Unfaulting Of Loops Research Articles

A Combined Atomistic-Continuum Study on the Unfaulting of Single and Multi-layer Interstitial Dislocation Loops in Irradiated FCC and HCP Metals

Unfaulting of single and multi-layer interstitial loops in face centered cubic (FCC) and hexagonal close-packed (HCP) metals, with Ni, Al, Mg and Zr as model systems, have been comprehensively studied employing molecular dynamics simulations and continuum modeling. A versatile approach properly accounting for atom insertion/removal and displacement field was developed, able to correctly construct single and multi-layer interstitial loop structures of arbitrary geometries. The unfaulting mechanisms and corresponding dislocation reaction intermediates in different unfaulting scenarios were clarified. Critical conditions necessary for unfaulting have been identified, and an analytical model has been developed to accurately predict these conditions. Particularly for unfaulting in FCC materials, the role of Double Shockley (D-Shockley) partial in unfaulting has been explicitly elucidated, and the mechanisms of unfaulting initiating from periphery of the Frank loop and induced by a moving straight edge dislocation have been clarified. For unfaulting in HCP, despite certain similarity to its FCC counterpart, clear difference in unfaulting behaviors due to the HCP stacking sequence has been demonstrated. A comprehensive mapping of unfaulting routes for double-layer interstitial loops in HCP has been established to reveal the relationships between the various loop morphologies observed in experiments. The present work provides important mechanistic insights to address key knowledge deficits regarding interstitial Frank loop unfaulting, essential for better understanding of deformation and irradiation-induced failure in structural metals. The new modeling techniques and predictive tools offered are expected to be of great merits to not only the study of unfaulting of dislocation loops, but also future investigation of other complex dislocation or defect ensembles.

Comprehensive study of vacancy frank loop unfaulting: atomistic simulations and predictive model

In this study, vacancy Frank loop unfaulting via Shockley partial loop(s) in two model material systems (Al and Ni) has been investigated through comprehensive atomistic simulations combining continuum modeling. A general approach has been outlined and has been demonstrated to enable reliable construction of different dislocation loop configurations. The unfaulting of vacancy Frank loop is shown to occur when the enclosed Shockley loop exceeds a threshold size, independent of the Frank loop radius. A continuum model is then established to quantitatively predict the geometrical parameters of the Shockley loop and threshold condition for unfaulting. The continuum model prediction yields excellent agreement with simulation results, and has been shown to apply in various different unfaulting scenarios involving either single or multiple Shockley loops. Experimentally observed Butterfly and Mercedes hexagon morphologies have been reproduced from our simulations, with the underlying dislocation reactions fully clarified, and a mechanism of competitive growth identified as the cause for their subsequent transformation into normal prismatic dislocation loops. The present study offers generic and flexible computational tools towards understanding defect reactions and evolution mechanism of Frank loops, and more generally secondary defects in quenched and irradiated materials.

Dislocation loop evolution and radiation hardening in nickel-based concentrated solid solution alloys

Effects of chemical composition, ion irradiation dose and temperature on unfaulting of irradiation induced Frank dislocation loops to perfect loops in two nickel based single-phase solid solution alloys, Ni–20Fe and NiFe–20Cr, have been studied. The fraction of Frank loops decreases with irradiation dose from 7.2 to 38.4 dpa at 500°C, but with more Frank loops remaining in the ternary alloy. However, perfect loops and dislocation networks become the dominant features of defects at 580°C in both alloys. The results indicate a thermally assisted loop unfaulting process that may be hindered by more sluggish defect motion in the alloy with more chemical components. Nano-indentation with both continuous stiffness method and single indentation method are used to measure radiation hardening. Loop unfaulting in both alloys irradiated at 580°C reduced radiation hardening while significant hardening is observed after irradiation at 500°C. The quasi-static single indentation method exhibits lower hardness results compared to continuous stiffness method, because dislocations induced from the cyclic loading in the latter method get relaxed and stabilized, resulting in higher resistance to the indenter.

