MICROSTRUCTURAL CHARACTERIZATION OF FERROMAGNETIC MATERIALS USING MAGNETIC NDE TECHNIQUES

Magnetic NDE techniques, namely, the acoustic Barkhausen noise, the magnetic Barkhausen noise and the magnetic hysteresis curves, were simultaneously used for microstructural characteriza- tion of nickel and steels. Results showed that, in nickel, the non-180(DEGREES) domain walls interact more strongly with dislocations than the 180(DEGREES) domain walls. A study of the grain size effect on the magnetic and acoustic Barkhausen noises showed a great potential as a NDE grain size measurement tool. Moreover, the Barkhausen signals indicate that the 180(DEGREES) domain walls in nickel seem to have a stronger inter- action with grain boundaries than the non-180(DEGREES) domain walls, as indicated by the acoustic Barkhausen signal. Based on the experi- mental observations, a theoretical model is being proposed to explain the grain size effect on the Barkhausen signals. The model takes into account the density of magnetic domain walls and their initial velocity, with both quantities being strongly dependent on microstructure. The acoustic and magnetic Barkhausen noises were also found to be very sensitive to the change in carbide morphology. It is proposed that the magnetic Barkhausen peak signal is caused by; mainly domain nucleation and the acoustic Barkhausen peak signal due to domain growth.; ('1)DOE Report IS-T-1131. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.