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The High Strain Rate Deformation of Tungsten Single Crystals.
Final rept. Aug 92-Apr 94,
ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD
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Samples of single-crystal tungsten, in three orientations 100, 110, 111, were compressively deformed at high strain rates 1,000-2,000 s using a Kolsky split-Hopkinson type pressure bar apparatus. Samples of the single-crystal tungsten were also deformed at low strain rate 0.0001 s using a conventional load frame. The 110 orientation exhibited constraint of flow consistent with twofold crystal symmetry about this axis, resulting in skewed flow and noncylindrical specimens after the Kolsky bar test. Both the 111 and 110 orientations exhibited a yield or load drop phenomenon. This load drop may have been associated with either 1 lack of screw dislocation cross slip at high strain rate 2 twinning of the crystal lattice or 3interstitial solute atoms such as nitrogen or carbon pinning dislocations. As a result of this load drop for the lower symmetry 111 and 110 orientations, the quasi-static 0.0001s test had higher flow stress for a given strain than the higher strain rate tests with the Kolsky bar. The 100 orientation crystals exhibited uniform deformation and high rate of work-hardening at both low and high strain rates. The unique behavior of these crystals is postulated in terms of the crystal symmetry, structural evolution at various strain rates, and dislocation flow mechanisms.
APPROVED FOR PUBLIC RELEASE