Effects of phase constitution on magnetic susceptibility and mechanical properties of Zr-rich Zr–Mo alloys

The effects of the microstructures and phases of Zr-rich Mo alloys on their magnetic susceptibilities and mechanical properties were investigated in order to develop a Zr alloy with low magnetic susceptibility for use in magnetic resonance imaging (MRI). The magnetic susceptibility was measured with a magnetic susceptibility balance, while mechanical properties were evaluated by a tensile test. The microstructure was evaluated with an X-ray diffractometer, an optical microscope, and a transmission electron microscope. Evaluation of the microstructures revealed that the α′ phase was the dominant form at less than 2% Mo content in the as-cast alloy. The ω phase was formed in as-cast Zr–3Mo but disappeared with aging at 973 K. Magnetic susceptibility was reflected in the phase constitution: the susceptibility showed a local minimum at Zr–(0.5–1)Mo with mostly α′ phase and a minimum at Zr–3Mo with mostly β and ω phases. The magnetic susceptibility of as-cast Zr–3Mo increased at 973 K due to disappearance of the ω phase. However, the susceptibility was still as low as that of as-cast Zr–1Mo. The ultimate tensile strength of α′-based Zr–Mo alloys was tailored from 674 to 970 MPa, and the corresponding elongation varied from 11.1% to 2.9%. Because Zr–Mo alloys containing ω phase were found, through tensile tests, to be brittle this phase should be avoided, irrespective of the low magnetic susceptibility, in order to maintain mechanical reliability. Elongation of the Zr–3Mo alloy was dramatically improved when the phase constitution was changed to α and β phases by aging at 973 K for 86.4 ks. The magnetic susceptibilities of the α′-based Zr–Mo alloys are one-third those of Ti–6Al–4V and Ti–6Al–7Nb, and thus these Zr alloys are useful for medical devices under MRI.