Measurement of Degree of Order in Ordered Alloys by Convergent Beam Electron Diffraction Patterns

Abstract

Long-range-ordered (LRO) alloys, with (Fe, Ni)3V compositions, are being developed at this laboratory for possible use in fusion reactors and other elevated-temperature applications. It would be advantageous to monitor the long-range-order parameter (S) in small samples of these alloys after irradiation, heat treatment, welding, etc. A convenient method which uses convergent beam electron diffraction (CBED) for measuring S has been developed. The technique combines the ability of CBED to detect relatively small changes in lattice parameter (a0) with the fact that the lattice dimensions of an alloy usually change with degree of order. A small change in a0 causes shifts of the holz lines which appear in the central disk of a CBED pattern. In addition, small changes in the accelerating voltage (E) will similarly change holz line positions and are related to changes in a0 by:2 Δa0/a0 = ΔE/2E. The instrumentation used was a Philips EM400T electron microscope equipped with a field emission gun and a continuous accelerating voltage control. This latter device permits the operator to make small, precise changes in E and enables one to measure a0 and S without actually recording the CBED patterns. However, the technique does require the use of calibration standards for which a0 and S are known.