Microstructure, fabrication and properties of quasicrystalline Al–Cu–Fe alloys: a review

The Zr57Cu20Al10Ni8Ti5 alloy solidifies into an amorphous phase upon rapid quenching or casting at low cooling rates. However, the amorphous alloys formed at different cooling rates exhibit different crystallization behavior. The slowly cooled bulk amorphous alloys prepared by copper mold casting reveal a first crystallization peak at 715 K upon heating at 0.33 K/s and crystallize via precipitation of an icosahedral quasicrystalline phase in the first crystallization step. The rapidly quenched ribbons exhibit a first crystallization peak at 720 K and crystallize by simultaneous precipitation of the quasicrystalline phase together with Zr2Cu and Zr2Ni intermetallic phases in the first stage of crystallization. It is supposed that the undercooled Zr57Cu20Al10Ni8Ti5 melt has a tendency to develop an icosahedral short-range order, which is favored by low cooling rate. As a result, the bulk amorphous alloy has a short-range order close to a quasicrystalline phase. In contrast, the structure of the ribbon is more similar to that of the melt at high temperature and the atoms are oriented more randomly.

Effect of cooling rate on the precipitation of quasicrystals from the Zr-Cu-Al-Ni-Ti amorphous alloy

A dodecagonal quasicrystalline phase was grown in nano-sized Ag-Pt multilayers upon solid-state reaction and the existence of the respective metastable state in the Ag-Pt system was also confirmed by ion-beam mixing experiments. The major driving force for intermixing between the nano-sized Ag and Pt layers, as well as for the crystal-to-quasicrystal transition, was thought to be the interfacial free energy, which elevated the initial Ag-Pt multilayers up to an energetic state corresponding to the dodecagonal phase.

Formation of a dodecagonal phase in the nano-sized Ag-Pt multilayers upon solid-state reaction

Scientists have studied the mechanical properties of quasicrystalline materials for quite some time. However the difficulty in obtaining material of reasonable quality hampered systematic investigations. The progress in materials preparation in recent years has triggered new activity in this field. Furthermore the new ternary and multicomponent alloys have demonstrated great promise for use as coatings with good wear resistance and low coefficients of sliding friction. However the physical reasons for these properties and their correlation with the particular structure of quasicrystals are still not understood. As in conventional alloys, experiments under well-defined conditions are required that can serve as a basis for understanding the intrinsic mechanical properties of quasicrystals. Such studies are now increasingly possible after the development of techniques to grow larger single quasicrystals up to a few centimeters in size directly from the melt.Since the mechanical behavior of quasicrystalline alloys is to a great extent determined by a brittle-to-ductile transition at about 70% of the absolute melting temperature, it is useful to discuss the mechanical properties with reference to appropriately defined low-temperature and high-temperature regions.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400034461

Mechanical Behavior of Quasicrystals

Results of mechanical micro- and nanotesting of single-quasicrystalline phases in the Al–Co–Ni-, Al–Pd–Mn- and RE–Mg–Zn-systems (RE=Y, Ho or Dy) are reported. Particular emphasis was devoted to the icosahedral phase in the Y–Mg–Zn-system, since — owing to its low melting point — high homologous temperatures Th can be attained almost without heating. At room temperature indentation of quasicrystals with icosahedral structure results in extended formation of type-III cracks, i.e. shear cracks with different height levels of the crack banks. This indentation-induced surface fragmentation is considerably less pronounced with decagonal phases, indicating that the quasicrystalline structure has a strong influence on brittleness. In contrast to the fracture behaviour the Meyer hardness value HM does not seem to follow a sequence HM (three-dimensional quasicrystal)》HM (two-dimensional quasicrystal)》HM (crystalline matter) but is mainly determined by the sequence of the solid→liquid transformation (or decomposition) temperature Tm, i.e. HM(Al73Ni14Co13)>HM(Al70Pd21Mn9)>HM(Y10Mg30Zn60). The hardness seems to be primarily governed by the complexity of the structure and by type and strength of bonding. At room temperature, all quasicrystals exhibit a strong indentation size effect with the hardness increasing with decreasing load. This effect becomes inverted at higher temperatures. The hardness variation with temperature depends on load: negligible variation for very small loads (nanotesting), very strong variation for moderate loads, and less drastic variation for higher loads. This observation is interpreted in the frame of two models: indentation-induced fragmentation, and dislocation involved deformation.

On the temperature dependence of the hardness of quasicrystals

The results of experimental studies on the plastic mechanical behavior of single-quasicrystals of Al-Pd-Mn at temperatures between 680 and 800 °C are reviewed. The stress-strain curves are characterized by a pronounced yield drop followed by a continuous decrease of the flow stress with increasing strain. The analysis of the microstructure of the deformed material and in-situ straining experiments in the electron microscope show that plastic deformation is based on a dislocation mechanism. The glide geometry of the dislocations can be derived from experiments in which the six-dimensional Burgers vectors and the glide planes are determined. Measurements of the thermodynamic deformation parameters, in particular, the activation volume and the activation enthalpy, indicate that dislocation motion is thermally activated and controlled by localized obstacles. These obstacles can be provided by Mackay-type clusters which form the basic structural elements according to current structure models of icosahedral Al-Pd-Mn. The decrease of the flow stress with increasing strain is explained as deformation softening caused by destraction of the structural and chemical order of the material by the motion of dislocations.

Dislocations and plastic deformation of quasicrystals

The experimental aspects of the plastic deformation of icosahedral quasicrystals are reviewed. Macroscopic experiments, which involve the general investigation of stress-strain curves and the determination of the thermodynamic activation parameters of the deformation process are described. Investigations of the microstructure of plastically deformed samples, studied by means of transmission electron microscopy are presented. Important parameters such as the dislocation density, the Burgers vectors of dislocations, and slip systems are analyzed. Additionally, the results of in-situ straining experiments giving direct insight into the dynamics of the deformation process are presented. Direct conclusions on the nature of the plastic deformation process are drawn, and the current view of the deformation mechanism based on the specific structure of this class of materials is consistently discussed in terms of a qualitative ‘cluster friction model’.

The plasticity of icosahedral quasicrystals

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Dislocations and plastic deformation of quasicrystals

The plasticity of icosahedral quasicrystals