Work softening in shock-loaded nickel
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THE
term " w o r k softening" was originally introduced by Polakowski I to designate the peculiar yielding behavior of a metal in a tensile test, in opposition to the normally encountered " w o r k hardening." It is solely due to substructural r e a r r a n g e m e n t s taking place during deformation, produced by dynamic r e covery or dislocation elimination through dynamic r e crystallization. In contrast, the more general t e r m "flow softening ''~ encompasses both geometric softening (due to the rotation of single crystals towards an orientation with a l a r g e r Schmid factor) and adiabatic softening (due to adiabatic heating during deformation), in addition to work softening. Work softening was initially found3 when aluminum was first deformed at a temperature and then at a higher temperature, the two temperatures having different characteristic substructures. More recently, Longo and Reed-ttill 4-6 have shown that a number of metals can exhibit work softening. Luft et a f found work softening in predeformed Mo single crystals. Work softening has also been found when both deformations are imparted at the same temperature, but different strain rates.~'11 Specifically, it takes place when dynamic r e c o v e r y is inhibited during p r e s t r a i n ing (the f i r s t deformation stage). This is possible at high strain rates. Of particular importance to the present study is the work of Giilec and Baldwin. n They predeformed nickel 270 in a Magneform unit, achieving strain r a t e s of the order of l0 s sec -t. Upon r e straining at conventional strain rates for tensile tests, they were able to detect d e c r e a s e s in the work hardening rates and yield point formation; these changes were, however, modest in comparison to the ones obtained by straining at two different temperatures. Since shock loading provides the fastest possible means of d e f o r m a t i o n - strain rates of 10~ sec "1 or more can be r e a c h e d - a n d since substructures differing substantially from conventional deformation substructures can be achieved, it was thought worthwhile to verify whether preshocked metals could exhibit work softening upon subsequent deformation. MARCA. MEYERS.formerly with the Department of Materials Science and Engineering,Instituto Mihtar de Engenharia. Rio de Janeiro, Brazil, is now Assistant Professor, Department of Metallurgical Engineering, South Dakota School of Mines and Technology,Rapid City, SD 57701. Manuscript submitted February 14, 1977. METALLURGICALTRANSACTIONSA
EXPERIMENTAL PROCEDURE Nickel was chosen for the following reasons: a) Longo and Reed-Hill 6 showed that predeformation at 77 K and reloading at 370 and 510 K promoted work softening. b) The substructure of shock-loaded nickel does not present any complicating features such as deformation twins (up to 25 GN//m2) and phase transformations. A 6 mm thick nickel plate (impurities in wt pct: Si: 0.04, Fe: 0.13, S: 0.09, C: 0.05, Cu < 0.01) was cold rolled down to 3 mm. Rectangular strips, with 100 x 15 mm and having the largest dimens
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