Premium
Optical diffraction: applications to lamellar microstructures
Author(s) -
Andrews K. W.,
Keown S. R.
Publication year - 1980
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1111/j.1365-2818.1980.tb04131.x
Subject(s) - lamellar structure , materials science , pearlite , eutectic system , diffraction , anisotropy , electron diffraction , micrograph , optics , microstructure , crystallography , scanning electron microscope , composite material , austenite , physics , chemistry
SUMMARY Optical diffraction techniques have a wide range of applications which extend from the further interpretation of electron micrograph images to aerial photographs. Useful metallographic applications are based on optical or electron micrographs. A brief account is given of a typical system for obtaining optical transforms or diffraction patterns and reconstituted images. Structures which appear frequently in eutectics or eutecoids show laminations arising from the alternating formation of different constituents with planar interfaces. The diffraction patterns are relatively simple but may be affected by variations in spacing and orientation. Account should also be taken of geometrical and other factors due to the tilt of the lamellae with respect to the surface. Besides recording the spacings it is also useful to take account of the directions of traces in the surface in view of the possible presence of structural anisotropy. The eutectic which is the sole constituent in alloys containing aluminium with 33% copper provides a useful illustration of the principles. Some typical results are briefly presented. A second application refers to ‘pearlite’, the eutectoid which forms in iron‐carbon alloys and many carbon and low alloy steels. The alternating lamellae are α‐iron and the carbide Fe 3 C. Two examples are given. The first refers to the use of spacing measurements in order to examine the dependence on temperature of transformation. The second shows how statistical spacing variations in a micrograph may reveal whether there is one or more basic spacing. The variation of the trace directions reveals structural anisotropy, and the relation of the spacing to this anisotropy is also examined.