Prismatic Dislocation Research Articles

Fracture mechanism of Zr2Si precipitate equilibrated in a solution-treated Si-modified Zircaloy-4

Secondary phase precipitates are vital to control corrosion, mechanical and irradiation response of multi-phase metallic alloys. As a model system, this work analysed hcp-Zr to fcc-Zr transformation-induced failure in a C16 type body-centred tetragonal Zr2Si binary precipitate using transmission electron microscopy and related techniques. The configurations of two types of dislocations, 〈001〉 {110} and 〈110〉 {001}, were approximated which interlocked along the major axis of Zr2Si precipitate constituting the sessile junctions. Threading dislocations resisted further glide, and failure occurred along the minor axis. Peierls stress and dislocation energy were estimated for the most favourable <001> {110} slip system and, in turn, probability of dislocation climb in Zr2Si precipitate was evaluated as a high temperature phenomenon, also evidenced by a prismatic dislocation loop along (11¯0) plane.

Synchrotron X-Ray Topography Studies for Defect Formation at the Early Stage of PVT-Grown 4H-SiC Crystals

Silicon Carbide (SiC), typically 4H-SiC, is replacing conventional silicon materials for high power and high frequency applications due to its excellent electrical and thermal properties [1]. However, high quality SiC wafers with low defect density are still in urgent need for automotive and energy saving applications. SiC wafers are obtained from bulk growth by physical vapor transport (PVT) method and considerable efforts have been expended to optimize growth process to lower defect densities. One major approach is the study of initial stages of crystal growth as most defects are nucleated at this stage and propagate into the boule. Ailihumaer et al [2] demonstrated the behaviors of threading edge dislocations (TEDs), threading screw dislocations (TSDs)/threading mixed dislocations (TMDs), and basal plane dislocations (BPDs) across the seed/newly grown layer interface in PVT-grown 4H-SiC. Sanchez et al [3] revealed the nucleation of TEDs and TSDs at the initial growth stage in PVT-grown SiC.However, more detailed investigations are still in demand for PVT grown 4H-SiC crystals during initial stage of growth. This study investigates 4° off-axis 4H-SiC wafers with several hundred microns of initial-stage growth PVT method. Synchrotron X-ray topography (SXRT) technique is employed and shows dramatically modified defect distribution across the seed/newly grown layer interface (Figure 1). The formation of Shockley stacking fault starting at initial stage of crystal growth is suggested as partial dislocations running along <11-20> directions on basal plane are observed to form rhombus shapes (Figure 2). Prismatic slip is observed to take place near the wafer edges and prismatic dislocations undergo cross slip to form unique dislocation configurations. A statistical analysis on defect changes across the seed/newly grown layer is being carried out and the results will be reported. These results will be complemented by observations from Nomarski optical microscopy (NOM) and Raman Spectroscopy studies to propose the formation mechanism of defects across the seed/newly grown layers.

A theoretical modelling of strengthening mechanism in graphene-metal nanolayered composites

Graphene-metal nanolayered composites (GMNCs) are a new generation of nano-structural composites characterized by a very high density of graphene reinforced interfaces (GRI) between metal nanolayers. Compared to traditional graphene flake reinforced composites, GMNCs have much higher strength, toughness and ductility due to the excellent ability of GRI on constraining dislocation motion and crack propagation. Despite numerous experimental and numerical studies on the mechanical behavior of GMNCs, the underlying strengthening mechanism is still not fully understood due to the absence of appropriate theoretical model. This paper proposes a continuum mechanics based theoretical model to explain the strengthening mechanism in GMNCs. In this model, the metal matrix and the GRI are simulated as homogenous elastic medium of infinite extend and inextensible thin membrane of zero thickness, respectively. Using the theoretical model, two boundary value problems namely (i) A circular prismatic dislocation loop approaching to the GRI and (ii) A mixed mode I/II penny-shaped crack near the GRI are formulated to reveal the two key strengthening mechanism: dislocation blocking and crack shielding, respectively. The two problems are solved analytically by the Generalized Kelvin's Solution (GKS) based method for 3D elasticity and Fredholm integral integration technique. Exact closed form solution for the Peach-Koehler (P-K) force on the dislocation loop is obtained. An efficient numerical scheme is developed to solve the Fredholm integral integration for the crack problem with very high accuracy. It is shown that our theoretical model can well capture and explain the strengthening mechanism observed in experiments. Moreover, the dominant role of Poisson's ratio on the strengthening efficiency is also revealed by our model. This finding implies the exciting possibility that the strength of GMNCs can be tailored by controlling the Poisson's ratio of the metal matrix. The present theoretical modeling can provide valuable insights into the mechanics-based design of GMNCs.