The effects of loop size on the unfaulting of Frank loops in heavy ion irradiation

When stainless steels are exposed to high irradiation dose, a fraction of Frank loops will unfault and transform into perfect loops. After unfaulting, perfect loops can glide and impinge into other loops to form dislocation networks, and dislocation networks will act as sinks for point defects and solute elements. Besides, unfaulting will influence the extent of irradiation hardening as well. Based on previous theories, the size of Frank loops was considered to be a key factor in unfaulting. To better understand the unfaulting process, 316L stainless steel model alloy was irradiated to 5dpa at 350 °C, 400 °C and 450 °C by 3 MeV Ni ions. Method was developed to quantitatively compare the size and density between Frank loops and perfect loops through relrod and weak-beam dark field techniques. Results showed that unfaulting barely occurred at 350 °C but was distinct at 400 °C and 450 °C. The majority of perfect loops were in the size range of 8–16 nm at 400 °C and were in the size range of 12–20 nm at 450 °C. The existence of a critical loop size for unfaulting was not supported by our results. Moreover, by our results, the energy of Frank loops does not play a decisive role in unfaulting, and the simple approximation of geometric constraint in the rate theory to calculate unfaulting diameter needs to be revisited. Irradiation hardening was also analyzed by nano-indention. It was found that loop unfaulting played an important role in the hardening decrease from 350 °C to 450 °C.

Molecular dynamics investigation of the interaction of an edge dislocation with Frank loops in Fe–Ni10–Cr20 alloy

The inhibition of dislocations motion by irradiation-induced defects, such as dislocation loops, is one of the main mechanisms of irradiation hardening of austenitic stainless steels. In this work, Molecular Dynamics (MD) simulations of interaction between an edge dislocation and Frank loops in Fe–Ni10–Cr20 ternary alloy mimicking austenitic stainless steels are carried out to investigate and model dislocation behavior. An empirical interatomic potential developed recently for a ternary FeNiCr system is used for the MD calculations. The interactions are calculated at different temperatures, loop orientations, loop size and solute atom configurations. The results show that the loop strength and the interaction processes depend on the solute atom configuration, the geometrical configurations between the dislocation and the loop and temperature. It is also demonstrated that a small Frank loop is not so weak an obstacle in the alloy. The interaction leads microstructural change such as loop shearing, loop unfaulting and loop absorption in the dislocation. In the former two cases, the loop remains after the interaction, however in some cases an absorption of the remaining loop by subsequent interactions with successive dislocations is observed.

Interaction of a screw dislocation with Frank loops in Fe–10Ni–20Cr alloy

The interaction of 2 and 5nm Frank loops with a moving screw dislocation is studied in Fe–10Ni–20Cr alloy (a model of austenitic 304 and 316 steels) employing the newly developed Fe–Ni–Cr interatomic potential in molecular dynamics simulations. The applied potential ensures full stability of FCC phase and smooth evolution of stacking fault energy (SFE) as a function of chemical composition, fitted to be in a close agreement with the CALPHAD database. A model of Fe–10Ni–20Cr random alloy closely reproduces elastic properties and SFE of 316-type austenitic steels. The results reveal a number of interaction mechanisms depending on loop orientation and ambient temperature. Half of the observed reactions lead to loop unfaulting despite a low SFE of the alloy. The unfaulting reactions are enhanced with temperature and the critical stress for the unfaulting is regularly higher in comparison with the loop shear interaction. By comparing present results with a recent study done in a low SFE Fe–50Ni alloy, we reveal that a magnitude of local variation of SFE is an important factor controlling the formation of dislocation constrictions. In the Fe–50Ni alloy, characterized by strong variations of local SFE, the constrictions are almost never observed so that the loop shear interaction prevails, while absorption is rare. In the Fe–10Ni–20Cr alloy, characterized by small variations of local SFE, the constrictions are regularly formed resulting in frequent loop unfaulting.

