In-Situ Neutron Diffraction Study of Strain-Induced Martensite Formation in 304L Stainless Steel at a Cryogenic Temperat
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In-Situ Neutron Diffraction Study of Strain-Induced Martensite Formation in 304L Stainless Steel at a Cryogenic Temperature Kaixiang Tao1, James J. Wall1, Donald W. Brown2, Hongqi Li1, Sven C. Vogel3, Mark A. M. Bourke2, Hahn Choo1,4 1. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 2. MST-8, Los Alamos National Laboratory, Los Alamos, NM 87545 3. LANSCE-12, Los Alamos National Laboratory, Los Alamos, NM 87545 4. Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ABSTRACT In-situ, time-of-flight neutron diffraction was performed to investigate the martensitic phase transformation during quasi-static uniaxial compression testing of 304L stainless steel at 300K (room temperature) and 203K. In-situ neutron diffraction study enabled the bulk measurement of intensity evolution for each hkl atomic plane during the austenite (fcc) to martensite (hcp and bcc) phase transformation. The neutron diffraction patterns show that the martensite phases started to develop at about 2.5% applied strain (600 MPa applied stress) at 203K. However, at 300K, the martensite formation was not observed throughout the test. Furthermore, from changes in the relative intensities of individual hkl atomic planes, the selective phase transformation can be well understood and the grain orientation relationship between the austenite and newly-forming martensite phases can be determined. The results show that the fcc grain families with {111} and {200} plane normals parallel to the loading axis are favored for the “fcc to hcp” and “fcc to bcc” transformations, respectively. INTRODUCTION The austenitic stainless steels typically have a metastable face-centered cubic (γ) structure. Upon cooling below the martensite starting temperature (Ms), the martensitic transformation occurs. With the aid of stress or plastic strain, the transformation may happen at temperatures higher than Ms [1-3]. The deformation-induced martensitic transformation in 304 austenitic stainless steel has been extensively studied [4-6] and the newly-forming martensite phases have hexagonal close packed (ε) and/or body-centered cubic (α') structures [7-8]. The formation of martensitic phases can significantly affect the mechanical properties of stainless steel by increasing its hardening, ultimate tensile strength, and the toughness [3, 9-10]. However, the micro-mechanism regarding the mechanical interactions between the austenite and martensite still remains unclear. Neutron diffraction is an ideal and powerful tool to investigate the microscopic deformation behavior of both the austenitic matrix phase and the newly-forming martensitic phases. Recent work by Oliver et al. [11] shows that the martensitic transformation in TRIP (transformation-induced plasticity) steel is grain-orientation dependent meaning that only grain families with particular orientations are preferred for transformation while other grains remain untransformed. It is consequently anticipated that the martensite forma
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