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The plastic deformation behavior of solution treated Co-27Cr-5Mo cast alloys with C contents up to 0.31 wt% were studied at 298 K by means of uniaxial tension and compression tests. In addition, the effect of initial grain size on the plastic flow behavior of annealed Co-27Cr-5Mo-0.05C wrought alloy was investigated. Optical and scanning electron microscopy and X-ray diffraction techniques were used to characterize the microstructures produced by plastic deformation.
Carbon contents greater than 0.05 wt% and furnace cooling after solution treatments at 1250 °C were found to inhibit the occurrence of strain-induced fcc(metastable)=>hcp phase transformation and lead to high strain hardening rate, high strength and low ductility. A similar behavior was found in annealed, 0.05 wt% C alloy for grain sizes smaller than 10 μm. In contrast, a fast cooling rate after the solution treatment and grain sizes in the range 100–2000 μm for this later alloy were found to promote phase transformation during deformation which lead to a rapid decrease of the rate of strain hardening. The yield strength and the strain hardening behavior of both cast-solution treated and wrought-annealed materials were found to depend on the type of loading during the tests.
The results allowed to conclude that the strain-induced fcc(metastable)=>hcp phase transformation plays an important role during large strain plastic deformation of low carbon Co-27Cr-5Mo implant alloys. However, when the carbon C in these alloys is larger than 0.05 wt%, the ductility and the fracture behavior are controlled by the size, morphology and distribution of secondary phase carbide particles in the microstructure.
Plastic deformation, strain hardening, microstructure, cobalt-base alloys
Professor, CINVESTAV-Unidad Saltillo, Coahuila,