Continuing the papers on alloys of Sc, Ti, V, Cr, Mn and Fe we review in the present paper, on the basis of the model developed by Miedema and co-workers, experimental and predicted enthalpies of formation of ordered binary intermetallic compounds of Co with arbitrary metal partners. The enthalpy effects are also reviewed for liquid Co alloys. The predicted values agree quite satisfactorily with the available experimental data for binary systems of Co with a transition metal. In the binary systems where Co is alloyed with a non-transition metal larger differences are sometimes observed. Since Co with other transition metals may often form solid solutions the enthalpy of forming solid solutions is dealt with in a concise way distinguishing between magnetic and non-magnetic matrix metals.

Enthalpies of formation of liquid and solid binary alloys based on 3d metals: IV. Alloys of cobalt

By means of constant-temperature, constant-pressure molecular dynamics, we investigate the crystal-to-amorphous transformation of the intermetallic alloy ${\mathrm{NiZr}}_{2}$ resulting from the introduction of antisite defects. We constructed an n-body potential in the framework of the second-moment approximation of the tight-binding description of the electronic density of states. This modeling of the interatomic forces is successful in reproducing both static and thermodynamic properties of the real material. The imposition of chemical disorder quantified by the appropriate value of the long-range-order parameter, S, engenders a volume expansion followed by relaxation to a stationary state characterized by lower density and higher potential energy. The behavior of the pair distribution functions, g(r), reveals that amorphization takes place for values of S\ensuremath{\le}0.6, the corresponding volume expansion being of the order of 2%. Moreover the thermodynamic states obtained by chemical destabilization and rapid quenching from the liquid state are nearly identical. On the time scale of our simulations (${10}^{\mathrm{\ensuremath{-}}10}$ s), no detectable long-range diffusion of either species follows the introduction of chemical disorder. Some relevant features of the pair distribution functions (first and second peak positions, number of nearest neighbors) are in good agreement with those obtained experimentally from amorphous ${\mathrm{NiZr}}_{2}$ samples generated by rapid quenching.

Molecular-dynamics study of amorphization by introduction of chemical disorder in crystalline NiZr 2

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

A semiempirical theory is presented to calculate free-energy diagrams of glass-forming alloys. The theory is based on the enthalpy of formation of the solid solution expressed as a sum of two contributions: (a) a chemical contribution due to electronic redistribution in forming the alloy, and (b) an elastic contribution arising from the difference in size between solute and solvent atoms. The enthalpy of formation of the amorphous phase has only the chemical contribution. The theory is used to successfully describe the glass-forming concentration range of some Zr-based alloys, which have been experimentally analyzed by several distinct techniques.

/pdf/determination-of-the-glass-forming-concentration-range-in-3716idx845.pdf

Determination of the glass-forming concentration range in binary alloys from a semiempirical theory: Application to Zr-based alloys

We have utilized differential scanning calorimetry to monitor the solid-state reaction of crystalline metals to form an amorphous alloy in multilayered thin-film diffusion couples of elemental Ni and Zr. The heat of formation of amorphous Ni68Zr32 alloy from the elemental metals is measured directly and found to be 35 ± 5 kJ/mole. The kinetics of these reactions has been examined. The activation energy for interdiffusion in the amorphous phase is determined to be E=1.05±0.05 eV.

/pdf/calorimetric-study-of-amorphization-in-planar-binary-2kiy6nma4k.pdf

Calorimetric study of amorphization in planar, binary, multilayer, thin-film diffusion couples of Ni and Zr.

The enthalpies of formation were measured in the intermetallic compounds ZrNi5, Zr2Ni7, Zr7Ni10, ZrNi, and Zr2Ni, and the amorphous Zr1−xNix alloys with x=0.25, 0.35, and 0.65 by means of a solution calorimeter based on liquid aluminium. For the amorphous system Zr1−xNix, the concentration dependence of the crystallization enthalpy was studied by means of differential scanning calorimetry on 12 different compositions. The concentration dependence of the formation enthalpy of the crystalline materials was analyzed in terms of Miedema’s model. The enthalpy effects observed in some of the amorphous alloys point to the occurrence of compositional short‐range ordering.

Study of the enthalpies of formation and crystallization in the system Zr‐Ni

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

We present experimental determinations of the enthalpies of formation of various amorphous and crystalline alloys in the Gd‐Ni and Y‐Ni systems. For the amorphous alloys, crystallization enthalpy is studied by means of differential scanning calorimetry. The enthalpy effects observed in amorphous alloys point to the occurrence of chemical short‐range order. This manifestation is analyzed using microscopic calculations of chemical short‐range order based on a tight‐binding Hamiltonian.

Short-range order and stability in Gd-Ni and Y-Ni systems

The enthalpies of formation are measured in the intermetallic compounds ZrNi5, Zr2Ni7, Zr7Ni10, ZrNi, Zr2Ni and the amorphous Zr1-xNix alloys with x = 0.25, 0.35 and 0.65. The enthalpy effects observed in some of the amorphous alloys point to the occurrence of compositional short range order and are related to the crystallization enthalpies.

EXPERIMENTAL AND THEORETICAL STUDY OF THE CRYSTALLIZATION ENTHALPIES IN AMORPHOUS Zr-Ni ALLOYS

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