Material Removal Mechanism and Evolution of Subsurface Defects during Nanocutting of Monocrystalline Cu

Multigroup large-scalenanocutting models of monocrystalline Cu were established by molecular dynamics simulations to investigate the influence of cutting parameters on the material removal mechanism. The formation and distribution of subsurface defect structures were revealed, and the evolution behavior of the complete prismatic dislocation loop was analyzed in depth. It was demonstrated that the chips and machined surface of monocrystalline Cu were mainly formed under the coupling effect of shearing and extrusion forces. A diamond tool with a larger edge radius or a negative rake angle could produce a noticeable suppression on the chip formation. The corresponding relationship between the location of defect atoms and the distribution of von Mises stress was studied, which indicated that the shear stress would become larger at the subgrain boundaries, dislocation lines, and the amorphous atoms than that in their nearby regions. The complete prismatic dislocation loop was formed by cross-slip between two sets of stacking faults; meanwhile, the generated multiple Lomer–Cottrell locks hindered its movement and promoted the work-hardening phenomenon. These research results are of great theoretical value to enrich the nanocutting mechanism and technology of plastic materials.

Atomic study on deformation behavior and anisotropy effect of 3C-SiC under nanoindentation

3C silicon carbide (3C-SiC) has great potential and value for innovated high-precision devices. However, the high hardness make it a daunting task to understand its deformation behavior and anisotropy effect, which impedes its processing and application. In this study, we performed molecular dynamic simulations of nanoindentation to study the deformation behavior and anisotropy effect of 3C-SiC. It is found that the elastic deformation and incipient plastic deformation are orientation-dependent, exhibiting fourfold symmetry, central symmetry and threefold symmetry while indenting on 3C-SiC (010), (110) and (111) surfaces. The dislocations nucleate and propagate due to the high shear stress induced by indentation, which in turn leads to the dissipation of shear stress in the sample. In the plastic stage, the dislocations evolve from embryonic dislocations to ribbon dislocations, and finally to prismatic dislocations. The prismatic dislocation loops are surrounded by stacking faults in {111} planes, most of which are enclosed by four stair-rod dislocations in <110> orientations. Moreover, we find that Thompson tetrahedron is suitable for understanding the anisotropy effect of ceramics with cubic diamond structure, and it is applied to investigate the formation and evolution of dislocations systematically in 3C-SiC. This research is meaningful for understanding the deformation behavior and anisotropy effect of 3C-SiC. The results may provide theoretical support for the processing and application of 3C-SiC.

The impact of oxygen-clustering on the transformation of electrochemically-derived graphite oxide framework

Oxygen-clustering in graphene oxide is the activated process of diffusion of oxygen functionalities: hydroxyls and epoxides, to form clusters and to trigger sp2 percolation, a key element for electrical conductivity. Impact of oxygen-clustering is thoroughly analyzed in electrochemically-derived graphite oxide (EGO), amine-functionalized frameworks (EGOF), and their pyrolyzed forms. Amine pillars within EGOF were suspected to amplify oxygen-clustering in EGOF as onset temperature of thermal decomposition was smaller in EGOF than in EGO. Thermochemical analysis proves that pillars within EGOF significantly augment oxygen-clustering aiding the reduction mechanism as inferred by the activation energy of thermal decomposition. Defective oxygen-clustering mechanism is evidenced using multiple techniques, including XRD, and is harnessed to enhance nitrogen doping. The pyrolytic morphological transformation from lamellae of EGOF into ravioli is revealed through SEM. A labyrinthic network of array prismatic dislocations provides stability to pyrolyzed EGOF ravioli whilst confining long chains of electron delocalization (localized π-orbitals), as unprecedently confirmed by HR-TEM/EELS and conductive AFM (C-AFM). Oxygen-clustering not only improves nitrogen doping in ravioli, but also improves density of array prismatic dislocations, which ultimately boosts Faradaic redox activity as observed in cyclic voltammetry (CV). Results show that Faradaic response originating from redox-active nitrogen groups predominantly relies on abundance of extended localized π-orbitals, which are geometrically and chemically confined within ravioli array prismatic dislocations. Our first-of-a-kind HR-TEM/EELS, C-AFM, and CV comparative and correlative analysis of pyrolyzed forms of EGO, EGOF, and their oxygen-clustered derivatives, confirms emergent role of confined electron delocalizations in amplifying Faradaic redox activity.

