Shape memory alloys — characterization techniques |
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Authors: | Jayagopal Uchil |
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Institution: | (1) Department of Materials Science, Mangalore University, 574 199 Mangalagangothri, India |
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Abstract: | Shape memory alloys are the generic class of alloys that show both thermal and mechanical memory. The basic physics involved
in the shape memory effect is the reversible thermoelastic martensitic transformation. In general, there exists two phases
in shape memory alloys, viz., a high-temperature phase or austenitic phase (A) and a low-temperature phase or martensitic
phase (M). In addition, an intermediate R phase exists in some special cases. The M↔A transformation is associated with a
recoverable strain of about 6.5–8% and the R↔A transformation is associated with a recoverable strain of about 1%. The former
transformation has been widely used in the applications like antenna deployment of satellite, aerospace couplings, orthodontic
arch wires, medical guide wires for diagnostic and therapeutic catheters and other industrial applications. Our group has
been giving emphasis to the characterization techniques for R phase, using differential scanning calorimetry (DSC), electrical
resistivity probe (ER) and thermomechanical analyzer (TMA). R phase is found to have attractive features like stability against
thermal cycling, a small thermal hysteresis and a negligible strain recovery fatigue. DSC has been used successfully to characterize
the recoverable strain parameters, apart from the determination of transformation temperatures. ER is used, for the systematic
study of the dependence of various phases on heat-treatment temperatures. TMA has been effectively employed for the study
of the mixed phases. A space-rotating platform is designed and fabricated, using an actuator of shape memory spring, for obtaining
controlled rotations. The efficiency and the reliability of this actuator has been tested, over a million thermal cycles. |
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Keywords: | Shape memory alloys characterization techniques austenitic phase martensitic phase R-phase |
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