Structural Features of Martensitic Transformations in Shape Memory Ferromagnetic Fe-Ni-Co-Ti Alloys

The Fe-Ni-Co-Ti alloy system belongs to the family of new smart ferromagnetic materials exhibiting the shape memory effect and superelasticity. The Fe-Ni-Co-Ti alloys with different compositions have been studied by metallographic and X-ray diffraction analyses at room temperature. The characteristic temperatures of martensite transformation were determined by low field magnetic permeability. It has been shown that hysteresis of the martensite transformation in the investigated alloys is rather strongly controlled by the aging time and its lowest value was 65, 100 and 185 K for alloys 1, 2 and 3, respectively. The corresponding degree of tetragonality of martensite was 1.15; 1.13 and 1.05 for alloys 1, 2 and 3, respectively. The martensite in these alloys is thin plates with straight edges, which indicates the high mobility of the latter and the absence of relaxation of the deformations that occur with the growth of the plates due to the coherent connection of austenitic and martensitic lattices. In an austenite grain, the relaxation occurs due to the energy advantageous directions of martensite plates growth and the formation of parallel plates. Narrower hysteresis of martensite transformation corresponds to the martensite with more thin plates. In addition, the relief of martensite on prepolished specimens becomes lower, indicating a lesser deformation of the shape during the transformation, while the volume changes are approximately equal.