified Quasichemical Model is used for the liquid phases which exhibit a high degree of short-range ordering. A number of solid solutions in the binary systems are modelled using the Compound Energy Formalism. All available and reliable experimental data such as enthalpies of mixing in liquid alloys, heats of formation of intermetallic phases, phase diagrams, etc. are reproduced within experimental error limits. It is shown that the Modified Quasichemical Model reproduces the partial enthalpy of mixing data in the liquid alloys better than the Bragg–Williams random mixing model which does not take short-range ordering into account.

Critical evaluation and thermodynamic optimization of the Al–Ce, Al–Y, Al–Sc and Mg–Sc binary systems

The standard enthalpies of formation of the compounds of early transition metals with late transition metals and with noble metals as determined by Kleppa and co-workers at the University of Chicago : A review

Molar enthalpies of formation of LnAl2 compounds

The enthalpies of formation of intermetallic phases of the Gd-(Fe, Co, Pd, Pt) binary systems were measured by a calorimetric method involving dissolution of the compounds in molten aluminum. These experimental values are compared with model predictions obtained from a tight-binding approach. It is shown that the difference between the energy levels of the two elements as the mean value of their bandwidths are the dominant factors in the determination of the enthalpy of formation. The highly asymmetric shape of the enthalpy of formation as a function of composition observed in some alloys is discussed considering thed-band characteristics of the components and the densities of states of the compounds.

Study of the enthalpies of formation in the Gd-(Fe, Co, Pd, Pt) systems

The enthalpies of formation of seven intermetallic phases of the Gd-Ni binary system were measured by a calorimetric method involving dissolution of the compounds in molten Al. These experimental values are compared with model predictions obtained from a tight-binding approach. It is shown that the relatively large difference between the energy levels of the two elements is a dominant factor in the determination of the formation energy. The highly asymmetric shape of the formation energy as a function of composition is discussed considering the particular shape of the density of states of these compounds.

Study of the enthalpies of formation in the Gd-Ni system

The isomer shifts and quadrupoole splittings of amorphous Fe x B 1−x alloys (10 a/o ⪅ × ⪅ 90 a/o) were studied by 57 Fe Mossbauer spectroscopy. The isomer shift vs. composition can be described by the ▵φ ∗− and ▵n WS -terms of Miedema's cellular model to predict the heat of formation of binary alloys and an additional volume mismatch term. This term is necessary for boron-rich alloys ( x ⪅ 50 a/o). We conclude that these samples have a strained atomic structure. The values of the quadrupole splittings at the boron-rich side increase strongly, indicating that the Fe-atoms are squeezed into asymmetric atomic surroundings. Implanted iron into crystalline boron has a comparable isomer shift and quadrupole splitting as the boron-rich alloys.

Mossbauer isomer-shifts and quadrupole splittings in the amorphous iron-boron system

The crystallization of amorphous alloys of the approximate composition Zr 8 M 2 , where M represents Cu, Ni, Co, Fe, Rh and Ir was studied by means of differential scanning calorimetry X-ray diffraction and also, in the case M = Fe by 57 Fe Mossbauer effect spectroscopy. It is shown that the first step in the crystallization of these alloys involves the precipitation of a metastable modification of Zr metal (ω-Zr).

K.H.J. Buschow

Papers

Molar enthalpies of formation of LnAl2 compounds

Study of the enthalpies of formation in the Gd-(Fe, Co, Pd, Pt) systems

Study of the enthalpies of formation in the Gd-Ni system

Mossbauer isomer-shifts and quadrupole splittings in the amorphous iron-boron system

Crystallization of amorphous zr-rich alloys