Dislocation and Grain Boundary Interactions in Deformed Titanium Foils (original) (raw)
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Interfacial dislocation arrays in twin boundaries of deformed titanium
Scripta Metallurgica et Materialia, 1994
Mechanical deformation of hexagonal close-packed (hcp) metals presents great interest. At low temperatures, the dominant deformation mechanism in these metals is mechanical twinning. The crystallography and core structure of twinning dislocations for the principal twin systems of hcp metals have been analysed by Serra, Bacon and Pond (1). The properties of these twinning dislocations have been investigated by atomic scale computer simulation (2), These models can be used as a reference in order to investigate the properties of hcp metals during deformation. From a geometric point of view, different behaviour is expected among distinct hcp metals. This could be attributed to the deviation of the individual c/a ratios from the ideal close-packed ratio (c/a =(8/3)1/2), which also results in the different shear moduli of hcp metals.
Basal slip of ⟨a⟩ screw dislocations in hexagonal titanium
Scripta Materialia
Basal slip of a screw dislocations in hexagonal closed-packed titanium is investigated with ab initio calculations. We show that a basal dissociation is highly unstable and reconfigures to other structures dissociated in a first order pyramidal plane. The obtained mechanism for basal slip corresponds to the migration of the partial dislocations and of the associated stacking fault ribbon in a direction perpendicular to the dissociation plane. Presented results indicate that both basal and pyramidal slip will operate through the Peierls mechanism of double-kink nucleation and will be equally active at high enough temperature.
Acta Materialia, 2011
A new constitutive plasticity model for prismatic slip in hexagonal α-titanium is developed. In the concept pure edge and screw dislocation densities evolve on the {101¯0}〈12¯10〉 slip systems. The model considers that the screw dislocation segments have a spread out core, leading to a much higher velocity of edge compared with screw dislocations. This enables the model to describe the observed transition in strain hardening from stage I to stage II in single crystals oriented for prismatic slip. Good agreement is found between the experimentally observed and simulated stress–strain behavior.
A dislocation density-based crystal plasticity constitutive model for prismatic slip in α-titanium
Acta Materialia, 2011
A new constitutive plasticity model for prismatic slip in hexagonal a-titanium is developed. In the concept pure edge and screw dislocation densities evolve on the f1 0 1 0gh1 2 1 0i slip systems. The model considers that the screw dislocation segments have a spread out core, leading to a much higher velocity of edge compared with screw dislocations. This enables the model to describe the observed transition in strain hardening from stage I to stage II in single crystals oriented for prismatic slip. Good agreement is found between the experimentally observed and simulated stress-strain behavior.
Physical Review B, 1990
The dynamic organization of dislocations into spatially heterogeneous substructures is demonstrated by applying the principles of dislocation dynamics that were outlined in the preceding paper. Here it is shown that the formation of persistent slip bands is a consequence of the competition between dipole formation and annihilation of dislocations of opposite Burgers vectors in the absence of temperature-enhanced climb recovery under cyclic stress conditions. Planar arrays, which are also uniaxial structures, are shown to arise from enhanced dislocation multiplication and the formation of stable dipole configurations along a slip plane at lower temperatures where climb is unimportant. Biaxial dislocation systems experience additional degrees of freedom compared with uniaxial systems because of available motion along additional slip systems. It is demonstrated that for a systern of orthogonal slip directions at high temperatures in which climb and glide motion are competitive, dislocation cellular structures form as a result of immobile dipole and junction formation and by the internal elastic strain energy minimization caused by long-range self-shielding. It is shown that the internal elastic strain energy is reduced by the self-organization process. However, the short-range nonlinear processes (i.e. , dipole and junction formation) are shown not to allow absolute elastic energy minimization.
Acta Materialia, 1998
ÐThe eect of prism slip on cyclic hardening and deformation substructure in Ti 3 Al single crystals cyclically deformed was examined. Fatigue tests were carried out in a symmetrical tension/compression mode at a ®xed total strain amplitude (De) of 20.2% to 20.4% at room temperature. As De and the number of cycles increased, the stress amplitude rose depending on crystal orientation. The cyclic hardening was more accelerated for specimens deformed by double prism slips than for those deformed by single prism slip. At the saturated stage, the edge dipoles and/or multipoles gathered at a localized region and formed bundles with a high residual stress ®eld in specimens deforming by double prism slips, while a high density of dislocations existed the bundled structure did not develop under operation of single prism slip. The peculiar structure containing bundles was called saturated bundled structure (SBS). The formation process of the SBS and the role of the bundled structure on the plastic behaviour are described.
