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I.

INTRODUCTION

NICKEL aluminum bronze (NAB) alloy is widely used in marine environments for critical items such as propellers, impellers, pumps, valves, and heat exchangers due to a combination of its mechanical properties and excellent erosion–corrosion resistance. NAB is more cathodic (more corrosion resistant) relative to steel,[1] and this is the main reason why most of the marine engineering companies will favor NAB as their material for their marine components due to its capability to better withstand extreme sea conditions. Nevertheless, the material still suffers from the effects of corrosion and cavitation which results in annual costs of millions of dollars in repairs and replacement. NAB alloys contain approximately 83 pct copper and 9 to 12 pct aluminum, with the remainder consisting of iron, nickel, and some impurities. The presence of Ni and Fe in the Cu-Al alloys increases the terminal FCC a phase area and suppresses the formation of  -phase in the Cu-Al alloys. Further, Al4Cu9 are damaging to the AZMAN AHMAD and JOSEPH POLDEN, Ph.D. Students, HUIJUN LI, Associate Professor, ZENGXI PAN, DOMINIC CUIURI, and STEPHEN VAN DUIN, Senior Research Fellows, and NATHAN LARKIN and NATHAN LANE, Research Fellows, are with the Faculty of Engineering and Information Science, University of Wollongong, Wollongong, NSW, Australia. Contact e-mail: [email protected] Manuscript submitted December 2, 2013. Article published online August 19, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B

corrosion resistance of the alloys due to high aluminum content.[2] Cast NAB consists of coarse Widmansta¨tten a phase, nickel-iron-aluminum K phases and island martensite b phase, and the transformation phase.[3] The poor casting properties of NAB can result in porosity, defects, and imperfections that degrade the mechanical and the corrosion properties of the final component. In order to improve the erosion–corrosion resistance of NAB, various metallic and organically based coatings have been tested.[4] While these coatings reduce the corrosion effects, the mechanical properties of the material are not enhanced. Friction stir processing has been successfully used to reconstruct the microstructure of heterogeneous metallic materials. The FSP technique has been shown to refine the coarse microstructure of cast NAB, eliminate porosities, and homogenise the microstructure thus increasing hardness, tensile properties, and fatigue strength.[5] Oh-ishi et al.[6] report that the main characteristic of the stir zone (SZ) of FSP NAB was inhomogeneous microstructures in various subregions, including Widmansta¨tten structure, equiaxed fine grain structure, or lamellar structure. Ni et al.[7] state that the inhomogeneous microstructure was parameter dependent in which the tensile and the hardness were increased significantly.[7] A particular function of FSP is able to create localised modification and control of the microstructure in the surface region without altering the overall shape and dimensions of the components. Having established that FSP can