Processing and Characterization of Cu-Al-Ni Shape Memory Alloy Strips Prepared from Elemental Powders via a Novel Powder
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IN recent decades, copper-based shape memory alloys have emerged as a potential material for a variety of applications, such as high damping capacity material, sensors, and actuators.[1–3] In particular, Cu-Al-Ni alloys have received considerable attention because of their ability to be used as a high-temperature shape memory alloy owing to their high thermal stability.[3–6] Cu-Al-Ni alloys produced by conventional casting route are brittle resulting from large elastic anisotropy and large grain sizes, which restricted their large-scale practical applications.[6–8] To overcome the problem of brittleness and to improve mechanical properties, several attempts have been made to refine the grain size by adding various alloying elements, such as Ti, Zr, V, and B.[9–12] The grain size refinement resulted in a significant improvement in the mechanical properties. However, limitations to achieve the desired level of grain refinement hindered further development of these alloys. It is well established that the powder metallurgy route provides much better control over composition and grain size. Therefore, processing of Cu-based shape memory alloys via the powder metallurgy route opens up a way to produce fine-grained material with a better chemical homogeneity and improved mechanical properties. In recent years, only a few efforts have been made MOHIT SHARMA, Graduate Student, SANJAY KUMAR VAJPAI, Research Scholar, and RAVINDRA KUMAR DUBE, Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, Uttar Pradesh, 208016 India. Contact e-mail: [email protected] Manuscript submitted February 3, 2010. Article published online July 1, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
to develop Cu-Al-Ni shape memory alloys from elemental powders via powder metallurgy routes.[13–16] These processing routes mainly involved mechanical alloying of elemental Cu, Al, and Ni powders in a planetary ball mill to produce Cu-Al-Ni alloy powders followed by the consolidation of milled powder. The mechanical alloying was carried out under inert gas atmosphere by varying the milling time between 40 hours and 200 hours. The mechanical alloying offered better chemical homogeneity in the alloy powders produced. However, the requirement of an inert environment during the milling of elemental Cu, Al, and Ni powders and of prolonged milling time make the process industrially unattractive. The most important aspect of processing via the powder metallurgy route is the consolidation of powders into useful products, such as strip, rod, and wire, with a near full density on a large scale. For Cu-Al-Ni shape memory alloys, negligible porosity, excellent interparticle bonding, and fine grain size are primary requirements to realize desired shape memory and mechanical properties. One of the important challenges in the powder metallurgy processing of shape memory alloys is the achievement of near full density in the products. The traditional powder metallurgy processing based on the ‘‘compaction-sin
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