Component Dislocations Research Articles

In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of < c >-component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy.

Activation of non-basal slip in coarse-grained Mg-Sc alloy

Mechanical properties of a coarse-grained hexagonal close packed (hcp) single phase Mg-Sc alloy with two different textures, randomized (through body centered cubic (bcc) to hcp transformation) or weakened (through only cold rolling) texture, was investigated. Alloys of both the textures showed high elongation of over 30% with a moderately high ultimate tensile strength of about 240 MPa. Transmission electron microscopy (TEM) observations showed that both a- and c- component dislocations were activated in the grain interiors, over 20 µm away from grain boundaries. Thus, pyramidal <c+a> slip can be activated in grain interiors of a coarse-grained Mg-Sc alloy.

Formation of polytypes structures in Mg single crystals

We report studies of polytypes structures (PSs) formed in the microstructure of deformed Mg single crystals. PSs emerge as co-products of lattice distortion developed in dislocations’ tangles due to reactions and rearrangements of dislocations to the lower energy configurations. TEM observations indicate that entanglements containing an excess content of 〈c〉-component dislocations that maintain dislocation interactions and the evolving pattern in a confined space nucleate PSs. The distortion and rotation of the hcp lattice induce the transition to the triclinic lattice originating PSs. The overlapping strain fields from mutually interlocked dislocations and stacking faults promote local shears and shuffles that develop polytypes domains within the PSs. Ab initio and molecular statics simulations permit understanding the processes governing triclinic and polytypic transformations in Mg and the role of dislocations in PSs’ self-assembly.

History-independent cyclic response of nanotwinned metals.

Nearly 90 per cent of service failures of metallic components and structures are caused by fatigue at cyclic stress amplitudes much lower than the tensile strength of the materials involved. Metals typically suffer from large amounts of cumulative, irreversible damage to microstructure during cyclic deformation, leading to cyclic responses that are unstable (hardening or softening) and history-dependent. Existing rules for fatigue life prediction, such as the linear cumulative damage rule, cannot account for the effect of loading history, and engineering components are often loaded by complex cyclic stresses with variable amplitudes, mean values and frequencies, such as aircraft wings in turbulent air. It is therefore usually extremely challenging to predict cyclic behaviour and fatigue life under a realistic load spectrum. Here, through both atomistic simulations and variable-strain-amplitude cyclic loading experiments at stress amplitudes lower than the tensile strength of the metal, we report a history-independent and stable cyclic response in bulk copper samples that contain highly oriented nanoscale twins. We demonstrate that this unusual cyclic behaviour is governed by a type of correlated 'necklace' dislocation consisting of multiple short component dislocations in adjacent twins, connected like the links of a necklace. Such dislocations are formed in the highly oriented nanotwinned structure under cyclic loading and help to maintain the stability of twin boundaries and the reversible damage, provided that the nanotwins are tilted within about 15 degrees of the loading axis. This cyclic deformation mechanism is distinct from the conventional strain localizing mechanisms associated with irreversible microstructural damage in single-crystal, coarse-grained, ultrafine-grained and nanograined metals.

Effect of nitridation on polarity, microstructure, and morphology of AlN films

The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was ∼1×108 and 2×1010 cm−2 for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {11̄02} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost free of screw-component TDs and that the density of pure edge TDs was ∼4×109 cm−2.

Twinning nucleation in Cu-8 at. % Al single crystals

Abstract Transmission electron microscopy observations of the dislocation substructure in Cu-8at.% Al single crystals deformed in tension have been carried out to elucidate the origin of the twinning dislocation which produces conjugate twinning in this alloy. It is argued that the ‘correct’ Heidenreich-Shockley dislocation is obtained in the reaction of the primary dislocation with the faulted (Frank) dipole. These interactions are expected to occur frequently during the deformation. Different configurations of the twinning sources are analysed with respect to the nucleation and the growth of the twin. Geometrical issues related to the pinning of the component dislocations at the nodes formed, the motion of the single twinning dislocation and the non-coherent twinning front and the passing barriers encountered during twin growth are analysed. It is shown that there are no geometrical and energetic obstacles to the development of a macroscopic twin by the pole mechanism from the source proposed here.

