Some effects of prestraining nickel at various rates on its subsequent tensile properties
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R E C E N T L Y , Longo and Reed-Hill I have shown that low temperature (77 K) prestraining followed by high temperature (370 K) restraining results in work softening in polycrystalline nickel. That is, the restraining s t r e s s - s t r a i n diagram exhibits a yield drop, whose magnitude depends on the amount of prestrain. Also, considerably less total tensile strain occurs than if deformation is accomplished solely at the straining temperature. The present work evolved from an attempt to verify that similar results could be produced by prestraining nickel at a high rate, then reloading at a lower rate. Appleton and Waddington 2 have already shown that conventional tensile tests of shock-loaded nickel show discontinuous yielding. Their tensile samples were cut from plates shocked in the thickness direction, hence their tensile s t r e s s axis was t r a n s v e r s e to the p r e straining s t r e s s axis. In the present work, it was intended to induce a high-speed tensile prestrain, uniaxially, along the same direction as the subsequent tensile axis. Also of concern in undertaking the present experiments was the broader question of mechanical prop"erty degradation due to high forming rates. Here, r e sults .have ranged from virtually no sensitivity to forming rate, as reported by Orava, et al., for Aluminum Alloy 2014 s to the severe hardening and loss of ductility known to occur in nickel subjected to shock loading. 4 The electromagnetic loading method employed for the current work was expected to give an indication of the magnitude of property degradation in nickel due to deformation at high speeds.
EXPERIMENTAL PROCEDURE 1) Material Ni-270 (99.9 pct Ni; 0.020 pct C, max; and 0.005 pct Fe, max) was supplied by Huntington Alloys Division of International Nickel for these experiments. The nickel was received in the form of sheet 1.42 mm thick. This was sheared into blanks 14.3 by 152 mm with the latter dimension perpendicular to the original rolling direction of the sheet. The blanks were reduced to 0.56 mm by rolling parallel to their long dimension, then machined by conventional methods to the configuration shown in Fig. 1. After machining, all samples were annealed in argon for 30 min at 700 K. The r e sultant m i c r o s t r u c t u r e consisted of equiaxed grains with an average lineal intercept of 30/~m. 2) Deformation The present experiments involved deformation in two steps. The first, which we will call 1)restraining, were performed at widely differing strain rates. The second, restraining, were simply the tensile tests p e r formed at conventional speeds to evaluate effects of the various prestraining treatments. A) PRESTRAINING Two methods were employed, depending on the strain rates desired. High-speed prestraining was
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1 A. S. GULE(~, formerly Visiting Assistant Professor, Department of Materials Science and Engineering, University of Florida, Gainesville, Fla. 31601, is now with the Technical University of Istanbul, I
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