Prismatic Dislocation Research Articles

Sequential transmutation of prismatic dislocations during [formula omitted] twin-slip interaction in titanium

Atomistic simulations are performed to investigate the interaction between prismatic dislocations and {112¯2} twin boundary in hexagonal close-packed titanium. Very unusual and interesting dislocation transmutation is observed. When a (101¯0)13[1¯21¯0] dislocation interacts with the twin boundary, it is first transmuted into a (011¯2)13[21¯1¯0] dislocation in the twin. But the (011¯2) is not a common slip plane and the transmuted dislocation is unstable which is then further transmuted into a prismatic dislocation (011¯0)13[21¯1¯0] in the twin with a lower line energy. This behavior, along with the transmutations of other prismatic dislocations, can be understood from the perspective of lattice correspondence in deformation twinning.

Hardening-softening of Al0.3CoCrFeNi high-entropy alloy under nanoindentation

The competition and balance mechanism between work hardening resulting from the surge in dislocations and material softening caused by plastic deformation during contact loading in Al0.3CoCrFeNi high-entropy alloy is unclear. The structural transformation and strain localization of Al0.3CoCrFeNi high-entropy alloy during nanoindentation are investigated using molecular dynamics, and the hardening-softening mechanism is discussed. The simulations demonstrate that the prismatic dislocation loop with independent nucleation is discovered for the first time in the [111] orientation. Dislocation multiplication and cross-slip lead to an increase in indentation resistance, resulting in work hardening. Free dislocation slip and dislocation annihilation accommodate plastic strain will reduce indentation resistance, resulting in plastic softening. Twin boundaries can effectively block dislocation propagation, which contributes to hardening. However, twin boundaries cause a softening effect in the later stage of plastic deformation owing to two reasons: (1) the formation of steps and the partial slip where the slip plane and Burgers vector are parallel to the twin boundary, (2) steps and local damage zones in twin boundaries become new nucleation sites for dislocations. The phase-field-crystal method confirms that Al0.3CoCrFeNi high-entropy alloy has the highest hardness when the twin spacing is 2.467 nm.

Characterization of Prismatic Slip in SiC Crystals by Chemical Etching Method

In 4H-silicon carbide crystals, basal plane slip is the predominant deformation mechanism. However, prismatic slip is often observed in single crystals grown by the physical vapor transport method as the diameter expands to 6 inches or larger. Thermal modeling has shown that occurrence of prismatic slip is attributed to increased radial thermal gradients. While X-ray topography can be used to characterize the presence and extent of prismatic slip, the feasibility of using the chemical etching method to assess the extent of prismatic slip in an industrial setting is investigated. The distribution of scallop shaped etch pits oriented along the directions that correspond to prismatic dislocations, correlate well with the results of the thermal model that predicts the occurrence of prismatic slip dislocations. This capability of the etch pit method to characterize prismatic slip can be used to manage radial thermal gradients during PVT growth.

Compact A15 Frank-Kasper nano-phases at the origin of dislocation loops in face-centred cubic metals

It is generally considered that the elementary building blocks of defects in face-centred cubic (fcc) metals, e.g., interstitial dumbbells, coalesce directly into ever larger 2D dislocation loops, implying a continuous coarsening process. Here, we reveal that, prior to the formation of dislocation loops, interstitial atoms in fcc metals cluster into compact 3D inclusions of A15 Frank-Kasper phase. After reaching the critical size, A15 nano-phase inclusions act as a source of prismatic or faulted dislocation loops, dependent on the energy landscape of the host material. Using cutting-edge atomistic simulations we demonstrate this scenario in Al, Cu, and Ni. Our results explain the enigmatic 3D cluster structures observed in experiments combining diffuse X-ray scattering and resistivity recovery. Formation of compact nano-phase inclusions in fcc structure, along with previous observations in bcc structure, suggests that the fundamental mechanisms of interstitial defect formation are more complex than historically assumed and require a general revision. Interstitial-mediated formation of compact 3D precipitates can be a generic phenomenon, which should be further explored in systems with different crystallographic lattices.

