Constrained Dendritic Growth and Solute Concentration Effects in Rapidly Solidified Co-Cr Alloys
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BIOCOMPATIBLE metallic alloys need to satisfy some basic requirements to be considered as viable materials for the development of bioengineering devices.[1,2] Depending on the particular application within the human body, some properties might also need to be modified or improved.[3,4] Among the various biocompatible materials, Co-Cr-Mo-C alloys have been widely employed in the development of prosthetic implants. These devices are commonly produced using investment casting techniques, or in some cases through wrought metal work.[5,6] In alloy casting, the processing conditions in general lead to relatively low cooling rates during solidification. As a result, the exhibited microstructure consists of coarse dendrites with significant interdendritic segregation and second phases. It is
A.L. RAMIREZ-LEDESMA is with the Department of Mechanical Engineering, Politecnico di Milano, Campus Bovisa Sud via La Masa 1, 20156, Milano, Italy. Contact e-mail: [email protected] H.F. LOPEZ is with the Materials Science and Engineering Department, CEAS University of Wisconsin Milwaukee, 3200 N. Cramer Street, Milwaukee, WI 53211. J.A. JUAREZ-ISLAS is with the Instituto de Investigaciones en Materiales, Universidad Nacional Auto´noma de Me´xico, Circuito Exterior S/N, Cd. Universitaria, C.P. 04510, Mexico, Ciudad de Me´xico, Mexico. Manuscript submitted March 19, 2018. Article published online February 15, 2019 2272—VOLUME 50A, MAY 2019
well known[7–9] that coarse interdendritic phases such as carbides or other intermetallics promote brittle alloy behavior in Co-Cr-Mo-C alloys. In addition, inherent casting defects, such as macro-segregation and gas porosity, could lead to inadequate mechanical properties and poor alloy performance.[10,11] Currently, biomedical applications have been focused on Co-Cr-Mo-C (F-75, biomedical grade) alloys used in the manufacture of orthopedic prostheses such as knee, shoulder, and hip implants. Further applications include the use of cobalt alloys as fixation devices in fractured bones and in the fabrication of cardiovascular devices.[12–14] Specifically, for the latter application Co-Cr alloys (L605) are preferred than titanium alloys.[15] Yet, in some of these applications relatively high formability is required which can only be achieved through alloy design where carbon and other interstitials are not involved. Alloying additions of substitutional elements such as Ni or Mo provide solid solution strengthening with Ni also promoting the stability of the c-phase. However, Ni is currently avoided as it leads to allergic reactions when it leaches in the human environment.[16,17] Hence, when high ductilities are desirable such as in the design of stents, binary Co-Cr alloys can be considered as prospective candidates. The performance of cobalt base alloys for biomedical applications can be further improved through microstructural alloy design and novel casting technology.[18,19] Among the potential alloy design options, rapid solidification (RS) can be considered a part of the casting tech
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