Analysis of local variations of internal stresses in cyclically deformed nickel crystals by Barkhausen noise measurements

A new method for producing and monitoring Barkhausen noise (BN) was applied to analyse local variations of internal stresses in the surface layer of nickel crystals which have been cyclically deformed at room temperature into the saturation stage. For different sites on the crystal surface both the integral BN signal and the BN signal height distribution along the magnetizing half-cycle were mapped. The evaluation of the spatial variation of the integral BN leads to the conclusion that macroscopic compressive internal stresses exist in the persistent slip bands (PSBs) which do not reverse their sign during the loading half-cycle. The observed cyclic variation of the BN signal distribution stemming from PSB surface regions shows that cyclic changes of the internal stresses also occur. With the growth of the microcrack population the cyclic variation of the BN signal distribution, and hence of the internal stresses in the surface layer, becomes smaller and finally fades completely.