Prismatic Dislocation Research Articles

Investigating the mechanical and fracture behaviour of Ti-based nanocomposites reinforced with single and bi-crystalline hBN nanosheets

The design and manufacturing of graphene and hBN-based nanocomposites is taking the era of material design to new horizons. The present article employs MD simulations to investigate the mechanical, fracture, and interfacial behaviour of the Ti-based nanocomposites reinforced with pristine as well as defective single and bi-crystalline hBN nanosheets. The nanocomposites exhibited over ∼100 % improvements in the failure strengths as compared to pristine Ti matrices. Reinforcement of the Ti matrices with single and bi-crystalline hBN nanosheets improved the failure strengths of the nanocomposites from 4.06 GPa to 9.74 GPa and 9.80 GPa, respectively. However, an increase in vacancy defect (Single or Di-vacancy) concentration (0–6%) resulted in a successive reduction of the failure strength of the nanocomposites. Moreover, the deformation mechanisms in Ti matrices reinforced with pristine and defective nanosheets were observed to be governed by {101‾1} <101‾2‾ > compression twin and {101‾0} <112‾0 > prismatic slip dislocations, respectively. Furthermore, the pull-out and pull-up velocities models of interfacial shear and cohesive strengths, respectively, were employed to confirm the observed results.

Understanding the mechanism of bimodal texture evolution and DRX behavior of AZ31 magnesium alloy during shear deformation

Shear deformation in commercial magnesium alloys frequently resulted in an additional atypical texture where the basal plane aligned nearly parallel to the extrusion direction, affecting the alloy's mechanical anisotropy. This study prepared AZ31 magnesium alloys with both typical shear texture and atypical texture using a two-stage shear method at 250 °C, 290 °C, and 340 °C. The formation mechanism of the bimodal texture and their relationship with dynamic grain refinement and deformation parameters were comprehensively investigated. While temperature-dependent activation of <c+a> slip usually contributed to the atypical texture, this study identified a {101¯2} twin associated microstructure that reinforced the abnormal texture at lower processing temperature. It was found that the abundant nucleation of the multi-orientation {101¯2} twin bands in the initial stage and their differential slip activations facilitated the evolution of an alternating banded structures characterized by bimodal texture. These microstructures inherited the grain boundary rotation axis of {101¯2} twin and was potentially assimilated into specialized θ[112¯0] symmetric tilt grain boundaries with lower interface energy during continuous shear, stabilizing the bimodal texture. During shear deformation, grain nucleation strengthened the [101¯0]−[112¯0] fibers with bimodal texture by generating ∼30°[0001] grain boundaries through the preferential accumulation of prismatic dislocations. However, more continuous dynamic recrystallization, discontinuous dynamic recrystallization, and micro-shear bands facilitated the nucleation of grains with dispersed basal orientations via dominant non-prismatic dislocations, weakening the overall texture. Besides, the mechanical anisotropy of the alloys was comprehensively influenced by atypical texture and microstructure. At higher deformation temperatures, the reduction in twinning bands and the increase in dynamic recrystallization level relieved the alloy’s tension-compression asymmetry, but deprived its yield strength. This study supplemented the formation mechanisms of bimodal texture during shear deformation, providing valuable insights for optimizing wrought magnesium alloys with superior mechanical properties.

Investigation of Dislocation Behaviors in 4H-SiC Substrate during Post-Growth Thermal Treatment

Dislocation behaviors after post-growth thermal treatment were investigated by X-ray topography and KOH etching. Generation of prismatic dislocations were observed in X-ray topography, and density of basal plane dislocations (BPDs) increases with annealing temperature and radial temperature gradient. Distribution of newly generated BPDs in the wafer after thermal treatment is correlated to the resolved shear stress arising from radial temperature gradient.

Effect of crystallographic orientation on the deformation and mechanical behavior of CoCrFeNi in Berkovich nanoindentation

