Magnetic Field Effects on Twin Dislocations

Deformation twinning is an important mechanism occurring in certain types of diffusionless shear phase transformations for which the shear processes involve a non-lattice vector. In these cases, the deformation occurs through the nucleation and motion of partial dislocations or the glide of already present partial dislocations, which induce a structural change in the material. The generation and motion of twin dislocations can be accomplished by heating or cooling through the phase transformation temperature, or may also be driven by an applied stress and/or a magnetic field. Thus far, the effect of a magnetic driving force on the nucleation and motion of twin dislocations has seen very little research (1). However, with the recent development of magnetically driven shape memory alloys (2–6), for which deformation twinning is controlled by the application of a magnetic field, a large interest is likely to arise in this field. The exact mechanism by which the magnetic field induces the shape memory effect in these alloys is not known but it is believed that reorientation of certain twin variants of the martensite phase is involved (4,7). This process of rearrangement of the martensite plates may induce large amounts of strain which can be used for engineering purposes, in particular for the development of fast actuators. The integration of these actuators with sensing and control capabilities are at the heart of smart systems which are becoming relevant in modern machine design (8). This paper attempts to determine the influence of a magnetic field on the motion of twin dislocations in order to understand the mechanism involved in magnetically driven shape memory alloys. In this paper, the nucleation of dislocations was not considered and thus the pre-existence of dislocations prior to the application of a magnetic field was assumed. The procedure used here is a general one, although for simplicity, we will apply the concepts to the (3 10 15)F transformation in ferrous alloys (9–12). The reason for this choice is the fact that twinning plays a crucial role in this phase transformation and the phenomenological crystallographic theory provides a reasonable description for this phase transition.