Strain Localisation and Fracture of Nuclear Reactor Core Materials

The production of prismatic dislocation loops in nuclear reactor core materials results in hardening because the loops impede dislocation motion. Yielding often occurs by a localised clearing of the loops through interactions with gliding dislocations called channeling. The cleared channels represent a softer material within which most of the subsequent deformation is localized. Channeling is often associated with hypothetical dislocation pileup and intergranular cracking in reactor components although the channels themselves do not amplify stress as one would expect from a pileup. The channels are often similar in appearance to twins leading to the possibility that twins are sometimes mistakenly identified as channels. Neither twins nor dislocation channels, which are bulk shears, produce the same stress conditions as a pileup on a single plane. At high doses, when cavities are produced (either He-stabilised bubbles at low temperatures or voids at high temperatures), there can be reduced ductility because the material is already in an equivalent advanced stage of microscopic necking. He-stabilised cavities form preferentially on grain boundaries and at precipitate or incoherent twin/ε-martensite interfaces. The higher planar density of the cavities, coupled with the incompatibility at the interface, results in a preferential failure known as He embrittlement. Strain localisation and inter- or intragranular failure are dependent on many factors that are ultimately microstructural in nature. The mechanisms are described and discussed in relation to reactor core materials.

Dislocation climb driven by lattice diffusion and core diffusion

Diffusion of material has a crucial influence on dislocation motion, particularly at elevated temperatures. It is generally believed that, in a single crystal, lattice diffusion prevails when the temperature is high and core diffusion dominates at relatively low temperatures. Due to the complexity of modeling the coupling between core and lattice diffusion, a given physical problem is often simplified into two extremes where only one of the two diffusion regimes is considered. However, a quantitative definition of the conditions under which each of the diffusion mechanisms is dominant is still lacking. In the present work, we employ a variational principle for the analysis of microstructure evolution; we demonstrated how finite element (FE) based analysis can be developed from it, in which the competition and synergy between core diffusion and lattice diffusion can be naturally taken into consideration. A dislocation climb model is further developed by incorporating the FE analysis into the nodal based three-dimensional dislocation dynamics framework, which also considers glide and cross-slip processes.A systematic study of the coalescence of prismatic dislocation loops (PDLs) at various conditions is conducted based on the proposed method; together with the analytical solutions of the motion of a circular PDL controlled by core and lattice diffusion, a diffusion mechanism map is constructed, which provides useful guidance on determining the dominant diffusion mechanism for given loop sizes, spacing, and temperature. The results show that, in a practical loop coarsening process, core diffusion provides a fast short circuit for local atomic rearrangement, so that it is dominant when loop size or the distance between loops is small, particularly at temperatures lower than 0.5Tm (Tm is the melting point of a given material). While, at high temperatures, when the distance between loops is large or when the loop size is large, lattice diffusion becomes more efficient. The present findings indicate that simultaneous consideration of both core and lattice diffusion is necessary to quantitatively understand the microstructure evolution for dislocation climb related physical processes, such as creep and post-irradiation annealing.

Transition between a nano-sized prismatic dislocation loop and vacancy cluster in α-iron: An atomic scale study

Plastic deformation and the presence of hydrogen enhance the concentration of vacancy-type defects in α-iron, wherein monovacancies can accumulate as a planar vacancy cluster (VC). The planar VC can then be nucleated into a corresponding prismatic dislocation loop (PDL), which can also be directly generated through cross-slip mechanisms of a screw dislocation. Inversely, a PDL can be converted to a corresponding VC. High densities of VCs and PDLs can significantly influence the mechanical properties of iron-based materials. Here, a quantitative energy barrier is established for the transition between nano-sized 1/2 〈111〉 PDLs and VCs using the nudged elastic band method for different applied strains. It was concluded that a cluster size of more than 19 vacancies can easily nucleate into a PDL if a VC with planar configuration is formed in α-iron. Further, the nano-sized VC to PDL transition is enhanced under compressive strain while the nano-sized PDL to VC transition is enhanced under tensile strain. This result indicates that nano-sized VCs generated by tensile deformation are converted to a PDL after unloading and diffusing away. Inversely, nano-sized PDLs that form by deformation can easily diffuse to high tensile stress regions near stress singularities, such as crack tips and triple-junctions of grain boundaries, and then accumulate as high-density VCs.

