Investigation On The Microstructure And Corrosion Resistance Of Shot-Peened Nickel-Aluninum Bronze

Nickel-Aluminum bronze?NAB?is widely used in marine application especially in propeller due to their high mechanical strength and corrosion resistance.The as-cast NAB is easily subjected to selective phase corrosion,cavitation erosion,fatigue corrosion and so on in the complex ocean condition because of coarse and inhomogeneous cast microstructure.SP treatment,as a severe surface plastic deformation process,is an effective way of homogenizing the surface microstructure,improving the mechanical properties of metals,etc.Therefore,in this study,the as-cast NAB alloy is shot-peened at three different shot-peening intensities?SPI?.The grain refinement mechanisms of various phases including?,?'and three intermetallic phases(?_II,?_III and?_IV)in as-SP NAB alloy are systematically studied with the optical microscope?OM?,transmission electron microscope?TEM?and scanning electron microscope?SEM?.Besides,the effect of SP treatment on corrosion and cavitation erosion is also discussed.The results are shown as below:?1?After SP treatment,the surfaces become quite uneven and rough with indentations.The severe deformation layers and refined microstructures are formed in the surface of matrixes.Increasing SPI can result in the formation of more grain refinement.Besides,the SP treatment can produce high compressive residual stresses?CRS?into surfaces.The microhardness after SP treatment increase with increasing SPI due to the grain refinement and CRS.?2?Various phases in NAB alloy including?,?'and three intermetallic phases(?_II,?_III and?_IV)display different grain refinement mechanisms.The grain refinement in the?phase involves dislocation segmentation in the precipitation-free zone and the twinning intersection in the precipitation zone.The formation of nanotwins is found to be responsible for the grain refinement in the?'phase.As for the?phases,the strip-shaped?_III phases are fragmented after severe deformation,and the?_IV and?_II phases exhibit the twinning and dislocation deformation modes,respectively.?3?The corrosion resistance of NAB alloy is enhanced after SP treatment.It is apparent that the passive film formed on as-SP samples have the higher growth rate and tends to be more protective and corrosion resistant.The SP-0.20 samples show the highest corrosion resistance and is followed by SP-0.25 and SP-0.15 samples implying that the corrosion resistance is not only related to the SPI but also to the roughness.?4?After SP treatment,the erosion resistance of NAB alloy is also improved,which is attributed to grain refinement,higher CRS and microhardness.The SP-0.20 samples show the best cavitation erosion resistance.