The hardness and plastic deformation behavior of single-phase concentrated solid solution alloy CoCrFeNi were studied for selected crystallographic orientations (⟨001⟩, ⟨011⟩ and ⟨111⟩) using Berkovich nanoindentation. The integration of transmission electron microscopy (TEM) and molecular dynamics (MD) simulations was carried out to elucidate comprehensively the distinction in the deformation mechanisms, the evolution of dislocation structures, and the pile-up of differently oriented CoCrFeNi alloys subjected to indentation by a Berkovich indenter. In MDs, a Berkovich indenter, meticulously crafted to replicate its real geometry, was effectively employed in investigating the nanoindentation. The ⟨001⟩-oriented CoCrFeNi exhibited the highest hardness and the lowest magnitude of pop-in events, followed by the ⟨011⟩ and then the ⟨111⟩ orientations. The pile-up behavior was strongly dependent on the crystallographic orientation and occurred by three sequential processes. The deformation twinning was activated only in the ⟨001⟩ orientation and inclined toward the center direction of the indent due to the force inclining toward the radial direction of the indent. The clear manifestation of the evolution of dislocations in the three orientations during indentation was elucidated. More SFTs or Stair-rods were observed in the ⟨001⟩ orientation, attributable to the intensified reaction of Shockley partials stemming from its higher density of Shockley partials, followed by the ⟨011⟩ and then the ⟨111⟩ orientations. The anisotropic nanoindentation-hardness was correlated with the microstructures. The highest hardness and lowest magnitude of pop-in events of the ⟨001⟩ orientation could be ascribed to its intense dislocation interactions, high-density SFTs and the formation of twins. In the ⟨111⟩ orientation, the three-fold symmetry emission of prismatic dislocation loops along the three slip directions not parallel to the surface leaded to a weak interaction between dislocations, and then resulted in the lowest hardness. This research may be helpful to enhance our comprehension of the intricate mechanical behaviors exhibited by different oriented FCC high entropy alloys under Berkovich indentation.

Mechanisms and kinetics of prismatic dislocation loop removal during graphitization

Molecular dynamics simulations are used to study the structure and removal of prismatic dislocation loops during graphitization. The carbon models contain a mixture of screw and edge dislocations, and are created by self-assembly at high temperature. Four mesh-based analysis tools are used to track the time-evolution of the dislocation loops, providing insight into loop structure and allowing quantification of kinetics. We find that the loop structure is complex, being dispersed in three dimensions with alternating screw and edge components in multiple slip planes. Loop removal involves edge glide through kink formation and propagation, followed by a slower screw glide mechanism. All analysis tools yield similar activation energies, in the range of 2.9 ± 0.3 eV, consistent with recent experimental work by ourselves. Literature values for the kinetics of graphitization fall into two brackets, a lower range of 2.8–3.9 eV, and a higher range of 7.8–11.6 eV. This work supports the lower range and suggests that prismatic dislocation loops are the key defect removed during graphitization.

The dependence of strength and strain hardening on dislocation-interface interaction in dual-phase titanium alloys

The slip transmission across an interface is essential for the mechanical properties of dual-phase alloys like Ti-6Al-4 V. However, the correlation between the dislocation-interface interaction and the strength and strain hardening anisotropy remains unclear due to the lack of direct experimental evidence. Via in situ scanning electron microscopy micropillar compression, prismatic plane dislocations were preferentially activated and interacted with an individual α/β interface at different angles. Based on transmission electron microscopy characterization, this study suggests that α/β interface shows a more pronounced strengthening effect when the coordinated slip system is more difficult to be activated and the slip deflection angle is larger. Differently, its higher strain hardening rate is initially determined by the larger Burger vector magnitude of interfacial residual dislocation after slip transmission. These results provide a unique basis for understanding the contribution of the interface to the mechanical properties of dual-phase alloys.

Molecular dynamics of monocrystal copper nano-scratched with tungsten tip

The utilization of ultra-sharp tungsten tips for nano-scratching has been employed in the fabrication of copper micro/nanostructures. Understanding the material removal mechanisms and the evolution of subsurface defects during this process is crucial for optimizing manufacturing processes. Nevertheless, the mechanisms of material removal and subsurface defect evolution of ultra-sharp tungsten tips nano-scratching monocrystal copper remain elusive. This study utilizes molecular dynamics simulation to investigate the material removal mechanism, subsurface defect, and dislocation evolution of monocrystal copper nano-scratched with tungsten tip. Furthermore, the formation mechanism of stress-induced stacking faults is revealed through spatially distributed hydrostatic pressure analysis. Finally, the effect of scratching parameters on subsurface damage and workpiece machinability is explored. The results indicate that plastic deformation mechanisms of nano-scratching monocrystal copper include amorphization, dislocation slip, and atomic phase transition. During the nano-scratching process, V-shaped dislocation loops and prismatic dislocation loops are formed in the shearing-plowing region, and atomic clusters and stacked fault tetrahedrons are generated in the subsurface region. Scratching along the [100] crystal orientation, coupled with relatively increased scratching distance, velocity, and appropriate depth facilitates the attainment of shallow subsurface deformed layer and excellent machinability.

