M 23 C 6 carbide dissolution mechanisms during heat treatment of ASTM F-75 implant alloys
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INTRODUCTION
IN surgical implant materials, a metal powder coating is commonly applied to enhance the implant fixation to bone and muscle by tissue ingrowth.[1–5] In cobalt-based alloys, such as ASTM F-75, the porous coating is formed through a sintering process at temperatures higher than those applied in conventional solution heat treatment (1300 ⬚C vs 1220 ⬚C).[2] In addition, to improve wear[6] and mechanical properties, and prevent fatigue failure of implant cast alloys, different types of treatments are applied. Table I highlights the three main kinds of treatments that are applied to these materials, namely, conventional solution or homogenization treatments,[7–10] hot-isostatic pressing (hipping),[11,12] and carbide refining treatments.[13,14,15] Several authors[11,12] have reported that the best elongation values were obtained by hipping treatments and carbides refining by melt additions. Annealing generally produces undesired results, and conventional solution treatments require very close control of the treatment conditions. Regarding costs and technological level required by different kinds of treatments, the most expensive and higher demanding technology are hipping treatments, while conventional solution treatments have the lowest cost. Due to intrinsic characteristics of the alloys, all of the aforementioned processes present technical difficulties that require a rigorous control of manufacturing processes in order to achieve the quality demanded on implants to be used in the human body. In conventional solution heat treatments, the temperature range available to dissolve most of carbides is very narrow. At low temperatures, spheroidization and coarsening of carbides are observed, but they are left out of the solution in the matrix of austenite.[16–19] Elevated temperatures cause carbide fusion, whereas low temperatures fall in a two-phase domain preventing complete H. MANCHA, J.I. ESCALANTE, G. MENDOZA, and M. ME´NDEZ, Researchers, and E. CARRANZA, M.Sc., are with the Centro de Investigacio´n y de Estudios Avanzados del IPN, 25000 Saltillo, Coah, Mexico. F. CEPEDAL and E. VALDE´S, Researchers, are with the Instituto Tecnolo´gico de Saltillo, 25000 Saltillo, Coah, Mexico. Manuscript submitted March 17, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
homogenization and decreasing, considerably, the carbide dissolution kinetics.[9] Moreover, high-temperature conditions employed in the application of the metal powder coating causes incipient fusion of M23C6 carbides, producing materials of poor mechanical properties. In this work, the M23C6 carbide dissolution of ASTM F75 alloys was experimentally followed during a liquid-phase homogenization treatment, in as-cast and pretreated for partial carbide dissolution (PTPCD) specimens. The microstructural features observed are explained in terms of a solutal diffusion-driven mechanism leading to the growth of the matrix by consuming the liquid phase formed by the carbide fusion.
II. MATERIALS AND METHODS Specimens from investment casting alloys conforming t
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