Physical and Structural Characterization of a Monocrystalline Cu-13.7Al-4.2Ni Alloy Subjected to Thermal Cycling Treatme
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MONOCRYSTALLINE Cu-Al-Ni alloys based on the Cu3Al intermetallic compound belong to an extensive family of metallic materials that suffer reversible martensitic transformation (RMT) associated with nonelastic effects including the shape memory effect (SME).[1–3] In these alloys the RMT takes place within a relatively narrow range of temperatures, usually close to room temperature (RT). This transformation is activated not only by heating and cooling procedures but also by the application of an external stress. It has been observed that the high temperature stable b1 phase, with an ordered type DO3 cubic structure, may be transformed to both martensitic phases: c¢1 with type Cu3Ti order and/or b¢1 with type 18R order, depending on the aluminum content.[1–4] The incorporation of nickel into the monocrystalline Cu-Al alloys, in addition to the afore-mentioned martensitic phases, could also introduce another metastable phase with a type R-3m ordered rhombohedric structure and Al7Cu4Ni stoichiometric composition.[5] These RMT induced ELAINE CRISTINA PEREIRA, Researcher, and LIOUDMILA ALEKSANDROVNA MATLAKHOVA, Professor, are with the Laboratory for Advanced Materials (LAMAV), State University of Northern Rio de Janeiro (UENF), Avenida Alberto Lamego, 2000, Parque Califo´rnia, Campos dos Goytacazes, RJ CEP 28013-602, Brazil. Contact e-mail: [email protected], [email protected] ANATOLIY NIKOLAEVICH MATLAKHOV, formerly Professor with the Laboratory for Advanced Materials (LAMAV), State University of Northern Rio de Janeiro (UENF), is now deceased. CARLOS YUJIRO SHIGUE, Professor, is with the Scholl Engineering Lorena (EEL-USP) of the University of Sa˜o Paulo (USP), Lorena, Brazil. SE´RGIO NEVES MONTEIRO, Professor, is with the Military Institute of Engineering (IME-RJ), Rio de Janeiro, Brazil. Manuscript submitted December 18, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
microstructural changes affect the properties and consequently the practical applications, especially those related to SME. SME alloys are currently used in engineering systems and medical devices. The typical application requires a repetitive operational regimen in which the alloy is subjected to alternated heating and cooling cycles enclosing the RMT critical temperatures of austenite starting (As) and finishing (Af) interval, as well as martensite starting (Ms) and finishing (Mf). Each cycle is associated with a thermal hysteresis during which the participant phases remain either coherent or semicoherent between themselves.[2–4] One important limitation for the practical use of SME alloys is the structural change resulting after a thermal cycling treatment (TCT). Indeed, during RMT the alloy experiences modifications in both its phase composition and microstructural aspects that control the physical and mechanical properties.[3,6–9] Actually, every thermal cycle introduces additional crystalline imperfections, such as dislocations and residual phases, that interfere with the RMT characteristics causing variation in the critical temperatures and de
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