Grain-orientation dependent nitriding behaviour of cubic and hexagonal substrates

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Previous nitriding investigations performed on polycrystalline austenitic stainless steels and ferritic Fe-based alloys revealed that in the presence of residual macrostresses on the surface of nitrided specimen, the nitriding rate depends on the orientation of surface grains. That is, the surface nitrogen concentration and the nitriding depth decrease upon increasing the angle between the surface normal of the grain and the normal of a {100} plane. This phenomenon can be ascribed to the heterogeneous distribution of residual macrostress developed during nitriding on the differently-oriented surface grains due to the elastically anisotropic nature of cubic substrates. Up to now, this phenomenon is only studied for a limited range of cubic materials with Zener´s anisotropy ratio larger than one. In order to confirm the current idea for the grain-orientation dependence of the nitriding behaviour of polycrystalline alloy, and in order to improve the understanding of this phenomenon, it is desired to broaden the range of materials for which data on orientation-dependent nitriding exist. Therefore, the primary purpose of this research is to investigate the grain-orientation dependent nitriding behavior of a broader range of metallic substrates with different elastic properties or different crystal structures. To do so, controlled gaseous nitriding experiments will be performed on the cubic W-based alloys (W-V and W-Ti) with Zener´s anisotropy ratio equals to one, on the cubic Mo-based alloys (Mo-Cr, Mo-V and Mo-Ti) with Zener´s anisotropy smaller than one and on the hexagonal Co-based alloys (Co-Cr and Co-V). Studying internal nitriding of refractory metal-based substrates (Mo- and W-based alloys) will be carried out at moderate temperature of 900 °C using NH3 gas or a mixture of NH3 and H2. This is the first time that the internal gaseous nitriding of these materials will systematically be studies at moderate nitriding temperatures using NH3-based media. Also, low-temperature nitriding of hexagonal Co-based alloys which may result in development of expanded phases will be performed using temperature range of 350-450°C. Previous low-temperature nitriding experiments revealed only the development of austenitic expanded phases upon nitriding of stainless steels. Thus, in this research the possibility for the formation of hexagonal expanded phase, for the first time, will be investigated.