In-Situ Neutron Diffraction Study of the Bauschinger Effect in B2 Structured CoZr
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THE Bauschinger effect, first reported in 1886, is primarily concerned with an observed reduction in yield strength during reverse loading following prestraining. Its origin was initially attributed to residual stresses due to inhomogeneous deformation of individual grains in polycrystalline materials.[2–4] However, observations of the Bauschinger effect in single crystals provided evidence for dislocation-based explanations.[5,6] It was proposed that back stresses created by dislocation pileups aid dislocation motion in the reverse direction.[7] Orowan[8] suggested an alternative dislocation-based theory, centered on a directional resistance to dislocation motion. Experiments were carried out to test these theories,[9–11] and features of both dislocation-based theories were observed. Later research has revealed that dislocation-based internal stresses are also present in dislocation wall and cell structures, which are formed due to heterogeneous distribution of dislocations, and have been shown to be present under applied stress as well as in the unloaded states.[12–15] In spite of the extensive research carried out, the cause of the Bauschinger effect is still debated, more than 100 years after its discovery. By examining and modeling the internal stress evolution directly, the contributions of [1]
R.P. MULAY, Graduate Research Assistant, and S.R. AGNEW, Associate Professor, are with the Materials Science and Engineering Department, University of Virginia, Charlottesville, VA 22904-4745. Contact e-mail: [email protected] B. CLAUSEN, Technical Staff Member, is with Los Alamos National Laboratory, LANSCE LC, Los Alamos, NM 87545. Manuscript submitted March 17, 2010. Article published online August 17, 2010 60—VOLUME 42A, JANUARY 2011
intergranular stresses and dislocation-based mechanisms can be parsed. Recent research on the anomalous ductility of select B2-structured intermetallic compounds, including CoZr, reveals them to be ideal model materials for this study of the Bauschinger effect. Wollmershauser et al.[16] reported a transition in the plastic deformation at approximately 250 MPa for CoZr tested in compression at room temperature (Figure 1). They showed that, prior to this transition, only the h001if110g slip mode is active, providing three independent slip systems. Beyond the transition, new deformation mechanisms are activated, which permit the grains to undergo arbitrary strains, as would five independent slip systems. However, there is some ambiguity regarding precisely which new deformation mechanism is activated (polycrystal plasticity modeling showed that either h111i 110 or h110i 110 type slip can capture the experimental observations[16]). The intergranular stresses before and after this transition point are markedly different, and therefore, in-situ cyclic loading experiments were performed within both regimes. In this way, the role of intergranular stresses in the Bauschinger effect may be examined under conditions where they are large and small, before and after the transition, respectively.
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