Controlling Shape Setting of Shape Memory Alloys in the Furnace: An Electric Resistance Method for Monitoring Material Characteristics During Heat Treatment

Wednesday, May 22, 2013: 15:15
Congress Hall 2 (OREA Pryamida Hotel)
Dr. Simone Pittaccio , CNR IENI Institute for Energetics and Interphases - Italian National Research Council, Lecco, Italy
Lorenzo Garavaglia , CNR IENI Institute for Energetics and Interphases - Italian National Research Council, Lecco, Italy
Stefano Viscuso , CNR IENI Institute for Energetics and Interphases - Italian National Research Council, Lecco, Italy
Shape setting is one of the most peculiar and fundamental aspects of Shape Memory Alloy (SMA) processing. Many different strategies have been devised to adjust treatment parameters and optimise material characteristics and are currently in industrial use. Often these methods have to be calibrated a priori and the results checked at the end of the ageing procedure, while it is not possible, in general, to monitor continuously the state of the alloy during shape setting. This paper presents a method to monitor the advancement of shape setting non-destructively in the furnace during the whole process.

A test bench was devised where 200mm-long NiTi wires of different diameters (0.25mm to 1mm), compositions (solution-treated Af  3°C to 13°C) and cold-working hardness (30% and 40%) could be straight annealed under a constant pre-strain of 0.02. A test current density of 1A/0.78mm2 was injected into each wire and the resulting tension was measured continuously over a length of 93mm of wire, and recorded. The resulting curves were very repeatable and showed an initial drop in voltage, followed by a definite rise, a plateau and a long slow decrease. The characteristic onsets of these phases were identified during a 3600s ageing test at 430°C and were used to set the durations of subsequent shape setting experiments. Mechanical tensile tests, DSC and XRD measurements were carried out on all specimens.

It was possible to correlate consistently relative voltage (or electric resistance) values with mechanical performance of and structural evolution in the material. Furthermore, it was possible to predict reliably the type of pseudoelasticity of the annealed wire from the point along the electric resistance curve where the heat treatment was stopped.

This method is being tested in the shape setting of more complex NiTi structures with favourable preliminary results and an outlook to possible industrial applications.

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