Continuous Measurements of Recrystallization and Grain Growth in Cobalt Super Alloys
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COBALT-BASED superalloys are common materials for the manufacturing of various components used in aerospace and biomedical applications. Their stability and high strength at elevated temperatures are beneficial in the design of blades or combustion chambers in gas turbines[1,2] They are also used in biomedical applications such as in hip, ankle, or knee implants, as well as in the manufacturing of biomedical stents.[3] In the latter case, meshed biocompatible metallic structures are subjected to a severe plastic deformation during their insertion into the human body cavity and must resist the high stresses induced by external pressure during their use.[4] Two grades of cobalt-based superalloys are of particular interest as they combine the high ductility required for the deployment of stents while having sufficiently high strength to sustain external pressure exerted by the human body during their lifetime of use.[5] The L605, also known as Haynes No. 25 alloy,[6] contains Cr, W, and Ni which provide strong solid solution strengthening.[7] Ni is added to limit the formation of brittle martensitic structures during cooling.[8] The alloy has a total elongation higher than 40 pct in the annealed state with an ultimate tensile stress MAHSA KEYVANI, THOMAS GARCIN, and MATTHIAS MILITZER are with the Centre for Metallurgical Process Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] DAMIEN FABRE`GUE is with the Universite` de Lyon, CNRS, Villeurbanne 69621, France, and also with the INSA-Lyon, MATEIS UMR5510, Villeurbanne 69621, France. KENTA YAMANAKA and AKIHIKO CHIBA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan. Manuscript submitted August 5, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
(UTS) of 1 GPa and a yield stress (YS) of almost 500 MPa.[9] Although the L605 alloy barely satisfies the required specifications for UTS and YS, it has been used in the stent application because of its ductility. A second grade called CCM became of recent interest for these applications.[10] In this alloy, the Ni content is decreased (or is completely eliminated) to reduce the risk of allergic reactions with the human tissue. CCM has a UTS of 1.2 GPa and a YS of 600 MPa but a 10 pct lower ductility than L605. Despite its high strength, it cannot yet be considered as a potential alloy for stent applications due to its lower ductility. Therefore, attention is currently focused on the design of processing paths that will bring their mechanical properties in the range of specifications for stent applications in terms of strength and ductility combinations.[5,11] Due to the Hall–Petch effect, the reduction of grain size in these alloys offers an appropriate method to increase the YS. The fundamental problem with this classical approach is that the decrease in grain size has a dramatic influence on the ductility.[12] It was recently proposed by Favre et al. that the formation of a bimodal grain size dis
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