Formation Of Prismatic Loops Research Articles

Insights into Prismatic Loop Formation in Irradiated Fe–Cr Alloys from Hypothesis-Driven Active Learning and Causal Analysis

Insights into Prismatic Loop Formation in Irradiated Fe–Cr Alloys from Hypothesis-Driven Active Learning and Causal Analysis

In-situ observation of ‘chemical’ strengthening induced by compositional fluctuations in Nb-Mo-Ta-W

The mechanical response of refractory high-entropy alloys (RHEA) can be greatly influenced by local chemical fluctuations. Here, we perform in-situ transmission electron microscopy (TEM) nanocompression tests at room temperature of body-centered cubic Nb-Mo-Ta-W RHEA single-crystal nanopillars to understand the reasons behind this at atomic scale. We find that (i) chemical composition is not uniform in space, following certain oscillatory patterns and (ii) that specimen near-surface regions with local ‘soft’ spots favor dislocation nucleation over ‘hard’ ones. Under stress these dislocations glide inwardly, inevitably running into hard local regions adding hardening stresses of up to 400 MPa to the nominal yield stress. These hard local regions act as ‘chemical’ obstacles as revealed by the occurrence of double cross-slip and the formation of prismatic loops. Using electron dispersive spectroscopy along with atomic resolution TEM, we demonstrate that the obstacles correspond to atomic environments presumably rich in W surrounded by misfit edge dislocations.

An improved method to model dislocation self-climb

Dislocations can provide short circuit diffusion paths for atoms resulting in a dislocation climb motion referred to as self-climb. A variational principle is presented for the analysis of problems in which fast dislocation core diffusion is the dominant mechanism for material redistribution. The linear element based self-climb model, developed in our previous work [1] Liu, Cocks and Tarleton (2020 J. Mech. Phys. Solids 135 103783), is significantly accelerated here, by employing a new finite element discretisation method. The speed-up in computation enables us to use the self-climb model as an effective numerical technique to simulate emergent dislocation behaviour involving both self-climb and glide. The formation of prismatic loops from the break-up of different types of edge dislocation dipoles are investigated based on this new method. We demonstrate that edge dipoles sequentially pinch-off prismatic loops, rather than spontaneously breaking-up into a string of loops, to rapidly decrease the total dislocation energy.

Atomistic simulation study of influence of Al2O3–Al interface on dislocation interaction and prismatic loop formation during nano-indentation on Al2O3-coated aluminum

A large-scale molecular dynamics (MD) simulation of nano-indentation was carried out to provide insight into the influence of the Al-Al2O3 interface on dislocation evolution and deformation behavior of Al substrate coated with Al2O3 thin film. Adaptive common neighbor analysis (a-CNA), centro-symmetry parameter (CSP) estimation, and dislocation extraction algorithm (DXA) were implemented to represent structural evolution during nano-indentation deformation. The absence of elastic regime was observed in the P-h curve for this simulated nano-indentation test of Al2O3 thin film coated Al specimen. The displacement of oxygen atoms from Al2O3 to Al partly through the interface greatly influences the plastic deformation behavior of the specimen during nano-indentation. Prismatic dislocation loops, which are formed due to pinning of Shockley partials (1/6 < 112>) by Stair-rod (1/6 < 110>) and Hirth dislocation (1/3 < 001>), were observed in all cases studied in this work. Pile-up of atoms was also observed and the extent of the pile-up was found to vary with the test temperature. A distorted stacking fault tetrahedron (SFT) is formed when a nano-indentation test is carried out at 100K. The presence of a prismatic dislocation loop, SFT and dislocation forest caused strain hardening and, consequently, there is an increase in hardness as indentation depth increases. Graphical abstract Figure illustrates nano-indentation model set up along with load vs. depth curve and distorted stacking fault tetrahedron.

