We report the experimental observation of an increase in the elastic anisotropy of tungsten upon He-ion implantation, probed optically using transient grating spectroscopy. Surface acoustic wave (SAW) velocity measurements were performed on a (110) oriented tungsten single crystal as a function of in-plane propagation direction for unimplanted and implanted samples. Our measurements allow us to finely resolve the remarkably small elastic anisotropy of the samples investigated. SAW velocity calculations are used to interpret the experimental data and to extract the Zener anisotropy parameter η and the elastic constant C44. Upon ion implantation, we observe an increase in the quantity (η-1) by a factor of 2.6. The surprising increase in elastic anisotropy agrees with previous theoretical predictions based on ab initio calculations of the effect of self-interstitial atoms and He-filled vacancy defects on the elastic properties of tungsten.
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We thank C. A. Dennett for assistance with numerical SAW velocity calculations and associated coding. R.A.D., A.A.M., J.K.E., and K.A.N. acknowledge support from the National Science Foundation (NSF) Grant No. CHE-1111557. A.V.-F. appreciates support from CINVESTAV and CONACYT through normal, mixed, and PNPC scholarships. A.G.E. acknowledges financial support from the South African National Research Foundation (NRF). This work has in part been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training program 2014-2018 under Grant Agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. This research used beamline 34-ID-E at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC-02-06-CH11357.