U. Messerschmidt

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.

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.

Dislocation density evolution upon plastic deformation of Al-Pd-Mn single quasicrystals

Dislocation density studies have been performed on icosahedral Al-Pd-Mn single quasicrystals after plastic deformation and after subsequent heat treatment. The deformation tests were carried out at a constant strain rate of 10- 5 s-1 at temperatures between 695 and 820 C. The heat treatments were performed at 730 C, corresponding to one of the deformation temperatures. The development of the dislocation density during heat treatment and that during plastic deformation are compared. The experimental data are interpreted using a kinetic equation, which describes the evolution of the dislocation density during deformation. Numerical values for the dislocation multiplication constant and the annihilation rate for icosahedral Al-Pd-Mn are presented.

Creep mechanisms of ferritic oxide dispersion strengthened alloys

Abstract A study of creep mechanisms of ferritic ODS alloys was based on high temperature tensile/compression tests combined with electron microscopy analysis of deformed specimens and in situ observations of dislocation motion under stress. The flow stress, its strain-rate sensitivity and its temperature dependence were discussed in terms of solid solution hardening, mutual dislocation interaction, the Orowan process, a thermally activated detachment model and solute drag effects. The mechanisms controlling the flow stress of ferritic ODS alloys were identified.

Plastic deformation of decagonal Al± Ni± Co quasicrystals

Plastic deformation experiments have been performed on Czochralski-grown decagonal Al-Ni-Co single-quasicrystals at temperatures between 780 and 860°C. Compression tests at a strain rate of 10-5s-1 with different orientations of the compression axis relative to the tenfold quasicrystal direction show an anisotropy of the plastic behaviour. If the compression axis is oriented parallel to the tenfold direction multiple slip and weak work hardening is observed. If the compression axis is tilted by 45°, single-slip conditions and deformation softening are found. Microstructural investigation by transmission electron microscopy indicates that plastic deformation is mediated by a dislocation mechanism. The results are interpreted in terms of a model in which the chemically ordered columnar clusters of the decagonal structure represent rate-controlling obstacles.

Dislocation processes during the plastic deformation of γ-TiAl

In situ straining experiments in a high-voltage electron microscope have been performed on coarse-grained g -Ti± 52 at.% Al at room temperature and elevated temperatures, in addition to macroscopic compression tests. At all temperatures examined, ordinary dislocations, superdislocations of h 101] Burgers vectors and microtwins carry the deformation, with ordinary dislocations dominating. The processes controlling the deformation diA er greatly for the temperature ranges below and above about 850 K. At low temperatures, ordinary dislocations as well as superdislocations move jerkily between positions where they are locally pinned, which can best be described by a precipitation-hardening mechanism. At high temperatures, the h 101] superdislocations show a shape typical of the locking± unlocking mechanisms. The ordinary dislocations are created and move in a very instantaneous event. Later, theyare smoothly curved and move in a viscous way. The nonplanar arrangement of these dislocations indicates the importance of diA usion processes. The dynamic behaviour and the results of macroscopic deformation tests are explained by the formation of intrinsic atmospheres around the dislocations.

Friction mechanism of dislocation motion in icosahedral Al—Pd—Mn quasicrystals

Abstract Results from the literature and unpublished ones of the present authors are summarized which are relevant to the mechanisms governing the mobility of dislocations in icosahedral Al-Pd-Mn quasicrystals. These results concern macroscopic deformation tests, conventional transmission electron microscopy, in situ straining experiments in a transmission electron microscope, and computer simulation experiments. These experiments can best be interpreted by assuming that the dislocation motion is controlled by the thermally activated overcoming of Mackay-type clusters. The present paper gives an estimate of the activation volume of this process. It turns out that the activation volume of overcoming the clusters individually is one order of magnitude smaller than the experimentally observed one. The experimental observations can be interpreted in a consistent way in terms of the Labusch-Schwarz theory of solution hardening in crystals, which considers the interpenetrating clusters as ‘extended’ obstacles, ...

Plastic deformation of zirconia single crystals: a review

Abstract The high-temperature deformation behaviour of zirconia single crystals stabilised with yttria is reviewed. Cubic or fully stabilised zirconia (FSZ), which is considered the matrix of high-strength partially stabilised zirconia (PSZ), deforms plastically down to 400°C without confining hydrostatic pressure. The relevant deformation behaviour above about 1200°C is characterised by athermal dislocation motion mainly on cube slip planes and diffusion-controlled recovery. Tetragonal polydomain zirconia or t′ zirconia consists of plate-like tetragonal domains alternately stacked to form large colonies. These colonies are arranged in a characteristic way to fill the whole crystal volume, t′ zirconia shows ferroelastic behaviour preceding dislocation plasticity. In tension, a tetragonal single crystal forms containing residual defects. The data available indicate that the coercive stress strongly depends on temperature. In situ straining experiments in a high-voltage electron microscope show an instantaneous switching of the individual tetragonal domains with the domain boundaries moving sidewise through the domains. Dislocations moving after the ferroelastic deformation are strongly bowed. The nature of the pinning agents is not clear yet. PSZ crystals are thought to consist of a cubic matrix and precipitates of the tetragonal phase of a structure similar to that of the colonies in t′ zirconia. Complete dislocations in the cubic matrix moving on cube planes are partial dislocations in two of the three tetragonal variants of the precipitates. They have then to produce a stacking fault or antiphase boundary like defect. The strong age-hardening and overageing experimentally observed can be explained by a decreasing width of the matrix channels between the precipitates and an increasing width of the domains within the colonies. Recent in situ studies in a high-voltage electron microscope have shown that the precipitates in PSZ may undergo ferroelastic deformation, too. Besides, in a number of cases the matrix of PSZ crystals turned out to be tetragonal rather than cubic. Thus, the formation of a tetragonal single crystal containing residual defects as during the ferroelastic deformation in t′ zirconia should affect the subsequent dislocation plasticity. The stacking fault or antiphase boundary like defects have experimentally been observed on 110 planes, however, not on the usual 100 slip planes. Thus, in spite of numerous experimental results a number of questions are still open particularly concerning the plastic deformation of PSZ crystals.