Structural properties of metastable phases in Zr-Nb alloys II. Systematicsof the atomic volumes and interatomic distances

This paper presents an analysis of the atomic volumes and interatomic dista nces (IDs) of three metastable structures in the Zr-Nb system, which are ob served upon quenching the stable, high temperature bcc phase, viz., the ome ga (omega) and hcp (alpha') phases formed from the parent phase by diffusio nless transformations, and the untransformed bcc (beta) phase. The analysis is based on the database with lattice parameter (LP) measurements develope d by Benites et al. using a neutron diffraction technique, which was report ed in paper I of this series. Alloys with Nh contents between 4 at.% and 31 at.% are considered. Deviations from Vegard's-law type of behaviour in the variation of the atomic volume of the omega phase with the at.% Nh are dis cussed, which are related to the peculiar behaviour of the a(omega) LP of t his phase in a narrow composition range, close to pure Zr. The LPs, togethe r with other relevant structural data, are used to establish and analyse th e composition dependence of the key IDs of the various metastable phases. I n particular, the behaviour of the shortest LD (SID) of the omega phase is investigated in detail. We find that the SID values for the omega phase for med in the composition range 0 less than or equal to at.% Nb less than or e qual to 10 are even smaller than Pauling's single bond-length for Zr, a qua ntity which is involved in the so-called Jamieson correlation for the omega phase in elements and alloys. On the basis of the present results, a new p icture of the effect of alloying upon the SID of the omega phase is propose d, which extends and refines previous ideas by Grad et al. (C) 2000 Elsevie r Science S.A. All rights reserved.