Neutron diffraction determination of the number and displacement of the atoms in the diffuse ω-phase of Zr 0·8Nb 0·2

• Published in: The Journal of physics and chemistry of solids Vol. 35; no. 7; pp. 879 - 891
• Main Authors: Keating, D.T., LaPlaca, S.J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1974
• ISSN: 0022-3697; 1879-2553
• DOI: 10.1016/S0022-3697(74)80270-2

Alloys of Zr and Nb with Nb concentrations between 5 and 17 per cent undergo an athermal displacive transformation from b.c.c. to the hexagonal ω-phase. The ω start temperature depends on Nb concentration. The ω structure is derived from b.c.c. by the displacement of atom rows parallel to [111]. For Nb concentrations greater than ≅ 17 per cent the Bragg reflections characteristic of the ω-phase never develop under athermal conditions but a diffuse scattering in their vicinity is present. The alloy in this state is referred to as diffuse omega phase and omega domains cannot be imaged in the electron microscope. Coherence of the displaced atoms with the b.c.c. lattice allows the determination of the number of displaced atoms and their mean displacement from intensity measurements of the b.c.c. Bragg reflections. At 300°K some 32 per cent of the atoms in the Zr 0·8Nb 0·2 alloy are displaced 56·2 per cent of the way to the ideal ω positions as represented by the AlB 2 structure. Both the number of atoms and their mean displacement increase as the temperature is lowered. We believe these results are the first to quantify the diffuse omega phase.