Enhanced plasticity and superplasticity of ultrafine-grained nickel

Abstract

Although superplasticity has intensively been studied for half century, few observations have been reported for pure metals due to fast grain growth at temperatures required for superplasticity. With developing of nanocrystalline materials, there was a hope that superplasticity could be obtained in a number of pure metals. Indeed, low temperature superplasticity in pure nickel was reported in pioneering work in 1999, later superplastic feature of nanonickel was attributed to sulfur presence in grain boundaries. Recently, it was concluded that superplasticity it is not related to the presence of sulfur at grain boundaries or a liquid phase at grain boundaries. Thereby, the phenomenon of superplasticity in pure metals is still far away for our understanding and it requires future work. This report is devoted to reassessment of superplastic behavior of nano-nickel and it provides new results on enhanced plasticity of pure nickel processed by HPT consolidation of rapid quenched ribbons. Bulk ultrafine-grained nickel was successfully processed by RT consolidation of rapid quenched ribbons using high-pressure torsion. RQ nickel possesses equiaxed grain structure with mean grain size of 1−2 μm showing well defined grain boundaries. Upon HPT consolidation mean grain size of ~0.2 μm and high dislocation density have been achieved. However, consolidated Ni specimens show enhanced plasticity >140% at testing temperature of 450°C and strain rate of 10-3 s-1. No superplastic regime has been attained for given testing conditions.