STP1365: Metallurgy, Microstructure, Chemistry and Mechanical Properties of a New Grade of Cobalt-Chromium Alloy Before and After Porous-Coating

    Mishra, AK
    Senior Research Projects Manager, Research Engineer I, and Research Engineer II, Smith and Nephew Inc., Memphis, TN

    Hamby, MA
    Senior Research Projects Manager, Research Engineer I, and Research Engineer II, Smith and Nephew Inc., Memphis, TN

    Kaiser, WB
    Senior Research Projects Manager, Research Engineer I, and Research Engineer II, Smith and Nephew Inc., Memphis, TN

    Pages: 18    Published: Jan 1999


    Abstract

    Titanium alloy (Ti-6Al-4V) and cast cobalt-chromium-molybdenum alloy (CoCr) have a long history of successful clinical use. However, a higher strength material would be useful to manufacture certain implants, such as smaller hip-stems, with more extensive porous-coating to facilitate improved fixation. A new grade [high carbon (C), proposed grade 2] of CoCr, within the ASTM specification for wrought cobalt - 28 chromium - 6 molybdenum alloy for surgical implants (F 1537), has been developed to meet this objective. One producer's version of this new material was compared with cast CoCr. The typical C content of this material is similar to that of cast CoCr. In the mock-sintered condition, the tensile and yield strength of bar and forgings of this new CoCr were similar to each other and greater than that of cast CoCr, and all three materials had similar ductility. In both porous-coated and mock-sintered conditions, the average grain size of bar and forgings of this new CoCr was 63±19 to 145±12 μm, while that of two commercially available porous-coated cast CoCr hip-stems from two manufacturers was 2540±395 and 3394±876 μm respectively. Metallography revealed that the cast CoCr implants contain the eutectic phase [face centered cubic (fcc) Co, the intermetallic sigma (σ) phase, and metal carbides (M23C6 and M7C3)] in the interdendritic regions. The 10 million cycle fatigue strength was 207 MPa for cast CoCr and 241 MPa for the new CoCr bar in the porous-coated condition, and 345 MPa for cast CoCr and at least 448 MPa for the new CoCr bar in the mock-sintered condition. Fatigue testing was performed only on bar since the tensile properties and microstructure of bar and forgings were very similar, both in the porous-coated and the mock-sintered condition. Thus, the new grade of wrought CoCr exhibited higher fatigue strength than cast CoCr in both porous-coated and mock-sintered conditions. This may be attributed to the finer grain size of this material.

    Keywords:

    Cobalt-chromium alloy, high carbon, bar-stock, forging, porous-coated, fatigue strength, tensile strength, microstructure


    Paper ID: STP14264S

    Committee/Subcommittee: F04.12

    DOI: 10.1520/STP14264S


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