Annihilation Of Screw Dislocations Research Articles

Stress effects on interaction modes and cross-slip annihilation distance of screw dislocation interactions

Phase field microelasticity model is used to study the cross-slip and annihilation of screw dislocations in face-centered cubic crystals, revealing the non-negligible effects of Escaig stress on dislocation interactions. We find that the Escaig stress (in the direction that expands the stacking fault width) promotes the partial dislocation interactions and the Fleischer cross-slip mechanism. In addition to three fundamental dislocation interaction modes, three new dislocation interaction modes emerge with the formation of parallel stacking fault bands. Furthermore, with the increasing of Escaig stress, the critical annihilation distance in dislocation interactions can be appropriately reduced. This research provides novel insights for manipulating strain hardening and understanding the formation of dislocation patterns.

Quantifying internal strains, stresses, and dislocation density in additively manufactured AlSi10Mg during loading-unloading-reloading deformation

The plastic deformation of the AlSi10Mg alloy manufactured via laser powder bed fusion (LPBF) is incompatible at the microscale, which causes residual strains/stresses and dislocation pile-ups at the Al/Si interfaces and grain boundaries. Hence, it is of fundamental significance to clarify these microscopic properties during plastic deformation. Here, in-situ neutron diffraction is employed to explore the residual strains, stresses, and dislocation density in the LPBF AlSi10Mg during loading-unloading-reloading deformation. It is found that the maximum residual stresses of the Al and Si phases in the loading direction reach up to about −115 (compressive) and 832 (tensile) MPa, respectively. A notable dislocation annihilation phenomenon is observed in the Al matrix: the dislocation density decreases significantly during unloading stages, and the amplitude of this reduction increases after experiencing a larger plastic deformation. At the macroscale, this dislocation annihilation phenomenon is associated with the reverse strain after unloading. At the microscale, the annihilation phenomenon is driven by the compressive residual stress in the Al matrix. Meanwhile, the annihilation of screw dislocations during unloading stages contributes to the reduction in total dislocation density.

Internal stresses and dislocation dynamics in cyclic plasticity and fatigue of metals

Abstract Theories of the role of internal stresses in cyclic plasticity and in the origin of fatigue cracks are discussed in the light of a quantitative phenomenological characterization of the shapes of hysteresis loops and recent advances in atomistic modelling of cross slip and annihilation of screw dislocations. Part of the phenomenological ‘back stress’ derived from hysteresis loop shapes by the Cottrell procedure can be substantiated as a real stress associated with lattice strains. According to the present ‘static-dynamic’ theory this stress can destabilize accumulated edge dipole arrays. Subsequent recovery by annihilation of screw dipoles with atomistically calculated heights then accounts quantitatively for the observed slip heterogeneity. Brown’s theory of homogeneously slipping PSBs with a surface stress singularity makes no indications of the observed loop shapes or PSB slip heterogeneity. The implications for fatigue of industrial materials are discussed.

Influence of surface treatment on creep behaviour of NiAl single crystals

Abstract Stoichiometric [110] oriented NiAl single crystals were deformed by compression creep tests at 300 °C up to 3.6%. To ascertain the role of surface conditions, deformation samples with mechanically polished, rough and oxidised surfaces were used. The polished samples were most easy to deform. An oxide layer of about 600 nm thickness did not give rise to softening due to poor oxide adherence. The dislocation substructure in the bulk and near the surfaces was investigated by TEM. The different arrangements were discussed with regard to dislocation generation at the surfaces, cross-slip and mutual annihilation of screw dislocations and edge dislocation trapping to form dipoles.

Formation of antiphase-domain boundary tubes in B2 ordered alloys by cross-slip and annihilation of screw dislocations

Abstract A detailed analysis of the cross-slip and annihilation of screw dislocations in B2 ordered alloys is carried out using anisotropic elasticity. The paths of the partial dislocations are calculated and it is shown that antiphase-domain boundary tubes with complex structures may be formed by the cross-slip and annihilation of screw dislocation dipoles with a suitable range of heights (e.g. for Fe-30 at.% A1 alloy from 70 to 200 A). APB tubes thus formed usually have a height of a few multiples of the elementary jog height, and widths of 7–50A in agreement with experiment. Image calculations (including the effects of the limited aperture, defocus and the lens aberration) for APB tubes with a complex cross-section give a good match with experimental results. The analysis also explains why APB tubes of such small heights are generally connected to edge dislocation dipoles of much larger heights.

Experimental characterization of surface film softening in iron: Effects of prestraining, temperature and orientation

Surface film softening was studied in nickel-plated iron single crystals by varying the amount of prestraining at room temperature, the test temperature and the tensile axis orientation. The combination of nickel plating and prestraining reduced the yield stress to as much as 40% of that for the control specimen and increased the uniform elongation by a factor of 2. The softening effect was largest at 4.2 K, decreased with temperature and disappeared at room temperature. The softening effect was most prominent in the specimens oriented in the [110] direction and was not found in the specimens oriented near [100]. In view of these experimental results the mechanism for surface film softening is examined. The annihilation of screw dislocations by cross-slipping which sustains the massive generation of edge dislocations after the initiation of cracks in the nickel films appears to determine the yield stress of the nickel-plated iron specimens.

Segmented helices in high purity aluminium

Abstract A high density of helical dislocations has been observed in zone-refined aluminium. The specific conditions of quenching and deformation required to produce this result indicate that helices are not normally seen in pure metals, partly because of the presence of effective competing sinks and partly because of the annihilation of screw dislocations by cross-slip during plastic deformation. The helices are not circular helices but are approximately the geodesics on a right prism. The base of the prism is a rhombus, the generator is the Burgers vector b and the feces are the (111) planes containing b. This result is explained as an effect of dislocation core energy, the core energy being lowest if the dislocation lies in a {111} plane containing b, i.e. a glide plane.