Experimental studies of structurally incommensurate crystal phases

This paper is meant to be a report on the experimental work on dilute Pd-based alloys with Co, Fe and Mn. These alloys exhibit the phenomenon of giant moments. The importance of measurements on paramagnetic alloys is emphasized. From these measurements the conclusion can be drawn that Co and Fe dissolved in Pd does not behave like a normal paramagnet, i.e. according to a Brillouin function. This result makes it possible to explain the existing discrepancy in the interpretations of magnetic measurements on one hand and of specific-heat experiments on the other. The main conclusions of this paper are: The giant moment should be accounted for by ‘normal’ values of the magnetic quantum number (3/2 for Co, 2 for Fe and 5/2 for Mn) and a large value of geff. Paramagnetic alloys of Mn in Pd behave according to Brillouin functions, but alloys of Co or Fe in Pd do not. Hence, a number of interpretations of magnetic measurements should be considered as incorrect. The localized model for ferromagnetism can ...

Magnetic behaviour of cobalt, iron and manganese dissolved in palladium

An overview of the thermophysical properties of segregating and demixing liquid binary alloys is presented encompassing the simple approaches that are commonly used by physicists, chemists and metallurgists in general. The various experimental and theoretical information available for such systems are put together to establish a respectable understanding between the experimental results, theoretical approaches and the empirical models. The key to understanding is the deviation that the properties exhibit from Raoult's law and the marked change in the liquid phase as a function of composition, temperature and pressure. The characteristic behaviour is ascribed to an outcome of the interplay of the energetic and structural re-adjustment of the constituent elements on mixing. After summarizing the experimental technique and some results, a comprehensive microscopic approach, based on statistical, electronic and hard sphere-like theory, is undertaken to further the understanding of the origin of the intriguing processes that are associated with the immiscible and phase-separating liquid alloys. We conclude by providing a brief account of the kinetic aspects of phase separation with some intended industrial applications.

https://iopscience.iop.org/article/10.1088/0034-4885/60/1/003/pdf

Segregation and immiscibility in liquid binary alloys

The sections in this article are


Introduction
General Considerations
General Course of an Isothermal Precipitation Reaction
Thermodynamic Considerations–Metastability and Instability
Decomposition Mechanisms: Nucleation and Growth versus Spinodal Decomposition
Thermodynamic Driving Forces for Phase Separation


Experimental Techniques for Studying Decomposition Kinetics
Microanalytical Tools
Direct Imaging Techniques
Scattering Techniques


Experimental Problems
Influence of Quenching Rate on Kinetics
Distinction of the Mode of Decomposition




Precipitate Morphologies
Experimental Results
Factors Controlling the Shapes and Morphologies of Precipitates


Early Stage Decomposition Kinetics
Cluster-Kinetics Approach
Classical Nucleation–Sharp Interface Model
Time-Dependent Nucleation Rate
Experimental Assessment of Classical Nucleation Theory
Non-Classical Nucleation–Diffuse Interface Model
Distinction Between Classical and Non-Classical Nucleation


Diffusion-Controlled Growth of Nuclei from the Supersaturated Matrix
The Cluster-Dynamics Approach to Generalized Nucleation Theory
Spinodal Theories
The Philosophy of Defining a ‘Spinodal Alloy' —Morphologies of ‘Spinodal Alloys'
Monte Carlo Studies


Coarsening of Precipitates
General Remarks
The LSW Theory of Coarsening
Extensions of the Coarsening Theory to Finite Precipitate Volume Fractions
Other Approaches Towards Coarsening
Influence of Coherency Strains on the Mechanism and Kinetics of Coarsening–Particle Splitting


Numerical Approaches Treating Nucleation, Growth and Coarsening as Concomitant Processes
General Remarks on the Interpretation of Experimental Kinetic Data of Early Decomposition Stages
The Langer and Schwartz Theory (LS Model) and its Modification by Kampmann and Wagner (MLS Model)
The Numerical Model (N Model) of Kampmann and Wagner (KW)
Decomposition of a Homogeneous Solid Solution
General Course of Decomposition
Comparison Between the MLS Model and the N Model
The Appearance and Experimental Identification of the Growth and Coarsening Stages
Extraction of the Interfacial Energy and the Diffusion Constant from Experimental Data


