First In Situ Lattice Strains Measurements Under Load at VULCAN
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TRODUCTION
VULCAN, a new engineering diffractometer, recently began commissioning at the Spallation Neutron Source (SNS), Oak Ridge National Laboratory.[1,2] With VULCAN, users can conduct mapping of residual stresses, chemistry, microstructure, and texture, as well as dynamically measure the mechanical response, chemical transformation, and microstructure evolution under applied load, temperature, or severe processing conditions. With special sample environments, including a vacuum furnace and unique multiaxial tension-torsion load frame, VULCAN extends neutron scattering studies to broader areas of materials science and solid mechanics.[1] The optics design features a focusing neutron guide system.[3] However, by employing an interchangeable neutron guide-collimator system, the instrument has the flexibility to be configured for individual experiments, depending on the desired instrument resolution and flux. In the high-resolution (HR) mode, the instrument resolution is 0.2 pct to 0.3 pct, with an incident beam flux of approximately 2.5 9 107 n/cm2/s. In the high-intensity (HI) mode, the incident beam flux reaches 1.2 9 108 n/cm2/s, whereas the resolution remains better than 0.5 pct. With such a high flux, the detector count rate is estimated to reach several thousand counts per pulse. Indeed, with a few grams of diamond powder, diffraction peaks can be identified in a single pulse even with the source power at 500 kW. The use of a double-disk chopper allows the easy selection of KE AN, Research Staff and Instrument Scientist, HARLEY D. SKORPENSKE, Scientific Associate, ALEXANDRU D. STOICA and DONG MA, Research Staff, and XUN-LI WANG, Distinguished Research Staff, are with the Neuron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Contact e-mail: [email protected] ERCAN CAKMAK, Graduate Student, is with the Neuron Scattering Science Division, Oak Ridge National Laboratory, and with the Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37996. Manuscript submitted May 13, 2010. Article published online October 19, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
neutron energy bandwidths by changing the chopper phase and rotating speed. At the SNS, neutrons are recorded in so-called event-based data acquisition mode, where each neutron carries a time stamp.[4] This provides the means for a new way of conducting measurements of transient behaviors. Instead of the traditional stepwise scan, neutron diffraction measurements can be made continuously and data synchronization can be performed later according to neutron arrival time at the sample position. In this paper, we report initial in situ neutron diffraction measurements of lattice strain evolution in a model face centered cubic (fcc) 316 stainless steel during continuous tensile loading, using the new event-based data acquisition system. The lattice strain and peak width evolutions are presented for samples measured under two incident beam modes (HR and HI), as well as different strain rates.
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EXPERIMENTA
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