Mechanistic investigation of highly bendable magnesium alloy sheet fabricated by short-process manufacturing

In this study, a commercial magnesium alloy AZ31 (Mg-3Al-1Zn-0.3Mn) sheet through a short manufacturing process was found to be ductile and highly formable in bending. Despite possessing a strong basal texture, the short-processed sheet without any annealing can be bent at a small radius, only 0.2 times its thickness in the 90° bending test. Additionally, it could withstand direct deformation by repeated folding-flattening. The in-situ microstructural characterization reveals that extension twin bands with strain localization appear in the bending area. During subsequent flattening, these twin bands underwent detwinning, reducing local strain concentrations and enabling further bending deformation. Such outstanding bend formability originated from the significant 〈a〉-type dislocation loops slipping on the prismatic crystal planes within dynamic-recrystallized grains. These grains underwent a uniform refinement to several microns in the short manufacturing process and exhibited low residual strain. The active prismatic dislocation slip within refined grains was due to its much lower relative activation stress to basal slip (CRSSprism/ CRSSbasal of only ∼1.6) owing to the effective grain boundary hardening. Furthermore, the prismatic dislocation activity was further enhanced when bypassing Al-Mn nano-particles during motion, leaving debris and loops that facilitated easy multiplication.

Embedding strain-rate sensitivities of multiple deformation mechanisms to predict the behavior of a precipitate-hardened WE43 alloy under a wide range of strain rates

A rare earth Mg alloy, WE43, exhibits high strength, good ductility, low anisotropy, and moderately high strain rate sensitivity. As such, the alloy is a viable candidate for high strain rate applications. In this work, a comprehensive set of mechanical and microstructure data recorded during quasi-static, high strain rate split Hopkinson bar (SHB), and impact tests on specimens of WE43 Mg alloy reported in (Savage et al., 2020b) is simulated and interpreted using an advanced Taylor-type crystal plasticity finite element (T-CPFE) model. The T-CPFE model is formulated physically to embed two sources of strain-rate sensitivities inherent to each slip and twinning mode in WE43, one that occurs under constant structure and another that affects structure evolution. The model parameters are established for the alloy by achieving agreement in the stress-strain response and microstructure evolution under quasi-static and SHB tests. Density functional theory calculations of anti-phase boundary (APB) energy are carried out to explain origins of the unusually large initial slip resistance for basal dislocations, which shear precipitates in the alloy. The initial slip resistances of the prismatic and pyramidal dislocations are, instead, rationalized by Orowan looping around precipitates. After calibration and validation, the model is shown to successfully predict WE43 response at much larger strain rates than those used for model calibration. Specifically, mechanical response, specimen geometry changes, twin volume fractions, and texture evolution are predicted for different orientations of the Taylor cylinders. Details of the modeling framework, comparison between simulation and experimental results, and insights from the results are presented and discussed.

New understanding of one-dimensional thermal glide of a nano-sized prismatic dislocation loop in bcc iron: an atomic scale study

Vacancy-type prismatic dislocation loops (PDLs) can be formed directly through cross-slip mechanisms of screw dislocations or via the accumulation and transformation of high-density vacancy-type defects. The concentration of vacancy-type defects in body-centered cubic (BCC) iron can be increased through plastic deformation and in the presence of hydrogen. Nano-sized PDLs can act as high-speed vacancy transport in BCC iron, causing embrittlement. Based on molecular dynamics simulations, this study comprehensively analyzes the one-dimensional diffusion of vacancy-type PDLs composed of 37–169 vacancies (V37-V169) at temperatures ranging from 100 to 1000 K. The activation energy of diffusion increased with decreasing PDL size for small loops (V37–V61) but remained almost constant for larger loops (>V91). This behavior was found to be associated with the tensile stress produced by the opposing edge dislocation segment in the loop where the distance is closer for a smaller PDL. That is, the tensile stress along the slip direction of edge dislocation causes the unstable stacking fault energy to increase, and the change of dislocation core structure further contributes to an increase in the activation energy for the diffusion of PDL. In contrast, an external compressive stress reduces the activation energy and pre-exponential factor of diffusion in small PDLs, as exemplified in V37 PDL.

