Improvement of cavitation erosion resistance of structural metals by alternating magnetic field treatment

Results of cavitation erosion tests for EN8 steel, nickel-aluminium bronze (NAB), 70/30 brass and aluminium alloy AA2014-T6 following alternating magnetic field (AMF) treatment are presented. These alloys were selected because of their magnetic nature; EN8 steel is ferromagnetic, NAB and 70/30 brass are diamagnetic and AA2014 alloy is paramagnetic. The indirect cavitation erosion tests (ASTM G32–10 standard) were fulfilled at a frequency of 20 kHz in deionized water which was maintained at room temperature and ambient pressure for a predetermined time. The results show significant decrease in the mass loss for all samples that had underg1 AMF treatment. The eroded samples were characterised by means of scanning electron microscopy, while microhardness measurements showed an increase in the surface hardness as a result of the AFM treatment. The results of X-ray diffraction indicated formation of more compressive residual stresses following treatment, while examination by transmission electron microscopy showed evidence of dislocation movement away from grain boundaries. In the case of the NAB and 20014-T6 alloys, there was evidence of new precipitation. By considering the deformed state and the magnetic nature of each alloy, mechanisms explaining the increase in the cavitation erosion resistance due to the treatment are proposed and discussed.