Effects of Section Size, Surface Cooling Conditions, and Crucible Material on Microstructure and As-Cast Properties of I
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THE biomedical grade cobalt alloy investigated in this work is Co-28Cr-6Mo (wt pct) with the composition as specified in ASTM F75. This alloy is widely used in orthopedic implants due to its excellent wear and corrosion resistance. Due to the complexity of the shape of the implants and the poor workability of the alloy, investment casting is often the preferred method of manufacture, attaining net-shaped components with little requirement for machining. Unfortunately, the mechanical properties obtained via this technique may fail to meet the requirements set out in the standards. This is a cause of concern as it could lead to early implant failure. Inadequate mechanical properties are directly related to inherent casting defects and the microstructure presented. It is generally accepted that the as-cast microstructures of ASTM F75 alloys produced by investment casting exhibit a dendritic, face-centered cubic (fcc), cobalt-based matrix, with secondary phases precipitated at grain boundaries and interdendritic zones RUTH KAISER, Ph.D. Student, and DAVID J. BROWNE, Senior Lecturer, are with the School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland. Contact e-mail: [email protected] KENNY WILLIAMSON, Materials Development Engineering Team Leader, CLAIRE O’BRIEN, Senior Materials Engineer (Metallurgy), are with the DePuy(Ireland), Loughbeg, Ringaskiddy, Co. Cork, Ireland. Manuscript submitted August 28, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
where the major secondary phase is an M23C6 carbide.[1–6] The nature of grain size, porosity, and secondary phase precipitates such as carbides all contribute to the mechanical properties and characteristics of the castings. Grain size in investment castings tends to be large with great variation between the outer and inner zones of the casting due to the nature of the process,[1,3,7] and porosity is an inherent characteristic of castings that is very hard to eliminate, but may be controlled. Secondary phase precipitates such as the M23C6 carbide are thought to be a major strengthening mechanism,[8] but depending on their morphology and type they can also contribute to the low mechanical properties observed in the as-cast state of the alloy.[4,9–11] A lamellar carbide has been detected at grain boundaries, while the carbides found within the dendritic matrix have a ‘‘blocky’’ appearance.[1–4] Subsequent thermo-mechanical treatments are carried out to homogenize the structure, relieve inherent stresses, eliminate porosity, and partially dissolve the carbide network resulting in superior mechanical properties. Numerous researchers have shown that carbide type and morphology are sensitive to casting parameters.[1,3,4,6,12,13] It may therefore be possible that controlled carbide precipitation could lead to consistently adequate mechanical properties conforming to the previously mentioned standard with reduced requirement for thermo-mechanical treatments. Controlled precipitation may be possible through controlled solidifi
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