Mechanism of thermally-activated prismatic slip in Mg

Prismatic slip of the screw <a> dislocation in magnesium at temperatures ≳ 150 K is understood to be governed by double-cross-slip of the stable basal screw through the unstable prism screw and back to the basal screw, with the activation energy controlled by the formation energy of two basal-basal kinks. However, atomistic studies of the double-kink process predict activation energies roughly twice those derived experimentally. Here, a new mechanism of prism glide is proposed, analyzed theoretically, and demonstrated qualitatively and quantitatively via direct molecular dynamics (MD) simulations. The new mechanism is intrinsically 3d, and involves the nucleation of a single kink at the junction where a 3d prismatic dislocation loop transitions from the basal screw segment to non-screw prismatic character. The relevant kink energies are calculated using recently-developed Neural-Network Potentials (NNPs) for Mg that show good agreement versus DFT for basal and prism <a> dislocations, enabling a parameter-free analytic model for the activation barrier. Direct MD simulations show both operation of the precise proposed mechanism and a stress-dependent activation barrier that agrees reasonably with the analytic model. Predictions of dislocation velocity compare very well with in-situ TEM data, and macroscopic strength versus temperature can also be understood. Overall, the new intrinsically 3d mechanism for dislocation glide due to single kink nucleation rather than double-kink nucleation explains key features of the prismatic slip in Mg and may have broader applicability in other metals where kink nucleation processes control thermally-activated flow.

A phase field model for the motion of prismatic dislocation loops by both climb and self-climb

We study the sharp interface limit and the existence of weak solutions of a phase field model for climb and self-climb of prismatic dislocation loops in periodic settings. The model is set up in a Cahn–Hilliard/Allen–Cahn framework featured with degenerate phase-dependent diffusion mobility with an additional stabilizing function. Moreover, a nonlocal climb force is added to the chemical potential. We introduce a notion of weak solutions for the nonlinear model. The existence result is obtained by approximations of the proposed model with nondegenerate mobilities. Lastly, the numerical simulations are performed to validate the phase field model and the simulation results show a big difference for the prismatic dislocation loops in the evolution time and the pattern with and without self-climb contribution.

Plastic deformation in nanoindentation of Alx(CuCrFeNi)1−x high entropy alloy

The molecular dynamics approach is utilized to examine the deformation mechanism and evolutionary patterns of mechanical behavior in the Alx(CuCrFeNi)1−x high-entropy alloy (HEA) during nanoindentation. A comprehensive investigation is carried out on how temperature, grain size, and alloy composition impact the mechanical attributes and structural changes in the Alx(CuCrFeNi)1−x HEA (with x representing the molar ratio, x = 0.04–0.3). Alterations in the proportion of aluminum within the alloy content reveal a correlation wherein an increase in the Al percentage leads to a reduction in the indentation force. In terms of plastic deformation, the interplay between Al concentration, grain size, and temperature influences the conversion of local stress and shear strain into the bulk of the substrate, as evidenced by the presence of diverse slip bands. Additionally, the concentration of aluminum and larger grain sizes facilitate the extension of shear bands into the substrate, thereby augmenting the overall ductility of the alloys. In the realm of microstructure development, the movement of mobile prismatic dislocations within the substrate significantly contributes to the deformation process. Notably, the aluminum content governs the displacement of these dislocation rings into the substrate, while the growth of these rings is hindered by the grain size.