Strengthening of edge prism dislocations in Mg–Zn by cross-core diffusion

The activation of prismatic slip in Mg and its alloys can be beneficial for deformation and forming. Experiments show that addition of Zn and Al solutes have a softening effect at/below room temperature, attributed to solutes facilitating basal-prism-basal cross-slip of prismatic screw dislocations, but a strengthening effect with increasing temperature. Here, the dynamic strain aging mechanism of cross-core diffusion within the prismatic edge dislocation is investigated as a possible mechanism for the strengthening at higher temperatures. First-principles calculations provide the required information on solute/dislocation interaction energies and vacancy-mediated solute migration barriers for Zn solutes around the dislocation core. Results for Mg–0.0045Zn show that cross-core diffusion notably increases the stress for prismatic edge dislocation glide but that the strengthening remains roughly 30% of the experimental strength. Other possible strengthening mechanisms of (i) solute drag of the prism edge dislocation and (ii) solute interactions and/or diffusion within the prismatic screw core, are then briefly discussed with some quantitative assessments pointing toward areas for future study.

Molecular dynamics study of tribological properties of Ni–Cr alloy coatings with different pore shapes

The presence of nanopores with different shapes cannot be avoided during the coating preparation process, and the pore shape is an important factor affecting the coating friction mechanism. This paper applies molecular dynamics to study the tribological properties of Ni–Cr alloy coatings containing spherical, rectangular and ellipsoid-shaped pores. It is found that the coating with spherical pores has the smallest fluctuation of friction coefficient and the shallowest depth of wear scar. The atoms near the rectangular pores are more likely to move towards the corners of the pores due to shear strain, and the stress concentration in the spherical pores is less than that in the rectangular and ellipsoidal pores. The spherical pore has the least effect on the potential energy inside the coating, and the interatomic bond strength is optimal. The ellipsoidal pore is also more effective in insulating the heat transfer to the bottom of the coating, protecting the substrate underneath the coating and prolonging the coating life. Coatings with rectangular pores generate more interstitial atoms during friction, producing Frank dislocation loop and prismatic dislocation loops.

Analysis on the compressive creep behaviors of Mg–Y alloys with various grain sizes

The creep behaviors of a hot-rolled Mg-4.5 wt%Y alloy sheet in the grain size ranging from d = 10 μm to d = 160 μm were carefully investigated in this work. It was shown that both loading direction and grain size exhibited effects on the creep behavior. The creep resistance along the ND was always greater than that along the RD, and the increased grain size contributed to the improved creep resistance along both directions. For the fine-grained samples (d = 10 μm to d = 25 μm), the dominant mechanisms were independent of the loading direction, including grain boundary migration (GBM), cross-slip made by basal dislocations and prismatic dislocations, and <c + a > dislocations slipping on the prismatic planes. The undifferentiated dominant mechanisms led to the extremely low creep anisotropy. For the coarse-grained samples (d = 60 μm to d = 160 μm), the dominant mechanisms were different along two loading directions. Specifically, all RD samples shared the dominant mechanisms of cross-slip and {101‾2} twinning, while the dominant mechanisms of all ND samples were dislocation climb and <c + a > dislocations slipping on pyramidal planes. The easy activation of {101‾2} twinning replaced GBM and induced the creep anisotropy, resulting in a low incremental speed of creep resistance along the RD. Differently, the primary dislocation climb and pyramidal <c + a > slip brought a high incremental speed of creep resistance along the ND. Consequently, the coarsen-grained samples had a strong creep anisotropy.

Effect of texture on the room temperature tensile and creep properties of Ti–6Al–4V plate with large thickness

Texture is inevitably generated during the rolling process of titanium alloys, which has a significant impact on mechanical properties. The relationship between microstructure, texture and mechanical properties of the rolled Ti–6Al–4V alloy plate with large thickness was systematically investigated in this study. The texture type varied with the thickness of the plate. The basal pole concentrated in the transverse direction (TD) at the surface of the plate and tilted from TD towards the rolling direction (RD) at the quarter thickness of the plate. For the center of the plate, the basal pole had peaks concentrated close to both TD and RD. Tensile tests indicated that TD samples not only have high strength but also maintain good plasticity compared with RD and the normal direction (ND) samples. The yield anisotropy could be explained by the difficulty of prismatic and basal dislocation slip. The cold creep behavior of the thick plate depended on both Schmid factor of dislocation slip and grain size. Moreover, the strain hardening exponent was positively correlated with cold creep strain. Abundant prismatic <a> and pyramidal <c+a> dislocations were activated, and slip transfer and cross-slip made the dislocation slip distance longer, which contributed to creep strain and thus reduced the creep resistance of ND sample.

Molecular dynamics simulation study of void collapse mechanisms in cubic metals under shock compression

Molecular dynamics simulations have revealed the collapse mechanism of a void in cubic metals under shock compression along [100] direction. The results show that the void collapse is caused by emitting dislocations from its surface. The type of cubic metal plays a decisive role in the subsequent microstructural evolution around the void. Void in face-centered-cubic metals collapses by emitting Shockley dislocations from its surface when piston velocity exceeds the threshold; while void in body-centered-cubic metals collapses by emitting prismatic dislocation loops, which are formed by the reaction of edge dislocations. Simulation results also reveal that the type of dislocation that induces void collapse is independent of void size. Finally, based on the elastic theory, we find that the maximum resolved shear stress along the slip direction determines the types of dislocation emitted from the voids.

