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Session 3164

NiTi – Magic or Phase Transformations?

Katherine C. Chen Materials Engineering Department California Polytechnic State University, San Luis Obispo, CA 93407

Abstract

NiTi alloys possess exciting properties and are staples in materials demonstrations. The shape memory effect and superelasticity property of NiTi fascinate people, but actually require significant materials knowledge to fully understand the phenomena. A laboratory dealing with phase transformations was thus developed to capitalize on the allure of NiTi for a junior/senior level “Thermodynamics and Kinetics of Materials” course.

Students examine and characterize the shape memory and superelastic properties of NiTi wire, and then realize the difference is in the transformation temperature (i.e., Af - the austenitic finish temperature). They then use the phase diagram and TTT diagram to develop the appropriate annealing treatments to change a sample from superelastic into shape memory behavior. Concepts of precipitation, kinetics, nucleation and growth are incorporated into the lab. Students can then also “train” the NiTi wires into desired shapes. Several different conceptual levels of phase transformations may be investigated according to learning objectives for different audiences.

Introduction

The shape memory effect and superelastic properties of NiTi alloys serve as fun demonstrations or “party tricks” to amuse the young and old. Often times people will claim that the materials are “magic,” but in reality, these materials demonstrate fascinating processing-structure-properties relationships for materials science and engineering students!

A laboratory has been developed to thoroughly understand the mechanisms and the origins for the properties. Students examine and characterize the shape memory and superelastic properties of NiTi wire, and then realize the difference is in the transformation temperature (i.e., Af - the austenitic finish temperature). They then use the phase diagram and TTT diagram to develop appropriate annealing treatments to change “as-received” samples from superelastic into shape memory conditions. Concepts of precipitation, nucleation and growth, and kinetics are incorporated into the lab. The lab is somewhat open-ended and asks the students to formulate explanations for the observed trends.

“Proceedings of the 2003 American Society for Engineering Education Annual Conference & Exposition Copyright © 2003, American Society for Engineering Education”

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Chen, K. (2003, June), Niti Magic Or Phase Transformations? Paper presented at 2003 Annual Conference, Nashville, Tennessee. 10.18260/1-2--12379