Perfect Dislocations Research Articles

Dislocation behavior in initial stage of plastic deformation for CoCrNi medium entropy alloy

The effects of crystal orientation on the plastic deformation behavior for typical CoCrNi medium entropy alloy under tensile and compressive loading were investigated by molecular dynamics simulations. The Lomer-Cottrell (L-C) structure and parallel hexagonal close packed (HCP) bands are produced in the [111] and [11̅0] orientations at the initial stage of tensile deformation, respectively. However, perfect dislocations and two types of non-slip dislocations (Hirth and Stair-rod) are generated in the [111] and [11̅0] orientations during compressive deformation, respectively. The L-C structure produced by tensile loading in the [111] orientation and Hirth/Stair-rod dislocations produced in the [11̅0] orientation under compression act as a dislocation pinning and limit the continued glide of the dislocations. The dislocation reactions under various conditions were analyzed, which is helpful to understand the plastic deformation mechanism of CoCrNi alloy.

Effect of twins on scratching behavior of nanotwinned diamond: A molecular dynamics simulation

Nanotwinned diamond is a promising candidate material for cutting tools, high-pressure physics, and other fields due to its excellent properties of much higher hardness and toughness as compared to single-crystal diamond. Remarkable achievements have been achieved in understanding the mechanical properties and deformation mechanism when nanotwinned diamonds are subject to compression, fracture, or indentation. In this work, the response of nanotwinned diamonds under scratching was studied using molecular dynamics simulation. The morphology of chips and subsurface damage, the distribution of temperature and stress, the formation process of subsurface damage, and the plastic deformation mechanism were systematically investigated. Compared with single-crystal diamonds, more obvious plastic deformation was found in nanotwinned diamonds. Unlike single crystal diamonds, in which perfect dislocations mainly slipped on those {111} planes inclined to the scratching direction, for the nanotwinned diamond, partial dislocation was found to be the dominant dislocation type and only slipped on the {111} planes parallel to the twin boundaries. Furthermore, detwinning was also observed during subsurface damage formation besides amorphism. These results show the mechanical response of nanotwinned diamonds under scratching and provide theoretical guidance for the application of nanotwinned diamonds.

Inverse Hall-Petch Behavior in Nanocrystalline Aluminum Using Molecular Dynamics

This work investigates the mechanical behavior of nanocrystalline aluminum, with special focus on deformation mechanisms, using molecular dynamics simulations with an interatomic potential parameterized by the authors. To this end, four nanocrystalline samples with grain sizes ranging from 8,2 to 14,2 nm were constructed, each with a volume of 15 x 15 x 20 nm3. As expected, the data from the tensile tests at a strain rate of 1,0 x 109 s−1 showed an inverse Hall-Petch relationship. The work hardening behavior revealed no significant gain in mechanical strength. The dislocation analysis indicated that perfect dislocation density decreases during tensile testing, while the Shockley partials increase. Grain boundary-mediated plasticity was evidenced with atomic diffusion along grain boundaries, as well as by grain rotation. Thus, it is concluded that the conventional plastic deformation mechanisms of metals are not preponderant for nanocrystalline aluminum.

Effects of pre-existing twins on the mechanical properties of Laves phase V2Hf-based alloy

Twins have attracted much attention due to the significant effects on strengthening and toughening metallic and ceramic materials, while relevant research on twins in intermetallic Laves phases is rare despite their wide applications as structural and functional materials. Herein, a Laves phase V2Hf-based alloy V-14Hf-22Nb (at.%) was designed and twins were incorporated by precompression for probing the effects of pre-existing twins on the mechanical properties of Laves phase alloy. The results showed that the pre-existing twins not only prompted the twin growth during the secondary compression but also reacted with perfect dislocation in the V2Hf. Consequently, the compression strength of the alloy decreased from 2.13 GPa to 1.95 GPa, and the plastic strain of the alloy dropped from 13.75% to 12.20%. The results obtained were helpful for better understanding of the effects of pre-existing twins on the mechanical properties of Laves phase alloys.

