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，将研究金属合金纳米晶结构在大电流密度和强磁场下的稳定性。用溅射方法在硅上制备了间隙纳米晶铁基和镍基合金(铁和碳，镍和氧)，并作为模型系统测量了晶粒度和相分离随电流密度和磁场强度的变化。硅作为衬底的高导热系数允许施加大电流密度，从而使间隙原子能够长距离移动。结果表明，电流密度高达4 mA/cm2时，铁颗粒沿电流方向长大，促进了晶粒的生长。这些大颗粒的形成是由碳化物首先形成，然后被电迁移溶解所触发的。通过开启平行于铁膜表面的大磁场，导致在铁膜平面内晶体取向的巨大排列。要同时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。对这些新现象的深入理解，将为相关新型微结构的应用铺平道路。除了项目的基本方面外，Fe-C合金还将通过场辅助烧结技术或放电等离子烧结(FAST/SPS)以不同的方式制备为纳米晶粉末。其技术目标是在足够低的温度下获得有用的块体纳米晶铁基或镍基合金(即纳米晶钢)，在这种温度下，颗粒生长受到抑制或各向同性。此外，这些致密的样品将被用来研究强磁场中的晶粒生长与薄膜中的二维生长。通过比较FAST/SPS制备的样品的微观结构和模型体系中的相应变化，可以解释粉末压制过程中温度、电流密度和磁场强度的影响。因此，可以揭示FAST/SPS过程中存在的电流密度和常规烧结过程中没有的电流密度的影响，并将其与本研究的基本部分中对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.