Effect of casting conditions on dendrite-amorphous/nanocrystalline Zr–Nb–Cu–Ni–Al in situ composites

High-strength in situ composites with enhanced ductility were prepared by various casting methods from Zr–Nb–Cu–Ni–Al alloys related to bulk metallic glass compositions. The composites exhibit a bimodal size distribution consisting of bcc β-Zr dendrites stabilized by the addition of Nb and a nanocrystalline/amorphous matrix. The dendrite and matrix crystallite size and the morphology depend sensitively on casting conditions. The cooling rates estimated from the secondary dendrite arm spacing of the Zr73.5Nb9Cu7Ni1Al9.5 alloy vary between ϵ≈2.6×103 K/s to 4.0×101 K/s. The effect of processing parameters on the mechanical properties is less significant for the Zr66.4Nb6.4Cu10.5Ni8.7Al8.0 alloy while highly pronounced for the Zr73.5Nb9Cu7Ni1Al9.5 alloy, where optimum properties like fracture stress of 1754 MPa and 17.5% strain to failure were achieved for cast rods 10 mm in diameter. The deformation of the composite proceeds partly through dislocation movement in dendrites and a shear banding mechanism in the nanostructured matrix.