Superplasticity and microstructure evolution in nanonickel

Although superplasticity has been studied for decades, few observations of this behavior have been reported for pure metals. A problem generally attributed to the difficulty of maintaining a grain size small enough for superplastic flow at the temperatures required for superplasticity. With the produce of nanocrystalline (NC) materials it has been hoped that superplasticity could be obtained in a number of pure metals due to the large volume fraction of grain boundaries in nanocrystalline material should enhance the tendency for superplastic behavior. Two methods of nanocrystalline materials producing (severe plastic deformation and elctrodeposition) allow making fully dense samples available for testing. In this paper, superplasticity was obtained from electrodeposited nickel with initially nanocrystalline structure at the lowest normalized superplastic temperature reported for any crystalline material. A maximum elongation of 895% was obtained at 0.4·TM. Although a mean grain size of ultrafine-grained (UFG) microstructure obtained by severe plastic deformation is comparable with grain size of NC nickel at the beginning of tensile test (about 0.5 µm), no superplastic behavior of UFG samples was detected. This controversy is discussed from viewpoint of non-homogeneity of microstructure in UFG pure nickel.