Interdiffusion and phase formation at iron-tungsten interfaces

Abstract

Low-activation steels are attractive candidates for wall materials in future nuclear-fusion power plants. Through a process called preferential sputtering, an enriched tungsten (W) layer is expected to develop on these steels, lowering erosion and thus increasing their lifetime and reducing contamination of the fusion plasma. However, the process of preferential sputtering may be counteracted by interdiffusion of W and iron (Fe). In this article, we investigate a simplified model system of such low-activation steels with a W-rich layer on the surface, by sputter depositing a thin W layer on top of pure Fe substrates. We investigate the processes that are activated when this model system is subject to temperatures relevant in the context of nuclear fusion reactors and assess the temperatures at which interdiffusion is expected to influence W surface concentrations. This is done by annealing a binary W-Fe system and analyzing the resulting concentration profiles by means of Rutherford backscattering spectrometry (RBS) and focused ion beam cross-sectioning (FIB). For annealing temperatures above 1000 K, an intermediate phase was observed to have formed, both between the Fe and W layer as well as on the surface of the W layer. This intermediate phase was determined to be Fe2W using Sputter X-ray photoelectron spectroscopy (XPS) and time-of-flight Rutherford backscattering spectrometry (ToF-RBS). The laterally averaged growth rate of this phase was determined to be (1.0±0.1)×10−18m2s at 1050 K and (2.8±0.2)×10−18m2s at 1100 K. Keywords: Diffusion, Iron, Tungsten, Rutherford backscattering spectrometry, X-Ray photoelectron spectroscopy, Eurofer