Mobility of grain boundary dislocations during the conservative untwisting of
Physical review. B, Condensed matter, 1996
We modeled the mobility of grain boundary dislocations ͑GBD's͒ during the untwisting of the ͓001͔ twist boundaries. Instead of assuming two semi-infinite crystals in calculating the grain boundary energy ͑i.e., the Read-Shockley approach͒ and therefore the driving force for untwisting, we assume equally spaced GBD's moving in the ͑001͒ boundary plane with the dislocations closest to the surface being pulled out by the image force. Experimental results from crystallite rotation in fcc gold were used to investigate the mobility of the GBD's. Two types of GBD motion were tested: viscous and thermally activated. The observed motions of the GBD's during untwisting can be described only as thermally activated. The Hirth-Lothe approach, which involves a thermally activated process overcoming the Peierls barrier, was applied to describe the mobility of GBD's during untwisting into the ⌺5 cusp/minimum ͑⌺ is the reciprocal of the density of the lattice sites in coincidence between two lattices at a misorientation͒ and the mobility of lattice dislocations ͕100͖ ͗110͘ during untwisting into the ⌺1 cusp/minimum. The Peierls barrier for GBD motion confined to the glide plane of the boundary ͑001͒ is significantly higher than that for lattice dislocations glide on ͕111͖ planes. From the untwisting rates, we estimate the energy barriers for GBD motions as 1.69 eV for ⌺1 and 1.84 eV for ⌺5 ͓001͔ twist boundaries. These results can explain the high yield stress and its sharp temperature dependence during plastic deformation of nanoparticle compacts of fcc metals. These results can also be used to estimate the largest size of crystallites that will rotate.
Acta Materialia 54 (2006) 2181
"We suggest a dislocation based constitutive model to incorporate the mechanical interaction between mobile dislocations and grain boundaries into a crystal plasticity finite element framework. The approach is based on the introduction of an additional activation energy into the rate equation for mobile dislocations in the vicinity of grain boundaries. The energy barrier is derived by using a geometrical model for thermally activated dislocation penetration events through grain boundaries. The model takes full account of the geometry of the grain boundaries and of the Schmid factors of the critically stressed incoming and outgoing slip systems and is formulated as a vectorial conservation law. The new model is applied to the case of 50% (frictionless) simple shear deformation of Al bicrystals with either a small, medium, or large angle grain boundary parallel to the shear plane. The simulations are in excellent agreement with the experiments in terms of the von Mises equivalent strain distributions and textures. The study reveals that the incorporation of the misorientation alone is not sufficient to describe the influence of grain boundaries on polycrystal micro-mechanics. We observe three mechanisms which jointly entail pronounced local hardening in front of grain boundaries (and other interfaces) beyond the classical kinematic hardening effect which is automatically included in all crystal plasticity finite element models owing to the change in the Schmid factor across grain boundaries. These are the accumulation of geometrically necessary dislocations (dynamic effect; see [Ma A, Roters F, Raabe D. A dislocation density based constitutive model for crystal plasticity FEM including geometrically necessary dislocations. Acta Mater 2006;58:2169–79]), the resistance against slip penetration (dynamic effect; this paper), and the change in the orientation spread (kinematic effect; this paper) in the vicinity of grain boundaries."
Some correlations between slip band emergence and dislocation pattern
IOP Conference Series: Materials Science and Engineering, 2009
Various forms of the plastic deformation in single crystals are studied on pure nickel and nickel alloys oriented for single slip [135] and multiple slip [001]. Particular attention is paid to the heterogeneity of deformation observed at two distinct scales: the slip bands and the dislocation organizations. The slip bands emerging at the surface can be studied using the atomic force microscopy (AFM). The height of extrusions and inter-band spacing depends on the orientation of tensile axis, the strain level and the nature of the alloy. At another scale, dislocation organizations typical of f.c.c. crystal have been observed, which depend on the orientation of tensile axis and on the stacking fault energy. A study by transmission electronic microscopy (TEM) has enabled us to approach the dimensional characteristics of these structures. In the case of mono-crystal oriented for single slip strained in stage III (γ ≈ 0.8) we observed a correlation between the inter-band spacing (d) and the inter-wall spacing (λ) of the type I dislocation pattern. This result suggests that this kind of walls act as a screen to the mobility of dislocations unlike equiaxed cells that would be only an obstacle to the dislocation mobility. This internal length is lower for Ni16%Cr alloy than for nickel. Consequently, stacking fault energy is probably a parameter which affects the internal length in relation with cross-slip capability. On the other hand, results, obtained of the [001] direction in nickel, are more complex due to multiple slip. Indeed, only equiaxed cells are observed for this orientation with cell size magnitude (λ) far lower than those observed for inter-band spacing (d). As in the case of samples oriented for single-slip, the equiaxed cells observed for samples oriented for multiple-slip seem to be only obstacles to the mobility of dislocations. However, there are probably walls associated with this kind of cells which act as barriers to the movement of dislocations.