5. Cobalt-chrome\" is the acronym used to describe a Co-Cr-Mo alloy used for wed

Question

5. Cobalt-chrome" is the acronym used to describe a Co-Cr-Mo alloy used for wedding rings, orthopedic implants, dental implants, and gas turbines. The amount of Mo is small and it forms a complete solid solution with CrThe Co-Cr binary diagram is shown below 1906 C 1800 Liquid 1495 C 2.5 BCC 269 583 1200 FCC SIGMA 39732 HCP 1o 90 100 a) What material has a face-centered cubic crystal structure? b) What material has a body-centered cubic crystal structure? c What material has a hexagonal close-packed crystal structure? d) Give the composition, temperature, and the reaction (upon cooling) for the cutectic e Give the composition, temperature, and the reaction (upon cooling) for the eutectoid. Give the chemical formula for the sigma phase g) The Co/Cr weight ratio is 2.2 for the "cobalt-chrome" alloy. Identify the phase(s) present in cobalt-chrome, the composition of the phase(s), and how much of the (each) phase is present at 800°C. What is the strengthening mechanism for this alloy?

Explanation / Answer

Following material has FCC Crystal Structure: Cobalt (Co), Following material has BCC Crystal Structure: Molybdenum (Mo) Following material has HCP Crystal Structure: Chromium (Cr) Eutectic: the alloy should solidify omitting the eutectic transition at 1397 °C. The phase (FCC, A1) that solidifies directly from liquid is stable till ca. 950 °C, when the martensitic-like transf Eutectectoid: At lower temperatures, at chromium concentration range including that of the examined alloy, the eutectoidal decomposition Co3Cr+Co2Cr is observed. At higher Cr concentration, its equation is Co2Cr+Co3Cr2 Sigma: Co5Cr3

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