Molecular dynamics simulation in single crystal 3C-SiC under nanoindentation: Formation of prismatic loops

We investigate the formation mechanism of prismatic loops in a 3C-SiC single crystal subjected to nanoindentations on its (111) and (110) surface with a fictitious spherical indenter using molecular dynamics simulation. On the (111) surface, the simulated plastic deformation proceeds with the nucleation and expansion of shear loops, which react with each other to form prismatic loops eventually. Further analysis reveal that the prismatic loops form from one single shear loop or two independent shear loops. For the former, the two screw components of one shear loop undergo cross-slip and form a prismatic loop by the "lasso"-like mechanism. For the latter, two independent shear loops intersect and evolve into a prismatic loop by an extended "lasso"-like mechanism, which is also manifested in the nanoindentation on 3C-SiC (110). These results could provide a further understanding for plastic deformation in 3C-SiC nanocrystal materials.

Formation of prismatic loops in AlN and GaN under nanoindentation

The formation of prismatic loops in metals has been studied extensively, but the corresponding research related to ceramics can rarely be found in the literature. In this article, the formation of prismatic loops in B4 ceramics under nanoindentation were investigated with molecular dynamics (MD) simulations. The indentation directions are respectively in the normal of the basal plane and the two prismatic planes in AlN and GaN single crystals. Two mechanisms for the formation of prismatic loop in B4 ceramics are found. One is the “lasso”-like mechanism, which is similar to that observed in metals, and the other is “nested-loops” mechanism, which has never been reported in the literature. MD simulations show that the screw segments of the two different shear loops, be adjacent to each other, can intersect with each other and finally produce a prismatic dislocation loop. The pile-up symmetry after unloading, associated with dislocation loops propagation, can be found, which is consistent with experimental results. More detailed analysis of dislocation patterns and directions indicate that plastic deformation is dominated by the shuffle-set dislocations along 13〈112¯0〉, and the glide-set dislocations along 13〈1¯010〉 are asymmetric.

Formation of prismatic loops from C15 Laves phase interstitial clusters in body-centered cubic iron

This work reports the transformation of C15 Laves phase interstitial clusters to prismatic loops in body-centered cubic iron. Molecular dynamics simulations and elasticity analysis show that, within a certain size range, C15 clusters are more stable than loops, but the relative stabilities are reversed beyond this range. C15 clusters can consistently grow by absorbing interstitials at small sizes and transforming into loops later. Both 〈 1 0 0 〉 and 〈 1 1 1 〉 / 2 loops may result from the transformation, revealing a new formation mechanism for 〈 1 0 0 〉 loops.

Dislocation nucleation and defect formation in copper by stepped spherical indenter

We investigated dislocation nucleation and defect formation underneath a spherical indenter which possesses atomic steps on its surface. Atomic-scale simulations of Cu (111) nanoindentation were performed. Our simulation results reveal that dislocations nucleate from surface ledges formed by atomic steps on indenter surfaces. We found that stepped indenters promote concurrent activation of three inclined {111} planes, which leads to an increased probability of forming threefold symmetric defects and punching prismatic loops along threefold symmetric directions. A new junction structure was observed and found to unzip during the formation of prismatic loops. The formation and destruction of defect structures can be explained using a conventional theory of dislocation reactions.

Atomistic simulation of the deformation mechanism during nanoindentation of gamma titanium aluminide

In this paper we present a large-scale molecular dynamics simulation that describes the deformation mechanism of an ordered intermetallic compound (TiAl) during a nanoindentation procedure. Using a totally rigid spherical indenter we were able to address the question on which mechanism underlies its plastic deformation, namely homogeneous defect nucleation followed by the expansion of dislocation loops. By means of the calculated local pressure, local shear stress and spatial rearrangements of atoms beneath the indenter, it was possible to quantify the indentation damage on the crystalline structure. Our results show that both emission and interaction of dislocations are mediated by expansion of glide loops on the {111} planes resulting in the formation of prismatic loops. Moreover, through the load-penetration depth response we estimated the elastic modulus and hardness values of the system, which are in good agreement with experimental results.