Room-temperature plastic deformation of single crystals of α-manganese - hard and brittle metallic element

The deformation behavior of single crystals of α-manganese has been investigated by micropillar compression at room temperature as a function of crystal orientation and specimen size. When the specimen size is reduced to the micrometer-range, single crystals of α-manganese are found to plastically deform by dislocation motion at room temperature for the first time, accompanied by very high yield stresses of the range of 4–6 GPa. Slip along [111] and [001] are identified to operate for compression axis orientations near [001] and near [011] and [1¯11], respectively. Any low-indexed planes cannot be designated as the slip plane for both slip along [111] and [001], because of the significantly wavy nature of slip lines caused by the occurrence of frequent cross-slip. Slip along [111] tends to prefer the slip plane of {112} rather than {110}. Slip along [001], on the other hand, tends to occur on the maximum resolved shear stress plane. The 1/2[111] dislocation carrying slip along [111] moves as a perfect dislocation without dissociating into partials and does not have any preferred orientation. The [001] dislocation carrying slip along [001] also moves as a perfect dislocation without dissociating into partials. Although the Peierls stress for the motion of these dislocations must be very high, there seems no deep Peierls valleys along particular directions, unlike the screw direction for the 1/2[111] dislocation in body-centered cubic metals.

Origin of strain softening in a nanograined Al alloy

One-step and multi-step nanoindentation experiments were conducted to investigate the mechanical response of an Al-Mg-Li alloy. Opposite to the strain hardening effect observed in the coarse-grained (CG) alloy, the nanograined (NG) alloy exhibits strain softening. Compared with multi-step nanoindentation experiments, the ultralow dislocation density and the grain boundary (GB) segregation promote a higher stress level during one-step nanoindentation experiments. Residual perfect dislocations induced by a previous loading and unloading during multi-step nanoindentation experiments lower the stress required for further plastic deformation. In addition, the storage of partial dislocations also serves to render plastic flow to commence at lower stresses. This work provides new insights for the fabrication and property optimization of NG alloys.

Dislocation mechanism of Ni47Co53 alloy during rapid solidification

Dislocations and other atomic-level defects play a crucial role in determining the macroscopic properties of crystalline materials, but it is extremely difficult to observe the evolution of dislocations due to the limitations of the most advanced experimental techniques. Therefore, in this work, the rapid solidification processes of Ni47Co53 alloy at five cooling rates are studied by molecular dynamics simulation, and the evolutions of their microstructures and dislocations are investigated as well. The results show that face-centered cubic (FCC) structures are formed at the low cooling rate, and the crystalline and amorphous mixture appear at the critical cooling rate, and the amorphous are generated at the high cooling rate. The crystallization temperature and crystallinity decrease with cooling rate increasing. Dislocations are few at the cooling rates of 1 × 1011 K/s, 5 × 1012 K/s, and 1 × 1013 K/s, and they are most abundant at the cooling rates of 5 × 1011 K/s and 1 × 1012 K/s, in which their dislocation line lengths are both almost identical. There appear a large number of dislocation reactions at both cooling rates, in which the interconversion between perfect and partial dislocations is primary. The dislocation reactions are more intense at the cooling rate of 5 × 1011 K/s, and the slip of some dislocations leads to the interconversion between FCC structure and hexagonal close packed (HCP) structure, which causes the twin boundaries (TBs) to disappear. The FCC and HCP are in the same atomic layer, and dislocations are formed at the junction due to the existence of TBs at the cooling rate of 1 × 1012 K/s. The present research is important in understanding the dislocation mechanism and its influence on crystal structure at atomic scales.

Interaction of an edge dislocation with a 〈110〉 tilt boundary in nickel: molecular dynamics simulation

The interaction of a lattice edge dislocation with a 〈110〉 tilt boundary in nickel was studied by the molecular dynamics method in the case when the dislocation glide plane is parallel to the grain tilt axis. The dependence of the local threshold stress on the grain misorientation angle is obtained. It is found that with increasing misorientation angle, the local threshold stress increases, but the growth rate for low- and high-angle boundaries is different: for high-angle boundaries, this growth is slower. When studying the mechanism of overcoming the low-angle 〈110〉 tilt boundary by an edge dislocation in the considered orientation, it was found that the dislocation, in fact, does not pass through the boundary, but exchanges places with a grain-boundary dislocation, which are ordinary perfect edge dislocations in the low-angle 〈110〉 tilt boundaries. When studying the interaction of a dislocation with high-angle boundaries, a peculiarity was noticed, which consists in pushing two or three grain-boundary dislocations out of the boundary at once.

