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INTRODUCTION

THE precipitates in biomedical cobalt (Co)-chromium (Cr)-molybdenum (Mo) alloys are known to affect their mechanical[1–3] and corrosive[4] properties. Therefore, knowledge of the precipitation of secondary phases in Co-Cr-Mo alloys is essential for establishing appropriate production processes and improving the alloys’ properties. The main precipitated phases observed in biomedical Co-Cr-Mo alloys are the intermetallic r-phase,[5] M23C6-type carbide,[6–10] g-phase (M6C-M12C-type carbide),[11–13] and M7C3-type carbide.[14] In previous studies, the presence of the p-phase (M2T3X type, a b-Mn structure with M and T: metallic elements and X: carbon and nitrogen)[15–18] and v-phase (intermetallic compound, with an a-Mn structure)[19] in the Co-Cr-Mo alloy was found, and the conditions under which the M7C3 type carbide formed were revealed.[15] In recent years, nitrogen has been attracting attention as an effective alloying element for improving the mechanical properties and workability of Co-Cr-Mo alloys.[20–26] Several authors[3,18,27–30] have studied the

SHINGO MINETA and ALFIRANO, Graduate Students, KYOSUKE UEDA, Assistant Professor, and TAKAYUKI NARUSHIMA, Professor, are with Department of Materials Processing, Graduate School of Engineering, Tohoku University, 6-6-02, Aza Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan. Contact email: [email protected] SHIGENOBU NAMBA, Senior Research Metallurgist, is with Materials Research Laboratory, Kobe Steel, Ltd., Kobe 651-2271, Japan. TAKASHI YONEDA, President, is with Yoneda Advanced Casting Co., Ltd., Takaoka 933-0951, Japan. Manuscript submitted March 26, 2012. Article published online September 22, 2012 494—VOLUME 44A, JANUARY 2013

microstructures formed in Co-Cr-Mo alloys with nitrogen content. In our previous studies, the microstructures of as-cast and heat-treated Co-28Cr-6Mo-0.25C-(0,1)Si(0,1)Mn-0.175 ± 0.25N alloys (mass pct) were investigated, and the formation of an M2X type (Cr2N structure) precipitate was reported.[18] However, the phase formation/dissolution of the secondary phases were not fully understood in those Co-Cr-Mo-C-N alloys where both the nitrogen and carbon contents were varied. In this study, the precipitate behavior in as-cast and heat-treated Co-28Cr-6Mo-(0 to 0.35)C-(0.15 to 0.25)N alloys (mass pct) was investigated, where the upper limits for the nitrogen and carbon contents were 0.25 and 0.35 mass pct, respectively, according to the ASTM F75 standards. II.

EXPERIMENTAL PROCEDURE

A. Specimens The chemical compositions of the Co-Cr-Mo-C-N alloys used in this study are listed in Table I. The target contents of carbon and nitrogen were 0, 0.15, 0.25, and 0.35 mass pct and 0.15, 0.20, and 0.25 mass pct, respectively. Hereafter, the alloys are referred to using the notation in the left column of Table I. The chemical analyses of the studied alloys are given in mass percent. The alloys were melted at 1773 K to 1873 K (1500 °C to 1600 °C) using an induction furnace, and then poured into copper (Cu) molds at around 1823 K (155