Effect of tungsten crystallographic orientation on He-ion-induced surface morphology changes

In order to study the early stages of nanofuzz growth in fusion-plasma-facing tungsten, mirror-polished high-purity tungsten was exposed to 80 eV helium at 1130 degrees C to a fluence of 4 x 10(24) He m(-2). The previously smooth surface shows morphology changes, and grains form one of four qualitatively different morphologies: smooth, wavy, pyramidal or terraced/wide waves. Combining high-resolution scanning electron microscopy (SEM) observations to determine the morphology of each grain with quantitative measurement of the grain's orientation via electron backscatter diffraction in SEM shows that the normal-direction crystallographic orientation of the underlying grain controls the growth morphology. Specifically, near-< 0 0 1 > parallel to normal direction (ND) grains formed pyramids, near-< 1 1 4 > to < 1 1 2 > parallel to ND grains formed wavy and stepped structures and near-< 1 0 3 > parallel to ND grains remained smooth. Comparisons to control specimens indicate no changes to underlying bulk crystallographic texture, and possible explanations of the structure growth, particularly loop-punching, are discussed. Future developments to control tungsten texture via thermomechanical processing, ideally obtaining a sharp near-< 1 0 3 > parallel to ND processing texture, may delay the formation of nanofuzz. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.