Low current electroplastic effect in ferromagnetic materials

The effect of an electric current on the indentation of nickel and copper samples was studied. Rectangular bars of nickel (99.7% pure) or copper (99.9% pure) with dimensions of 3×5×70 mm were polished and mounted on a sample holder which provided electrical contacts at the sample ends. A dc current I in the range of 0–15 A was injected parallel to the bar axis during Vickers indentation tests with loads of 0.25, 0.5 and 1 N. The tested surfaces were examined by scanning electron microscopy, and by x-ray diffraction. The nickel microhardness increased as a function of the current up to 10% for 1⩽3–6 A, depending on the indenter loading, and then decreased monotonically by as much as 20–25%. The microhardness of the copper samples varied by less than 5% with I, except at a load of 0.25 N, where it increased up to 17% for I⩽3 A, then decreased for 3<I⩽9 A, and then was approximately constant with further increasing of I. A theoretical model is proposed in which the decrease of the Ni hardness is explained by the interaction between the current-induced magnetic field and magnetic domain walls.