F. Zapoticla, California Polytechnic State University, San Luis Obispo, San Luis Obispo, CA
Manufacturing of NiTi for biomedical devices requires precise control of the transformation temperature and mechanical properties. This study investigates the effects of applied strain of 0-10 % and heat treatments of 350-600ºC for times of 2-30 minutes during shape-setting of Ti–50.8 at% Ni wire with a nominal diameter of 0.495 mm and an initial transition temperature (Af) of 12ºC. Strain was applied prior to heat treatments by coiling NiTi wire, essentially producing extension springs, around different diameter steel mandrels to obtain different strain levels. The samples of NiTi wire under applied strain were heat treated in a salt bath, followed by a rapid quench. Transformation temperatures and shear moduli were characterized using the differential scanning calorimeter and tensile testing. Changes in Af and shear modulus due to aging processes and applied strain were observed. Following theory, precipitation rates of Ni-rich phases generally increased with increased temperature, time and applied strain levels. Thus, reaction rates to achieve a particular Af might be faster than expected if the sample is under strain during the shape-setting process. The general trends in Af due to applied strain and heat treatment are summarized in a three-dimensional time-temperature-transformation-like diagram, with contours of Af as a function of heat treatment time, temperature, and applied strain.KEYWORDS: Materials Engineering, NiTi, wire, applied strain, transformation temperature, Af, shear modulus, tensile testing, TTT-like diagram, DSC
Summary: Frank Zapoticla, K.C. Chen and A.R. Pelton*
*NDC, 47533 Westinghouse Dr., Fremont, CA 94539
