Production of CoAl based ferromagnetic shape memory alloys and investigation of their physical properties

SUMMARY Production of CoAl Based Ferromagnetic Shape Memory Alloys and Investigation of Their Physical Properties In this study, Co-based two different Ferromagnetic Shape Memory alloys group as Co86-x Al14 Crx (x = 0 and 4 by atomic percentage) and Co35 Al25 Ni40-y Cry (y = 0 and 4 by atomic percentage) were produced in Arc Melter under vacuum. Re-melting was applied for a few times so as to increase the homogeneity of alloys. All the produced alloys were heat treated at 1200 oC for 24 hour. Thermal properties of alloys such as transformation temperatures, entalphy changes, entopry changes and Curie temperatures were determined by using DSC (Differantial Scannig Calorimetry) and TG/DTA (Thermogravimetric Analyzer) instruments. XRD diffraction patterns, optical images and Vickers hardness measurments were performed to identify crystal structure and surface morphology. Additionally, the magnetization properties of alloys were measured by using PPMS (Physical Properties Measurement System) at room temperature. As a result of thermal measurements, first group of alloys as Co86Al14 and Co82Al14Cr4 indicated high temperature shape memory property. The transformation temperature of the CoAl alloy increased with adding the Cr element. On the other hand, the entalphy change and entrophy change decreased by addition of Cr. ε and γ phase were seen in XRD patterns in both of alloys. At the same time, peak intensities of the CoAl alloy were decreased the Cr addition. ε and γ phase were also seen In the optical images, it was found that the ε phase region was diminished. Vickers harnedness value of CoAl alloy was increased by addition of Cr. Finally, the Cr addition lead to a decrease in magnetization saturation and Curie temperature of CoAl alloy. Transformation temperatures of second group alloys as ( Co35 Al25 Ni40 and Co35 Al25 Ni36Cr4 ) decrased by addition of Cr. Two phases, γ and β were seen in XRD diffraction patterns of alloys. In CoAlNiCr, the intensities of these phase decreased and became broader than that of CoAlNi alloy. In the second group of alloys, dendirite type structure was seen in the optical image. While Vickers hardness of CoAlNi alloy increased, magnetization saturation and Curie temperature of CoAlNi alloys decreased with increasing the Cr content in the samples. Key Words: Co-based Shape Memory Alloys, Curie Temperature, Martensitic Transformation, Ferromagnetic Shape Memory Effect.