Formation of the reaction product layer on the surface of biomedical titanium alloys, Ti 29Nb 13Ta 4.6Zr (TNTZ) and Ti 6Al 4V ELI (Ti64), during gas nitriding was investigated. These alloys were exposed to nitrogen atmosphere at 1023, 1073, 1123 and 1223 K. After the gas nitriding, a reaction product layer was observed on the surface of both alloys, and was analyzed using an X ray diffraction (XRD), Auger electron spectroscopy (AES) and X ray Photoelectron spectroscopy (XPS). The layer was comprised of two phases, which were outer oxide layer (mainly TiO 2 ) and inner nitride layer (mainly TiN or Ti 2 N). In these layers, the thickness of the oxide layer particularly depended on the kinds of alloys. According to the thermodynamics and point defect theory, the growth rate of oxide layer is expected to be increased by the presence of Al in TiO 2 . Namely, the dissolution of Al into TiO 2 may increase the number of oxygen vacancies, resulting in acceleration of oxygen diffusion inward. On the other hand, the elements that accelerate the growth of the oxide layer are not contained in TNTZ. Thus, the oxide layer formed on Ti64 was thicker than that of TNTZ. In a similar way, the elements that accelerate the growth of the nitride layer are not contained in both TNTZ and Ti64. Thus, the nitride layers with similar thicknesses may be formed on TNTZ and Ti64 during gas nitriding.
Synchrotron X-ray microtomography has been utilized for the in-situ observation of ductile fracture in cast Al-7%Si alloys exposed at a high temperature for 0-10 6 s. A high resolution experimental configuration and deflection contrast imaging technique have enabled the reconstruction of silicon particle images with an isotropic voxel with a 0.474 mm edge. The variations in particle shape, size and spatial distribution at the high temperature are readily observed, along with the existence and growth of high-density micro-pores. Three-dimensional image analysis is applied and its feasibility is confirmed. It is clarified that in the case of an as-cast material void nucleation and growth have been observed as has been reported for the general ductile fracture of metallic materials. When it is exposure at the high temperature for a long time, however, ductile fracture is found to be attributable mainly to the growth of pre-existing micro-pores and not the nucleation of new voids at silicon particles. Since such tendency has been also confirmed for other materials, more detailed analysis might be expected in a near future in order to understand actual ductile fracture process in practical materials.
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