ELSEVIER Physica B 213&214 (1995) 433 435 PHYSICA[ Systematics of lattice parameters and bonding distances of the omega phase in Zr-Nb alloys G.B. Grad ~, J.J. Pieres, A. Fern/mdez Guillermet ~, G.J. Cuello 1'2, J.R. Granada *'x, R.E. Mayer Comision Nacional de Energia Atdmica. Centro Atdmico Bariloche and lnstituto Balseiro, 8400 Bariloche (RN), Argentina Abstract Neutron-diffraction measurements and experimental data from the literature are combined in a study of characteristic changes in the structural parameters a~,, % and z,o, of the omega phase in the Zr-Nb system, and in systematizing relations between the omega and the parent BCC phases. The interatomic distances in the omega phase reveal remarkable regularities in their composition dependence, which point to peculiarities in the bonding characteristics of this phase. 1. Introduction Recently, there has been a renewed fundamental inter- est in the elements Ti, Zr and Hf, and both experimental [1] and theoretical [2] studies have been reported on the cohesive properties and structural stability of the omega (co) and BCC phases. The physical metallurgy of these phases was studied extensively in the past [3], but addi- tional information is needed to make them clear in rela- tion with the basic research work on the elements [1,2]. This motivated us to a systematic investigation of the Zr-Nb system, comprising its thermodynamics [4] and structural properties [5, 6]. In the present paper we re- port new neutron scattering data for the o~-phase. atmosphere, and remelted several times to favour homo- geneity. They were encapsulated in quartz under argon, annealed for 2h at 1273 K and quenched in water. The chemical composition was determined by wave-disper- sive electron-microanalysis, using pure elements as the standards. The average O and N content of the alloys was 1240 and 130 wt ppm, respectively. The neutron-dif- fraction measurements were performed on a time-of- flight (TOF) diffractometer [7], which uses an electron LINAC and a Pb target as a pulsed source. The data were analysed using the formalism from Ref. [8], and the structural parameters were determined by a least-square fit [5]. 2. Experimental Three Zr alloys with 15.3at% were made in Nb contents of 7.7, 13.3 and an arc furnace, in an argon * Corresponding author. CONICET, Argentina. 2Also at CRUB, Universidad del Comahue, Argentina. 3. Lattice parameter tendency in the BCC/~o transforma- tion The BCC structure can be described by a rhombohed- rally centered hexagonal cell with atomic positions (0,0,0), + (3, z3, 3), and lattice parameters aBcc and CBcc, which are related to the lattice parameter ap of the 0921-4526/95/$09.50 C 1995 Elsevier Science B.V. All rights reserved SSDI 0921 -4526(951001 80-8