Introduction: Metal devices in human body cause metals artifact in magnetic resonance imaging (MRI). The main cause of metals artifact is a susceptibility artifact induced by volume magnetic susceptibility (χv) mismatch between the metal and human tissue. The authors have developed Au-6Nb alloy with χv value similar to that of human tissue [approximately -9 × 10-6, (-9 ppm)]. However, Vickers hardness (HV) of the alloy (approximately 120HV) was insufficient for various biomedical applications. Increasing of Nb content was effective to increase HV of Au-Nb alloys, however, χv of the alloys increased simultaneously, and χv mismatch enhance. In this study, the χv and HV of Au-xNb alloys (x ≥ 6) were investigated, and the effects of heat treatment on them were clarified.
Experimental Methods: Au-xNb (x = 6, 8, 12, 15 and 19.1) alloy ingots were fabricated by argon-arc-melting. The ingots of lower Nb content (Nb = 6 and 8) were rolled at room temperature, and the ingots of higher Nb content (Nb = 12 and 15) were rolled at 573 K in air to a thickness of approximately 1-1.5 mm. The total rolling reduction was 50-70%. The rolled alloy plates were homogenized at 1273 K for 20 h. The plates after homogenization were then cut into pieces for evaluation. Au-19.1Nb alloy ingot was homogenized at 1273 K for 2 w, and then cut into pieces without rolling. Aging treatments were performed at 1073 K. The χv values were measured using a magnetic susceptibility balance; the HV was measured under a load of 100 g for 30 s; and the phase constitution was analysed using X-ray diffractometry (XRD).
Results and Discussion: In XRD profile of Au-19.1Nb alloy, only peaks of Au2Nb were observed. Thus, χv value of Au-19.1Nb alloy (-23 ppm) was assumed to be that of Au2Nb. χv value of Au-xNb alloys (x ≤ 12) increased linearly with increasing Nb content, however, χv value of Au-15Nb alloy (1 ppm) indicated reduction from Au-12Nb alloy (8 ppm). XRD profiles of Au-15Nb alloy showed the precipitation of Au2Nb. These results suggested that χv value of Au-Nb alloys of higher Nb content can be controlled by controlling amount of Au2Nb precipitation using aging treatment. 1073 K aging treatments for Au-12Nb alloy and Au-15Nb alloy certainly precipitated Au2Nb in the alloys and reduced χv values close to -9 ppm, which demonstrated that χv values of Au-Nb alloy with higher Nb content could be adjusted to -9 ppm. The maximum HV of Au-12Nb alloy and Au-15Nb alloy indicating χv value close to -9 ppm after aging treatment was approximately 230HV. This is nearly twice as much as that of Au-6Nb alloy also indicating χv value close to -9 ppm.
Conclusion: Aging treatment at 1073 K can reduce χv value of Au-Nb alloys with higher Nb content, and helps achieve a MRI artifact-free Au-Nb alloy with higher hardness.
A part of this work was financially supported by JSPS KAKENHI, Japan Science and Technology Agency, Tanaka Kikinzoku Research Fund and Terumo Foundation for Life Sciences and Arts.; JSPS KAKENHI; Japan Science and Technology Agency; Tanaka Kikinzoku Research Fund; Terumo Foundation for Life Sciences and Arts