Structure and composition of Laves phases in binary Cr–Nb, Cr–Zr and ternary Cr–(Nb, Zr) alloys

The Laves phase structure in three Cr-based two-phase alloys, Cr–12Nb, Cr–8Zr and Cr–2Nb–1Zr was characterized using transmission electron microscopy. Whereas in the as-cast condition, Cr2Nb exhibits the twinned C15 structure, Cr2Zr even after hot extrusion and annealing shows a heavily faulted C36 structure (or a higher order polytype such as a 6H structure). The ternary Cr2(Zr,Nb) compound exhibits the C36 structure in the as-cast condition but reverts to the C15 structure after annealing, the structural transformation being accompanied by a composition change. Solid-state precipitates of the Laves phase Cr2(Nb, Zr) in the ternary alloy that result in the matrix after annealing are always associated with dislocations. These dislocations either link to the interdendritic precipitates or link the solid-state precipitates to each other. Since the solubility of Zr is low in the matrix, we propose that pipe diffusion along these dislocations serves as the principal mechanism to redistribute the Zr from the interdendritic precipitates to the solid-state precipitates.