The γ phase Co-Cr alloy can be converted into the ε phase at high pressures, which shows a HCP crystal structure. FCC crystal structure is commonly found in cobalt rich alloys, while chromium rich alloys tend to have BCC crystal structure. FCC crystal structure is found in the γ phase, and the γ phase shows improved strength and ductility compared to the σ phase. The σ phase, where the alloy contains approximately 60–75% chromium, tends to be brittle and subject to a fracture. Structure ĭepending on the percent composition of cobalt or chromium and the temperature, Co-Cr alloys show different structures. ASTM F75, ASTM F799, ASTM F1537 are Co-Cr-Mo alloys with very similar composition yet slightly different production processes, ASTM F90 is a Co-Cr-W-Ni alloy, and ASTM F562 is a Co-Ni-Cr-Mo-Ti alloy. There are several Co-Cr alloys that are commonly produced and used in various fields. These elements stabilize the γ phase, which has better mechanical properties compared to other phases of Co-Cr alloys. In order to overcome the low ductility, nickel, carbon, and/or nitrogen are added. This is a concern as the alloys are commonly used in hip replacements. However, Co-Cr alloys tend to have low ductility, which can cause component fracture. Moreover, tensile and fatigue strength increases radically as they are heat-treated. The hardness of Co-Cr alloys varies ranging 550-800 MPa, and tensile strength varies ranging 145-270 MPa. Good mechanical properties that are similar to stainless steel are a result of a multiphase structure and precipitation of carbides, which increase the hardness of Co-Cr alloys tremendously. Biocompatibility also depends on the film and how this oxidized surface interacts with physiological environment. Īs its wide application in biomedical industry indicates, Co-Cr alloys are well known for their biocompatibility. Properties Ĭo-Cr alloys show high resistance to corrosion due to the spontaneous formation of a protective passive film composed of mostly Cr 2O 3, and minor amounts of cobalt and other metal oxides on the surface. Recently, in 2010, scientists at the University of Cambridge have produced the alloy through a novel electrochemical, solid-state reduction technique known as the FFC Cambridge Process which involves the reduction of an oxide precursor cathode in a molten chloride electrolyte. However, synthesis of Co-Cr alloy through the method mentioned above is very expensive and difficult. ASTM F75, a Co-Cr-Mo alloy, is produced in an inert argon atmosphere by ejecting molten metals through a small nozzle that is immediately cooled to produce a fine powder of the alloy. Due to the chemical reactivity of metals at high temperature, the process requires vacuum conditions or inert atmosphere to prevent oxygen uptake by the metal. Pure metals are then fused together under vacuum either by electric arc or by induction melting. Chromium usually goes through aluminothermic reduction technique, and pure cobalt can be achieved through many different ways depending on the characteristics of the specific ore. Both of the ores need to go through reduction process to obtain pure metals. The common Co-Cr alloy production requires the extraction of cobalt and chromium from cobalt oxide and chromium oxide ores. Recently, due to excellent resistant properties, biocompatibility, high melting points, and incredible strength at high temperatures, Co-Cr alloy is used for the manufacture of many artificial joints including hips and knees, dental partial bridge work, gas turbines, and many others. In the 20th century, the alloy was first used in medical tool manufacturing, and in 1960, the first Co-Cr prosthetic heart valve was implanted, which happened to last over 30 years showing its high wear-resistance. Under the name Stellite, Co-Cr alloy has been used in various fields where high wear-resistance was needed including aerospace industry, cutlery, bearings, blades, etc.Ĭo-Cr alloy started receiving more attention as its biomedical application was found. Haynes reported his alloy was capable of resisting oxidation and corrosive fumes and exhibited no visible sign of tarnish even when subjecting the alloy to boiling nitric acid. The alloy was first discovered with many other elements such as tungsten and molybdenum in it. History Ĭo-Cr alloy was first discovered by Elwood Haynes in the early 1900s by fusing cobalt and chromium. Cobalt-chrome has a very high specific strength and is commonly used in gas turbines, dental implants, and orthopedic implants. Cobalt-chrome disc with dental bridges and crowns manufactured using WorkNC DentalĬobalt-chrome or cobalt-chromium ( CoCr) is a metal alloy of cobalt and chromium.
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