Thermal diffusivity degradation and point defect density in self-ion implanted tungsten

TL;DR

This study uses transient grating spectroscopy and kinetic modeling to quantify how self-ion irradiation creates point defects in tungsten, significantly reducing its thermal diffusivity, with invisible defects playing a major role.

Contribution

It introduces a combined experimental and theoretical approach to accurately estimate irradiation-induced point defect densities in tungsten, including defects too small for TEM detection.

Findings

Thermal diffusivity decreases by about 55% at 0.1 dpa.

Invisible point defects significantly impact thermal diffusivity degradation.

TGS and kinetic models effectively estimate defect densities across various doses.

Abstract

Using transient grating spectroscopy (TGS) we measure the thermal diffusivity of tungsten exposed to different levels of 20 MeV self-ion irradiation. Damage as low as 3.2 x 10^-4 displacements per atom (dpa) causes a measurable reduction in thermal diffusivity. Doses of 0.1 dpa and above, up to 10 dpa, give a degradation of around 55% from the pristine value at room temperature. Using a kinetic theory model, the density of irradiation-induced point defects is estimated based on the measured changes in thermal diffusivity as a function of dose. These predictions are compared with point defect and dislocation loop densities observed in transmission electron microscopy (TEM). Molecular dynamics (MD) predictions are combined with the TEM observations to estimate the density of point defects associated with defect clusters too small to be probed by TEM. When these "invisible" defects are accounted for, the total point defect density agrees well with that estimated from TGS for a range of doses spanning 3 orders of magnitude. Kinetic theory modelling is also used to estimate the thermal diffusivity degradation expected due to TEM-visible and invisible defects. Finely distributed invisible defects appear to play a much more important role in the thermal diffusivity reduction than larger TEM-visible dislocation loops. This work demonstrates the capability of TGS, in conjunction with kinetic theory models, to provide rapid, quantitative insight into defect densities and property evolution in irradiated materials.