Theoretical Approaches to Understanding the Properties of Modulated Structures — A Review

Abstract

Experimental studies on compositionally modulated alloys in systems like Cu-Ni, Cu-Pd, Ag-Pd and Au-Ni have shown remarkable increases (factors of 2 to 4) in the bi-axial elastic modulus when the modulation wavelength is close to a critical wavelength (on the order of 20Å) depending on the system. This has been sometimes called the “supermodulus effect”. In contrast, compositionally modulated Cu-Au alloys appear to have no enhancement of elastic modulus at any wavelength.

These results present a very great challenge to our theoretical understanding of material properties because elastic moduli have heretofore been found to be almost totally insensitive to microstructural changes. It is generally believed that only if the nature of the forces in the solid change in some significant way can the elastic moduli alter appreciably. There are substantial anomalies in the magnetic properties of some of the compositionally modulated alloys which also strongly suggest fundamental alterations in the electron distribution within the solid.

The several theories which have been offered to date to explain the “supermodulus effect”, ranging from Fermi surface-Brillouin zone interactions to coherency strain phenomena, will be reviewed and critically examined. The general outlines of future theoretical developments in this field will also be suggested.