An experimental investigation of intermittent flow and strain burst scaling behavior in LiF crystals during microcompression testing

Abstract

The first experimental statistical study is reported of intermittent plastic deformation of LiF microscopic samples having low initial dislocation densities, in both as-grown and gamma-irradiated conditions. The investigations used the microcompression testing method. Data sets were evaluated independently for the loading and flow deformation stages for each material. Investigations selectively examined evolution of the strainburst response in both the spatial and temporal domains. A revised analysis technique provided advances in quantitative evaluations of the statistical experimental data relative to previous studies. Platen displacement event cumulative probability distributions exhibited both Gaussian regimes at small displacements and power law regimes for event displacement, duration and average velocity at larger sizes. However, the observed event size scaling exponents did not follow the expectation from mean-field theory, revealing scaling exponents in the range 1.8-2.9. Additionally, extraordinarily large displacement events were observed that exceeded the sizes of those found in previous studies by at least 10 times. Quantitative clarification of the power-law exponent values and their dependence on deforming sample conditions demands both further experimental studies with larger numbers of samples and a wider range of sample conditions. Such studies would benefit from better matching of the time scales of dislocation processes and observation and, still further improvements to the data analysis methods.

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.…”

“…Two sets of slip systems, {110}{110) and {100}{110), which are known to be responsible for the plastic deformation in LiF were included in the crystal plasticity model (Dimiduk et al, 2010;Gilman, 1959). …”

Micro-plasticity and intermittent dislocation activity in a simplified micro-structural model

“…Such experiments measure the acoustic energy released by intermittent dislocation activity during constant stress deformation (in the tertiary creep regime). Indeed, via single sensor acoustic emission signals, such dislocation activity can be well characterised in time revealing power-law behaviour with exponents of 1.6 to 1.8, which is very similar to that seen via the micro-compression stress-strain curves 6,7,11 . Furthermore, via multiple sensor monitoring, time and space clustering of dislocation avalanches could be observed 19 .…”

“…As a result, the strain associated with the discrete dislocation activity is increased to an easily detectable magnitude. Whilst this more contemporary work has been primarily motivated by the "smaller-is-stronger" size-effect paradigm 4,5,[8][9][10] , an extensive analysis of the statistics of the discrete dislocation activity has revealed power-law behavior in the distribution of strainburst magnitudes giving an exponent of ≈ 1.6 − 2.2 6,7,11 . Similar exponents can also be found in bulk samples via detailed analysis of load displacement signals, where structural evolution is also seen to occur 12,13 .…”

“…Here, advantage is taken of a nano-indentation platform equipped with a flat punch tip to compress micro-/nano-crystals [4][5][6][7] . In addition to the sub-nanometer displacement resolution of the system, which notably has a lower strain resolution than the above mentioned torsion experiments, the sample size is decreased to the micron range and below.…”