Grain-orientation dependent nitriding behaviour of cubic and hexagonal substrates

以往对多晶奥氏体不锈钢和铁素体铁基合金的渗氮研究表明，当渗氮试样表面存在残余宏观应力时，渗氮速率取决于表面晶粒的取向。即随着晶粒表面法线与{100}面法线夹角的增大，表面氮浓度和渗氮深度减小。这一现象可归因于立方基体弹性各向异性的特性导致渗氮过程中在不同取向表面晶粒上产生的残余宏观应力分布不均。到目前为止，这种现象只在有限范围内研究了齐纳各向异性比大于1的立方材料。为了证实目前关于多晶合金氮化行为的晶粒取向依赖的观点，并为了提高对这一现象的理解，需要扩大存在取向依赖氮化数据的材料的范围。因此，本研究的主要目的是研究具有不同弹性性能或不同晶体结构的更广泛的金属衬底的晶粒取向依赖的氮化行为。为此，将对齐纳各向异性比为1的立方w基合金（W-V和W-Ti）、齐纳各向异性比小于1的立方mo基合金（Mo-Cr、Mo-V和Mo-Ti）和六方co基合金（Co-Cr和Co-V）进行可控气体氮化实验。研究难熔金属基衬底（钼基和钨基合金）的内氮化将在900°C的中等温度下使用NH3气体或NH3和H2的混合物进行。这是第一次系统地研究这些材料在中等渗氮温度下使用nh3基介质的内部气体氮化。此外，六方钴基合金的低温氮化可能导致膨胀相的发展，将在350-450°C的温度范围内进行。以往的低温渗氮实验只揭示了不锈钢在渗氮过程中奥氏体膨胀相的发展。因此，本研究将首次探讨六方膨胀相形成的可能性。

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.