D. Caillard

Prismatic slip in beryllium: I. The controlling mechanism at the peak temperature

Abstract An in situ study of prismatic glide in beryllium is made to provide a simple model for understanding the anomalous increase in elastic limit as a function of temperature. This effect is also observed in several ordered alloys such as superalloys. In this paper, Part I, the mechanism controlling the deformation is studied at the stress peak temperature (300 K). It consists of alternate cross-slip events between the basal and the prismatic plane, leading to a ‘locking-unlocking’ mechanism which is studied in detail. This new mechanism appears to be a variant of the Peierls mechanism, where screw dislocations exhibit a metastable spreading in the prismatic plane, in addition to the classical stable spreading in the basal plane. Quantitative in situ measurements give local values of several parameters (such as stress, probabilities of locking and unlocking and strain rate), and the velocity of dislocations and the local strain rate are theoretically expressed. The exact origin of the stress anomaly i...

An in situ study of prismatic glide in α titanium at low temperatures

Abstract In situ experiments have been performed in two titanium ingots with different purities in a view to studying the glide mechanism of a dislocations in prismatic planes between 100 and 300 K. The observation of the jerky movement of rectilinear screw dislocations leads to the conclusion that the strain rate is controlled by the locking-unlocking mechanism. Local stress measurements reproduce well the variation of the macroscopic CRSS with temperature. The stress dependence of the locking-unlocking mechanism and the core structure of dislocations are discussed.

Transmission electron microscopy study of dislocations and extended defects in as-grown icosahedral Al-Pd-Mn single grains

Abstract Analyses of transmission electron microscopy observations of dislocations and faults oriented in mirror planes of as-grown single grains of icosahedral Al—Pd—Mn show that the Burgers vectors and associated fault vectors are perpendicular to the fault planes. These results suggest that climb processes have to be considered as realistic deformation modes at high temperatures in these materials.

Dislocation mechanisms in Ni3Al at room temperature. In situ straining experiments in TEM

Abstract Transmission electron microscopy in situ straining experiments were performed under weak beam conditions, in Ni3Al(0.25 at.% Hg) at room temperature. They revealed rectilinear screw superdislocations dissociated either in the two octahedral or in the cube cross-slip plane, and fully reversible cross-slip between these planes. Screw superdislocations have the lowest mobility. They glide in octahedral planes through series of jumps and waiting times in sessile positions for which the antiphase boundary (APB) is contained in the glide plane. The jump amplitude can either be large, or equal to the superpartial separation. In order to explain these experimental results, a new sessile configuration of screw superdislocations has been introduced, with the APB ribbon in {111}, and both superpartials cross-slipped in {100} on a short distance (about 20 A). This configuration has some common points with that already proposed by Paidar, Pope and Vitek (1984), but is different in several aspects. It is consi...

Prismatic and basal slip in Ti3Al I. Frictional forces on dislocations

Abstract In situ straining experiments have been conducted in polycrystalline Ti3Al, with the aim of analysing the mechanisms controlling dislocation motion and interpreting themechanical properties. This first part is devoted to the glide process of 1/3 〈1120〉 dislocations in prismatic and basal planes. Antiphase boundary (APB) energies have been measured in these planes. Two different APB energies and two different frictional forces have been observed in parallel prismatic planes, corresponding probably to two different cutting planes. The high Peierls-type frictional forces which have been observed in the basal and in one type of prismatic plane are shown to originate from non-planar dislocation cores, in agreement with recent atomistic calculations.

From dislocation cores to strength and work-hardening: A study of binary Ni3Al

Abstract A quantitative model for the peak temperature in work-hardening in L1 2 intermetallics is proposed. It is based on the competition between the exhaustion of mobile dislocations by the Kear Wilsdorf mechanism and the yielding of incomplete locks at high stress. The model is assessed by a set of experimental data measured in binary Ni 3 Al polycrystals of three different compositions. These include, in particular, the planar fault energies of the dislocation cores measured by weak-beam electron microscopy, combined with computer image simulations and macroscopic data about flow stress, work-hardening and mobile dislocation exhaustion rates. These parameters are measured as a function of alloy composition. The model also fits successfully data published for other L1 2 compounds.

Martensite nucleation on dislocations in Cu-Al-Ni shape memory alloys

In the present work, the martensite nucleation on dislocations has been observed. Cu–Al–Ni shape memory alloys have been superelastic cycled inside the transmission electron microscope. The in situ experiences show that the dislocations in β phase can be a nucleation site for γ3′ and β3′ martensites, which at the same time have been characterized by electron diffraction. The martensite plates can nucleate on the dislocations when the stress is applied and retransform to the βL21 phase when the sample is unloaded. The results are discussed in terms of the atomic configuration of the dislocation core, which facilitates the martensite nucleation.In the present work, the martensite nucleation on dislocations has been observed. Cu–Al–Ni shape memory alloys have been superelastic cycled inside the transmission electron microscope. The in situ experiences show that the dislocations in β phase can be a nucleation site for γ3′ and β3′ martensites, which at the same time have been characterized by electron diffraction. The martensite plates can nucleate on the dislocations when the stress is applied and retransform to the βL21 phase when the sample is unloaded. The results are discussed in terms of the atomic configuration of the dislocation core, which facilitates the martensite nucleation.

An in situ study of cube glide in the γ′-phase of a superalloy: I. The controlling mechanism

Abstract An in situ study of the plastic deformation of single crystals of the γ′-phase of a CMSX2 superalloy (L12 structure) has been performed between 120 and 1150 K, in order to analyse the role and the mechanisms of glide of superdislocations in {001} cube planes. Extensive cube glide and dislocation sources are observed from very low-temperatures (140 K) to high-temperatures (1150 K), for a tensile axis close to ⟨110⟩. Glide on cube planes is controlled by a locking—unlocking mechanism acting on screw dislocations, in the whole temperature range. This mechanism is described in detail and compared with that presented earlier for prismatic glide in beryllium. It corresponds to a series of cross-slip events, on super partial dislocations which are alternately extended in one intersecting octahedral plane, and in the cube plane. On the other hand, non-screw dislocations interact with small clusters or solute atoms. The explanation for the activation of cube glide at very low-temperatures is given in part...

Prismatic glide in divalent h.c.p. metals

Abstract In situ transmission electron microscopy observations have been made in magnesium and beryllium of the glide process in the prismatic plane. The observations are in accordance with a jog-pair mechanism at high temperatures and a locking–unlocking cross-slip mechanism at low temperatures. Improved calculations of the corresponding activation energy and activation volume are reported which take full account of anisotropy. Agreement with the experimental values in Mg can only be reached when it is assumed that the jog in the glide process has twice the elementary height.

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

Dislocation motion in icosahedral Al–Pd–Mn quasicrystals has been observed in situ in a transmission electron microscope between 700 and 750°C. Contrast analyses show that it takes place by pure climb in twofold, threefold and fivefold planes. Moving dislocations exhibit polygonal shapes with edges parallel to twofold directions, in agreement with a difficult jog-pair nucleation. Dislocation multiplication, annihilation and local pinning are described and discussed, as well as the role of the phason faults trailed in the wake of dislocations, at the lowest investigated temperatures.