Carbide Formation and Dissolution in Biomedical Co-Cr-Mo Alloys with Different Carbon Contents during Solution Treatment
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INTRODUCTION
THE Co-Cr-Mo alloys are currently applied as primary materials for orthopedic implants, in addition to stainless steels and titanium and its alloys, owing to their advantageous properties such as excellent mechanical properties, high corrosion resistance, and high wear resistance.[1–4] Standard cast Co-Cr-Mo alloys, registered as ASTM F75, have been used to manufacture biomedical devices such as artificial joints and denture bases because of their high wear resistance, which can be attributed to precipitation strengthening by carbides formed in the matrix.[5–7] Cast Co-Cr-Mo alloys are subjected to heat treatment in order to remove casting defects and improve their mechanical properties. Therefore, the behavior of carbides formed in Co-Cr-Mo alloys during heat treatment has been investigated in some studies.[8–18] Several studies have focused on investigating the dissolution behavior of carbides during solution treatment.[8–16] Although the precipitation or dissolution behavior of carbides is essential knowledge to establish an effective and low-cost manufacturing process for Co-Cr-Mo SHINGO MINETA, Graduate Student, KYOSUKE UEDA, Assistant Professor, and TAKAYUKI NARUSHIMA, Professor, are with the Department of Materials Processing, Tohoku University, Sendai, 980-8579, Japan. Contact e-mail: [email protected] SHIGENOBU NAMBA, Senior Research Metallurgist is with the Materials Research Laboratory, Kobe Steel, Ltd., Kobe, 651-2271, Japan. TAKASHI YONEDA, President, is with the Yoneda Advanced Casting Co., Ltd., Takaoka, 933-0951, Japan. Manuscript submitted November 4, 2009. Article published online May 7, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
alloys, a systematic explanation of the behavior of carbides during solution treatment has not been reported thus far. Therefore, the objective of this study is to investigate the microstructural changes that take place during the solution treatment in Co-Cr-Mo-C alloys with various carbon contents. On the basis of the observations and analyses, the phase, shape, and dissolution behavior of precipitates in the Co-Cr-Mo-C alloys are discussed. II.
EXPERIMENTAL PROCEDURE
A. Specimens Cast Co-Cr-Mo alloys with various carbon contents were investigated in this study; their chemical composition was Co-28Cr-6Mo-xC (x = 0.12, 0.15, 0.25, and 0.35 mass pct). Table I lists the chemical compositions of these alloys. Hereafter, these alloys with carbon contents of 0.12, 0.15, 0.25, and 0.35 mass pct are referred to as C12, C15, C25, and C35, respectively. Ingots of these alloys were induction melted under Ar atmosphere and cast in a copper mold. The size of the fabricated ingots was 33 mm in diameter and 100 mm in height. The ingots were cut into disks having a diameter and thickness of 33 and 5 mm, respectively. Plate specimens were then prepared by dividing each disk into six equal parts. B. Solution Treatment The plate specimens were heat treated at 1473 to 1548 K for 0 to 43.2 ks in Ar atmosphere with a gas flow rate of 1.7 9 10–6 m3Æs–1; they were then w
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