Quasicrystal Formation, Phase Selection and Crystallization Kinetics in Zr-Cu-Ni-Al Based Metallic Glasses

摘要

Quaternary Zr-Cu-Ni-Al is one of the best glass forming alloys known. In a narrow concentration range icosahedral quasicrystals are formed upon annealing; in Zr_(69.5)Cu_(12)Ni_(11)Al_(7.5) the metastable primitive icosahedral phase is probably oxygen stabilized. At slightly higher oxygen contents (about 1 at.%) the formation of a metastable fcc "big-cube" phase (NiTi_2-type) is competing with very high nucleation rates. The aim of this paper is to investigate in detail the formation of quasicrystals as well as microstructures consisting of the "big-cube" phase in order to design in particular nanocrystalline structures by controlled crystallization. Nucleation rates were measured by means of crystallization statistics. By modeling the obtained nucleation rates in the framework of diffusion controlled classical nucleation interfacial energies as well as melting temperatures for the metastable phases could be derived. Atomic mobility was estimated from growth data. Using these data TTT-diagrams can be drawn and microstructures (for example nanocrystalline) predicted. The phase selection during the formation of the metastable structures was observed to depend strongly not only on the exchange of the Zr by Hf or Ti, but also on the late transition elements chosen, hydrogen or oxygen contamination as well as on alloying with small amounts of other elements for example Si, Sn, Y or Mo. Regarding the influence of the late transition metals there is some evidence that the quasicrystals are a hybrid of two structural elements, i.e. the tetragonal Zr_2Ni (Al_2Cu) and the tetragonal Zr_2Cu (MoSi_2) structure. The mentioned alloying elements as well as oxygen and hydrogen are assumed to influence the arrangement of the structural elements towards one or the other structure. Bulk metallic glasses exhibit unique properties, e.g. their extremely large elastic limit can be used for improving the elastic response of golf clubs. Zr-based alloys are among the most promising systems for bulk metallic glass formation; besides the Be-containing Vitreloy type glasses best glass forming ability was reported in Zr_(65)Cu_(17.5)Ni_(10)Al_(7.5). Any study on crystallization of these glasses are of utmost importance in order to understand their high thermal stability. But such studies are also of interest, since some of these glasses, e.g. Zr_(69.5)Cu_(12)Ni_(11)Al_(7.5) can be used as a precursor material for icosahedral quasicrystals. In a number of papers further alloying or exchange of elements were found to promote or suppress quasicrystal formation. Partially quasicrystalline Zr_(69.5)Cu_(12)Ni_(11)Al_(7.5) alloys were reported to absorb large amounts of hydrogen (up to H/M = 2.0), thus making these microstructures a candidate for a new class of hydrogen storage materials; the icosahedral structure of these quasicrystals provides similar to Laves-phases a high number of tetrahedrally coordinated sites for interstitial hydrogen. Only recently partially nanocrystalline Zr-based alloys were designed as promising new class of high strength bulk alloys. The observed increase in mechanical strength combined with good ductility is thought to result from a combination of the nanoscale particle size, the low interfacial energy and localization of deformation in the intergranular amorphous phase. The aim of this paper is review our recent knowledge on the crystallization kinetics as well as on the influence of contamination and controlled alloying on the phase selection in Zr-Cu-Ni-Al glasses. Such a study is a prerequisite for controlled design of nano-quasicrystalline alloys, but can also help to understand the controlling factors in the quasicrystal formation.