  Developing a comprehensive understanding of the modification of material
properties by neutron irradiation is important for the design of future fission
and fusion power reactors. Self-ion implantation is commonly used to mimic
neutron irradiation damage, however an interesting question concerns the effect
of ion energy on the resulting damage structures. The reduction in the
thickness of the implanted layer as the implantation energy is reduced results
in the significant quandary: Does one attempt to match the primary knock-on
atom energy produced during neutron irradiation or implant at a much higher
energy, such that a thicker damage layer is produced? Here we address this
question by measuring the full strain tensor for two ion implantation energies,
2 MeV and 20 MeV in self-ion implanted tungsten, a critical material for the
first wall and divertor of fusion reactors. A comparison of 2 MeV and 20 MeV
implanted samples is shown to result in similar lattice swelling.
Multi-reflection Bragg coherent diffractive imaging (MBCDI) shows that
implantation induced strain is in fact heterogeneous at the nanoscale,
suggesting that there is a non-uniform distribution of defects, an observation
that is not fully captured by micro-beam Laue diffraction. At the surface,
MBCDI and high-resolution electron back-scattered diffraction (HR-EBSD) strain
measurements agree quite well in terms of this clustering/non-uniformity of the
strain distribution. However, MBCDI reveals that the heterogeneity at greater
depths in the sample is much larger than at the surface. This combination of
techniques provides a powerful method for detailed investigation of the
microstructural damage caused by ion bombardment, and more generally of strain
related phenomena in microvolumes that are inaccessible via any other
technique.