Interaction of [formula omitted] prismatic screw dislocations with the [formula omitted] interface side face in [formula omitted] Ti alloys

Slip transmission across α−β interfaces is of great significance in understanding the strength of α−β Ti alloys for aerospace applications. Molecular statics (MS) and molecular dynamics (MD) simulations were conducted to investigate the mechanisms of slip transmission of 〈a〉 prismatic screw dislocations across the α−β interface side face. In these simulations, the α phase consisted of pure HCP Ti whereas the β phase was modeled as Ti60Nb40 BCC random alloy using a Ti–Nb interatomic potential. Firstly, the misfit dislocations structure on the α−β interface side face has been characterized from MS simulations. This predicted dislocation structure is in good agreement with experimental observations in Ti-alloys. Secondly, MD simulations of the interaction of [a1], [a2], and [a3] prismatic screw dislocations with the interface side face in an α−β bi-crystal were performed at different temperatures. A distinct barrier of the interface side face to different types of dislocation transmission was found. The origin of this anisotropy in the slip transmission is due to the relative misalignment of the slip systems between the α and the β phases from the Burgers orientation relationship. Finally, the mechanisms of slip transmission of each dislocation type were analyzed in a detailed atomistic description and compared to experimental observations.

Solute strengthening of prism edge dislocations in Mg alloys

The poor ductility of hcp Mg is attributed to the low activity of non-basal slip systems and so activation of prismatic slip can aid ductility in rolled sheets by providing three additional <a> Burgers vector slip systems. Experimental studies show that dilute additions of alloying elements such as Zn and Al leads to softening of prismatic slip at low temperatures but strengthening at higher temperatures. Here, the role of solute strengthening of prismatic edge dislocations is investigated as a possible explanation for the higher-T strengthening. Mg–Zn is studied using first-principles inputs in a parameter-free solute strengthening theory. First-principles DFT is necessary to accurately assess the strong solute chemical interaction energies in the core of the compact edge dislocation. Such calculations are subtle due to motion of the dislocation in the presence of the solute, and methods to obtain reliable results with acceptable computational cost are discussed. While interaction energies of Zn in the prism edge core can be quite large ( ± 0.34 eV), the edge solute strengthening of prismatic slip in dilute Mg–Zn remains well below experiments at high temperatures. However, the large difference (0.68 eV) in Zn/dislocation interaction energies across the core of the edge dislocation suggests strengthening by dynamic strain aging as an explanation for the higher-T strengthening.

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., (111)[101], were mainly formed. Dislocations of the primary coplanar slip systems such as (111)[011] and (111)[110] 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.

Evaluating the influence of non-glide strain on prismatic dislocation in Mg using density functional theory calculations and the Peierls–Nabarro model

Mg and Mg alloys frequently exhibit anisotropic mechanical properties. Various high strength Mg alloys have been developed recently. Since the effect of non-glide stress on slip deformation can be prominent under a large strain or stress, it is crucial to examine the effect of non-glide stress/strain on dislocation motion. Mg easily undergoes 〈a〉 dislocation at ambient temperature; hence, in this study, the generalized stacking fault (GSF) energy of 〈a〉 dislocation on prismatic {101¯0} planes under non-glide strains was examined using the density function theory. To comprehend how dislocation behaves under various conditions, the relation between GSF energy and Peierls stress based on the discrete Peierls–Nabarro model was established, and screw and edge dislocation behaviors in the prismatic plane were discussed to obtain deeper understanding. While the dislocation core width decreases with compression and increases with tension, the Peierls stress increases greatly with compression and reduces slightly with the application of tension.

Plastic deformation mechanism and defect patterning under nanoindentation in medium entropy alloy CoCrNi

In this work, molecular dynamics simulations are employed to study the mechanical response and plastic deformation evolution of single-crystal medium entropy alloy (MEA) CoCrNi under nanoindentation. The effects of lattice distortion (LD) and chemical short-range order (CSRO) on plasticity initiation and defect evolution including the patterning of dislocations, planar faults and prismatic dislocation loops (PDLs) are discussed with respect to anisotropy. The results show that CSRO increases Young’s modulus and critical nucleation stress of Shockley partial (SP), while LD and thermal vibration have the opposite effect. In MEAs, a preference of SP nucleation at CoCr clusters is observed, and the SP bundles develop into a compact plastic zone with dense dislocation networks due to frequent dislocation interactions and large slip resistance by LD and CSRO. Twin boundaries (TBs) and geometrically necessary dislocations (GNDs) play the dominant role in the accommodation of local plasticity, while the propagation of PDLs, dependent on the slip propensity of loading orientation and the degree of slip resistance, assist in the relief of local stress. Once the PDL nucleation is suppressed, dislocation pileup is triggered by dense dislocation networks with limited dislocation propagation tendency, leading to a higher strength of the material. The above results elaborate the relation between local plasticity and defect patterning, which sheds light on the design of MPEAs with heterogeneous deformation behavior.

