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College Park, Maryland      June 6 - 10 , 2004

T3-C4 (4:45 PM): Using inter- and intra-granular strains as a guide to understand fatigue behaviors of polycrystalline engineering materials

X.-L. Wang (Spallation Neutron Source, Oak Ridge National Laboratory; Metals and Ceramics Division, Oak Ridge National Laboratory), A. D. Stoica, Y. D. Wang (Spallation Neutron Source, Oak Ridge National Laboratory), D. J. Horton (Spallation Neutron Source, Oak Ridge National Laboratory; Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA.), H. Tian, P. K. Liaw (Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996, USA.), E. Maxey, J. W. Richardson (Intense Pulsed Neutron Source, Argonne National Lab)

Time-of-flight neutron diffraction technique provides reliable data on inter- and intra-granular grain-orientation-dependent strains generated during the plastic deformation of polycrystalline materials [1-2]. A study of 316 stainless steel samples at different stages of fatigue life provides experimental evidence of the effect of cyclic loading on grain-level micromechanics [3]. The experimental data show that residual inter-granular strains develop after each half loading cycle as a result of the elastic anisotropy, which induce a highly heterogeneous loading redistribution at the grain level. In the early stage of fatigue life the grain-orientation-dependent inter-granular strains oscillate between two extremes states corresponding to the ends of a tensile or compressive half loading cycle showing almost identical symmetry but a sign reversal [4]. In the late stage, the inter-granular strains vanish for tests ending in tension, but are surprisingly stable for tests ending in compression. The different behaviors in tension and compression are related to the crack initiation and could be linked to a crack closure mechanism. Separately, the diffraction line broadening analysis allows probing the intra-granular strains and evaluating the stored energy, which is a fingerprint of the material hardening process and can be directly related to the successive stages of deformation. As the intra-granular strains are mainly induced by immobile dislocations, the diffraction response to the lattice distortion is highly anisotropic and can be used to identify the average dislocation character and distribution. In the case of 316 stainless steel, neutron diffraction data have shown that the immobile dislocations generated by cyclic loading are mainly edge, rather than screw, type. This finding was corroborated by recent transmission electron microscopy studies.

This research was supported by Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under Contract DE-AC05-00OR22725 with UT-Battelle, LLC.

1) X.-L. Wang, Y. D. Wang, and J. W. Richardson, J. Appl. Cryst., 35, 533-537 (2002)
2) Y. D. Wang, X.-L Wang, A. D. Stoica, J. W. Richardson, and R. Lin Peng, J. Appl. Cryst. 36, 14-22 (2003).
3) Y.D. Wang, H. Tian, A. D. Stoica, X.-L. Wang, P. K. Liaw, and J.W. Richardson, Nature Materials, 2, 103-106 (2003).
4) X.-L. Wang, et al., Mat. Sci. Eng. (in press)

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