The Effect of Grain Size and Strain Rate on the Substructures and Mechanical Properties in Nickel 200

Abstract

The substructural developments taking place in nickel 200 with grain diameters of 47, 108, 141, and 274 μm have been studied at four different 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-defined cell boundaries increases with an increase in grain size at a given strain rate. The cell size refinement takes place throughout the entire range of percent strains (up to 30 pct) in tension for the nickel samples with grain diameters of 47, 108, and 141 μm at all four strain rates used in this article. However, nickel, with the largest grain diameter of 274 μm, shows refinement and then sat- uration for tensile strains greater than 25 pct. The cell size strengthening described by the mod- ified Hall-Petch (MHP) equation at the selected four strain rates of this article indicates that the flow stress is higher for smaller grain size samples at a given cell size. The effect of strain rate on the slope from the MHP plots is such that even though it does not change 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 grain 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 observed for all four grain diameter samples, the influence of grain size on the slope of the MHP plots is so small that it can be assumed that they may become grain size independent at extremely high strain rates.