Microstructural comparison of Zr73.5Nb9Cu7Ni1Al9.5 nanostructure-dendrite composites produced by different casting techniques

Zr73.5Nb9Cu7Ni1Al9.5 (numbers indicate at.%) nanostructure-dendrite composites were fabricated using three different casting techniques: suction casting, centrifugal casting and arc-melting. The microstructure of the suction casting sample consists of micrometer-scale dendrites in a nanostructured matrix. However, some areas in the matrix contain nano-scale crystals together with an amorphous phase revealing an inhomogeneity of the specimen. On the contrary, the microstructures of the centrifugally-cast and the arc-melted samples are overall homogeneous, consisting of micrometer-scale dendrites that are homogeneously distributed in a nanostructured matrix. The dendrites correspond to a body centered cubic (bcc) beta-Zr phase, whereas the nanostructured matrix consists of body centered tetragonal (bct) Zr2Cu-type and hexagonal close-packed (hcp) MgZn2-type phases. The comparison of the microstructures of these two alloys reveals that the formation of nano-scale twins and a disordered omega-phase in the beta-Zr dendrites only happens in the centrifugally-cast sample. The differences in the phases and the microstructures between the differently prepared samples significantly influence the corresponding mechanical properties of the specimens. (c) 2006 Elsevier B.V. All rights reserved.