M. Wollgarten

The plasticity of icosahedral quasicrystals

Abstract 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’.

In-situ observation of dislocation motion in icosahedral Al-Pd-Mn single quasicrystals

Abstract The plastic deformation of icosahedral Al-Pd-Mn single quasicrystals has been studied by in situ straining experiments in a high-voltage electron microscope at elevated temperatures. The results provide the first direct evidence for dislocation motion in quasicrystals. The dislocation velocity, for an applied stress of 390 MPa, was determined as 7×10−7ms−1. It was found that the dislocation motion takes place in planes which are perpendicular to threefold and fivefold lattice directions.

On the determination of the Burgers vector of quasicrystal dislocations by transmission electron microscopy

Abstract The contrast of dislocations in icosahedral quasicrystals observed by transmission electron microscopy is discussed in the framework of the kinematical theory of electron diffraction. It is found that, owing to the special structural features of dislocations in quasiperiodic lattices, there are two basically different imaging conditions for which the contrast can vanish. The conditions are established for contrast experiments in which the direction of the Burgers vector components can be determined in physical and orthogonal space. Provided that the dislocation displacement field fulfils certain requirements, contrast experiments employing both extinction conditions permit the defect to be characterized by the direction of a Burgers vector in the six-dimensional reference space of icosahedral quasicrystals.

Study of plastically deformed icosahedral Al-Pd-Mn single quasicrystals by transmission electron microscopy

Abstract Al[sbnd]Pd[sbnd]Mn single-quasicrystals have been plastically deformed at temperatures between 730 and 800°C up to different strain values and analysed in the transmission electron microscope. Dislocations were created during deformation. The dislocation density ranged between 1·7 × 107 cm−2 in undeformed samples and 7·8 × 108cm−2in material deformed at 732°C. Six-dimensional dislocation Burgers vectors were determined employing the convergent-beam electron diffraction technique. 87% of these Burgers vectors were oriented parallel to twofold directions in three-dimensional physical space. Their moduli were 0·113, 0·183 and 0·296nm. The ratio of the phason to the phonon component of the Burgers vectors was found to increase with increasing strain. A variety of slip systems was observed. In most cases the respective slip plane normals were parallel to fivefold and threefold directions.

Intrinsic deformation properties of icosahedral Al-Pd-Mn single quasicrystals

Abstract Plastic deformation experiments were performed on icosahedral Al-Pd-Mn single-quasicrystals to determine the thermodynamic activation parameters of the deformation process. The stress exponent and the strain-rate sensitivity of the flow stress were obtained by means of stress relaxation experiments. The activation enthalpy of the deformation process was measured by temperature change experiments. In the range from 730 to 800°C a nearly constant value of about 7 eV was determined.

Analysis of dislocations in icosahedral Al–Cu–Fe alloy by transmission electron microscopy

Abstract The electron diffraction contrast of dislocations in icosahedral Al65Cu20Fe15 quasicrystals has been analysed. The displacements in the translational degree of freedom were characterized by a Burgers vector parallel to a twofold quasicrystal axis. Evidence of displacements in the phason degree of freedom was obtained by high-resolution electron microscopy.

Diffraction contrast of quasicrystalline dislocations in the transmission electron microscope

Abstract The contrast of dislocations in icosahedral Al62Cu25·5Fe12·5 was investigated in the transmission electron microscope. It was found that the contrast vanishes under two basically different diffraction conditions. This is different from the behaviour in normal crystals and is due to the special structural features of dislocations in quasiperiodic lattices. Quantitative evaluation of the results indicates that the phason component of the dislocation strain field is substantially stronger than the phonon component.

Dislocations and plastic deformation of quasicrystals

Abstract 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.

Dislocation reactions in icosahedral Al-Pd-Mn quasicrystals

Abstract Possible dislocation reactions in face-centred icosahedral quasicrystals are considered from the geometric and energetic points of view. Some typical examples of the reactions between dislocations with different twofold Burgers vectors were confirmed experimentally in a plastically deformed Al-Pd-Mn icosahedral quasicrystal by identification of the Burgers vectors of dislocations in triple-node arrangements. Several examples were investigated in a transmission electron microscope using the defocus convergent-beam electron diffraction technique. On the basis of these dislocation reactions, the experimentally observed fact of an increase in the relative number of dislocations exhibiting a larger phason part of the Burgers vector with increasing plastic strain can be explained.

Mechanical Properties of Quasicrystals

Quasicrystals show mechanical properties which distinctly differ from those of crystals, particularly of metals. The plasticity of these materials shows a brittle-to-ductile transition at very high temperatures and a lack of work hardening. A review of the experimental work, mainly performed on icosahedral Al-Pd-Mn will be presented in this chapter. This includes results of macroscopic deformation experiments as well as microstructural characterizations of deformed samples in a transmission electron microscope. The interpretation of the results is presented in terms of a cluster-friction model, which assumes the Mackay-type clusters as rate controlling obstacles for moving dislocations.