In-situ measurement of diffusion and surface segregation of W and Ta in bare and W-coated EUROFER97 during thermal annealing

Abstract

The near-surface composition of polished EUROFER97 samples was measured by Rutherford backscattering spectrometry while maintaining the analyzed surface at temperatures up to 1188 K. A sample pre-coated with less than a monolayer of tungsten by magnetron sputter deposition was included in the study. A shifting equilibrium due to diffusion and segregation of tungsten and tantalum was observed, with segregation being most important between 900 K and 1040 K, which is near the known transition temperature between the martensitic and austenitic phases. Diffusion counteracted any increase of the near-surface concentration of the elements of interest both above and below the temperature range in which segregation dominated. An asymmetry in the diffusion-segregation balance was observed when approaching the same temperature from above and below. The largest segregation peak measured at elevated temperature in this work indicated an increased amount of tungsten and/or tantalum near the sample surface with an areal atom density corresponding to 70–80% of a monolayer. Additional measurements were performed on bare EUROFER97 after annealing at 1089 K and 1553 K followed by rapid cooling to near room temperature which, if carried out at a critical rate, is expected to restore a martensitic microstructure. In the former case a similar enrichment peak was seen as that measured while the sample temperature was elevated, while in the latter case an increased concentration of tungsten and tantalum from 0.4 to 0.55 atomic percent in a layer of at least 300 nm thickness near the surface was observed.