Effect of twin nucleation and twin on the recrystallization of a nickel-based superalloy during hot compression

The nucleation mechanism of deformation twin (DT) and annealing twin (AT), and the effect of twins on recrystallization were studied comprehensively for Nickel-based superalloys during dynamic recrystallization (DRX). The aim is to improve the understanding of microstructure evolution during hot deformation. Hot compression tests were carried out at the temperature of 900-1150 °C with the strain rate of 0.01-1 s -1 for a Nickel-base superalloy. The results show that the DT and the Ʃ3 boundary exist simultaneously in the DRX stage. DTs are nucleated by the rotation of Franck partial dislocations and the decomposition of perfect dislocations. The DTs hinder the movement of dislocations in deformed grains and promote the nucleation of DRX grains. Ʃ3 boundaries associated with stacking faults are formed at recrystallized grain boundaries. In the condition of continuous DRX (CDRX), the Ʃ3 boundary changes the grain orientation and restrain the CDRX nucleation. While for the discontinuous DRX (DDRX), the Ʃ3 boundary provides an additional nucleation site and increase the nucleation rate. In addition, the comprehensive effect of DTs and ATs can promote the DRX nucleation, for which increase at first and then decrease with nucleation mechanical transform from CDRX to DDRX. • Deformation twins are nucleated by the rotation of Franck partial dislocations and the decomposition of perfect dislocations. • The quantity of deformation twins decreasing with the increase of DRX volume fraction. • Deformation twins hinder the movement of dislocations and promote the nucleation of CDRX. • The Ʃ3 boundaries hinder the CDRX nucleation and accumulate the DDRX nucleation process.

Research on phase transition induced plastic deformation in nanoindentation of single crystal diamond

Nanoindentation experiments were performed on the {100} crystal plane of single crystal diamond using molecular dynamics simulation to study the mechanism of its plastic deformation. The relationship between phase transition and plastic deformation during the nanoindentation process was discussed using the load-displacement (P–h) curve and phase transition evolution process obtained from the experiment. It was observed that the initial plastic characteristic of diamond was because of the phase transition, rather than dislocation slip. The nucleation and growth of 1/2[110]{111} perfect dislocation ring during the plastic deformation of single crystal diamond were determined by observing pop-in events and dislocation evolution in P-h curves. The plastic deformation of single crystal diamond was attributed to the formation and growth of 1/2[110]{111} complete dislocation ring caused by the phase transition from disordered sp3 to ordered sp2 carbon atoms based on the further analysis of microscopic defects. It was considered that the phase transition during the nanoindentation of single crystal diamond is an important excitation source for plastic deformation at the atomic scale. Finally, it was observed through the analysis of bond length and angle during the phase transition process that the CC bond fracture of diamond was because of the transformation from sp3 to sp2 carbon atoms, and the fracture and recombination of adjacent carbon atoms, which led to graphitization. This conclusion confirmed that graphitization is the mechanism of plastic deformation of diamond and the pop-in event is caused by the stress release caused by the rupture of CC bond.

On continuum simulations of the evolution of faulted and perfect dislocation loops in silicon during post-implantation annealing

Predictive models of the damage evolution during post-implantation annealing are important for predicting the performance of final devices. An existing model of the thermal evolution of dislocation loops in silicon during post-implantation annealing has been recalibrated to better capture the distinction between faulted and perfect dislocation loops. The calibration was based on experimental findings from the literature on the simultaneous presence of both faulted and perfect dislocation loops after post-implantation annealing.Graphical abstract

The Challenges of Modeling Defect Behavior and Plasticity across Spatial and Temporal Scales: A Case Study of Metal Bilayer Impact

Atomistic molecular dynamics (MD) and a microstructural dislocation density-based crystalline plasticity (DCP) framework were used together across time scales varying from picoseconds to nanoseconds and length scales spanning from angstroms to micrometers to model a buried copper–nickel interface subjected to high strain rates. The nucleation and evolution of defects, such as dislocations and stacking faults, as well as large inelastic strain accumulations and wave-induced stress reflections were physically represented in both approaches. Both methods showed similar qualitative behavior, such as defects originating along the impactor edges, a dominance of Shockley partial dislocations, and non-continuous dislocation distributions across the buried interface. The favorable comparison between methods justifies assumptions used in both, to model phenomena, such as the nucleation and interactions of single defects and partials with reflected tensile waves, based on MD predictions, which are consistent with the evolution of perfect and partial dislocation densities as predicted by DCP. This substantiates how the nanoscale as modeled by MD is representative of microstructural behavior as modeled by DCP.

Molecular dynamics simulation on creep-ratcheting behavior of columnar nanocrystalline aluminum

Creep-ratcheting deformation behavior of columnar nanocrystalline (NC) Al varying in grain sizes have been studied using molecular dynamics simulations at three different temperatures (300 K, 467 K, and 653 K). The underlying deformation mechanisms of columnar NC Al for two stress ratios of ratcheting are also evaluated in this study. The highest strain accumulation and rapid hysteresis loop proliferation are attained at 653 K. The dislocation density is low at 653 K in contrast to the other two deformation temperatures. The perfect dislocations support the creep-deformation process followed by Shockley partial dislocations. It is observed that the cyclic hardening phenomenon at 300 K and the cyclic softening phenomenon at 653 K dominate during creep-ratcheting loading. The specimen fails earlier at higher temperatures owing to the change in the shape of hysteresis loops. The predominant grain boundary-based deformation mechanisms of columnar NC Al specimens are grain boundary (GB) migration, GB diffusion, GB widening, GB sliding, and GBs merging for all the deformation temperatures. The variation in the extent of deformation with respect to temperatures depends on strain accumulation in the plastic region under creep-ratcheting loading.

