F.R. de Boer

Model predictions for the enthalpy of formation of transition metal alloys II

Abstract We present a computer program in Algol 60 by means of which enthalpy effects can be calculated for binary alloys in which at least one transition metal is involved. Predictions can be made by means of this program for seven fixed ordered compositions in the solid phase and for the two limiting heats of solution and the heat of mixing at the equlatomic composition in the liquid phase. For solid alloys the predictions are representative for equilibrium compounds. As an example, we Include the output for combinations of iron and cobalt with all other metals. In addition, we include a complete table of heats of solution in liquid alloy systems. A discussion is given of both the accuracy of the predictions and the differences between predictions and the experimentally observed quantities that vary systematically with the position of the elements in the periodic table.

Cohesion in alloys — fundamentals of a semi-empirical model

Abstract A semi-empirical model of alloy cohesion involving two material constants for each element is introduced by means of the physical ideas underlying the scheme. The resulting expressions for the heat of formation of binary alloys are presented and their applicability in various extreme situations is discussed. The model is shown to reproduce a vast amount of experimental information on the sign of heats of formation. Detailed comparison with experiment for particular classes of alloys will be presented in the sequels to this paper.

On the heat of formation of solid alloys

Abstract We demonstrated recently that the available experimental data on the heat of formation of solid alloys of transition metals can be accounted for by means of a cellular model. The energy effect is derived from two contributions; a negative one, arising from the difference in chemical potential, ϑ∗, for electrons at the two types of atomic cells, and a second term, which reflects the discontinuity in the density of electrons, n ws , at the boundary between dissimilar atomic cells. Expressed as a formula, ΔH ~ [-Pe(Δϑ∗) 2 + Q(Δn ws ) 2 ] . In this paper we demonstrate that the second term is preferably to be written as Q 0 (Δn 1 3 ws ) 2 . Values for P and Q 0 can be derived from basic arguments. The advantage of this alteration is that the values for P and Q 0 are now nearly the same for widely different alloy systems (i.e., as different as intermetallic compounds of two transition metals, and liquid alloys of two non-transition metals). It is demonstrated that the description (and hence the predictions) for heats of formation of alloys of transition metals is sufficiently accurate to be of practical interest. The present model conflicts strongly with descriptions of heats of formation of transition metal alloys in terms of the Engel-Brewer theory.

Physics of Magnetism and Magnetic Materials

1. Introduction. 2. The origin of atomic moments. 3. Paramagnetism of free ions. 4. The magnetically ordered state. 5. Crystal fields. 6. Diamagnetism. 7. Itinerant-electron magnetism. 8. Some basic concepts and units. 9. Measurement techniques. 10. Caloric effects. 11. Magnetic anisotropy. 12. Permanent magnets. 13. High-density recording media. 14. Soft-magnetic materials. 15. Invar alloys. 16. Magnetostrictive materials. Author index. Subject index.

Magnetic and crystallographic characteristics of rare-earth ternary carbides derived from R2Fe17 compounds

Abstract We have studied the magnetic properties of the rhombohedral R 2 Fe 17 C compounds with R = Ce, Pr, Sm, Gd, Tb, Dy, Ho or Y and the hexagonal R 2 Fe 17 C compounds with R = Er, Tm or Lu. For all compounds the lattice parameters were determined. The Curie temperatures were found to be considerably enhanced with respect to the C-free counterparts. The magnetic anisotropy of the R 2 Fe 17 C compounds was studied on magnetically aligned powders in field strengths up to 35 T. The rare-earth sublattice anisotropy is much stronger in R 2 Fe 17 C than in R 2 Fe 17 , leading to an easy c -axis anisotropy in Sm 2 Fe 17 C even at room temperature. The Curie temperatures and the high field data obtained at 4.2 K were analysed in terms of a mean field model.

Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal ferromagnets

The magnetostructural coupling between the structural and the magnetic transition has a crucial role in magnetoresponsive effects in a martensitic-transition system. A combination of various magnetoresponsive effects based on this coupling may facilitate the multifunctional applications of a host material. Here we demonstrate the feasibility of obtaining a stable magnetostructural coupling over a broad temperature window from 350 to 70 K, in combination with tunable magnetoresponsive effects, in MnNiGe:Fe alloys. The alloy exhibits a magnetic-field-induced martensitic transition from paramagnetic austenite to ferromagnetic martensite. The results indicate that stable magnetostructural coupling is accessible in hexagonal phase-transition systems to attain the magnetoresponsive effects with broad tunability.

Empirical description of the role of electronegativity in alloy formation

Empirical correlations are established, which demonstrate that the heat of formation of metallic binary alloy systems is predominantly determined by the electronegativity difference, the difference in electronic density at the boundary of the atomic cell and the size difference, the third term being absent for compounds. The work function of a metal is recommended as a measure for its electronegativity. The electron density at the cell boundary is shown to be related to the compressibility of the pure metal. The consequences of the energy effects for quantitative descriptions of charge transfer in metallic alloys are discussed extensively.

Exchange‐Enhanced Paramagnetism and Weak Ferromagnetism in the Ni3Al and Ni3Ga Phases; Giant Moment Inducement in Fe‐Doped Ni3Ga

Measuring the magnetization of various alloys in the Ni3Al and Ni3Ga phases, we found that going to higher Ni concentrations within these phases, a transition from strongly exchange enhanced paramagnetism to weak ferromagnetism takes place at 74.5 and 76 at.% Ni, respectively. The shape of the reciprocal magnetic susceptibility vs temperature curves measured between 4.2° and 300°K is of the Stoner type and suggests for these alloys a density‐of‐states curve varying appreciably in the Fermi region leading to a low effective Fermi energy. This conclusion is confirmed by the strong curvature of the magnetization in high magnetic fields. These results and the fact that an increase of the magnetic field applied on ferro‐magnetic samples effects a large increase of the magnetization even at the lowest temperatures (4.2°K) and in the highest fields (200 kOe) led us to a description in terms of the itinerant electron model. The use of this model seems justified by the fact that in agreement with calculations by E...

Magnetic properties of a series of novel ternary intermetallics (RFe10V2)

Abstract We have studied the magnetic properties of a series of novel ternary compounds of the composition RFe 10 V 2 (R ≡ Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y). X-ray diffraction showed that these materials crystallize in the tetragonal ThMn 12 structure. The Curie temperature falls into the range 483 to 616 K. From high-field magnetic measurements made at 4.2 K on aligned powders it was derived that the iron sublattice anisotropy is about 4 T, while the rare earth sublattice anisotropy is comparatively low in most of the compounds studied.

On the heat of mixing of liquid alloyS—II

Abstract In a previous paper, the authors demonstrated that the alloying behaviour of liquid metals can be described, in a qualitative way, by a simple cellular model. In the present paper, a quantitative analysis of heats of mixing and solution is given for liquid alloys; all parameters used in the calculations are tabulated. The values calculated from the model generally agree with available experimental data. Discrepancies between model and experiment mainly occur for systems with large negative heats of mixing and can be attributed to ordering effects in the liquid alloy. A complete list of calculated values of heats of solution of metals in liquid iron, uranium, and tin is presented. A comparison is made between the present model and existing models.