Size effects in LiF micron-scale single crystals of low dislocation density

Abstract

This study examines the size-dependent deformation response of pure LiF single crystals using microcompression testing. Microcrystals with an 〈001〉 orientation and sample diameter D ranging from 1 to 20 μm were fabricated by focused ion beam (FIB)-milling from bulk crystals having a low initial dislocation density. Both as-grown and γ-irradiated crystals were examined to characterize the effect of an increased point defect density on the size-affected plastic flow response. Similar to previously studied face-centered cubic (FCC)-derivative metals, both types of LiF microcrystals exhibit typical size-dependent plastic flow behavior: a dramatic size-dependent and statistically varying flow stress, atypically high strain hardening rates at small plastic strains, and fast intermittent strain bursts. The size-dependent strengthening obeys a power law, σ ∼ D−m, where m ≈ 0.8, and this rapid hardening results in engineering flow stresses of 650 MPa in 1-μm samples. The findings are evaluated against possible dislocation mechanisms that could be responsible for the observed size effects.