Co-Ni-Ga Alloys with Room Temperature Ferromagnetic Martensite Phase

Co-Ni-Ga and Co-Ni-Al alloys are expected to be good ferromagnetic shape memory alloys (FSMAs) owing to their higher ductility resulting from the presence of γ-phase precipitates and higher stability in preparation since they do not contain the highly volatile element Mn. Co-Ni- Ga alloys have a wide range of martensitic transformation and Curie temperatures. In order to explore the possibility of obtaining Co-Ni-Ga alloys with room temperature ferromagnetic martensitic phase, two series of compositions, viz., Co70-xNixGa30 (20 ≤ x ≤ 26) and CoxNi25Ga75-x (43 ≤ x ≤ 50) were taken up for investigation. Polycrystalline ingots of these alloys were prepared by arc melting followed by homogenization and quenching at ice water. Analysis of room temperature X-ray diffraction patterns revealed that most of these alloys had a single-phase (tetragonal) structure typical of a martensitic phase, while some of the alloys exhibited a two-phase (cubic and tetragonal) structure due to the presence of both martensite and austenite phases. All alloys having single martensite phase at room temperature showed martensitic transformation at elevated temperature as well as a linear change of the characteristic martensitic transformation temperatures (As, Af, Ms and Mf) with the number of valence electron to atom ratio (e/a). As, Af, Ms and Mf showed distinctive variations when aged in the martensite phase and austenite phase. All the alloys were ferromagnetic at room temperature and the Curie temperature was determined by high temperature ac magnetic susceptibility and magnetization measurements. The typical twin lamellar structure of martensite phase was observed by optical microscope and the development of the cubic γ-phase along with the parent β′-phase was investigated for different ageing temperatures and annealing temperatures. These studies provide useful information about the potential of these alloys for actuator applications.