Shape Memory Alloys for Classroom Demonstrations, Laboratories, and Student Projects
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Shape Memory Alloys for Classroom Demonstrations, Laboratories, and Student Projects Katherine C. Chen1, Wendy C. Crone2, and Eric J. Voss3 1 Materials Engineering, California Polytechnic State University, San Luis Obispo, CA 93407, [email protected], 2Engineering Physics, University of Wisconsin – Madison, WI 53706, 3 Chemistry, Southern Illinois University Edwardsville, IL 62026 ABSTRACT Shape memory alloys (SMAs) are unique materials that effectively capture the attention of students due to their dramatic phase transformations that result from temperature or stress. The fascinating properties and intriguing applications of SMAs can entice students to learn about the materials field, as well as relatively complicated materials concepts. SMAs have been incorporated into a range of courses under a variety of topics, such as crystal structures, phase transformations, kinetics, constitutive relations, and smart materials. The concepts can be presented at different levels of knowledge, appropriate to the learning objectives for the particular audience. Several educational activities using NiTi shape memory alloys have been developed, such as web-based videos, shape setting of new designs, classroom demonstration of actuation, visualization of stress-induced transformations, and heat treatments to change transformation temperatures. BACKGROUND The unique behavior of NiTi is based on a thermoelastic phase (or crystal structure) transformation. The high temperature (or parent) phase is known as “austenite” and has the ordered intermetallic crystal structure, B2 or CsCl (Pm 3 m). At particular temperatures or strains, the austenite transforms into “martensite,” which has a monoclinic crystal structure (P21/m). If the parent phase is cooled below Mf (the martensite finish temperature), the austenite completely transforms to martensite, yet the bulk macroscopic shape is left intact. However, on the atomic scale, several different martensite variants have been created and are twinned to maintain the original bulk shape. There are a total of 24 possible crystallographically-equivalent habit planes of martensite. Once in the martensite form, the material is easily deformable through twinning. Particular variants grow at the expense of others to accommodate the deformation and produce a global shape change. While most metals deform by slip or dislocation movement, NiTi responds to stress by simply changing the orientation of its crystal structure through the movement of twin boundaries. Particular crystallographic orientations are favored to accommodate the strain. The shape memory effect (Figure 1a) takes place when shape change occurs through martensitic twin reorientation and then the material is heated above Af (the austenite finish temperature) to induce the phase transformation. Since there is only one possible parent phase (austenite) orientation, all martensitic configurations revert to that single defined structure and shape upon heating. Superelasticity refers to the ability of NiTi to return to its original
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