In-situ tensile testing of single-crystal molybdenum-alloy fibers with various dislocation densities in a scanning elect
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Andreas Sedlmayr Karlsruhe Institute of Technology, Institute for Materials Research II (IMF II), 76021 Karlsruhe, Germany
P. Sudharshan Phani Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996
Reiner Mönig and Oliver Kraft Karlsruhe Institute of Technology, Institute for Materials Research II (IMF II), 76021 Karlsruhe, Germany
Easo P. George and George M. Pharra) Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 27 June 2011; accepted 17 August 2011)
In-situ tensile tests have been performed in a dual beam focused ion beam and scanning electron microscope on as-grown and prestrained single-crystal molybdenum-alloy (Mo-alloy) fibers. The fibers had approximately square cross sections with submicron edge lengths and gauge lengths in the range of 9–41 lm. In contrast to previously observed yield strengths near the theoretical strength of 10 GPa in compression tests of ;1–3-lm long pillars made from similar Mo-alloy single crystals, a wide scatter of yield strengths between 1 and 10 GPa was observed in the as-grown fibers tested in tension. Deformation was dominated by inhomogeneous plastic events, sometimes including the formation of Lüders bands. In contrast, highly prestrained fibers exhibited stable plastic flow, significantly lower yield strengths of ;1 GPa, and stress–strain behavior very similar to that in compression. A simple, statistical model incorporating the measured dislocation densities is developed to explain why the tension and compression results for the as-grown fibers are different.
I. INTRODUCTION
Knowledge of the mechanisms by which plastic deformation occurs is the key to improving structural materials and pushing them to their performance limits. Although continuum descriptions of plasticity are well suited to describing the bulk mechanical response of materials, plastic deformation at small scales is often dominated by discrete events that are not yet well understood (e.g., dislocation processes, surface interactions, and test geometry effects).1–10 Experimental methods for characterizing and observing small-scale deformation phenomena have advanced significantly in recent years, largely as a result of new in-situ mechanical testing methods (see, e.g., Gianola and Eberl11). However, our understanding of the mechanisms of small-scale plasticity is still at an early stage and requires further experimental observations, microstructural analyses, and development of new models that can account for the discrete and often stochastic nature of the deformation. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.298 508
J. Mater. Res., Vol. 27, No. 3, Feb 14, 2012
This work reports new observations of the tensile mechanical behavior of Mo-alloy single-crystal fibers with dimensions in the submicron range. Although the compressive deformation behavior of
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