Mechanisms of plasticity and fracture of partially lamellar titanium aluminum

The deformation and fracture mechanisms active in the γ- and α 2-phases of a binary Ti–48 at% Al intermetallic compound during tensile loading at room temperature and 800°C have been determined by scanning and transmission electron microscopy. A change of the dominant fracture mode from transgranular at room temperature to mostly intergranular at 800°C has been observed. At both room and elevated temperature the γ-phase accommodated most of the strain by activation of essentially the same modes of dislocation slip and ordered twinning, whereas a profound change in the plastic behavior of the α 2-phase has been detected. At room temperature the α 2-phase exhibited inhomogeneous slip of < a> dislocations, b=< a>=1/3<11 2 ̄ 0>, on both {1 1 ̄ 00} and {2 ̄ 201}, whereas < c>-component dislocations, e.g. b=<2 c+ a>=1/3<11 2 ̄ 6>, have not been observed. After straining at 800°C the α 2-phase contained more uniform, denser populations of < a> dislocations, and notably also < c>-component dislocations. Climb occurred in both phases during tensile tests at 800°C. It has been proposed that the observed change in fracture mode may in part be related to the reduction in the plastic incompatibility between the two phases due to the increased level of plasticity exhibited by the α 2-phase at 800°C.

Activation of slip in lamellae of alpha2-Ti3Al in TiAl alloys

The activation of slip at room temperature in lamellae of α2-Ti3Al in a Ti- 48 at.% Al alloy has been studied using transmission electron microscopy. In common with other studies, it appears that slip activity occurs mainly at, or near to, the intersection of certain cross-lamellar twins, formed by deformation in γ-TiAl, with the α2 lamellae. In polycrystalline samples, heat treated to a duplex microstructure with a significant lamellar component, only dislocations with Burgers vectors given by b = 1/3<1120> have been activated, and essentially no dislocations with b = 1/3<1126>, that is c -component dislocations, have been observed after deformation in tension at room temperature. It has been found that, for these samples slip activity in α2-Ti3 Al may occur either by a slip transmission process or by stress-induced activation of sources in the interfaces between the two phases (α2-Ti3Al and γ-TiAl). The resultant defects are superdislocations with Burgers vectors b = 1/3<1120> gliding on prismatic {1100} and/or pyramidal {2201} planes. In contrast with the observations made on polycrystalline material, in samples of polysynthetically twinned crystals deformed in compression parallel to [111]//[0001α2, activity of c-component dislocations has been observed. It is shown that, in all cases studied, the experimental results are consistent with the stress-induced activation of sources in the interfaces, rather than transmission of slip from γ-TiAl. The results are discussed with reference to the influence of this shear transfer across α2-lamellae on tensile ductility in alloys of γ-TiAl.

Accessing the Excess: An Atomistic Approach to Excesses at Planar Defects and Dislocations in Ordered Compounds

The concept of thermodynamic excesses at interfaces in multicomponent systems is generalised to crystalline materials in a way which makes it useful for practical atomistic calculations. It is necessary to terminate the region around the defect on the atomic scale in a way which is consistent with macroscopic thermodynamics, and this problem is solved by introducing a tapered termination, equivalent to averaging over an ensemble of terminations distributed broadly on the atomic scale. The approach leads to a unique definition of excesses which generalises the notion of a charged interface or polar interface. Five different situations are distinguished: (i) grain boundaries, (ii) a heterogeneous interface with more than two components, (iii) a heterogeneous interface in a two component system, (iv) free surfaces, and (v) dislocations. The simple example of a magnetite (Fe304) (111) surface is treated. General formula are also given for excess free energies of extended planar defects and their dependence on the chemical potentials and excesses of the components.

