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 strain-burst 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.

Keywords:

• size effect
• microcompression
• intermittency
• scale-free flow
• dislocation mechanism

Acknowledgements

The Air Force Research Laboratory and the Air Force Office of Scientific Research supported the effort. PAS acknowledges support from the AFRL Science and Technology for the 21st Century program. We thank T.A Parthasarathy, S.I. Rao, and R. LeSar for technical discussions. Finally, the authors gratefully acknowledge the work by Clauset et al. described in reference 32 and thank them for the MATLAB analysis codes available for public use.

Notes

Note

1. for this study, “α” and “τ” represent event platen displacement and event time duration scaling exponents, respectively. The definitions happen to be reversed from those used elsewhere. (See Equation (1).)