Improvement of the shape memory effect in Fe-based alloys by training

Abstract

Shape recovery in iron-based shape memory alloys is known to be incomplete. Thermomechanical cycling or training is recognized to improve the shape memory effect. In the present work, different aspects of training are studied: the number of cycles, the amount of deformation and the annealing temperature. As the number of cycles increases, the epsilon martensite plates become thinner and within the different grains they tend to align in the same direction. Still, different variants are present. XRD measurements showed a strong increase in the E martensite volume from cycle 1 to cycle 2 and a slight decrease for further cycles. There exists an optimum deformation for obtaining stress induced E martensite which is reversibly transformable to austenite during annealing. When the amount of deformation is too low, the fraction of stress induced epsilon martensite is very low, therefore, the shape memory effect is small. If the deformation is too large, slip may occur and different E; martensite variants are formed, which hinders the reverse epsilon ->gamma transformation. The recovery annealing temperature is critical for obtaining a good shape memory, especially during training. The recovery annealing temperature of 400 degrees C is too low for completing the reverse transformation; annealing at 600 degrees C is most commonly used