Temporally resolved LEIS measurements of Cr segregation after preferential sputtering of WCrY alloy

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Abstract

The dynamic behaviour of thermally driven segregation of Cr to the surface of WCrY smart alloy is studied with low energy ion scattering (LEIS). Sputtering the WCrY sample with 500 eV D2+ ions at room temperature results in preferential removal of the lighter alloy constituents and causes an almost pure W surface layer. At elevated temperatures above 700 K the segregation of Cr atoms towards the surface sets in and prevents the formation of a pure W layer. The simultaneous heating and sputtering of the sample leads to a surface state which reflects the balance between sputter removal and segregation flux, and deviates from the equilibrium due to thermally driven segregation. Stopping the sputter ion beam allows the system to relax and develop toward the segregation equilibrium. The time constants for the temporal changes of W and Cr surface coverage are obtained from a series of LEIS measurements. The segregation enthalpy is determined from the time constants obtained for various sample temperatures.

Introduction

The investigation of candidate materials for the first wall of a future fusion power plant is an ongoing topic in nuclear fusion research. One of the most suitable materials is tungsten due to its high melting point and small sputter erosion under particle bombardment [1], [2]. However, the tungsten wall cladding in a fusion environment is exposed to a high neutron flux. Stable tungsten isotopes can undergo transmutation forming e.g. rhenium and radioactive isotopes of tungsten. For example, the stable tungsten isotopes 182W and 186W have rather large cross sections for the (n,2n) reaction [3] and abundances of the resultant isotopes will increase with operational time of the power plant. In case of a loss-of-coolant accident accompanied by air or water ingress the temperature of the first wall can rise up to 900 °C to 1200 °C [4]. Under these conditions, tungsten and its main transmutation products rhenium and osmium form oxides which are volatile above 700 °C, and are released into the vacuum vessel.

A possible route to prevent the release of radioactive material is the application of self-passivating tungsten alloys [5]. The self-passivating alloy contains chromium (and other constituents). Under accidental conditions the chromium forms a stable oxide layer on the surface which prevents the further oxidation and release of tungsten, whereas preferential sputtering of the lighter alloy constituents by bombarding plasma particles causes the enrichment of tungsten on the surface which results in little erosion under normal operation.

A self-passivating WCrY alloy has been recently developed and the oxidation resistance has been verified in laboratory experiments [6]. Furthermore, the development of a tungsten rich surface has been confirmed by plasma exposure in the linear plasma device PSI-2 and subsequent secondary ion mass spectrometry analysis of the surface composition [7].

A recent investigation of WCrY alloy with LEIS has been focused on the thermal stability and the preferential sputtering [8]. The experiment confirmed that ion beam bombardment with 250 eV deuterons at room temperature produces an almost pure tungsten surface due to preferential sputtering. However, when the sample temperature of an unexposed sample is increased above 700 K the segregation of chromium towards the surface sets in, and at a temperature of 1000 K no tungsten is found in the topmost surface layer.

In this work we will continue the previous study and investigate the dynamic behaviour of the temperature driven segregation. For that purpose, sputter erosion and sample heating are applied simultaneously in order to produce a non-equilibrium state of the sample surface. It neither shows the full tungsten enrichment due to preferential sputtering, nor does the surface show the segregation equilibrium depending on the sample temperature. Stopping the ion bombardment allows the surface to relax under action of the segregating particle flux. A continuous LEIS measurement of the individual alloy constituents yields time-resolved surface concentrations.

Section snippets

Experiment

The LEIS apparatus has been described in detail in [9]. Singly charged ions (typically He+ or Ne+) are produced in a Bayard-Alpert type ion source, extracted with a weak electric field, and accelerated to an energy of typically 1 keV. The ion beam is focused and steered onto the sample by two sets of einzel lenses and deflection plates. The sample holder of type Prevac PTS 1000 RES/C-K is mounted at a four axis manipulator (VG Scienta). The sample can be rotated to allow for incident angles in

Results and discussion

In the following the temporal evolution of the Cr surface coverage due to thermally activated segregation will be investigated. For that purpose the WCrY sample is heated up to a temperature of 1000 K and kept at this temperature with simultaneous sputter erosion by 250 eV deuterons by irradiation with 500 eV D2+ ions. The sputter cycle lasts for 4300 s allowing the sample to reach the dynamic equilibrium. Immediately after stopping the sputter ion beam a series of LEIS spectra is measured.

Summary and conclusion

Simultaneous heating and preferential sputtering by D bombardment of a WCrY alloy sample results in a non-equilibrium state which is somewhere in between the states where either Cr is fully segregated to the surface, depending on the temperature, or a fully W covered surface is obtained at room temperature. Stopping the sputter erosion lets the sample relax to the segregation equilibrium.

The analysis of a series of LEIS measurements documents the temporal evolution of the surface concentrations

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Mr. Albert Hiller and Mr. Roland Bär for technical assistance and maintenance of the LEIS apparatus, and Mrs. Beatrix Göths for polishing the WCrY sample.

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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