Crack nucleation and dislocation activities in titanium alloys with the strong transverse texture: Insights for enhancing dwell fatigue resistance

Cold dwell sensitivity of near α titanium alloys has posed a significant challenge to the engineering safety within the aerospace industry. However, there exists inconsistency regarding the critical facet formation mechanism between basal and prismatic nucleation. A key revelation in this paper is that the controversy surrounding the facet nucleation plane primarily arises from variations in texture: when the loading direction is parallel to the c-axis or at a 45 ° angle, it leads to basal plane cracking, whereas loading direction perpendicular to the c-axis results in prismatic plane cracking. What's more, a large anisotropy is observed in the dwell fatigue performance in descending order: rolling direction (RD) > transverse direction (TD) >45° direction. To be specific, the dwell fatigue life of RD specimens was 2 times and 4.07 times longer than that of TD and 45 ° specimens, which exhibits excellent dwell fatigue resistance. Furthermore, the high prismatic dislocation density near the facet of RD specimen was confirmed through the Transmission Electron Microscope (TEM) and Transmission Kikuchi Diffraction (TKD) map, which is evidenced that prismatic slips lead to higher strain hardening during cyclic loading. The basal plane of facet grain is generally parallel to the facet line, indicating that the elliptical facet developed by the transgranular fracture through basal plane. Twinning activation in hard grain to accommodate deformation in Ti60 alloy is firstly observed under dwell fatigue tests, which is a product of high stress level in the hard grain. This paper unveils the significant impact of texture on dwell fatigue resistance, offering novel insights into enhancing dwell fatigue performance through texture control.

Nucleation of Nano-sized Prismatic Dislocation Loop from Spherical Vacancy Clusters in &lt;i&gt;α&lt;/i&gt;-iron: An Atomic-scale Study

Nucleation of Nano-sized Prismatic Dislocation Loop from Spherical Vacancy Clusters in &lt;i&gt;α&lt;/i&gt;-iron: An Atomic-scale Study

Molecular dynamics simulation of effects of loading parameters on fatigue crack growth behavior in titanium single crystal

AbstractThe fatigue crack growth behavior in single‐crystal titanium with a prismatic crack was simulated using molecular dynamics (MD) considering the effects of applied temperature, strain ratio, and strain rate. The results indicate that the material exhibits transient cyclic hardening behavior and dominant cyclic softening behavior. The peak tensile stress decreases with increasing temperature or decreasing strain rate. The deformation mechanism for crack growth involves prismatic dislocation emission and the formation of vacancy defects at different loading conditions. Deformation twinning occurs at the highest temperature, and a secondary crack emerges at the highest strain rate. The Mises stress concentration at the twin boundary and the coalescence between the initial and secondary cracks may accelerate crack propagation. The ΔJ shows good linear relationships with fatigue crack growth rate (FCGR), indicating that ΔJ possesses the potential to assess fatigue crack growth behavior at the atomic scale for ductile metallic materials.

In situ three-dimensional observation of plasticity onset in a Pt nanoparticle.

Defects in nanocrystals can dramatically alter their physical and chemical behavior. It is thus crucial to understand the defect behavior at the nanoscale to enhance material properties. Here, we report three-dimensional defect characterization at the onset of plasticity in a 550 nm Pt nanoparticle. By combining in situ nano-indentation with Bragg Coherent X-ray Diffraction Imaging (BCDI), we directly observe the strain field inside the Pt particle during indentation, revealing the nucleation and propagation of prismatic dislocation loops. Subsequent post mortem imaging of the complete dislocation network, coupled with multi-reflection BCDI, enabled us to determine the Burgers vectors of the defects revealing sessile dislocations. Finally, by measuring the elastic field inside the crystal during indentation, we estimate that the shear stress required to generate defects is 6.4 GPa, representing the upper theoretical limit of elasticity and setting an unprecedented standard for Pt nanoparticles. Our findings provide fundamental insights into defect dynamics in nanoscale systems, offering invaluable knowledge for advanced materials design and engineering.

〈112̄0〉-orientation-dependent crack initiation in a titanium alloy under dwell fatigue

Soft grains with a “rotation angle” between 15° and 30° are prone to induce cracking in hard grains, for the reason that prismatic dislocation slips in these soft grains exert a higher stress (higher κ value) normal to the basal plane of hard grains.

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.