BIAS FOR PRISMATIC DISLOCATION LOOPS IN ZIRCONIUM. NUMERICAL ANALYSIS

Using the analytical expression for the energy of elastic interaction of radiation point defects with a prismatic edge dislocation loop of zirconium (Burgers vector bD 1/ 3 <1120>, {11 2 0 }occurrence plane), the bias of loops of different nature (vacancy and interstitial) was calculated by the finite difference method. The toroidal geometry of the reservoir was used. It allowed one to calculate biases for loops of any size without any correction of the elastic field in its area of influence. In the dilatation center approximation the dependences of the loop bias on the loop radius were obtained. The principal possibility of coexistence of loops of different nature in the prismatic plane of zirconium is shown. A qualitative concept of the radiation growth (RG) mechanism was formulated within the framework of the classical elastic ideology.

Exploring the basal/prismatic slip transfer at grain boundaries in magnesium: A molecular dynamic simulation

This study utilized the molecular dynamics method to investigate the slip transfer behaviors of edge and screw dislocations in magnesium at basal/prismatic grain boundaries (BPGBs), where the Burgers vectors and the slip planes are parallel across the GBs. The results demonstrate that when shear stress increases, the initially introduced basal dislocation migrates toward, interacts with a BPGB, and is transmuted to a prismatic dislocation and vice versa. The critical resolved shear stress for the basal screw dislocation transmutation is considerably higher than edge dislocation. The slip transfer from basal to prismatic slip has a higher resistance than the slip transfer from prismatic to basal slip. The influence of normal strain perpendicular to the slip plane and temperature was also analyzed to better understand the process of the interactions. Moreover, as the tensile strain increases, the basal to prismatic slip transfer is more easily facilitated.

A novel strengthening mechanism in crystalline/amorphous dual-phase Mg alloys: A molecular dynamics study

The crystalline/amorphous dual-phase structure is a novel and advanced strategy to improve the mechanical properties of Mg alloys. Here, molecular dynamics simulation is employed to investigate the effect of the amorphous nanopillar size and the content of rare earth element Y on the interaction mechanism between prismatic 〈a〉 dislocation and amorphous nanopillar in the Mg alloys. The results show that the strengthening effect of amorphous nanopillar on the Mg alloys is significantly dependent on the size of the amorphous nanopillar. It is worth noting that an unconventional mechanism caused by extended dislocations appears when dislocation interacts with amorphous nanopillar, which is different from the traditional shear mechanism and Orowan mechanism of the interaction between dislocation and crystalline precipitates. The results also indicate that Y atoms can increase the difficulty of the 〈a〉 basal slip, thus preventing the perfect prismatic dislocation from decomposing into extended dislocation on basal plane.

Early-Stage Dislocation Structures inside the Dislocation Channels of Face-Centered-Cubic Metals with Point Defect Clusters

Early-stage dislocation structures inside dislocation channels of rapid-cooled and tensile-deformed aluminum single crystals were investigated by using the bright field imaging mode in a scanning transmission electron microscope (STEM-BF). Inside dislocation channels, arrays of the prismatic dislocation loops originated from dislocations of the primary slip system, i.e., , were mainly formed. Dislocations of the primary coplanar slip systems such as and were activated owing to internal stresses caused by the primary dislocations pile-up inside the cleared channels. The activated primary coplanar dislocations left the dislocation loops elongating along the edge dislocation directions behind them. Inter-dislocation-loop interactions occur especially at the arrays of the prismatic dislocation loops originated from dislocations of the primary slip systems and produce “butterfly shape” dislocation loops. As the “butterfly shape” dislocation loops have “sessile” junctions, they should act as “obstacles” against the following multiplications and glides of the dislocations. When the interactions proceed, “tangled structures” would be formed around the arrays of the prismatic dislocation loops originated from dislocations of the primary slip system.