Decomposition Kinetics in Alloys Pre-Decomposed During Quenching
Influence of the Loss of Particle Coherency on the Precipitation Kinetics
Combined Cluster-Dynamic and Deterministic Description of Decomposition Kinetics


Self-Similarity, Dynamical Scaling and Power-Law Approximations
Dynamical Scaling
Power-Law Approximations


Non-Isothermal Precipitation Reactions
Acknowledgements

Homogeneous Second‐Phase Precipitation

Magnetic resonance and relaxation in structurally incommensurate systems

Neutron diffraction measurements of liquid Na-Sn alloys provide evidence for appreciable ordering in the liquid. At small Sn concentrations (about 20 at.%) the liquid is similar to a simple ionic mixture with maximum distance between the negatively charged Sn atoms. At higher Sn concentrations, the Sn atoms tend to form aggregates, perhaps in the form of tetrahedra of covalently bonded Sn atoms. Elements of the solid structure seem to be preserved in the liquid state.

https://iopscience.iop.org/article/10.1088/0305-4608/13/12/007/pdf

Structure of liquid Na-Sn alloys

Neutron-scattering study of the incommensurate phase transition of Rb 2 ZnBr 4

The results of small-angle ($0.025lQl0.125$ ${\mathrm{\AA{}}}^{\ensuremath{-}1}$) neutron scattering measurements on dilute ferromagnetic $\mathrm{Pd}\mathrm{Fe}$ (0.25, 0.5, 1, and 2 at.%) and $\mathrm{Pd}\mathrm{Mn}$ (0.5 and 2 at.%) alloys are presented. Critical scattering has been clearly observed in a wide temperature interval around the Curie temperature, which for $Tg{T}_{C}$ could be described by the usual Ornstein-Zernike formalism, but for $Tl{T}_{C}$ exhibited an extra quasicoherent scattering. Both the temperature and magnetic field dependence of this additional scattering could be explained by the presence of randomly oriented ferromagnetic clusters or microdomains. It is found that a distribution of cluster sizes is necessary to describe the observed scattering. The included magnetic polarization associated with the individual giant moments in $\mathrm{Pd}\mathrm{Fe}$ is obtained from the anisotropic scattering in a magnetic field and gave an average spatial extent of about 50 \AA{}. A comparison of the macroscopic magnetization and the neutron scattering results is made. A modified percolation type of model has been employed to describe the ferromagnetic ordering process, and evidence is provided that the giant moment systems can be characterized as a special case among the more general spin-glass systems.

Inhomogeneous ferromagnetic ordering in Pd Fe and Pd Mn alloys studied via small-angle neutron scattering

Neutron diffraction measurements of liquid Li-Sn alloys provide evidence for appreciable ordering in the liquid, probably accompanied by charge transfer from Li to Sn. The effects are strongest in Li4Sn, which happens to be close to the 'zero-alloy' composition SCC(q) and rho CC(r) have been determined at the latter composition. For two other compositions, combination with available X-ray results has led to reasonable estimates of SNN(q) and SCC(q). The structural properties of the Li-Sn alloy system closely resemble those of the Li-Pb system. Some essential features of the diffraction pattern can be described by the mean spherical approximation (MSA).

https://iopscience.iop.org/article/10.1088/0305-4608/14/9/006/pdf

Structure of liquid Li-Sn alloys

The structure of liquid sodium-caesium alloys has been investigated for a large number of compositions covering the whole concentration range. X-ray as well as neutron diffraction techniques were employed. Special attention has been paid to the small-angle region of the diffraction pattern. The long-wavelength limit of the structure factor has been interpreted by applying a model proposed by Bhatia and March. Appreciable concentration fluctuations, indicating a tendency to phase separation, occur in the liquid. The size of the clusters was estimated using Guinier's model. For a few concentrations, molecular dynamics computer simulations have been carried out. Different models for evaluating the three partial structure factors have been considered.

/pdf/investigations-on-the-structure-of-liquid-na-cs-alloys-2g0bphhotz.pdf

C. van Dijk

Papers

Structure of liquid Na-Sn alloys

Neutron-scattering study of the incommensurate phase transition of Rb 2 ZnBr 4

Inhomogeneous ferromagnetic ordering in Pd Fe and Pd Mn alloys studied via small-angle neutron scattering

Structure of liquid Li-Sn alloys

Investigations on the structure of liquid Na-Cs alloys