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

将原位XRD与离子注入和离子溅射蚀刻相结合，可以直接获得扩散和相变过程。这种开发的实验室方法的主要优点是将几分钟的时间分辨率与优于50纳米的深度分辨率完美地结合在一起，并且可以分析厚至5-20微米的层（因为x射线的信息深度不再是限制因素）。此外，不同相的单独衍射峰的存在允许对深度分辨数据进行相位选择分析。本项目的重点是阐明双相不锈钢氮化过程中发生的过程，双相不锈钢是一种非常有吸引力和重要的工业用钢，由于奥氏体和铁素体相的混合物在传统的扩散实验中很难分离，因此对氮化的要求特别高。然而，这些相表现出不同的氮扩散率、氮溶解度和结构转变。初步实验表明，氮化过程中发生的现象的复杂性，很可能是文献非常有限且常常相互矛盾的原因。现在，将原位XRD与STEM、EBSD、3D-SIMS成像和深度剖面以及原子探针断层扫描等先进材料表征方法相结合，将使我们能够详细了解这些材料的氮迁移率和诱导的微观结构变化。此外，沉淀硬化钢的氮化是用原位和非原位方法的相同组合进行研究的，因为类似的实验障碍存在于另一类先进的-同样重要的技术-不锈钢中，没有文献中明确的数据。将这两类不锈钢与传统奥氏体不锈钢相结合的主要方面是Fe-Cr-Ni-N相的有限稳定性。该相开始衰变为具有不同晶粒尺寸的bcc或fcc结构的CrxN和Fe-Ni基体（根据XRD数据），因此，该相内的氮含量和氮通过该相的输运波动较大。所有这些影响都强烈地影响了氮化过程，在这种衰变开始之后，在XRD数据中就可以看到。因此，对这一现象的理解具有重要的、根本的意义。将研究这一过程背后的潜在转变，并利用原位XRD和非原位表征技术的先进组合，深入了解不同效应的复杂相互作用。一个预期的结果将是指导在工业环境中更有效和可重复的高级不锈钢氮化，即双相和沉淀硬化等级。

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.