The effect of nickel hardness and grain size on acoustic and electromagnetic Barkhausen emission

Acoustic Barkhausen emission (ABE) and electromagnetic Barkhausen emission (EBE) have been measured during the magnetization of nickel. Of particular interest is the effect on the Barkhausen emission envelope as the microstructure of the material is varied. Samples of nickel were heat treated at temperatures up to 1100°C and the effect on ABE and EBE measured. Measurements on ‘as machined’, unannealed material indicate that EBE is unrelated to ABE since EBE evaluates surface condition, while ABE is generated within the bulk of material, and the machined surface and bulk conditions are different for an unannealed sample. On heat treatment the correlation between surface condition and bulk condition returns (as indicated by Barkhausen emission envelopes) and the correlation is considered to be due to the relative energies of break away of 71°, 180° and 109° domain walls. Measurements were made of hardness and grain size and the characteristics of the Barkhausen emission signals compared to hardness and grain size. Parameters used to characterize the Barkhausen emission signals include the time corresponding to the peak in the EBE signal ( t p ) and the rms level of the ABE signal ( V rms ). These parameters correlate well with material hardness and grain size and offer some potential for measuring these quantities.