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 m were fabricated by focused ion beam (FIB)-milling from bulk crystals having a low initial dislocation density. Both as-grown and ␥-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, ∼ D −m , where m ≈ 0.8, and this rapid hardening results in engineering flow stresses of 650 MPa in 1-m samples. The findings are evaluated against possible dislocation mechanisms that could be responsible for the observed size effects.

Micropillar compression of LiF [111] single crystals: Effect of size, ion irradiation and misorientation

“…Our results contrast with previous studies that have shown strong size effects in the plastic deformation of fee metals (Uchic et al, 2004;Greer et al, 2005;Volkert and Lilleodden, 2006) and ionic compounds (Nadgorny et al, 2008;Dimiduk et al, 2010). In particular, Nadgorny et al (2008) reported a strong size effect in [100] oriented LiF micropiUars fabricated by FIB when the micropillar diameter was in the range 1-10 urn.…”

“…However, the fact that these micropiUars did not display a whisker-type behavior, controlled by the nucleation of dislocations (as the one observed in the Mo micropiUars of Bei et al (2008)), indicates the existence of an initial dislocation network caused by the thermal residual stresses developed upon cooling because of the differences in the coefficients of thermal expansion of both phases in the eutectic. So, both LiF micropiUars in Nadgorny et al (2008) and in this investigation contained a network of mobile dislocations prior to testing although the actual densities are unknown.…”

“…In particular, Nadgorny et al (2008) reported a strong size effect in [100] oriented LiF micropiUars fabricated by FIB when the micropillar diameter was in the range 1-10 urn. The discrepancies between these results and those reported in this paper can be attributed to three causes: differences in the initial dislocation density, FIB machining and crystal orientation.…”

“…The LiF [100] micropiUars of Nadgorny et al (2008) were machined from bulk crystals with a very low initial dislocation density (10 9 nr 2 ) but it should be noted that the real dislocation density on the FIB machined micropiUars was likely to be much higher than that of the bulk crystals because of the effect of ion irradiation. Otherwise, the micropiUars would have been virtually dislocation-free, displaying a whisker-type behavior, which was not the case.…”

“…In the case of [100] oriented LiF micropiUars, compression takes place by the activation of the soft slip systems, while LiF micropiUars perfectly oriented along the [111] deform in compression by plastic deformation along the hard slip systems. The critical resolved shear stress obtained by us in [111] LiF and by Nadgorny et al (2008) in [100] LiF are plotted in Fig. 10 as a function of micropillar diameter.…”