Microstructure and Mechanical Properties of Heat-Treated Co-20Cr-15W-10Ni Alloy for Biomedical Application
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I.
INTRODUCTION
AUstenitic stainless steels have been used as materials for balloon-expandable stents.[1,2] These stainless steels have excellent workability, but their strength is not necessarily sufficient for the stents, which are used in the plastic deformation region. In recent years, Co-20Cr-15W-10Ni (mass pct) alloys, whose composition is specified in the ASTM F90 standards, have been used as a substitute for stainless steels in stent applications because of their excellent strength, biocompatibility, and hot and cold workability.[2–4] The combination of a tubing process and laser cutting is the most popular manufacturing method for metallic stents.[3] In the tubing process, hot forging and a cyclic process of cold working and annealing are conducted for the alloys. During this process, microstructural changes such as precipitation and phase transformation may occur. Several studies have been reported on the precipitates in Co-Cr-W-Ni alloys.[5–11] M23C6-type (M: metallic element) and g-phase (M6C-M12C type) carbides,[5–9,11] and a and b-Co3W, Co2W (Laves phase), Co2W, and l-phase (Co7W6) intermetallics[6,7] have been reported as precipitates in the Co-Cr-W-Ni alloys. However, the phases of the precipitates were inconsistent with each other, even under the same heat-treatment conditions. As for the mechanical properties of the alloys, Teague et al. reported that the precipitation of the g phase
KOSUKE UEKI, Student, KYOSUKE UEDA, Associate Professor, and TAKAYUKI NARUSHIMA, Professor, are with the Department of Materials Processing, Tohoku University, 6-6-02, AzaAoba, Aramaki, Aoba-ku Sendai 980-8579, Japan. Contact e-mail: [email protected] Manuscript submitted October 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
caused a deterioration in the ductility.[6] The present authors reported the formation of M23X6-type (X: C and N) and g-phase precipitates in the alloy and showed that the formation of the g phase deteriorated the tensile properties of the alloys.[11] However, the effect of the heat treatment, in particular, at relatively low temperatures, on the microstructure and mechanical properties of the alloy has not been clarified. Therefore, studies on the heat-treatment behavior of this alloy for a wide range of heat-treatment temperatures and durations are required. In this study, a Co-20Cr-15W-10Ni (mass pct) alloy, with its composition specified in the ASTM F90 standards, which can be used for biomedical applications, was heat-treated at temperatures of 673 K to 1623 K (400 C to 1450 C) for 0.6 to 259.2 ks, and the microstructures and mechanical properties of the heattreated alloys were investigated.
II.
EXPERIMENTAL
A. Specimens A forged bar with a diameter of 22.5 mm (Carpenter Technology Corporation, PA, USA) having the chemical composition listed in Table I was used as the base alloy (as-received alloy) for heat treatment. Hereafter, the chemical compositions of the alloys are denoted by their mass percentages, and the notation ‘‘mass pct’’ is omitted. The as-received alloy was
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