Electrodeposition and characterization of sacrificial copper-manganese alloy coatings: Part II. Structural, mechanical,
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MANGANESE (Mn) is, with Cd and Zn, one of the few common metals with a redox potential sufficiently low to offer galvanic sacrificial protection to steel parts. Pure Mn coatings on steel, however, are highly reactive and exhibit a high corrosion current and quick dissolution.[1,2] Alloying of Mn with other metals such as zinc,[3–8] nickel,[9,10] iron,[11,12] chromium,[13] cobalt,[14] or tin[15] potentially enables tailoring of the corrosion potential and dissolution kinetics, improving the coating performance. Ideal sacrificial coatings should provide, in addition, suitable contact and soldering surfaces for any foreseeable application in the field and, therefore, should exhibit a low friction coefficient, low ductility, good electrical conductivity, and good solderability both at the nano- and microscale, where surface interactions will occur. All these properties can be optimized by alloying. Electrodeposited Mn grows as a metastable centered tetragonal (CT) phase.[1,2] This ductile metastable phase undergoes room-temperature recrystallization to a brittle bcc phase in a time frame of several weeks, which easily leads to cracking and flaking upon impact or abrasion and should be prevented or retarded. Cu additions to Mn, in particular, can stabilize electrodeposited Mn in its ductile form[2] and, potentially, could tune the redox potential of the alloy, thus allowing control of the dissolution kinetics. JIE GONG, Ph.D. Candidate, is with the Materials Science Program, and the Department of Metallurgical and Materials Engineering, University of Alabama, Tuscaloosa, AL 35487-0209. GUOHUA WEI, formerly with the Department of Metallurgical and Materials Engineering, University of Alabama, Visiting Scholar, is with the Nanotribology Laboratory for Information Storage and MEMS/NEMS, Ohio State University, Columbus, OH 43210. JOHN A. BARNARD, Professor, is with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261-2208. GIOVANNI ZANGARI, Associate Professor, is with the Department of Materials Science and CESE, University of Virginia, Charlottesville, VA 22904-4745. Contact email: [email protected] Manuscript submitted January 24, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
Although the crystal structure and phase transformation of electrodeposited Cu-Mn alloy coatings have rarely been studied, the Cu-Mn phase diagram has been investigated in detail in cast alloy systems.[16–20] Cu and Mn form an fcc solid solution ( phase) at high temperatures,[16] expanded with respect to fcc Mn.[17] In the quenched state, the fcc phase is stable when the Cu content is over 18 at. pct; a CT structure ( phase), however, is eventually obtained when Cu is less than 18 at. pct.[18,19] The fcc-CT (or ) transformation is of the diffusionless (martensitic) type.[17,19] This Mn-rich CT structure is unstable and further separates into a Cu-rich phase (fcc) and pure Mn (bcc). This transformation can be slowed down by increasing the copper content in the alloy.[20] As
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