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Abstract

Our research focused on the formation of Ti40Zr40-xNi20Cux (x=3, 5), Ti45Zr38-xNi17Cux (x=3, 5), Ti53Zr27-xNi20Cux (x=3, 5) and Ti58Zr24-xNi18Cux (x=3, 5) icosahedral (i-phase) quasicrystals by three different techniques and their subsequent characterization and high-pressure hydrogenation. The quasicrystals of this system are thermodynamically stable and they have a relatively good affinity for hydrogen absorption. The aim of this research was to study the influence of processing parameters on the structure, morphology and magnetic properties of the material, before and after hydrogen absorption. With a thermogravimeter and an attached mass-spectrometer we studied the weight percent of desorbed hydrogen and the distribution of desorption temperatures, which gave us an insight into the hydrogen bonding energies (sites) in the material, depending on the structure, composition and hydrogen content.
The icosahedral samples were produced by high-energy planetary ball-milling with subsequent annealing at temperatures around 500°C in vacuum. The temperatures and activation energies of crystallization were measured by differential scanning calorimetry (DSC) and calculated using the Kissinger equation, respectively. A strong decrease of the magnetization during the mechanical alloying was observed due to a modification of the nickel d-states. Scanning electron microscopy (SEM) revealed the formation of non-homogeneous agglomerates during the mechanical treatment, with an average size of 100 μm.
For the preparation of alloys with different structures, from crystalline, quasicrystalline and amorphous, we used the melt-spinning technique. First, the process parameters, i.e., pressure, temperature, distance between nozzle and cooling wheel, rotation speed, material and the geometry of the crucible, had to be optimized. We studied the dependence of the ribbons’ thickness, structure and magnetic properties on the cooling-wheel speed. As expected, the thickness of the ribbons decreases, from about 120 μm at a surface speed of 12 m/s to 30 μm at speeds above 40 m/s. Also, at low wheel speeds mainly the hexagonal C14 Laves phase and cubic solid solution of titanium and zirconium are formed, at middle speeds the icosahedral quasicrystalline phase is formed, whereas at speeds above 35 m/s we get fully amorphous ribbons. Regardless of the effect of rotation frequency on the structure, the magnetization (at 1 Tesla) is constant, within the experimental uncertainty. A linear decrease of quasi-lattice cell parameter due to higher amounts of titanium and copper instead of zirconium was observed. Using transmission electron microscopy (TEM) we confirmed that the ribbons (spun at 22 m/s) contain grains of icosahedral phase imbedded in an amorphous matrix with the same composition. The average particle size of the i-phase on the wheel side of the ribbon was approximately 50 nm, whereas on the argon side the maximum grain size was up to 500 nm. The 5-fold symmetry (besides 3- and 2-fold) was confirmed by selected-area electron diffraction and high-resolution TEM, which is basic characteristic of icosahedral quasicrystals. X-ray photoelectron spectroscopy (XPS) analysis showed that the thickness of an (zirconium and titanium) oxide layer on the surface of the ribbons was around 7 nm. Using temperature and loading time we have a rough influence on the extent of hydrogen in the sample, where hydrogen in the i-phase was determined from the shift in the XRD peaks and the linear dependence between the [H]/[M] ratio and the quasi-lattice constant aq. To measure the overall amount (wt. %) of hydrogen desorbed and to obtain the hydrogen bonding sites distribution thermo-gravimetry (TG) with an attached mass-spectrometer of these materials has been applied. We also studied how the loaded hydrogen affects the magnetic properties of these alloys.
At the end we did a series of vacuum-casting experiments with different sizes of copper cold molds, 3, 2 and 1.5 mm, for samples with the same compositions we used for the melt-spinning. Three milimetre quenched rods of various compositions were hydrided and studied by vibrating sample magnetometer (VSM) to see how the magnetic properties are affected by the absorbed hydrogen. Also, we compared the mass-spectra of hydrogen from the quenched rods with those from the melt-spun ribbons to see how the cooling regime affects the hydrogen-bonding energy distribution.