Evolution of dislocation loops and voids in post-irradiation annealed ThO2: A combined in-situ TEM and cluster dynamics investigation

The effect of isochronal annealing on the evolution of dislocation loop and void population in proton irradiated ThO2 has been investigated. Post-irradiation annealing in other actinide oxides like UO2 shows significant loop coarsening. ThO2 samples were irradiated with 2 MeV protons up to a dose of 0.1 dpa at 600 °C. Post-irradiation isochronal annealing was performed at 600, 800, 1000 and 1100 °C for 1 h at each temperature using in-situ TEM. Only faulted 1/3<111> type dislocation loops were observed, and their sizes and distribution were characterized. The population of self-interstitial atom (SIA) dislocation loops did not show any significant growth and coarsening. Additionally, nanometric voids were observed at annealing temperatures of 1000 and 1100 °C. Using cluster dynamics (CD), we have studied the nucleation and growth of point defects and defect clusters, i.e., SIA prismatic dislocation loops and nanometric and sub-nanometric voids in proton irradiated ThO2. The CD model was further utilized to predict the growth and coarsening of loops and voids during isochronal annealing at the experimental and higher temperatures. The model did not predict significant SIA loop growth which closely corresponds to the TEM observations. CD predicted SIA loop coarsening is insignificant even at high annealing temperature of 1500 °C because the model only considers the growth of defect clusters by absorption of like point defects, i.e., SIA loops absorb interstitials and voids absorb vacancies, and cannot account for their migration and coalescence due to elastic interaction. The CD model also predicts the evolution of nanometric voids having mean size within the error bounds of TEM observations.

Prediction of dislocation - grain boundary interactions in FCC aluminum bicrystals using a modified continuum criterion and machine learning methods

Mechanical properties of metals such as strength and toughness are strongly correlated to complex interactions between various defects in the crystalline structure. While elementary interactions between these defects have been investigated using recent micro- and nano-characterization techniques, understanding of the detailed interaction mechanisms has hardly been obtained. To understand defect-driven plasticity at various time and length scales, it is necessary to formulate a general guideline to predict both the interaction type (transmission or reflection) and the dislocation's subsequent slip system after the interaction. Many criteria based on the geometric alignment of the defects have been developed to predict this phenomenon, but these have yet to be found to be accurate when applied to general data sets of grain boundaries (GBs). With this motivation, we conduct a systematic study using molecular dynamics (MD) models of bicrystals to analyze defect interaction process between a prismatic dislocation loop and eleven different grain boundaries of the following character: three tilt, three twist, and five mixed. Based on the MD observations, two new prediction methods are developed: the first is a new data-driven parametric score function based on the classical geometric criteria, and the second is by applying Gaussian process machine learning methods to find the probability distribution of a hidden function. The proposed data-driven prediction methods could pave a new way to predict the unit interaction of dislocations with various GBs, which could show much higher accuracy compared to pre-existing geometric criteria.

Distinct nucleation and propagation of prismatic dislocation loop arrays in Ni and medium-entropy CrCoNi alloy: Insights from molecular dynamics simulations

This paper utilizes molecular dynamics simulations to investigate the mechanisms of nucleation and propagation of prismatic dislocation loop (PDL) arrays in single-crystal medium-entropy CrCoNi alloys and pure Ni, with a particular focus on understanding the different size-dependent plasticity. To explore this phenomenon, we varied the size of PDLs. Our findings suggest that the anticipated size effects (i.e., smaller is stronger) are observed in both the elastic and plastic stages of Ni samples, but they are less pronounced in CrCoNi samples. Furthermore, it is observed that PDLs in CrCoNi alloys are often trapped in stable pileups, which results in a surprisingly significant strengthening effect. In contrast, PDLs in similar Ni configurations glide easily and exit from the free bottom. The observed differences between the behavior of CrCoNi and Ni can be attributed to the dislocation-length-dependent lattice friction in CrCoNi, as well as a significant stress gradient resulting from localized loading during nanoindentation. A simple dislocation mechanics model is used to rationalize the abnormal size effect in CrCoNi. These findings provide an insightful understanding of the strengthening mechanisms due to complex dislocations in multi-principal element alloys.