In situ XRD Investigations for Understanding Nitrogen Mobility and Changes in the Microstructure of Advanced Stainless Steels during Nitriding

使用原位XRD结合离子注入和离子溅射蚀刻提供了直接访问扩散和相变过程。这种实验室方法的主要优点是几分钟的时间分辨率与优于50 nm的深度分辨率的完美结合，以及分析厚达5-20 µm的层的可能性（因为X射线的信息深度不再是限制因素）。此外，不同相位的单独衍射峰的存在允许深度分辨数据的相位选择性分析。该项目的重点是阐明双相不锈钢氮化过程中发生的过程-这是一种非常有吸引力和重要的工业用钢-这是特别苛刻的奥氏体和铁素体相的混合物是很难分离在传统的扩散实验。然而，这些阶段表现出不同的氮扩散率，氮溶解度，和结构转变。氮化过程中发生的现象的复杂性，如初步实验所示，是最可能的原因，强烈有限的文献往往是矛盾的。现在，将原位XRD与非原位先进材料表征方法（如STEM、EBSD、3D-SIMS成像和深度分析以及原子探针断层扫描）相结合，将使我们能够详细了解氮迁移率和此类材料微观结构的诱导变化。此外，采用原位和非原位方法相结合的方法，对沉淀硬化钢的渗氮进行了研究结合这两类不锈钢和常规奥氏体不锈钢的主要方面是Fe-Cr-Ni-N相的有限稳定性。该相开始衰变为CrxN和具有不同晶粒尺寸的bcc或fcc结构（根据XRD数据）的Fe-Ni基质，因此，该相内的氮含量和氮传输波动。所有这些效应都强烈影响氮化过程，在这种衰减开始之后，在XRD数据中变得可见之前。因此，对这一现象的理解具有重大的根本意义。将研究这一过程背后的潜在转变，并使用先进的原位XRD和非原位表征技术相结合，深入了解不同效果之间的复杂相互作用。一个预期的结果将是在工业环境中对高级不锈钢（即双相和沉淀硬化等级）进行更有效和可再现氮化的指导。

Using in situ XRD in combination with ion implantation and ion sputter etching gives direct access to diffusion and phase transitions processes. The main advantage of this developed laboratory method is the excellent combination of a time resolution of a few minutes with a depth resolution of better than 50 nm and the possibility to analyse thick layers up to 5–20 µm (as the information depth of the X-rays is no longer a limiting factor). Furthermore, the existence of separate diffraction peaks for different phases permits a phase selective analysis of the depth resolved data. The focus of this project is to elucidate the processes occurring during nitriding of duplex stainless steels – a very attractive and important class of steels for industrial use – which is especially demanding as the mixture of austenite and ferrite phases is hard to separate in conventional diffusion experiments. Yet, these phases exhibit different nitrogen diffusivities, nitrogen solubilities, and structural transformations. The complexity of the phenomena occurring during nitriding, as indicated by a preliminary experiment, is the most probable reason for the strongly limited literature which is often contradictory. Now, combining in situ XRD with ex situ advanced materials characterization methods as STEM, EBSD, 3D-SIMS imaging and depth profiling, as well as atomic probe tomography, will allow us detailed insights into the nitrogen mobility and induced changes in the microstructure of such materials.Furthermore, nitriding of precipitation hardening steels is investigated with the same combination of in situ and ex situ methods as similar experimental obstacles are present for this other class of advanced – and equally technologically important – stainless steels with no unequivocal data available in the literature.The main aspect combining these two classes of stainless steels and conventional austenitic stainless steel is a limited stability of the Fe-Cr-Ni-N phase. This phase starts to decay into CrxN and a Fe-Ni matrix in either bcc or fcc structure (according to XRD data) with varying grain size and, thus, fluctuating nitrogen content within and nitrogen transport through this phase. All these effects strongly influence the nitriding process after this decay started already before it becomes visible in the XRD data. Thus, the understanding of this phenomenon is of significant, fundamental interest. The underlying transformation behind this process will be investigated and a deep insight into the complex interplay of the different effects will be obtained using the advanced combination of in situ XRD and ex situ characterization techniques. One anticipated result shall be a guidance for a more efficient and reproducible nitriding of advanced stainless steels, i.e. duplex and precipitation hardening grades, in industrial environments.