Exploring specimen size effects in plastic deformation of Ni 3 (Al, Ta)
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Exploring specimen size effects in plastic deformation of Ni3(Al, Ta) Michael D. Uchic1, Dennis M. Dimiduk1, Jeffrey N. Florando2, William D. Nix3 Air Force Research Laboratory, Materials & Manufacturing Directorate, Wright-Patterson AFB, OH 45433-7817 2 Lawrence Livermore National Laboratory, Livermore, CA 94550 3 Stanford University, Department of Materials Science and Engineering, Stanford, CA 943052205 1
Abstract In this paper we present a mechanical test methodology to explore specimen size effects in Ni3Al, where the overall test sample dimensions artificially limit the volume for substructure evolution and hence the availability of jogs/kinks along individual dislocation lines. The test methodology consists of using Focused Ion Beam milling to micromachine cylindrical compression samples that have diameters ranging from 5 to 20 microns into the surface of a bulk sample, which is followed by nanoindentation using a flat-ended tip to measure the mechanical properties of the microsamples in uniaxial compression. The initial test results show that there is a strong increase in the flow stress with decreasing sample size, although misfit between the flat indenter tip and the top surface of the compression samples complicates complete interpretation of the mechanical test results at this time. Introduction It is generally agreed that the mechanical properties of the intermetallic Ni3Al are strongly influenced by the kinematics of dislocation motion, and there is considerable evidence in the literature to show that at low temperatures in the anomalous-flow regime dislocation glide is influenced by the motion of jogs or kinks moving laterally along screw dislocation lines (for a review of this subject, see [1]). It is likely that these jog/kink dynamics are strongly influenced by the total line length of the dislocations. For example, one study involving a statistical analysis of the probability of dislocation self-immobilization via fluctuations in the jog/kink population predicted that the rate of self-immobilization should be exponentially dependent on the line length of the dislocation [2]. One way to examine the effects of line length on the mobility of dislocations is to perform tests where the overall dimensions of the sample artificially limit the dislocation line length. For Ni3Al, one could expect that as the test sample size approaches the scale of jog/kink spacing along the screw dislocation lines, aspects of the flow stress anomaly should change dramatically. With the exception of in situ straining of TEM specimens, there have been no readily accessible methods for fabricating samples on a size scale where the physical dimensions of the sample can have an important effect on the dislocation activity (i.e. microns in length). However, the development of new nano-scale fabrication, manipulation, and actuation technologies potentially allows for the creation and testing of samples that can explore these size scale effects. From a practical perspective, the handling and testing of small samples is
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