Effects of Phase Constitution of Zr-Nb Alloys on Their Magnetic Susceptibilities

摘要

The magnetic susceptibilities and microstructures of Zr-Nb binary alloys were investigated to develop a new metallic biomaterial with a low magnetic susceptibility for magnetic resonance imaging (MRI). The magnetic susceptibility was measured with a magnetic susceptibility balance, and the microstructure was evaluated with an X-ray diffractometer (XRD), an optical microscope (OM), and a transmission electron microscope (TEM). Zr-Nb alloys as-cast showed a minimum value of magnetic susceptibility between 3 and 9 mass percent Nb, and the value abruptly increased up to 20 mass percent Nb, followed by a gradual increase with the increase of the Nb content. XRD, OM, and TEM revealed that the minimum value of the susceptibility was closely related to the appearance of the athermal omega phase in the beta phase. Since the magnetic susceptibility of Zr-3Nb alloy consisting of an alpha' phase was as low as that of Zr-9Nb alloy consisting of the beta and omega phases, that of the co phase was lower than that of the alpha' and beta phases. When Zr-16Nb alloy was heat-treated, the isothermal co phase appeared, and, simultaneously, the magnetic susceptibility decreased. Therefore, the omega phase contributes to the decrease of the magnetic susceptibility, independently of the formation process of the co phase. The magnetic susceptibility of the Zr-3Nb alloy as-cast was almost one-third that of Ti-6Al-4V alloy, which is commonly used for medical implant devices. Zr-Nb alloys are useful for medical devices used under MRI.