Effect of phase stability of FCC and HCP phases on tensile and low-cycle fatigue properties of Co–20Cr–10Mo–xNi medium-entropy alloys

Abstract

Co–Cr alloys are widely used in balloon-expandable coronary stents. However, early fracture of the stents caused by cyclic deformation after placement has been reported. In this study, Co–Cr–Mo–Ni alloys with long low-cycle fatigue (LCF) lives were designed based on the phase stabilities of the face-centered cubic (FCC) and hexagonal close-packed (HCP) phases and deformation modes. The deformation modes in the Co–20Cr–10Mo–(20–35)Ni alloys at room temperature were investigated by X-ray diffraction, electron-channeling contrast imaging, and electron backscattered diffraction. Deformation-induced HCP phase was observed in the Co–20Cr–10Mo–(20–29)Ni alloys, and deformation twins were observed in the Co–20Cr–10Mo–(23–35)Ni alloys after tensile deformation. The deformation modes continuously shifted from the deformation-induced FCC → HCP transformation to deformation twinning with increasing Ni content. The LCF lives of the Co–20Cr–10Mo–26Ni alloy and Co–20Cr–10Mo–29Ni alloy under a total strain amplitude of 0.01 were 6537 and 6330 cycles, respectively, which are 2.5 times higher than that of a conventional Co–20Cr–10Mo–35Ni alloy. These alloys also exhibited good strength–ductility balance owing to the transformation-induced plasticity and twinning-induced plasticity effects. Thus, these alloys are plausible candidates for fatigue-resistant Co–Cr alloys for balloon-expandable stents.