Clustering in Fe-Mo alloys

Abstract

A quantitative study was made of the diffuse scattering due to clustering at 550° C in an Fe-6.1 at. % Mo alloy. The intensity due to local order was separated from the effects of different atomic sizes of the species and first-order thermal diffuse scattering, making use of the symmetry of these contributions. The Warren local-order parameters have been derived and used in a computer program to generate the corresponding atomic distributions. Comparison of the atomic configurations for clustering in this alloy are made with those for our previous measurements on a more dilute alloy, Fe-3.9 at. % Mo at the same ageing temperature.

In both alloys there are irregularly shaped Mo-rich clusters, but these are very dilute. For the smallest clusters there is some resemblance to the equilibrium precipitate Fe₂Mo. This weak Mo concentration in these zones explains why previous investigators have found little hardening due to GP zones in this alloy. Also, the average displacement of Fe atoms from lattice sites is less in the alloy of 6.1 at. % Mo than in the more dilute alloy; this suggests there may be a relationship of these displacements to alloy-softening.

The available Mössbauer spectra for this alloy are re-examined, and the suggested assignment of the Fe peaks to specific numbers of Mo neighbours in the first two shells is revised on the basis of the computer-generated atomic configurations; the third neighbour shell appears to be important.