Deformation and fracture characteristics in TiAl at room temperature and 800 °C

The characteristics of deformation and fracture at room temperature and 800 °C of a binary ( γ-α 2)-Ti-48 at.%Al intermetallic with a duplex microstructure have been identified by scanning and transmission electron microscopy (SEM and TEM). A change in the predominant fracture mode from transgranular at room temperature to intergranular at 800 °C has been observed which coincided with profoundly different plastic behavior of the α 2-phase. The α 2-lamellae exhibited only localized planar slip of 〈a〉 dislocations during room temperature deformation, whereas at 800 °C 〈a〉 dislocations are activated more generally, and in addition motion of 〈c〉-component dislocations has been implied. The γ-TiAl phase deformed by ordered twinning and slip of both ordinary and superdislocations during loading at both temperatures.

HREM of 〈 c〉-type dislocations in high-dose neutron-irradiated and postirradiation annealed Zircaloy-2

High resolution electron microscopy (HREM) was performed on 〈 c〉-type dislocations in annealed Zircaloy-4 which had been neutron irradiated to 20 dpa at 690 K. Lattice images of the 〈 c〉-type dislocations, which lie on the basal plane, reveal unfaulted dislocation structure. Conventional TEM analysis of these dislocations in postirradiation annealed Zircaloy-2 shows that the 〈 c〉-component dislocations have Burgers vectors of 1 3 〈11 2 ̄ 3〉 and [0001]. Large bends in the 〈 c〉-component dislocation network are associated with multiple 〈 a〉-type dislocations. Initial results by electron holography of the 〈 c〉-type dislocation core region produces phase shifts having a length of ~ 3 basal planes in isolated regions along the length of the dislocations. The shift may occur due to large solute concentrations.

C-Component dislocations in sub-boundaries in a Zr-2.5 wt% Nb alloy

The nature of α-α sub-boundaries containing 〈 c〉-component dislocations in Zr-Nb alloys has been studied by electron microscopy. The character of the sub-boundaries is thought to be established by the extrusion operation. The boundaries are composed of 〈a〉 and 〈c〉 dislocations forming a variety of tilt, twist and mixed boundaries. The α-α boundaries are closely linked with the α-β interface structure in extruded or β-quenched alloys and are only formed during dissolution of the β particles. Rotation of 〈a〉-tilt boundaries is localized around 〈c〉 dislocations forming a step structure. The configuration of 〈c〉-dislocation steps is believed to be a fault bounded by two 1 2 〈c〉partial dislocations. The importance of these dislocation arrays in understanding the strengthening mechanisms and irradiation growth behaviour of the alloy is emphasized.

Microstructural characterization of neutron-irradiated and post-irradiation annealed Zircaloy-2

Annealed Zircaloy-2 was neutron irradiated to 20 dpa at 690 K. Irradiation-induced dislocation structures in the matrix consisted of dense loop and network dislocations, having Burgers vectors, b = 1 3 〈1120〉 , as well as a high density of c- component dislocations. Intermetallic precipitates underwent irradiation-induced dissolution resulting in higher solute concentration within the grains. There were solute concentration gradients and irradiation-enhanced precipitates at the grain boundaries and these were non-uniformally distributed. In-situ post-irradiation annealing in an HVEM enabled continuous observation of the evolving dislocation structure. Dislocations were found to have Burgers vectors of the pure c- type , 〈a + c〉-type and pure a- type . The formation of the c- component dislocations and their relevance to irradiation-induced growth are discussed. Based on these observations, recommendations are made as to possibly alterations to the chemistry of Zircaloy-2 that might reduce the magnitude of irradiation growth.