X-ray diffraction line broadening and radiation damage

The diffraction line profiles of single crystal Zr and Zr-2.5Nb pressure tubing have been measured and analysed before and after irradiation in a nuclear reactor. The irradiation produces damage in the form of prismatic dislocation loops. In all cases, Fourier analysis of the deconvoluted peak using the method of Warren and Averbach shows that the magnitude of the distortion (micro-strain) component is not consistent with the dislocation density measured by transmission electron microscopy. The discrepancy can be related to the reduced strain energy from the dipoles on opposite sides of the prismatic dislocation loops and loop alignment, which reduces the average strain in the same way as polygonization or sub-grain boundary formation. The Fourier analysis also gives a value for the coherent diffraction domain size that is consistent with the mean spacing between the dislocations and thus the dislocation density.

Three-dimensional crystal plasticity and HR-EBSD analysis of the local stress-strain fields induced during twin propagation and thickening in magnesium alloys

Present work focuses on analysis of the stress and strain fields inside and around the individual {10–12} twin in magnesium alloy. The 3D crystal plasticity model represents twin as an ellipsoidal inclusion surrounded by the matrix. Five different twin thicknesses and three different lateral twin lengths are used for stress/strain analysis. The simulations are complemented with experimental observations using high-resolution electron backscattered diffraction. The simulations and experiments show a similar distribution of the shear stress and the spatial activity of individual slip systems (basal, prismatic, pyramidal). Plasticity induced inside the twin is dominantly caused by the prismatic dislocations slip and does not influence twin back stress which is identical to pure elastic twin. The twin with larger lateral dimension requires lower equilibrium stress which suggests anisotropic twin propagation and increased thickness of such twins. The lateral twin propagation is mostly influenced by prismatic and pyramidal slip in the twin vicinity. The twin thickness can reach a maximal level that is driven by the critical resolved shear stress values for dislocation slip with the significant influence of basal slip.

Microstructural evolution of Ti2ZrHfV0.5Mo0.2 refractory multi-principal alloy under low-energy, high-flux He ion irradiation

The irradiation response of refractory multi-principal element alloys (RMPEAs) has gained increasing attention for the potential applications in future fusion devices. In controlled fusion reactors, plasma-facing materials have to suffer continuous low-energy high flux helium (He) ion irradiations. During high flux He irradiation, vacancy-interstitial pairs can be generated through trap mutation mechanism even without cascade collision. To simplify the model, Ti2ZrHfV0.5Mo0.2 RMPEA was irradiated with 50 eV He+ (less than the minimum He displacement energy of the alloy EdTi = 67 eV) under 1200 K to various fluences of 5 × 1024 - 1 × 1026 ions/m2, and the microstructural evolution was investigated. In the irradiated RMPEA, the intense sputtering of Ti, the growth and degradation of fuzz mainly composed of Zr, Hf elements on the surface, as well as the formation of V, Mo enriched band and high density large He cavities in the subsurface layer were observed. Although the sputtering threshold energy for He of V is similar to that of Ti (ETi = 24 eV, EV = 29 eV), V showed anomalous enrichment in the subsurface layer. Moreover, unlike the W fuzz growth process, the adatoms used to form fuzz in Ti2ZrHfV0.5Mo0.2 were derived from the Zr, Hf atoms diffusing through the vacancy-solute exchange mechanism, rather than from the atoms formed by the loop-punching mechanism. The anomalous enrichment of V atoms and the different fuzz growth mechanism were attributed to (i) different migration rates for different species of atoms in the alloy under vacancy diffusion mechanism and (ii) the formation of AB2 (A = Ti, Zr, Hf; B = V, Mo) phase blocking the continued formation of prismatic dislocation loops mainly composed of V and Mo.

