The impact of high current densities and magnetic fields on the microstructure of nanocrystalline iron- and nickel-based alloys and related effects during spark plasma sintering of these alloys.

在本项目中，将研究受高电流密度和高磁场影响的金属合金纳米晶结构的稳定性。间隙，纳米晶铁和镍基合金（铁与碳，镍与氧）通过溅射在硅上制备薄膜，并用作模型系统来测量晶粒尺寸和相分离作为电流密度和磁场强度的函数。硅作为衬底的高导热性允许应用大电流密度，使间隙原子能够长距离移动。第一个结果表明，当电流密度达到4 MA/cm2时，铁晶粒在电流方向上被拉长，从而促进了晶粒的生长。这些大晶粒的形成是由碳化物先形成，然后被电迁移溶解所触发的。通过打开一个平行于铁膜表面的大磁场，可以在铁膜平面内形成巨大的晶体取向排列。要以定性和定量的方式解释这些实验结果，需要进行对照实验。对新现象的深入理解将为相关新型微观结构的应用铺平道路。除了项目的基本方面，Fe-C合金将通过场辅助烧结技术或火花等离子烧结（FAST/SPS）以不同的方式制备纳米晶粉末。技术目标是在足够低的温度下获得有用的大块纳米晶铁或镍基合金（即纳米晶钢），在这种温度下，晶粒生长受到抑制或各向同性。此外，这些致密样品将用于研究高磁场下的晶粒生长，并与薄膜中的二维生长进行比较。温度、电流密度和磁场强度对粉末压实过程的影响可以通过比较FAST/SPS制备的样品的微观结构和模型系统的相应变化来阐明。因此，可以揭示在FAST/SPS过程中存在和在常规烧结过程中不存在的电流密度的影响，并将其与研究基础部分薄膜的结果进行比较。

In the present project the stability of the nanocrystalline structure of metallic alloys, which are subject to high current densities and high magnetic fields, shall be studied. Interstitial, nanocrystalline iron- and nickel-based alloys (iron with carbon, nickel with oxygen) are prepared as thin films on silicon by sputtering and used as model systems to measure grain size and phase separation as a function of electric current density and magnetic field strength. The high thermal conductivity of silicon as a substrate allows the application of large electric current densities enabling interstitial atoms to move long distances. First results reveal that current densities as high as 4 MA/cm2 lead to enhanced grain growth with iron grains being elongated in the direction of the electric current. The formation of these large grains is triggered by carbides forming first and then being dissolved by electromigraton. By switching on a large magnetic field oriented parallel to the surface of iron films leads to a colossal lining up of crystallographic orientation within the plane of the films. To interpret these experimental findings in both a qualitative and quantitative way requires controlled experiments. Deeper understanding of the novel phenomena will pave the way for application of the related novel microstructures. In addition to the fundamental aspects of the project, the Fe-C alloys will be prepared differently as nanocrystalline powders by field assisted sintering technique or spark plasma sintering (FAST/SPS). The technological aim is to obtain useful bulk nanocrystalline iron- or nickel-based alloys (i.e. nanocrystalline steels) at low enough temperatures, where grain growth is suppressed or is isotropic. In addition, these compact samples will be used to study grain growth in high magnetic fields compared to the 2-dimensional growth in thin films. The effect of temperature, current density and magnetic field strength during powder compaction can be elucidated by comparing the microstructures of samples prepared by FAST/SPS with corresponding changes in our model systems. Thus the effect of the current density present during FAST/SPS and absent during conventional sintering could be revealed and compared with the results obtained for thin films in the fundamental part of the study.