Investigation of crystallization behavior of nickel based metallic glasses

In this study, two different compositions (Ni35,116Co24,184W23,7B17 and Ni32,75Co22,55W23,7B15Nb6) in the Ni-Co-W-B metallic glass alloy family were discussed. The study was carried out on the crystallization behavior of these metallic glasses. After sample production, differential scanning calorimetry analysis (DSC) was performed at different heating rates (5, 10, 20, 40 and 50 K/min.). The characteristic temperatures (Tg, Tx and Tp) increased with increasing heating rate. Especially, it was determined that the crystallization steps of Ni32,75Co22,55W23.7B15Nb6 alloy are more sensitive to the heating rate. It has been determined that the subcooled liquid region range of the niobium-containing alloy is larger than the other alloy. Although this was not enough to evaluate the glassing ability, it showed improvement. In addition, the appearance of a larger supercooled liquid region was associated with the presence of the element Nb. This was evaluated as an indicator of the contribution made to the improvement of thermal stability. Thermal analyzes showed that the characteristic temperatures in the crystallization zones of the Nb-containing alloy shifted to higher temperatures compared to the Ni35,116Co24,184W23.7B17 alloy. Therefore, the impression has been formed that the thermal stability of the Ni32,75Co22,55W23.7B15Nb6 alloy has improved. Kinetic studies were performed using the Kissinger and Ozawa approaches. Activation energy values were evaluated by using the characteristic temperature-dependent functions of the approaches. It has been determined that the primary crystallization activation energy values of Ni35,116Co24,184W23,7B17 and Ni32,75Co22,55W23,7B15Nb6 alloys are close to each other. In addition, local activation energy values were determined in order to evaluate the crystallization initiation and growth process of the alloys locally. In addition, by using the Kissinger equation, the subsections of the continuous heating transformation (CHT) diagrams of Ni35,116Co24,184W23,7B17 and Ni32,75Co22,55W23,7B15Nb6 alloys were determined theoretically. The transformation diagrams of the theoretical calculations have been verified by the heat treatments of certain temperature-time relationship. For this purpose, heat treatments was applied to the samples produced in completely amorphous structure during randomly selected different temperatures and holding times. Through X-ray diffraction analysis (XRD), information was obtained about the phases precipitated in the microstructure of each sample. Analysis of the heat-treated amorphous samples showed that the nickel solid solution (Niss) and CoWB crystal phases precipitated, respectively, during the heat treatment process. In addition, it was observed that the crystal phases detected in the XRD analysis showed harmony with the phase boundary regions in the CHT diagrams. Finally, in order to obtain information about the mechanical properties of the amorphous and crystallized forms of Ni35,116Co24,184W23,7B17 and Ni32,75Co22,55W23,7B15Nb6 alloys, the microhardness values of the samples aged at certain temperatures and times were measured. The measurement result revealed that the nano-sized phases precipitated with temperature gradually increased the hardness value. The hardness values of Ni35,116Co24,184W23,7B17 alloy aged at 900K for 0-500 minutes heat treatment times were measured in the range of 1231-1448 HV. The microhardness values of the Nb-doped Ni32,75Co22,55W23,7B15Nb6 alloy were measured in the range of 1259-1530 HV. It was determined that the ultra-high hardness CoWB phase crystals, which precipitated in high amounts and nanoscale in the structure, caused an increase in the hardness values.