Ion-irradiation induced clustering in W-Re-Ta, W-Re and W-Ta alloys: An atom probe tomography and nanoindentation study

Alan Xu,David E.J Armstrong,Christian Beck,Michael P Moody,George D.W Smith,Paul A.J Bagot,Steve G Roberts

doi:10.1016/j.actamat.2016.10.050

Abstract

In tungsten plasma-facing fusion reactor components, Ta is the third most abundant element formed by transmutation (after Re and Os), yet little is known about the behaviour of W-Ta alloys under irradiation and any effects Ta might have on Re clustering in W-Re-Ta alloys. In this study, W−4.5 at.%Ta, W−2 at.%Re−1 at.%Ta and W−2 at.%Re alloys were exposed to 2 MeV W+ ions to a fluence of 2.64 × 1015 ions cm−2 at temperatures of 573 and 773 K. Atom probe tomography and nanoindentation were used to characterise the chemical and physical properties of the irradiation-induced clusters and the mechanical properties of the irradiated layer. In the W-Ta alloy, no evidence of irradiation-induced clustering was found. In the W−Re−Ta alloy, at both irradiation temperatures studied the presence of Ta reduced the W-Re cluster number density and volume fraction compared to that in the W-Re alloy; however it did not alter cluster composition as the Ta was rejected from the clusters. The reduced cluster density in the W-Re-Ta alloys was associated with a smaller degree of irradiation hardening than in the W-Re alloys. Possible mechanisms by which Ta hinders cluster development are discussed.

Highlights

Tungsten is the leading candidate plasma facing material for any future nuclear fusion reactors, in which it will have to withstand extreme conditions including up to ~1300 K operating temperature, 1e20 MW mÀ2 heat flux and 14 MeV neutron and up to 3.5 MeV Heþ ion bombardment [1e4]
Exact transmutation rates will depend on the neutron spectrum at any given location in a reactor but neutronics modelling by Gilbert and Sublet using a theoretical fusion neutron spectrum suggest 3 at% Re, 1.4 at% Os and 0.9 at% Ta could be produced in tungsten after 5 years in a typical first wall position
Neutron irradiation can lead to radiationinduced precipitation at solute concentrations where a solid solution would be expected at thermal equilibrium [7e11], giving rise to solute clustering and formation of ordered s and c phases [7,12]

Summary

Introduction

Tungsten is the leading candidate plasma facing material for any future nuclear fusion reactors, in which it will have to withstand extreme conditions including up to ~1300 K operating temperature, 1e20 MW mÀ2 heat flux and 14 MeV neutron and up to 3.5 MeV Heþ ion bombardment [1e4]. Neutron irradiation can lead to radiationinduced precipitation at solute concentrations where a solid solution would be expected at thermal equilibrium [7e11], giving rise to solute clustering and formation of ordered s and c phases [7,12] The presence of these clusters and precipitates can significantly harden the tungsten, may induce embrittlement and may severely reduce component lifetime [7,8,13,14]. In a WÀ1ReÀ1Os alloy, ion-irradiated to 33 dpa at 773 K, the solute cluster density was higher by a factor of 3, and the irradiation-induced hardening was 1.54 GPa higher, than in an identically irradiated W-2Re alloy [16,17] This shows that synergistic effects between various solute species within tungsten alloys are of major importance during the formation and growth of clusters and precipitates. The work reported here investigates the effects of Ta on Re clustering and precipitation in W-based alloys during ion irradiation, using Atom Probe Tomography (APT) to examine the microstructures and nanoindentation to study the associated hardness changes

Experimental

Atom probe analysis

Irradiation hardening

Clustering behaviour in W-Re-Ta and W-Ta alloys

Effect of Ta on irradiation hardening

Summary