POLYTYPIC STRUCTURES IN CLOSE-PACKED ZR(FECR)2 LAVES PHASES

The polytypic structures formed in bulk Zr(Fe(1-x)Cr(x))2 alloys (x = 0 to 1) in the as-cast and heat-treated conditions (30 min at 850 or 900-degrees-C) were determined by electron diffraction techniques, ZrCr2 was the only bulk alloy to exhibit a range of polytypic structures. The hexagonal polytypes 2H, 4H, 8H and 12H were present in as-cast ZrCr2. Upon heat treatment at 850 or 900-degrees-C the hexagonal polytypes were replaced with the cubic polytype 3R and 3R-twinned. The cubic polytype 3R was the only structure formed in as-cast or heat-treated ZrFe2. The hexagonal polytype 2H was the only structure formed in Zr(Fe(1-x)Cr(x)), alloys (x = 0.25, 0.45 and 0.75) in all metallurgical conditions. This behaviour is compared with that of Zr(Fe(1-x)Cr(x))2(x not-equal-to 1) precipitates in zirconium-rich alloys where a variety of polytypes are observed. The presence of the polytype structures in the precipitates is attributed to the lattice misfit and associated distortional stresses present between the precipitates and the alpha-Zr matrix in the Zr-rich alloys. No such stresses would be present in the bulk alloys.