D. Häussler

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.

HVTEM in situ observations of dislocation motion in the oxide dispersion strengthened superalloy MA 754

Abstract In situ straining experiments in a high-voltage transmission electron microscope have been performed on the oxide dispersion strengthened nickel-base superalloy INCONEL MA 754 at 1020 and 1065 K. The dislocations are pinned at the particles and bow out between them. After breaking free from the particles, the dislocations move in a viscous way. At low average dislocation velocities, the waiting time between full bow-out and detachment is larger than the time for the motion in-between the particles and the bowing-out, suggesting that the interfacial pinning mechanism is rate-controlling. At higher velocities, the waiting time is small compared to the time for bowing-out. Nevertheless, the particles impede dislocation motion due to the back stress which results from bowing. The spacings between the dispersoid particles along the curved dislocation segments, the curvature of the segments, the angles of attack as well as locally acting effective shear stress are estimated from the video recordings using the DeWitt–Koehler line tension model. The results are compared with earlier conventional post-mortem TEM data and macroscopic deformation data from the literature.

Interaction processes between dislocations and particles in the ods nickel-base superalloy INCONEL MA 754 studied by means of in situ straining in an HVEM

In situ straining experiments in a high-voltage electron microscope (HVEM) have been performed on the oxide-dispersion strengthened (ODS) nickel-base superalloy INCONEL MA 754 at 1020 and 1065 K. The results can be interpreted by interfacial pinning of dislocations at the dispersoids and, in some cases, by the Orowan mechanism. In addition to the thermally activated detachment of dislocations from particles, a viscous friction mechanism possibly due to point defect diffusion in the dislocation cores and Taylor hardening contribute to the flow stress.

Microprocesses of the plastic deformation of icosahedral Al–Pd–Mn single quasicrystals

Abstract Recent experimental observations on the plastic deformation of icosahedral Al–Pd–Mn single quasicrystals are described originating from conventional transmission electron microscopy in a high-voltage electron microscope (HVEM), from in situ straining experiments in an HVEM and from the determination of activation parameters from macroscopic compression tests at low temperatures. The moving dislocations are created by multiplication events initiated by cross slip. They trail planar faults of different electron microscopy contrast. Since recovery occurs at high temperatures, the dislocation mobility should be discussed on the basis of low-temperature data. While it was previously interpreted in terms of an extended cluster friction model, the present paper shows that it can equivalently be explained by the Peierls mechanism on the cluster scale.

In situ high-voltage electron microscope deformation study of a two-phase (α2 + γ) Ti-Al alloy

Abstract In situ straining experiments in a high-voltage electron microscope have been performed on two-phase Ti-47at.%Al-2at.Cr-0.2at.%Si at room temperature and at 900 K, Two microstructures were investigated which were produced by thermomechanical treatments: a so-called near-gamma and a nearly lamellar microstructure. The processes controlling the motion of individual dislocations in the γ phase are very similar in both materials. At room temperature, ordinary dislocations and superdislocations show a jerky motion which is impeded by localized obstacles and jogs. At high temperature, the dislocations are smoothly bent or arranged in preferred orientations and move in a viscous way. This mode of motions as well as the nonplanar arrangement of dislocations point to the action of climb as an essential process during the high-temperature deformation of Ti-Al. These observations are very similar to those on Al-rich single phase TiAl investigated earlier. The considerably higher flow stress of the two-phase alloys is an effect of their particular microstructures, i.e. of grain boundaries and lamella interfaces.

Plastic properties of icosahedral Al–Pd–Mn single quasicrystals

Abstract Plastic deformation properties of icosahedral Al–Pd–Mn single quasicrystals have been measured over a wide range of temperature in constant strain rate experiments including stress relaxation tests. The properties are different in the low and high-temperature ranges. At low temperatures, the deformation is controlled by the lattice friction of the gliding dislocations and a very effective work-hardening mechanism. At high temperatures, the deformation properties are discussed in terms of recovery-controlled plastic deformation.

Dislocation mobility versus dislocation substructure controlled deformation of icosahedral Al–Pd–Mn single quasicrystals

Abstract This paper compares new high-voltage electron microgaphs of the dislocation structure of icosahedral Al–Pd–Mn specimens deformed at a high temperature (≅800°C) with those deformed at a low temperature (610°C). At all temperatures, the dislocations consist of almost straight segments on different planes. However, planar faults trailed by the dislocations have been observed at low temperatures only. The temperature dependence of the steady-state flow stress is modelled based on an evolution law of the dislocation density proposed before including recovery, and by considering the contribution of long-range dislocation interactions to the flow stress. Owing to the thermally activated recovery of the microstructure, the activation energy of deformation measured experimentally turns out to be higher than the energy of the dislocation mobility, explaining the unreasonably high experimental values.

Dislocation processes during the deformation of NiAl–0.2at.%Ta

Abstract Macroscopic compression tests and in situ straining experiments in a high-voltage electron microscope were performed on NiAl–0.2at.%Ta at room temperature and at elevated temperatures. At room temperature in soft orientations, dislocations of a〈100〉 Burgers vectors bow out between jogs. In contrast to pure NiAl, the dislocations move in a viscous way between the pinned configurations. At 475°C in a hard orientation, dislocations with a〈110〉 Burgers vectors move in a viscous way in configurations strongly depending on the respective slip plane. Preferred orientations of dislocations are of mixed character, most pronounced as very straight dislocations oriented along 〈111〉 directions on {110} planes. These configurations cannot be explained on the basis of the existing atomistic theories. The flow stress is interpreted in terms of the back stress of the dislocations bowing out between jogs at room temperature, the statistical theory of solid solution hardening, and the formation of atmospheres containing Ta atoms at elevated temperatures.

One-dimensionally modulated quasicrystal phase related to icosahedral Al-Mn-Pd

Reference CIME-ARTICLE-1997-008View record in Web of Science Record created on 2007-02-15, modified on 2017-05-12

Dislocation dynamics during the deformation of intermetallic alloys and the flow stress anomaly

In situ straining experiments on NiAl, NiAl-0.2at% Ta, {gamma}-TiAl, and MoSi{sub 2} in a high-voltage electron microscope showed a transition from the obstacle controlled dislocation motion or the Peierls mechanism at low temperatures to either an unstable or viscous motion at high temperatures. It is suggested that the viscous motion is due to the formation of point defect atmospheres around the dislocations, which cause additional drag and may be responsible for the flow stress anomaly in some of these materials. The atmospheres may be of an extrinsic or an intrinsic nature. A new model is proposed for the origin of intrinsic atmospheres assuming that the energy of a dislocation in an intermetallic alloy may be lowest if the dislocations contain a number of point defects in their core. The dragging of atmospheres may lead to an inverse dependence of the strain rate sensitivity on the strain rate, as observed experimentally. The macroscopic deformation data of the studied materials are discussed in terms of the model.