Equilibrium and Dynamic Properties of Dislocation Dipoles

AbstractThis paper embraces several aspects related to dislocation self-organization in single slip. The formation of prismatic loops, seminal to the nucleation and growth of dislocation braids, is a dynamical process involving elastic dipolar interactions. Preliminary simulations of the concerted process of loop generation are presented. Elastic properties of dislocation dipoles are analysed as a function of dipole character. Equilibrium properties are investigated under the isotropic and anisotropic elastic approximations. The orientation dependence of the so-called dipole passing stress is examined, and its implication in the elementary mechanisms that take place under constrained deformation conditions, such as fatigue cycling, are discussed.

Dislocation Emission and Formation of Prismatic Loop in Single Crystalline Aluminum under Indentation (Simulations by Molecular Dynamics)

Recent new finding of unstable displacement burst observed in the relation between the indent load and indent depth of nanoindentation is much possible to be related to the collective dislocation emission. In the present paper, dislocation emission and prismatic dislocation loop formation of a single crystalline aluminum under nano-scaled indentation are simulated by the molecular dynamics. Effects of the stress distribution due to two different types of indenters and of the interatomic description by three different embedded atom type potentials are discussed with much emphasis. In result, the dislocations are emitted from the severely damaged surface atomic layer due to the indenter. Afterwards, the prismatic dislocation loops, which have the same gliding direction but not on the slip planes, are attained by both unstable reaction between the shear loops emitted from the surface and the cross slip mechanism.

Features of the plastic deformation of titanium alloys in impulse loading

1. An increase in the rate of impulse loading of VT1-0 α-titanium strengthens twinning in the {1123} planes but with a lower rate the material deforms both by slip in the (0001) plane and by twinning in the\(\{ 10\overline 1 2\} \) plane (with the formation of prismatic loops and cleaved dislocations). The described transition corresponds to deformation of the α-phase with a reduction in temperature, when the probability of cleavage of dislocations increases. 2. The results of tests of VT30 β-titanium alloy under conditions of impulse loading showed that an increase in the impulse loading rate, the same as for α-titanium, increases the probability of cleavage of dislocations. However, the structure of β-titanium is very sensitive to the presence of alloying elements in the alloy.

On the formation of prismatic loops in quenched F.C.C. metals and alloys

physica status solidi (a)Volume 41, Issue 1 p. K5-K8 Short Note On the formation of prismatic loops in quenched F.C.C. metals and alloys M. Condat, M. Condat Laboratoire de Métallurgie Structurale des Alliages Ordonnés, E.R.A. n° 221, E.N.S.C.P., Paris Search for more papers by this authorM. Fayard, M. Fayard Laboratoire de Métallurgie Structurale des Alliages Ordonnés, E.R.A. n° 221, E.N.S.C.P., Paris Search for more papers by this author M. Condat, M. Condat Laboratoire de Métallurgie Structurale des Alliages Ordonnés, E.R.A. n° 221, E.N.S.C.P., Paris Search for more papers by this authorM. Fayard, M. Fayard Laboratoire de Métallurgie Structurale des Alliages Ordonnés, E.R.A. n° 221, E.N.S.C.P., Paris Search for more papers by this author First published: 16 May 1977 https://doi.org/10.1002/pssa.2210410145Citations: 1 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume41, Issue116 May 1977Pages K5-K8 RelatedInformation

Yielding and Work-Hardening in Ni-ThO2Alloys

Theories of dispersion-phase-strengthening are briefly discussed and then critically examined in the light of experimental evidence obtained with nickel containing a fine dispersion of thoria. The results are in close agreement with Orowan's theory. The work-hardening theory of Fisher, Hart, and Pry, though in qualitative agreement with the experimental results, postulates the formation of concentric dislocation loops around particles of the dispersed phase. Transmission electron micrographs of the nickel-thoria indicate that the formation of prismatic loops near to particles is a more likely process, and this is borne out by analysis of the experimental results.

The behaviour of jogs in dislocations

Abstract It is shown that the glide motion of jogs in a screw dislocation traversing a dislocation forest is restricted by the presence of the forest and that these jogs tend to aggregate in cusps. A mechanism for the formation of prismatic loops during deformation is suggested.