Evaluation of a Phase Transformation in a Ferromagnetic Shape Memory Fe-Pd Alloy using a Magnetic Barkhausen Noise Technique

Rapid-solidified Fe-29.6 at%Pd alloy is a ferromagnetic shape memory alloy, in which giant magnetostriction is caused by the re-arrangements of the activated martensitic twin variants due to applying magnetic field. We have tried to evaluate the transformation phenomena from fct martensite twin phase to fcc austenite stable one in this alloy by use of a magnetic Barkhausen noise (MBHN) technique. The envelope of BHN voltage vs time in Fe-29.6 at%Pd alloy shows two peaks, where secondary peak on intermediate state of magnetic process decreases with increasing temperature, while the BHN envelope in iron and nickel does not change. The power voltage of high frequency part (8∼10 kHz) in BHN spectrum increases with increasing temperature and it shows a maximum value at As (∼370 K) and then, decreases. From the analysis of BHN, it can be concluded that the variation of BHN due to the phase transformation is apt to arise at the higher frequency part of the BHN spectrum during intermediate stage of magnetization process, and BHN seems to be a useful technique for the non-destructive evaluation of the martensite phase transformation in ferromagnetic shape memory alloy with activated twins.