Grain-orientation dependent nitriding behaviour of cubic and hexagonal substrates

以前的氮化研究进行多晶奥氏体不锈钢和铁素体铁基合金显示，在存在残余宏观应力的氮化试样的表面上，氮化速率取决于表面晶粒的取向。即，晶粒的表面法线与{100}面的法线的角度越大，表面氮浓度越低，氮化深度越低。这种现象可以归因于残余宏观应力的不均匀分布在氮化过程中开发的不同取向的表面晶粒由于弹性各向异性性质的立方衬底。到目前为止，这种现象只研究了有限范围的立方材料与齐纳的各向异性比大于1。为了证实目前的想法的晶粒取向依赖的多晶合金的氮化行为，为了提高对这种现象的理解，它是希望扩大的材料范围，其中存在的数据取向依赖的氮化。因此，本研究的主要目的是研究具有不同弹性性质或不同晶体结构的更广泛的金属基底的晶粒取向依赖的氮化行为。为此，将对齐纳各向异性比等于1的立方钨基合金（W-V和W-Ti）、齐纳各向异性比小于1的立方钼基合金（Mo-Cr、Mo-V和Mo-Ti）和六方钴基合金（Co-Cr和Co-V）进行受控气体氮化实验。研究难熔金属基基材（钼基和钨基合金）的内部氮化将在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，这些材料的内部气体氮化将系统地研究在温和的氮化温度下使用NH3基介质。此外，六方钴基合金的低温氮化可能会导致膨胀相的发展，将使用350 - 450 ° C的温度范围进行。以前的低温渗氮实验表明，不锈钢渗氮后只形成奥氏体膨胀相。因此，在这项研究中，六方膨胀相的形成的可能性，第一次，将进行调查。

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.