Preparation and Characterization of Co-Ni-Ga Ferromagnetic Shape Memory Alloys

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Shape memory alloys (SMA) have been engineered for various applications and devices since the first discovery of the shape memory effect in the 1930s. The advent of Nitinol (a Ni-Ti alloy) in 1962 established SMAs as a major area of research and development. . Unlike most conventional metals that recover less than 1% of the strain before plastic deformation, SMAs undergo a diffusionless thermo-elastic martensitic phase transformation that enable them to deform via the movement of twins or self accommodation process rather than by conventional dislocation slip mechanism and thereby allow recovery of strain as large as 8%. Diffusionless phase transformation in an SMA can be triggered by temperature change or application of stress or magnetic field. In conventional SMA, shape change or mechanical strain is achieved by applying a mechanical stress or by temperature variation. These inherently slow processes put an upper limit on the actuation speed of the SMA. Faster change of shape / volume or mechanical strain can be achieved in some magnetic alloys by applying an alternating magnetic field. Such alloys are called ferromagnetic shape memory alloys (FSMAs). Current research activities on FSMAs are mainly aimed at understanding the properties of FSMAs and developing FSMAs with properties desirable for actuator applications. Quite a few FSMAs had been developed and many of them have been proposed as potential candidates for sensor and actuator applications. But till now, no practical device has been reported with these FSMAs and the material is still being investigated intensively. Practical application of prototype Ni-Mn-Ga alloy is limited because of its extreme brittleness in polycrystalline state. Co-Ni-Ga solidifies in a peritectic reaction and forms a composite structure with fcc g-phase and bcc b matrix. Thus, controlled amount of -phase can be introduced in the -matrix by proper choice of composition and suitable heat treatment conditions. The hot workability and room temperature ductility of these alloys are significantly improved by the introduction of the -phase, which is a great advantage for practical applications. Understanding the evolution of various crystalline phase in FSMAs by different processing conditions and the resulting changes in properties of the alloys is crucial for evaluating these materials for actuator applications. Although the prototype FSMA, Ni- Mn-Ga has been well studied, other FSMAs such as Co-Ni-Ga, Ni-Fe-Ga etc. have not yet been investigated with so much rigour. In this thesis work, a systematic investigation of the processing conditions and physical properties of several Co-Ni-Ga alloys has been carried out. Three series of Co-Ni-Ga alloys have been prepared by a process consisting of arc melting technique followed by the homogenization at high temperature and quenching to low temperature. Care was taken to prepare alloys with ferromagnetic martensite near room temperature. The samples were characterized by powder X-ray diffractometer, energy dispersive spectrometer attached to a scanning electron microscope, optical microscope, differential scanning calorimeter, magnetic ac susceptometer, vibration sample magnetometer, Vickers microhardness tester, Universal testing machine (UTM) and strain gauge setup couple to an electromagnet, etc. The present thesis work has been presented in six chapters...
Supervisor: A. Srinivasan