Transformation of Lamellar Structures in Equal Channel Angular Pressing: Geometric Model and Application to Nickel Alumi
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INTRODUCTION
ALUMINUM bronze is a class of copper alloys highly favored for use in corrosive environments, thanks to a unique protective surface layer of dual copper and aluminum oxides.[1–3] Both strength and corrosion resistance of aluminum bronze are improved with increasing Al content, although the benefit is limited by the formation of an adverse c2 phase (Cu9Al4) at >8.8 wt pct Al which embrittles Al-bronze[4] and causes selective corrosion due to its higher Al content.[1–3] Nickel aluminum bronze (NAB) is the strongest of this class, as additions of iron and nickel suppress the formation of the c2 phase, allowing the maximum Al content to 11 wt pct.[5] However, these additions also lead to a highly complex array of precipitates in the primary a matrix, identified as jI to jIV depending on composition and morphology. Large rosettes of Fe3Al are designated as jI or jII (distinguishable by size and location[6]), the lamellar NiAl in the eutectoid structure as jIII and the fine Fe3Al-based precipitates in the primary a matrix as jIV.[2] (Detailed descriptions of the various phases can be found in References 2 and 7.) The nickel-rich jIII phase is of particular concern since, unlike the three iron-rich j-phases, it causes selective corrosion at the a-jIII interface, accelerating the loss of CAMERON J. BARR, Ph.D. Student, DANIEL T. MCDONALD, Research Fellow, and KENONG XIA, Professor, are with the Department of Mechanical Engineering, University of Melbourne, Melbourne, VIC 3010 Australia, and also with Defence Materials Technology Centre, Hawthorn, VIC 3122, Australia. Contact e-mail: [email protected] Manuscript submitted July 28, 2014. Article published online June 7, 2015 4202—VOLUME 46A, SEPTEMBER 2015
material during exposure to sea water owing to the continuous nature of the lamellar jIII.[8,9] The attack is especially severe under crevice corrosion conditions with deep penetration beneath the surface.[10] In addition, the presence of jIII leads to a highly inhomogeneous cast microstructure, providing a large number of sites for stress concentration and resulting in reduced ductility. Therefore, significant improvements in mechanical and corrosion performances can be gained by disrupting the continuity of the lamellar jIII. Severe plastic deformation (SPD) offers a unique opportunity to both break the lamellar structure[11–15] and greatly enhance the strength of NAB through grain refinement.[16] The ultrafine-grained (UFG) structure obtained by SPD has also been known to enhance corrosion resistance on its own right in several other alloys.[17] Of the available SPD processes, equal channel angular pressing (ECAP) is widely used for its ability to produce bulk UFG materials.[16] However, only limited studies have been conducted on the effect of ECAP on lamellar structures. Wang et al.[11] investigated the breakdown of lamellar structures during ECAP of a eutectic Al-33 wt pct Cu alloy. Shear was found to concentrate on and cut through the lamellae, forming highly distorted pathways which subdivided the lamella
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