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Structure, thermal and magnetic properties of ferromagnetic Co-Ni-Al alloys

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Ferromagnetic shape memory alloys (FSMAs) are being intensively studied because of their potential applications as smart materials. Martensitic transformations and lattice reorientation processes in FSMAs can be triggered not only by changes in temperature and stress, as in conventional SMAs, but also by applying an external magnetic field. The martensitic phase transformation of the ferromagnetic Co-Ni-Al alloy systems has been studied in several papers [1÷5]. The Curie and the martensitic transition temperatures of the β phase increase and decrease with increasing the Co content, respectively. The shape memory effect proceeds due to the thermoelastic martensitic transformation from the B2 parent phase into the martensite phase with L10 structure. The unique properties of the Co-Ni-Al alloy system is an improved ductility allowing hot rolling and cold rolling of a presence of γ phase with fcc structure. The β single-phase polycrystalline alloys show a poor ductility [2]. It has been reported that the hot fabricability of NiAl-based alloys could be improved by the introduction of γ phase [6, 7]. Since the composition range of β phase exhibiting the FSM is located near the β + γ two-phase region, the β-based alloys are able to introduce the γ phase by suitable choice of composition and heat treatment temperature [1, 2]. Several percent of γ phase significantly improves the ductility of the Co-Ni-Al two-phase alloys, which is of great advantage for practical applications [8]. Compounds with a strong coupling between crystallographic structure and magnetism usually exhibit a magnetic field dependence of the structural transitions. A typical example is the Gd5(SixGe1-x)4 alloys (0.24 ≤ x ≤ 0.5), where a transformation from the paramagnetic monoclinic phase to the ferromagnetic orthorhombic phase can be induced either by cooling or by the application of a magnetic field [9[...]

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