Probing Mesoscopic Strain Evolution during Creep Deformation: An In-Situ Neutron Diffraction Study
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Probing Mesoscopic Strain Evolution during Creep Deformation: An In-Situ Neutron Diffraction Study Hahn Choo1,2 , Donald W. Brown3, Mark A. M. Bourke3, and Robert W. Swindeman2 1. Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996, U.S.A. 2. Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A. 3. Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
ABSTRACT The development of lattice strain was studied using in-situ time-of-flight neutron diffraction during constant-load tensile creep deformation of an austenitic 316FR stainless steel at 180, 240, and 300MPa at 873K (a power-law creep regime) with time resolution of 900 seconds. The macroscopic (global) and mesoscopic (lattice) strains were measured simultaneously during creep using an extensometer and neutron diffraction, respectively. The hkl-specific lattice strains were measured to gain insights into the plastic anisotropy at various stages of creep deformation (i.e., primary, secondary, and tertiary regimes). Furthermore, the creep-induced lattice strain behavior was compared to the result obtained from a quasistatic tension test at 873K. The lattice strain evolution in the axial direction (direction parallel to the tensile loading axis) during the primary and secondary creep (dislocation creep) is quite similar to the quasistatic case (slip). However, in the tertiary creep regime, the creep-induced lattice strain accumulation is smaller than the quasistatic case at a given total strain, except the (111) reflection.
INTRODUCTION The development of lattice strain (or intergranular strain) during elastic-plastic deformation of polycrystalline alloys at ambient temperature has been widely studied using self-consistent modeling [1] and neutron diffraction measurements [2]. In-situ neutron diffraction during mechanical loading has provided measurements of strains of individual grain orientations in polycrystals, which offer insights on deformation mechanisms at the mesoscopic length scale. However, the in-situ measurements of intergranular strains at elevated temperatures are limited to date [3]. The evolution of lattice strain at various stages of creep deformation is currently under investigation to gain insights to the implications of the various deformation mechanisms (slip, dislocation creep, or diffusional creep coupled with grain boundary sliding) on elastic/plastic anisotropy. In this paper, we report the lattice strain evolution during tensile deformation of a 316FR stainless steel (single-phase fcc polycrystalline alloy) at 873K (about 0.48Tm). We will present: first, the lattice strains measured as a function of applied stress during in-situ quasistatic tension test; second, lattice strains measured as a function of time during in-situ creep deformation; and finally the comparison between the creep and the quasistatic tensile deformation results.
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EXPERIMENTAL DETAILS The material used in this study is 316FR
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