Origin of Microstructural Irreversibility in Ni-Ti Based Shape Memory Alloys during Thermal Cycling
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THE science and technology of shape memory alloys are based on (1) martensitic transformation with thermal or mechanical inducement(s) and (2) reversibility of the transformation when such inducement is removed.[1–14] The latter effectively decides the memory effect: reversibility of the shape. The subject is not old. Although shape memory effects were noted in Au-Cd and In-Tl alloys,[14] the popularity, both academic and applied, of shape memory alloys was arguably initiated with discovery of Nitinol (Nickel Titanium Naval Ordnance Laboratory).[9,14] Applications of shape memory alloys, today, range from actuators (coffee machine to robotics) to biomedical (orthopedic transplants to cardiovascular stents), from advanced aero-space (shrink-to-fit couplers in airplanes to collapsible space antennas) to simple consumer products (eyeglass frames to toys); the list can indeed be exhaustive.[8,9,15–22] Almost all such applications of shape memory alloys are based on the reversibility of austenite M martensite transformation.[8,9,12–14,23–27] This automatically stipulates[8,9,14,28] the thermoelastic nature of the transformation with twinning as the deformation/accommodation R. BASU and L. JAIN, Research Scholars, I. SAMAJDAR, Professor, and P. PANT, Assistant Professor, are with the Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai 400076, India. Contact e-mail: [email protected] B.C. MAJI, M. KRISHNAN, and K.V. MANI KRISHNA, Scientific Officers, are with the Materials Science Division, Bhabha Atomic Research Center, Mumbai 400085, India. Manuscript submitted December 5, 2010. Article published online November 19, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
mode. In other words, a small driving force with minimal possibility of introducing irreversible processes such as slip) is needed.[9,14] Introduction of slip is expected to create shape memory fatigue[8,9,14,23–35]: changes in transformation temperature, reduction/loss of memory or introducing irreversible strains, loss of pseudoelasticity, etc. The simplest form of shape memory fatigue is achieved through thermal cycling,[8,14,25–27,29–35] which is observed to increase dislocation density, depress transformation temperatures, and affect the R-phase and martensite transformation behavior. However, no study exists linking thermal cycling with direct microstructural observations: changes in grain size/shape, in-grain misorientations, retained martensite, etc. This was the motivation behind the current study. In this study, Ni50.6-Ti49.4 and Ni47-Ti50-Fe3 (in atomic percentage) were produced through vacuum arc melting. The alloys were then subjected to different marforming[36–38] operations (Subject of a separate study: PhD thesis of R. Basu, IIT Bombay; expected 2012), so that different starting microstructures could be obtained. The objective was to index the possible microstructural irreversibilities by direct electron backscattered diffraction (EBSD) measurements on austenite microstructures subjected to supposedly reversible austenite
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