Fabricating and characterization of nimngafe micro alloys

In this thesis, microstructure, mechanical, thermal and magnetic properties of Ni50.52Mn9.03Fe11.93Ga28.52 and Ni52.27Mn11.66Fe13.25Ga22.83 bulk alloys produced in an arc melting furnace have been investigated. In addition, the obtained bulk alloys were grinded by mechanical milling for different times and the effect of milling time on the magnetic properties of the material has been also investigated. The surface morphology of the bulk alloys was examined by scanning electron microscopy and it was determined that both alloys consist of austenite and martensite phases. The mechanical properties of the bulk alloys were determined by Vickers hardness test and it was observed that the hardness value of the material increased as the amount of Ga in the alloy decreased. In order to determine the thermal properties, the martensitic phase transition temperatures of the bulk alloys were determined by differential scanning calorimetry measurements at temperatures ranging from room temperature to 600 °C. The transition temperatures As, Af, Ms and Mf were found to be 75.22, 94.60, 73.12 and 64.61 °C for Ni50.52Mn9.03Fe11.93Ga28.52 and 145.01, 184.41, 132.55 and 119.13 °C for Ni52.27Mn11.66Fe13.25Ga22.83, respectively. Accordingly, the significant decrease in the amount of Ga and the larger valence electron concentration of Ni52.27Mn11.66Fe13.25Ga22.83 alloy increased the martensitic transition temperature. The magnetic properties of the alloys were investigated by vibrating sample magnetometer and Mossbauer measurements at room temperature. The Mossbauer spectra obtained for both bulk alloys revealed a sextet and a doublet. The sextet and doublet are related to ferromagnetic austenite phase and paramagnetic martensite phase, respectively. In the Ni52.27Mn11.66Fe13.25Ga22.83 alloy, the amount of sextet was less. This is in agreement with the scanning electron microscopy and hardness results. In the Mossbauer spectra taken from the particles grinded by mechanical milling for 20, 50 and 100 h, a sextet and a doublet were formed as in the bulk alloys as a result of 20 h grinding, while an additional sextet and doublet appeared as a result of 50 and 100 h milling. It is thought that the new sextet is caused by a new phase that emerges as a result of strong collision of small particles with each other. The new doublet is thought to be due to superparamagnetism associated with the decrease in the grain size of the particles.