Modeling of critical resolved shear stress for interaction between screw dislocation and prismatic dislocation loop in alpha-iron

Modeling of critical resolved shear stress for interaction between screw dislocation and prismatic dislocation loop in alpha-iron

Coupled effect of Cr and Al on interactions between a prismatic interstitial dislocation loop and an edge dislocation line in Fe-Cr-Al alloy

Segregation of alloying elements to a prismatic dislocation loop under irradiation is an important phenomenon in understanding the role of loops in radiation effects in alloys. In this work, various segregation sequences of Cr and Al atoms to a 〈100〉 and a 1/2〈111〉prismatic dislocation loop in a Fe-Cr-Al alloy are explored using molecular dynamics method and ab initio energy calculation. The results reveal that Cr can readily segregate to the loops while Al atoms cannot by themselves, but the segregated Cr atoms are able to promote Al segregation at the loops, defined as a coupled segregation of Cr and Al. After interacting with Cr and Al, a previously mobile 1/2〈111〉 loop becomes sessile. Al segregation through the coupled segregation process can further affect the pinning behavior of a dislocation loop to an edge dislocation motion. By varying the Al fraction in the segregated solutes, a nonlinear dependence of the pinning effect on dislocation motion is determined. At lower fractions, the pinning strength increases with the increasing Al fraction until Al fraction reaches a critical value of about 19%, above which the pinning strength decreases with further increase of Al fraction. All these results suggest that, in addition to the effect of Al on the surface resistance to oxidation, Al effect on the dislocation loop behavior is important to optimize the composition of Fe-Cr-Al alloys for their applications in accident tolerant fuel concepts.

The role of deviatoric stress and dislocations on the α to ω phase transformation in Ti

Under extreme conditions, α-Ti becomes unstable and transforms either into β-Ti at high temperature or into ω-Ti at high pressure. In what concerns the α to ω phase transformation (PT), there has been a wide range of experimentally reported transition pressures from approximately 2 to 15 GPa at room temperature. Deviatoric stresses and internal defects are often assumed to be the root cause of this variation. In this study, these postulates are revisited using both continuum mechanics and molecular dynamics (MD) simulations. First, a simple continuum model, assuming linear elasticity and isotropic plasticity, is developed to describe the effects of applied stress and dislocations on the stability of an ω nucleus in an infinite α domain. Second, a new MD simulation method is developed to generate an ω nucleus in the α domain utilizing the displacement field identified from the topological analysis. Results from MD simulations show that despite the fact that phase diagrams typically delineate the limits between two phases in terms of only P and T, deviatoric stress promotes the α to ω phase transformation by reducing the critical radius above which an ω nucleus is stable. Furthermore, the required deviatoric stress to nucleate and stabilize a nanoscale ω nucleus is likely emanating from the internal stress of defects such as dislocations. The MD-informed micromechanics models are used to identify favorable configurations where dislocations help favor the α to ω transformation. These configurations show that the interaction with a basal or prismatic dislocation reduces the critical radius of a ω nucleus by about 10 or 16 %, respectively. In addition, prismatic edge dislocations are found to promote the growth of ω nucleus when interacting with the (1¯1¯20)α//(0001)ω interface. Importantly, a simple model of the arrival of dislocation at an ω nucleus suggests that PT does not necessarily require a pile-up to be present but could alternatively be mediated by a constant rapid flow of dislocations.