Irradiation growth of zirconium single crystals: A review

The purpose of irradiation growth studies using single crystals of zirconium is to attempt to derive the underlying mechanisms of growth through the use of specimens with well characterized microstructures, where grain boundaries are absent. Early work at low fluences showed that growth of annealed crystals takes the form of an expansion parallel to the a- axes and a contraction along the c- axis . More recent experiments have shown that, at temperatures of 353 and 553 K, a growth plateau is reached at fluences of 10 25 n/m 2 and strains of ~10 −4. At higher fluences ( ~ 3 × 10 25 n/m 2 at 553 K) an increase in growth rate occurs, similar to the “breakaway” phenomenon observed with Zircaloy-2, which has been associated with the nucleation and growth of 〈c〉- component vacancy loops on the basal planes. The growth mechanism at high fluences is therefore somewhat analogous to that proposed by Buckley. Crystals that were severely deformed show exceptionally high growth rates that continue in a steady-state manner to high fluences. The growth is consistent with the annihilation of an excess of interstitials at the 〈a〉- type dislocations, together with the loss of the corresponding vacancies to a high density of oriented twin boundaries and to 〈c〉- component dislocations. Further experiments on single crystals with controlled microstructures could provide additional information on the mechanism of growth in zirconium. The examination of post-irradiation microstructures would also be of value, to confirm the relationship between growth and microstructure.

〈 [formula omitted]〉 Component dislocations in annealed Zircaloy irradiated at about 570 K

Annealed Zircaloy-4 irradiated at 550–580 K to fluences between 3.5 and 8.6 × 10 25 n/m 2, E > 1 MeV has been examined using transmission electron microscopy and shows the presence of long 〈 c〉 component dislocations not present prior to irradiation. Specimens of annealed Zircaloy-2 and 4 irradiated to lower fluences contain only 〈 a〉 type loops. The appearance of 〈 c〉 component dislocations coincides with the reported onset of accelerated irradiation growth in annealed Zircaloy-2 at 553 K, i.e. at 3–4 × 10 25 n/m 2, E > 1 MeV. This strongly suggests an important role of 〈 c〉 component dislocations in the growth process — possibly as vacancy sinks.

“C” component dislocations in deformed ti3ai

“C” component dislocations in deformed ti3ai

Dislocation Nucleation, Growth and Suppression During Cw Laser Annealing of Silicon

ABSTRACTOptical, X-ray and transmission electron microscopy plus preferential chemical etching have been used to examine the dislocations and lattice strain introduced during cw laser annealing of silicon. In addition to a substrate scanning mode we operate our cw Ar-ion laser in a “pulse” mode by using an electronically activated shutter located within the laser cavity. This permits accurate measurements to be made on isolated spots or large area scans with a dislocation density that can be deliberately varied. In particular we discuss surface slip traces, their component dislocations and resulting lattice strains, as well as submicron extrinsic dislocation loops which result from the condensation of ion-implantation produced interstitial silicon. Recommendations are presented for producing defect and strain-free material, as well as samples with specific densities of dislocations.

Formula omitted] dislocations in zirconium alloys

Dislocations with 〈c〉- component Burgers vectors have been found in abundance near deformation twins, and to a lesser extent near grain boundary junctions, in deformed Zircaloy-2 and Zircaloy-4. Both pure 〈c〉 and 〈c + a〉 dislocations have been identified by TEM contrast experiments. The segments of 〈c〉- component dislocations tend to be long, straight, and to lie on either basal or pyramidal planes. It is suggested that these dislocations are generated in order to maintain compatibility between crystallites which differ significantly in their ability to accommodate an imposed deformation by 〈a〉- slip . The manner in which 〈c〉- component dislocations can alter the partitioning of irradiation-produced point defects, and their influence on irradiation growth are discussed.

Observations of periodic clustering of low-angle grain boundary dislocations

Abstract A low-angle grain boundary has been observed by transmission electron microscopy in which there occurs a periodic clustering of the component dislocations. It appears that this cannot be interpreted using existing models for the change of low-angle grain boundary misorientation.

Four-layer defects in quenched aluminium

Abstract The form of vacancy clusters in quenched aluminium of 99.99% purity has been investigated using transmission electron microscopy techniques. A new four-layer planar vacancy cluster lying on parallel {111} planes has been observed and its structure has been characterized by observing both the unfaulting behaviour of the defect and the electron contrast behaviour of its component dislocations. It is argued that this defect is heterogeneously nucleated.