The face-centered-cubic (fcc) to hexagonal close-packed (hcp) martensitic transformation exhibited by an as-cast Co-Cr-Mo-C alloy was investigated in this work. The alloy was annealed at 1150°C, water quenched, and then isothermally aged at 700°C to 900°C. Quantitative measurements of transformed hcp martensite (also known as e-martensite) as a function of time and temperature were used in plotting C curves describing the transformation kinetics. Moreover, microstructural characterization indicated that the transformation exhibited two distinctive e-martensite morphologies. In the neighborhood of coarse carbides, multiple nucleation events were linked to the formation of e-martensite, while in the bulk of the dendrite matrix, the dominant morphology was as straight plates. Kinetic determinations of activation energies, Q, in either region indicated that in platelike e-martensite, Q = 57.24 kcal/mol, but in ''pearlite''-like morphologies, Q = 37.88 kcal/mol. Apparently, as the activation energy was reduced, multiple nucleation events were favored leading to the ''pearlitic'' appearance.

The tribological behavior of two cobalt-base alloys-an as-cast high-carbon and a wrought low-carbon Co alloy-that are used as hip implant materials is examined in this work. This work discusses the experimental results of cobalt-cobalt wear pairs, in wrought and as-cast conditions, where the amount of hexagonal phase is systematically modified through an isothermal aging treatment. Fully FCC and HCP Co alloys are tested versus alloys with various volume fractions of HCP phase (0.05 to 1.0 volume fractions). Preliminary results indicate that Co-Cr-Mo/Co-Cr-Mo alloy pairs both possessing an HCP matrix microstructure tend to exhibit outstanding wear properties.

Cobalt base (Co-Cr-Mo-C) alloys are known to exhibit two crystal structures, namely, face-centeredcubic (fcc) and hexagonal-close-packed (hcp). Accordingly, in this work, fcc and hcp lattice constants were measured at room and elevated temperatures in a Co-27Cr-5Mo-0.05C alloy using "in-situ" X-ray diffraction techniques. It was found that the lattice parameters corresponding to both the fcc and hcp phases increase linearly with temperature as a result of thermal expansion. Linear (␣ l ) and volumetric (␣ v ) coefficients of thermal expansion were computed for the main crystal structures found in this alloy. In particular, it was found that the fcc phase deviates from ideal isotropy as ␣ v was approximated by ␣ v ϭ 2.7 ␣ l . Also, the equilibrium fcc-hcp temperature was established to be approximately 970°C. In particular, the thermodynamic stability of the fcc phase and its effect on the exhibited lattice parameters were disclosed. Moreover, measurements of alloy density with temperature indicated a linear decrease as a result of thermal expansion. Finally, the (c/a) ratio found for the hcp phase exhibited a nonlinear trend with temperature. This, in turn, was accompanied by a contraction along the hcp c-axis.

A modification of the polyol method has been shown to result in improved efficiency and enhanced kinetics for the synthesis of silver nanocrystals when compared with the traditional polyol method. The Ag nanocrystals produced were characterized using Xray diffraction and transmission electron microscopy. Accordingly, the exhibited Ag crystal structure, corresponding lattice constants, and resultant particle sizes were determined by these means. In addition, using Fourier transform infrared spectroscopy, it was found that a solid-state reaction between the AgNO 3 and the Poly (vinylpyrrolidone) (PVP) takes place prior to their dissolution in ethylene glycol. Moreover, when crystals grow under total rest conditions, they do not develop a spherical morphology as in the traditional polyol method, but a well-defined geometric shape showing preferential crystallographic growth directions. Under the experimental conditions of this work, the exhibited nanocrystal shapes were quasi-planar hexagonal. Apparently, PVP interacts with the FCC crystal structure promoting growth on the {100} preferential direction by playing the role of an atomic arranger.

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