Development of Stress-Induced Martensitic Transformation in TiNi Shape Memory Alloy
Mechanical characteristics, infrared imaging and temperature changes accompanying the TiNi pseudoelastic behavior were working out.
The stress-induced exothermic martensitic forward and endothermic reverse transformations were studied using new fast (538 Hz) and sensitive (0.025 K) infrared camera.
Basing on the obtained mechanical and thermal data it was found that the initial, macroscopically homogeneous transformation occurs at the same stress level for all strain rates applied, regardless of the loading manner, while the stress of localized transformation increases with the strain rate.
An inflection point was noticed on the stress-strain curve, dividing the transformation range in two stages: the first heterogeneous, where transformation bands nucleate and evolve throughout the specimen; the second, where the bands overlap, related to significant temperature increase and upswing region of the curve.
At the stage of advanced martensitic forward and reverse transformation, a new generation of much thinner transformation bands was observed. They appear to nucleate in regular distances from the primary developed wider bands and evolve in perpendicular direction.
At final part of the loading, a decrease of the average specimen temperature reveals the saturation stage of the transformation.
At higher strain rate, more dynamic run of the transformation process was observed, revealed in creation of numerous fine transformation bands.
It was also observed that nucleation of the localized forward transformation takes place in the weakest area of the specimen, whereas the reverse transformation always initiates in its central part.
Acknowledgments: The research has been carried out with the support of the Polish National Center of Science - Grant No. 2011/01/M/ST8/07754 and the Japan Society for the Promotion of Science; Post-doc IDP04774. The assistance of Dr. M.Maj in obtaining the infrared measurements is greatly appreciated.