Evolution of deformation microstructure in a coarse-grained Fe-Mn-Al-C steel under bending fatigue

• Pre-saturation stage was fully austenitic under cyclic loading. • γ → ∊ martensitic transformation was detected. • Extended dislocations and channel-vein structures were observed. • Dislocation densities were relatively low. Microstructure evolution during cyclic deformation of a coarse-grained (∼25 μm) high-Mn steel under high-cycle bending fatigue at room temperature is reported. Vickers microhardness initially increased until 10,000 cycles indicating cyclic hardening but almost saturated at prolonged cycling. The cyclic deformation of austenite leads to formation of perfect dislocations and stacking faults, as well as shear bands in the pre-saturation stage, while the formation of minor amount of ∊ -martensite was observed at intersections of shear bands at the saturation stage. Dislocation densities estimated by X-ray diffraction were relatively low ∼10 14 m −2 .

In-situ TEM study on the evolution of dislocation loops and bubbles in CeO2 during Kr+ single-beam and Kr+-H2+ dual-beam synergetic irradiation

CeO2 is widely used as the nonradioactive surrogate fuel of UO2 when studying the irradiation performance of UO2. The evolution and characteristics of dislocation loops and bubbles in CeO2 foils were studied by in-situ transmission electron microscopy (TEM) observation during 400 keV Kr+ & 30 keV H2+ dual-beam synergetic irradiation and 400 keV Kr+ single-beam irradiation at 1073 K. The rotation of the habit plane of dislocation loops induced by above ion irradiation was found in the CeO2 for the first time, such as from [2¯11¯] to [3¯11¯] and then to [1¯00]. The rafted loops were first observed under Kr+ & H2+ dual-beam synergetic irradiation, which not only had similar [1¯11¯] direction, but also belonged to perfect dislocation loops (PDLs). The rafted loops were formed not only by the growth of loops that absorbed irradiation defects, but also by the combination of loops. But, this phenomenon of loop rafting was not obvious during Kr single-beam irradiation. It was first found that Kr+ irradiation induced the change of Burgers vectors of PDLs. The absorption of large PDLs to small PDLs was also observed. The average size and areal number density of dislocation loops and gas bubbles as a function of irradiation dose were constructed, which showed that the addition of H2+ obviously affected the characteristics of dislocation loops and gas bubbles and the swelling of CeO2.

On the deformation behavior of CoCrNi medium entropy alloys: Unraveling mechanistic competition

Recently, CoCrNi medium entropy alloys (MEA) have been the subject of numerous investigations due to their unique mechanical properties such as an exceptionally high strength-ductility combination. The resulting superior toughness of CoCrNi MEAs is attributed to an interplay of multiple deformation mechanisms, such as twinning, and partial and perfect dislocation glide. The current understanding of MEA deformation mostly stems from an indirect analysis of the defect evolution in deformed microstructures, where the contributions of individual mechanisms are assessed from the relative concentrations of associated defect structures. Here, we propose that the mechanistic contributions to microstructural deformation are more properly reflected by the percentages of total strain accommodation. Using atomistic simulations, the mechanical response of nanocrystalline CoCrNi MEA under uniaxial tension is investigated as function of grain size and chemical short-range ordering (SRO). The contributions of deformation mechanisms are resolved directly from the amount of strain accommodation by leveraging continuum based kinematic metrics. It is found that during initial loading, deformation occurs by partial dislocation slip, in agreement with experimental observations. Under continued loading, the governing deformation mechanisms transition to twinning and perfect dislocation slip. Furthermore, the grain size that corresponds to the maximum strength is found to decrease in presence of SRO.

