Microstructure and mechanical properties of as-cast Zr-Nb alloys

On the basis of the microstructures and mechanical properties of as-cast Zr-(0-24)Nb alloys the effects of phase constitution on the mechanical properties and magnetic susceptibility are discussed in order to develop Zr alloys for use in magnetic resonance imaging (MRI). The microstructures were evaluated using an X-ray diffractometer, an optical microscope, and a transmission electron microscope; the mechanical properties were evaluated by a tensile test. The alpha' phase was dominantly formed with less than 6 mass% Nb content. The omega phase was formed in Zr-(6-20)Nb alloys, but disappeared from Zr-22Nb. The beta phase dominantly existed in Zr-(9-24)Nb alloys. The mechanical properties as well as the magnetic susceptibility of the Zr-Nb alloys varied depending on the phase constitution. The Zr-Nb alloys consisting of mainly alpha' phase showed high strength, moderate ductility, and a high Young's modulus, retaining low magnetic susceptibility. Zr-Nb alloys containing a larger volume of omega phase were found to be brittle and, thus, should be avoided, despite their low magnetic susceptibility. When the Zr-Nb alloys consisted primarily of beta phase the effect of omega phase weakened the mechanical properties, thereby leading to an increase in ductility, even with an increase in magnetic susceptibility. The minimum value of Young's modulus was obtained for Zr-20Nb, because this composition was the phase boundary between the beta and omega phases. However, the magnetic susceptibility of the alloy was half that of Ti-6Al-4V alloys. Zr-Nb alloys consisting of alpha' or beta phase have excellent mechanical properties with low magnetic susceptibility and, thus, these alloys could be useful for medical devices used in MRI. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.