Use of Electrical Resistance Testing to Redefine the Transformation Kinetics and Phase Diagram for Shape-Memory Alloys Z. HE, K.R. GALL, and L.C. BRINSON The phase-transformation temperatures of a nickel-titanium–based shape-memory alloy (SMA) were initially evaluated under stress-free conditions by the differential scanning calorimetric (DSC) technique. Results show that the phase-transformation temperature is significantly higher for the transition from detwinned martensite to austenite than for that from twinned martensite (or R phase) to austenite. To further examine transformation temperatures as a function of initial state, a tensile-test apparatus with in-situ electrical resistance (ER) measurements was used to evaluate the transformation properties of SMAs at a variety of stress levels and initial compositions. The results show that stress has a significant influence on the transformation of detwinned martensite, but a small influence on the R-phase and twinned martensite transformations. The ER changes linearly with strain during the transformations from both kinds of martensite to austenite. The linearity between the ER and strain during the transforma- tion from detwinned martensite to austenite is not affected by the stress, facilitating application to control algorithms. A revised phase diagram is drawn to express these results. I. INTRODUCTION SHAPE-MEMORY alloys (SMAs) have received con- siderable attention for many applications because of their interesting practical characteristics. These materials can undergo large strain and return to their original shape under certain thermal conditions. [1,2] The strain-recovery features of SMAs are based upon a thermoelastic phase transformation between the austenite and martensite crystallographic struc- tures. A change of temperature in the absence of stress results in a reversible phase transformation between austenite (high T) and twinned/self-accommodated martensite (low T) with no net macroscopic strain. These stress-free transformation tem- peratures are typically termed A s and A f , respectively, for the start and finish temperatures for the transformation from martensite to austenite and M s and M f , respectively, for the transformation from austenite to martensite. Upon loading at low temperatures, the twinned martensite undergoes detwin- ning/reorientation, resulting in an oriented microstructure with accompanying macroscopic strain on the order of 10 pct. At low temperatures, after unloading, the material main- tains this strain level. However, heating causes the material to transform back to the original austenitic phase, thus recov- ering any induced strain and returning it to the original shape and size. At higher temperatures, loading induces a phase transformation from austenite to detwinned martensite, also accompanied by a large strain. Unloading at high temperatures results in recovery of the strain in a hysteresis loop as the material undergoes a reverse phase transformation back to austenite. In addition to the austenite-martensite phase trans- formation, there is another intermediate state known as the R-phase transformation, occurring in many NiTi alloys. In these materials, upon lowering the temperature, the austenite cubic structure gradually distorts to a rhombohedral lattice structure called the R phase, [1] with the martensitic lattice being obtained after additional cooling. A characteristic stress- strain curve at low temperature for the material studied in this work is shown in Figure 1. The strain-recovery feature of SMAs has been used in many places as an actuator. [3–9] For example, a method for non- contact motion control of a hard-disk-drive suspension has been developed using shape-memory-effect (SME) wires in order to prevent friction between the slider and disk; [3] SMA “muscle” fibers have been used as actuators driving an under- water robot; [4] and sputtered SMA thin films have been used as microactuators. [5] In addition to strain, the electrical resis- tance of SMAs is dependent on the phase. This phenomenon could be used to develop a feedback positioning control. [10,11,12] When SMAs are used as actuators, it is important to under- stand the relation between phase status, temperature, strain, and stress. Only then can the desired actuation response at a given stress-temperature condition be accurately controlled. To use SMAs as sensors, the relation between strain and electrical resistance under different stresses also needs to be determined quantitatively. To date, however, a complete understanding of the SMA state (interdependence of phase fraction, temperature, stress, strain, and resistance) is lacking, and this has impacted the ability to use SMAs in precision applications for actuation and sensing. The phase-transformation temperatures of SMAs are typ- ically measured under stress-free conditions by the differential scanning calorimetric (DSC) [13,14] method. This method mea- sures the heat flow through a test sample during a temperature cycle. When analyzing DSC data, the temperature at which a material transforms is indicated by a peak in the heat-flow values. Phase-transformation temperatures recorded by this method are commonly used to characterize the material in modeling/prediction algorithms. However, it has been recently discovered that these transformation temperatures change under different stress and strain states. [15–18] Standard DSC METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 37A, MARCH 2006—579 Z. HE, Graduate Student, K.R. GALL, Graduate Research Assistant, and L.C. BRINSON, Professor, are with the Mechanical Engineering Department, Northwestern University, Evanston, IL 60202. Contact e-mail: K–gall@northwestern.edu Manuscript submitted February 10, 2005.