Micromechanical response of pure magnesium at different strain rate and temperature conditions: twin to slip and slip to twin transitions

The strain rate (ε˙) and temperature (T) dependent mechanical response of single crystal magnesium (Mg) micropillars compressed along the [2¯110] direction (a-axis) is investigated from room temperature to 573 K and from 10-3 to 100 s-1. The loading direction was chosen to disfavour basal slip activation by a low Schmid factor, allowing the investigation of the rate-sensitivities of extension twinning and prismatic slip. For T ≤ 423 K, the plasticity was governed by extension twinning for the entire range of the applied strain rates. At T > 423 K and for ε˙ ≤ 10 s-1, extension twinning did not occur and a continuous plastic flow induced by prismatic dislocation mediated plasticity was observed: the twin to slip transition takes place due to the decrease of the critical resolved shear stress of non-basal slip. For ε˙ > 10 s-1, however, the accommodation of the plastic deformation by activation of prismatic slip is not enough to match the applied deformation rate, favouring again deformation twinning. The first part of this work provides a complete overview of the mutual effects of T and ε˙ on the transition points of deformation modes in Mg at the microscale. In a second stage, the influence of thermal and kinetic contributions on the evolution of the flow stress leading to the slip to twin transition at 573 K has been assessed in more detail. Within the slip-dominated plasticity regime, this work provides a quantitative assessment of the increases in the saturation stress (stage III) with ε˙ at high temperature, showing how the strain rate dependency of the dislocation generation rate in the pillar and escape rate at the free surfaces of the structure controls the stress evolution in Mg microcrystals.

Dislocation nucleation mechanisms during nanoindentation of concentrated FeNiCr alloys: unveiling the effects of Cr through molecular simulations

Fe-based alloys with high chromium and nickel concentrations are very attractive for efficient energy production in extreme operating conditions. We perform molecular dynamics (MD) simulations of nanoindentation on fcc FeNiCr multicomponent materials. Equiatomic FeNi, Fe55Ni19Cr26, and Fe74Ni8Cr18 are tested by using established interatomic potentials and similar conditions, for the elucidation of key dislocation nucleation mechanisms and interactions. Generally, we find that the presence of Cr in these alloys reduces the mobility of prismatic dislocation loops, and increases their area, regardless of crystallographic orientation. Dislocation nucleation and evolution is tracked during mechanical testing as a function of nanoindentation strain and Kocks–Mecking continuum modeling displays good agreement with MD findings. Furthermore, the analysis of geometrically necessary dislocations (GNDs) is consistent with the Ma–Clarke’s model at depths lower than 1.5 nm. The presence of Cr leads to a decrease of the GND density with respect to Cr-less FeNi samples, thus we find that Cr is critically responsible of increasing these alloys’ hardness. Post-indentation impression maps indicate that Ni–Fe–Cr compositions display strain localization and hardening due to high Cr concentration.

A low-temperature prismatic slip instability in Mg understood using machine learning potentials

Prismatic slip in magnesium at temperatures T≲150 K occurs at ∼ 100 MPa independent of temperature, and jerky flow due to large prismatic dislocation glide distances is observed; this athermal regime is not understood. In contrast, the behavior at T≳150 K is understood to be governed by a thermally-activated double-cross-slip of the stable basal screw dislocation through an unstable or weakly metastable prism screw configuration and back to the basal screw. Here, a range of neural network potentials (NNPs) that are very similar for many properties of Mg including the basal-prism-basal cross-slip path and process, are shown to have an instability in prism slip at a potential-dependent critical stress. One NNP, NNP-77, has a critical instability stress in good agreement with experiments and also has basal-prism-basal transition path energies in very good agreement with DFT results, making it an excellent potential for understanding Mg prism slip. Full 3d simulations of the expansion of a prismatic loop using NNP-77 then also show a transition from cross-slip onto the basal plane at low stresses to prismatic loop expansion with no cross-slip at higher stresses, consistent with in-situ TEM observations. These results reveal (i) the origin and prediction of the observed unstable low-T prismatic slip in Mg and (ii) the critical use of machine-learning potentials to guide discovery and understanding of new important metallurgical behavior.