Interaction of dislocations with Frank loops in Fe–Ni alloys and pure Ni: An MD study

Variation of the stacking fault energy in FCC alloys and austenitic steels is well known to influence the evolution of radiation damage and its effect on deformation mechanisms. The primary defects observed in austenitic steels under neutron irradiation are mainly Frank loops. Here, we study the interaction of edge and screw dislocations with Frank loops in low stacking fault energy Fe–Ni alloys. The interatomic potentials employed were specially developed to reproduce a number of properties of real austenitic steels. The influence of temperature and loop morphology on the interaction mechanism and the critical resolved shear stress for dislocations to overcome loops has been investigated. All investigated reactions have been subdivided into three classes depending on temperature, loop size and interaction geometry. It is shown that by decreasing stacking fault energy below a certain value the formation of constrictions on dislocations is suppressed so that loop unfaulting becomes a less favorable mechanism in comparison with loop shear. Additional effect of solid-solution alloying, causing a non-negligible friction stress, is expressed in the impedance of the propagation of dislocations in the secondary glide planes, which is another factor limiting the unfaulting process.

Transition pathways in the unfaulting of dislocation loops

In order to study the dynamic mechanism of loop unfaulting, we performed large-scale classical molecular dynamics simulations involving computational cells with several millions of atoms. To induce dislocation loop unfaulting, we launched 1 ps duration traction stress pulses at a free surface of the computational box. In many cases, we observe unfaulting to involve both intuitive and complex dislocation processes with multiple Shockley partial dislocations. However, in some instances, we observe unfaulting to occur by a sudden instability of the stacking fault without clear traces of dislocation reactions.

Localized deformation and hardening in irradiated metals: Three-dimensional discrete dislocation dynamics simulations

When irradiated, metals undergo significant internal damage accumulation and degradation of mechanical properties. Damage takes the form of a high number density of nanosize defect clusters (stacking-fault tetrahedrons (SFTs) or interstitial loops). The alteration of mechanical properties is manifested in a hardening behavior and localized plastic deformation in defect-free channels. This work uses discrete dislocation dynamics (DD) to capture these effects. It sets the framework for the elastic interaction between gliding dislocations and defect clusters and details a scheme for loop unfaulting and absorption into dislocations. Here, it is shown that SFTs represents weaker pinning points for dislocation motion than parent dislocation loops. It is also shown that appreciable yield drop can be attributed to high density of defects decorating the dislocations. Strong obstacles cause dislocations in Cu to continually double cross slip causing the formation of defect-free channels. Finally, the correlation between yield stress increase and defect number density is in excellent agreement with the experiment.

Role of solute atoms on microstructural evolution in neutron irradiated nickel

Fission reactor irradiation of nickel alloys with solutes of widely varying volume size factors 2 at% of Si, Cu, Ge and Sn, was performed for a wide range of irradiation temperature. At a lower temperature 473 K, a high density of small vacancy clusters are mixed with interstitial-type dislocation loops in all the alloys. At higher temperatures, in the alloys with solutes of negative volume size factor, Si, and extremely large positive factor, Sn, the unfaulting of interstitial-type dislocation loops during irradiation is suppressed, and the development of dislocation structures is moderate. In the alloys with solutes of positive but not large volume size factor, Cu and Ge, including pure Ni, unfaulting of loops strongly promotes the development of dislocation structures. As a consequence of the difference in these dislocation structure developments, void formation is almost completely suppressed in the first category and is enhanced strongly in the second.

Dislocation/obstacle interactions during low dose irradiation creep of nickel

Analyses of interactions between gliding dislocations and irradiation induced obstacles indicated that loop unfaulting, loop annihilation, and loop sweeping can control low dose irradiation creep in fcc metals. Dislocation reactions remove glide barriers from active slip planes and rationalize rapid creep rates observed in deuteron irradiated nickel. Irradiation creep experiments revealed that pre-irradiation reduced irradiation creep rates in annealed Ni whereas preirradiation had little effect on creep rates in cold-worked Ni. Dislocation/obstacle interactions and the role of the applied stress in barrier removal processes were related to observed irradiation creep responses.