A. Korner

Weak-beam study of superlattice dislocations moving on cube planes in Ni3(Al, Ti) deformed at room temperature

Abstract The dislocation structure resulting from the compression of single crystals of Ni3(Al,Ti) at room temperature has been studied using transmission electron microscopy imaging methods. Deformed samples of e = 1% and e = 9% true strain were sliced parallel to (111) and (010) planes. Weak-beam imaging methods reveal the (010) planes to be the dissociation planes of the screw superlattice dislocations. Transition of superlattice dislocations from (111) onto (010) planes by the Kear-Wilsdorf mechanism was identified. Remarkable dislocation movement on (010) planes was found. The flow stress is interpreted as a dislocation interaction on octahedral and cube planes. Measurements of the antiphase-boundary energies on (111) and (010) planes yielded values of γ111 = γ010 =250±30mJm−2

Transmission electron microscopy study of the stacking-fault energy and dislocation structure in CuMn alloys

Abstract Single crystals of copper-based manganese alloys (Cu−1.05, 1.3, 3.3, 5.05 and 11.6 at.%Mn) were grown and deformed in tension at room temperature. The weakbeam method of electron microscopy has been used to resolve the partial dislocations and to determine the stacking-fault energy γ. Contrary to most other Cu alloys, the value of γ does not decrease with increasing solute content. It stays the same for all the investigated CuMn alloys and corresponds to the value of pure Cu (γ = 41 mJm−2). It is difficult to include the results in a γ against e/a plot since the valency value of Mn in Cu cannot be specified. In Cu−11.6 at.%Mn a pronounced planar dislocation structure is observed and it is suggested that this is caused by the occurrence of short-range order. The large values of the critical resolved shear stress of the investigated CuMn alloys cannot arise from chemical locking (the Suzuki effect) since γ does not change with solute concentration.

Transmission electron microscopy study of cross-slip and of Kear-Wilsdorf locks in L12 ordered Ni3Fe

Abstract In L12 ordered alloys the cross-slip modes of the superlattice dislocations are of special interest since a sessile configuration (Kear-Wilsdorf (KW) lock) can be formed by cross-slip from {111} glide planes to {010} planes where the screws are assumed to be locked. Single crystals of ordered Ni3Fe were deformed at various temperatures below the critical order-disorder temperature; contrary to most Ll2 ordered alloys Ni3Fe does not show an increase of the flow stress with increasing temperature. At room temperature the density of screws is very low, indicating that they are annihilated by cross-slip. The superlattice screw dislocations show a large dissociation (antiphase-boundary (APB) fault) on (111) cross-slip planes but none on (010) planes. In specimens deformed at 600 K the weak-beam method of TEM was applied to analyse the superlattice dislocations. The four-fold dissociation of the superlattice dislocations with a character near edge orientation is clearly resolved. The screws are either ...

In situ tem study of the h.c.p. to f.c.c. martensitic phase transformation in CoNi single crystals

A single crystal of Co + 32% Ni was grown and transformed by deformation from the f.c.c. to the h.c.p. phase at room temperature. TEM specimens made from this h.c.p. crystal were used to study the h.c.p. to f.c.c. martensitic phase transformation by in situ annealing experiments. At about 180°C the growth of new f.c.c. lamellas is observed; they are several atom layers thick, emerge from the bulk region of the specimen and are not in connection with the lamellas of retained austenite present in the thin foil. Several new lamellas have the orientation of the f.c.c. twin variant. When the temperature is increased more lamellas grow into the thin foil and at about 350–400°C the transformation is complete since hardly any retained martensite is encountered. From a comparison of the results with those of bulk annealed specimens and those of pure Co it can be concluded that the results could be generally valid for thermally induced transformations from h.c.p. to f.c.c. The experimental observations can be compared with the various models of the phase transformation found in the literature. There is no agreement with models based on the random nucleation of individual stacking faults since the lamellas expand at a temperature when the stacking fault energy is still positive. Also a polar dislocation model seems unlikely. The results are in favour of the models based on the formation of lamellas; at least there seems to be no contradiction to these models.

Dislocations at grain boundaries in deformed silicon

Abstract Regions adjacent to Σ = 9 and Σ = 25 boundaries in silicon bicrystals are found to be in an advanced stage of hardening. Electron microscopy shows multiple slip, cross-slip, network formation and some transmission of slip across the boundary.

Exhaustion of a[101](111) slip to explain the strength anomaly in Ni3(Al, Ti)

Abstract The dislocation structure of Ni3(Al, Ti) deformed in the temperature range where the flow stress increases with increasing temperature (room temperature up to 450°C) has been investigated. Long straight screw superlattice dislocations transformed into Kear-Wilsdorf locks have been observed after room temperature deformation. Also frequently observed are dislocations curved on the cube (010) cross slip planes. With increasing temperature the tendency for cube cross slip and dislocation movement on cube planes increases. This is consistent with thermally assisted exhaustion of primary octahedral slip by cube cross slip which is responsible for the flow stress anomaly. Dislocations of the primary cube slip system (±a[110](001)) are activated at deformation temperatures already below the temperature of the flow stress maximum. Their density increases markedly with increasing deformation temperature. Dislocation interactions of octahedral (111) and cube (001) slip could also contribute to the flow str...

The temperature dependence of the antiphase boundary energy in Ni3Fe I. The equilibrium value

Abstract The equilibrium value γ of the antiphase boundary (APB) energy in Ni3Fe as function of temperature is determined by measuring the equilibrium separation of the two unit dislocations which border the APB. It is found that γ decreases more strongly at higher temperatures than predicted by theoretical estimates and γ vanishes approximately at the ordering temperature Te From the time interval to reach equilibrium it can be concluded that the decrease in γ is due to atomic rearrangement which is more pronounced at higher temperatures.

Core energies and core tensions of dissociated and partial dislocations in f.c.c. and h.c.p. metals

Abstract Using the anisotropic theory of elasticity and a semi-atomistic Peierls model, the core energies and core tensions of dislocations in f.c.c. and h.c.p. metals have been calculated. All the results were expressed in terms of Fourier expansions. A simple algorithm allowed the determination of the core energies and core tensions of individual Shockley partials which form the perfect dislocations in close-packed planes.

Use of weak Bragg reflections for analysing superlattice dislocations in Ni3(Al,Ti)

Abstract Electron microscope images of superlattice dislocations taken with g·b>2 can give rise to multiple image peaks whose position and separation are not necessarily directly related to dislocation dissociation schemes. This was proved by image simulations and is consistent with the results of the kinematical theory of image formation. A study of the separation widths of dissociated superlattice dislocations in Ni3(Al, Ti) was made using g·b = 4 and 8 and large |s|. Dissociation widths were determined to be in the range 3 nm 370mJm−2

On interactions of superlattice dislocations in ni3(al, ti) a weak-beam analysis

Abstract A single crystal of L12 ordered Ni3(Al, Ti) has been deformed in compression at 450°C which is the peak temperature of the flow stress anomaly. The dislocation structure in the cube (010) planes is analysed by using the weak-beam imaging technique. Superlattice dislocations (b = ± α[10]) glissile on (010) planes are found to interact with superlattice dislocations of the primary cube ± α[10](001) slip system. Dislocation reactions are produced by the intersecting slip systems. The analysis of such interactions which act as locked structures is the major subject of this paper. Characteristic resultant features are dislocation dipoles which break up into partial dipoles and dislocation loops. They are attached to the superlattice dislocations in a very specific manner. A model is proposed to explain the formation of such configurations. From the fact that these reactions are formed by intersecting slip systems we assume that they determine work-hardening properties. A more common meaning is derived...