| The synthesis of amorphous Ni-Nb and Ni-Zr alloys by mechanical alloying has been investigated. The microstructural and thermal properties of amorphous alloys have been studied by combining X-ray diffraction, scanning and transmission electron microscopy (STEM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The following results were obtained: (1) Amorphous Ni-Nb and Ni-Zr alloy powders have been synthesized by mechanical alloying under a controlled atmosphere. The formation range of the amorphous state is equal to a/o larger than amorphous alloys produced by other methods. The thermal stability of mechanically alloyed amorphous powders has been investigated and compared with that of amorphous alloys prepared by the rapid quenching technique. There are some discrepancies among the observed data. The presence of tiny crystallites as nucleation centers and high oxygen levels in the mechanically alloyed amorphous alloys might be responsible for the differences in crystallization behavior. Metastable oxide phases were observed during crystallization studies of mechanically alloyed amorphous powders. (2) The formation of amorphous alloys by mechanical alloying of pure nickel and niobium powders has been investigated. The driving force is believed to be the negative heat of mixing. The amorphization rate is proportional to the heat of mixing. The amorphization of Ni{dollar}sb{lcub}60{rcub}{dollar}Nb{dollar}sb{lcub}40{rcub}{dollar} powders by MA of pure crystalline Ni and Nb powder is a diffusion controlled process. (3) The microstructural change during MA of mixtures of the intermetallic compounds NiZr{dollar}sb2{dollar} and Ni{dollar}sb{lcub}11{rcub}{dollar}Zr{dollar}sb9{dollar} has been observed by STEM, SEM, and DSC. A specific "cauliflower" phase was formed during the early stages of MA. It is suggested that the solid state reaction between intermetallic compounds NiZr{dollar}sb2{dollar} and Ni{dollar}sb{lcub}11{rcub}{dollar}Zr{dollar}sb9{dollar} is not the only origin for the formation of the "cauliflower" phase. The driving force for the amorphization of intermetallics is believed to be either the steep rise in the free energy of the line compounds as material transfer moves their compositions off stoichiometry, or the creation of a critical defect concentration in the intermetallic compounds. (4) Mechanical alloying was carried out on mixtures of "line" intermetallic compounds (i.e. those having a narrow range of homogeneity) in the following binary alloy systems: Cr-Ti, Mn-Ti, Cu-Ti, Fe-Ti, Co-Ti, Ni-Ti, Cu-Zr, and Mn-Si. Amorphization by mechanical alloying was observed to be complete for intermetallic compound mixtures in all the above systems except Cr-Ti, which exhibited partial amorphization, and Mn-Si, which showed no amorphization. The amorphization tendencies could not be predicted from calculated enthalpies of mixing for the amorphous alloys, all of which were negative, or from calculated differences in free energy between the amorphous phase and the mixture of equilibrium intermetallic compounds. |
