Size Effects in LiF Micron-Scale Single Crystals of Low Dislocation Density
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0976-EE06-24
Size Effects in LiF Micron-Scale Single Crystals of Low Dislocation Density Edward M. Nadgorny1,2, Dennis M. Dimiduk3, and Michael D. Uchic3 1 Physics, Michigan Technological University, 1400 Townsend Dr, Houghton, MI, 49931-1295 2 Anteon Corporation, Dayton, OH, 45431-1231 3 Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH, 45433-7817
ABSTRACT This study examines the deformation response of 20, 5, and 1µm diameter samples fabricated by FIB-milling from bulk ultrapure LiF single crystals. The bulk crystals have a very low initial dislocation density as revealed by an etch-pit technique. Two types of microsamples were compressed preferentially by single slip inside a nanoindentation system. Similar to previously studied FCC-derivative metals, LiF microsamples demonstrate dramatic strengthening achieving the engineering flow stress σ ≈ 650 MPa in 1-µm samples. The stress-m diameter dependence obeys a power law, σ~D , where m ≈ 0.8. Stochastic variation of flow stress, fast intermittent deformation events (“avalanches”) and highly localized slip bands after avalanches - all characteristic of size effects in metals, are also observed in LiF. Possible dislocation mechanisms of the observed size effects are discussed. INTRODUCTION Selected recently-discovered [1-3] unusual properties of micron-scale deforming samples present major challenges for explaining the underlying mechanisms of size effects. In particular, such samples demonstrated the following unusual features: a dramatic increase and stochastic variation of the flow stress, strong intermittency in plastic flow, deformation avalanches and power-law avalanche-size distributions. Figure 1 illustrates the findings on four materials. and representative results obtained on Ni single crystals of different diameters [3].
Figure 1. Dependence of normalized shear flow stress as a function of sample diameter D for several materials, together with the results of 3d-DDS computation, where Ks is the anisotropic shear modulus, and stress-strain diagrams for Ni single crystals of different diameters [3].
However, to date size-effect investigations have been performed only on metals and alloys where the initial dislocation density is typically either unknown or as a rule rather high. One of the advantages of the LiF single crystals used in this study is their exceptional perfection, which can be kept during preparation of microsamples. Additional advantages are that dislocations in LiF can be easy revealed for measuring the dislocation density, high purity crystals are commercially available, the orientation for preferential single slip is achieved when they cleaved along a {100} plane and the bulk yield stress can be considerably modified by irradiation and successive annealing. Finally, bulk LiF crystals have been intensively studied and their dislocation dynamical and mechanical properties are well known (see, for example, [4, 5]). Therefore, LiF crystals permit verification of size effects in an alternative cl
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