THE EFFECT OF GRAIN-SIZE AND STRAIN-RATE ON THE SUBSTRUCTURES AND MECHANICAL-PROPERTIES IN NICKEL 200

The substructural developments taking place in nickel 200 with grain d iameters of 47, 108, 141, and 274 mum have been studied at four differ ent strain rates of 0.01, 0.25, 2.5, and 5/min during tensile testing at room temperature. The percent strain necessary to develop well-defi ned cell boundaries increases with an increase in grain size at a give n strain rate. The cell size refinement takes place throughout the ent ire range of percent strains (up to 30 pct) in tension for the nickel samples with grain diameters of 47, 108, and 141 mum at all four strai n rates used in this article. However, nickel, with the largest grain diameter of 274 mum, shows refinement and then saturation for tensile strains greater than 25 pct. The cell size strengthening described by the modified Hall-Petch (MHP) equation at the selected four strain rat es of this article indicates that the flow stress is higher for smalle r grain size samples at a given cell size. The effect of strain rate o n the slope from the MHP plots is such that even though it does not ch ange with an increase in strain rate up to 0.25/min for the four grain sizes, the actual value of the slope decreases with an increase in gr ain size at a given strain rate. Beyond this strain rate, even though an increase in the slope value as a function of strain rate has been o bserved for all four grain diameter samples, the influence of grain si ze on the slope of the MHP plots is so small that it can be assumed th at they may become grain size independent at extremely high strain rat es.