Microstructure and magnetic susceptibility of as-cast Zr–Mo alloys

The microstructures and magnetic susceptibilities of Zr–Mo alloys were investigated to develop a Zr alloy with a low magnetic susceptibility for magnetic resonance imaging (MRI). The microstructure was evaluated with an X-ray diffractometer (XRD), an optical microscope (OM) and a transmission electron microscope (TEM), and the magnetic susceptibility was measured with a magnetic susceptibility balance. The α′ phase with acicular structure was dominant in Zr–1Mo alloys, while the ω and β phases with the equiaxed and relatively flat (no acicular) microstructure was dominant in Zr–3Mo. The mixed microstructural features of Zr–1Mo and Zr–3Mo were observed in Zr–2Mo, which consists of the α′, ω and β phases. The β phase is stabilized when the Mo content exceeds over 3 mass% Mo. As-cast Zr–Mo alloys showed a minimum value of magnetic susceptibility at 3 mass% Mo, and the value abruptly increased up to 10% Mo before remaining stable up to 15 mass% Mo. XRD, OM and TEM revealed that the minimum value of the susceptibility was closely related to the appearance of the athermal ω phase in the β phase. As the Mo content decreases from 3 mass%, the α′ phase appears with the ω and β phases. On the other hand, as the Mo content increases from 3 mass%, the β phase increases and the ω phase decreases. Thus the appearance of the α′ and β phase leads to an increase in magnetic susceptibility. The magnetic susceptibility of as-cast Zr–3Mo alloy was almost one-third that of Ti–6Al–4V, which is commonly used for medical implant devices. Zr–Mo alloys are useful for medical devices used under MRI.