Grain size-dependent phase-specific deformation mechanisms of the Fe50Mn30Co10Cr10 high entropy alloy

The present work reports the effect of initial microstructures (i.e., the material annealed at 600 °C (A600) with the fine grain single FCC phase vs. the material annealed at 900 °C (A900) with the coarse grain FCC/HCP dual phase) on deformation mechanisms, damage resistance, and mechanical properties of the Fe50Mn30Co10Cr10 HEA. The strength of A600 is higher than A900, and the ductility of both materials is similar with the total elongation exceeding 50%. A900 shows a higher strain hardening rate than A600 below 0.2 true strain, which could be attributed to simultaneous deformation accommodation from the FCC/HCP dual phase. The FCC grains mainly exhibit deformation-induced HCP phase transformation while the HCP grains show multiple deformation mechanisms, i.e., various slip modes (Shockley partial dislocations, perfect dislocations, extended dislocations, and dislocations present on prismatic planes), the HCP to FCC reverse transformation, the formation of kink bands. The strain hardening rate of both materials is similar above 0.2 true strain, which could be related to higher TRIP kinetics of A600 than A900. The A600 shows the refinement of the crack size due to its fine size of grains and HCP phase. The superior damage resistance could lead to the large ductility of A600 even in the presence of the non-recrystallized areas in its initial microstructure. The present work could provide microstructure design solutions for metastable HEAs with superior strength-ductility combination and damage resistance.

Creep deformation behavior of the Ni–Fe-based GH984G alloy

The creep deformation behavior and microstructure evolution of the Ni–Fe-based GH984G alloy were investigated at temperatures of 700 °C–775 °C, with the stress ranging from 150 MPa to 400 MPa. The results demonstrate that the apparent stress exponent and apparent creep activation energy are 6.53 ± 0.74 and 546.7 ± 6.8 kJ/mol, respectively. The creep activation energy is higher than that of lattice self-diffusion, which is related to the interaction between dislocation and γ′ precipitates and can be corrected by threshold stress. The threshold stresses are 109.1 MPa, 51.5 MPa and 24.8 MPa at 700 °C, 750 °C and 775 °C, respectively. The true stress exponent is 4.95 ± 0.14 and the true creep activation energy is determined to be 288.3 ± 4.3 kJ/mol. The microstructure analysis demonstrates that the climbing of a/2 <110> perfect dislocation is the primary mechanism of creep at 700 °C. The dislocation climbing and a/6 <112> partial dislocation shearing into γ′ are both significant factors during creep deformation at 750 °C. The creep damage tolerance of 6.4 ± 1.1 indicates that microstructural degradation, including γ′ and M23C6 coarsening, is the primary creep damage mechanism for the GH984G alloy. The deformation mechanism map for GH984G alloy is established and the creep mechanism within the investigated conditions locates in the power-law creep domain controlled by dislocation climbing.

The evolution and interaction of irradiation defects in CeO2 during H2+ & He+ dual-beam synergistic irradiation investigated by in-situ TEM

As a surrogate material of UO2, non-radioactive CeO2 was investigated by in-situ transmission electron microscopy during 30 keV H2+ & He+ dual-beam synergistic irradiation at 1073 K. The relationships between the size/density of dislocation loops and irradiation fluence were established, as well as for gas bubbles. Based on the analysis of the Burgers vectors, the formation of perfect dislocation loop (PDL) through the reaction between PDLs and the direct transformation from Frank dislocation loop (FDL) to PDL were obtained. The shrinkage of FDLs at high dose was observed. It was not only found that the number density and size of bubbles near the grain boundary were lower than those in the crystal, but also found that no loop-bubbles complex was formed. The mechanism of irradiation-induced excess oxygen vacancies should be the reason for the shrinkage of dislocation loops at high dose and the depletion of bubbles in the dislocation loops.

Mechanical Load-Induced Atomic-Scale Deformation Evolution and Mechanism of SiC Polytypes Using Molecular Dynamics Simulation.

Silicon carbide (SiC) is a promising semiconductor material for making high-performance power electronics with higher withstand voltage and lower loss. The development of cost-effective machining technology for fabricating SiC wafers requires a complete understanding of the deformation and removal mechanism. In this study, molecular dynamics (MD) simulations were carried out to investigate the origins of the differences in elastic–plastic deformation characteristics of the SiC polytypes, including 3C-SiC, 4H-SiC and 6H-SiC, during nanoindentation. The atomic structures, pair correlation function and dislocation distribution during nanoindentation were extracted and analyzed. The main factors that cause elastic–plastic deformation have been revealed. The simulation results show that the deformation mechanisms of SiC polytypes are all dominated by amorphous phase transformation and dislocation behaviors. Most of the amorphous atoms recovered after completed unload. Dislocation analysis shows that the dislocations of 3C-SiC are mainly perfect dislocations during loading, while the perfect dislocations in 4H-SiC and 6H-SiC are relatively few. In addition, 4H-SiC also formed two types of stacking faults.