Understanding size effects on the strength of single crystals through high-temperature micropillar compression

Soler, Rafael; Wheeler, Jeffrey M.; Chang, Hyung Jun; Segurado, Javier; Michler, Johann; Llorca, Javier; Molina-Aldareguia, Jon M.

Acta Materialia 81 (2014)

Compression tests of 〈1 1 1〉-oriented LiF single-crystal micropillars 1-5 μm in diameter were carried out from 25 °C to 250 °C. While the flow stress at ambient temperature was independent of the micropillar diameter, a strong size effect developed with elevated temperature. This behavior was explained by rigorously accounting for the different contributions to the flow stress of the micropillars as a function of temperature and pillar diameter: the lattice resistance, the forest hardening; and the size-dependent contribution as a result of the operation of single-arm dislocation sources. This was possible because the micropillars were obtained by chemically etching away the surrounding matrix in directionally solidified LiF-NaCl and LiF-KCl eutectics, avoiding any use of focused ion beam methods, yielding micropillars with a controlled dislocation density, independent of the sample preparation technique. In particular, the role of the lattice resistance on the size effect of micrometer-size single crystals was demonstrated unambiguously for the first time. This result rationalizes the different values of power-law exponent for the size effect found in the literature for face-centered cubic and body-centered cubic metals as well as for covalent and ionic solids.