Peculiarities of the dislocation ensemble motion in doped alkali halide crystals

Abstract

Correlation between the dislocation mobility in the stress field of a concentrated load and other plastic deformation parameters for doped NaCl crystals was studied. The good correlation between all parameters under investigation was found in the case of the impurity hardening and the disturbance of this correlation was established in the case of the impurity softening. The influence of impurity state on the low-temperature anomaly of dislocation mobility was revealed.     

Magnetically stimulated hardening of NaCl(Pb) crystals

Dislocation motion in NaCl(Pb) crystals under a pulsed mechanical load with and without a magnetic field is investigated. It is found that the dislocation mobility decreases when these crystals are deformed in a magnetic field. It is inferred that the observed magnetically stimulated hardening of NaCl(Pb) is due to a characteristic feature of spin-dependent electronic transitions in the dislocation-lead impurity system which increase the barrier for dislocation motion. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 3, 226–228 (10 August 1999)     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

Evidence for a transition in deformation mechanism in nanocrystalline pure titanium processed by high-pressure torsion

Nanocrystalline titanium with an average grain size of about 60–70 nm was prepared by high-pressure torsion. The results of hardness and structural evolutions indicate that a strain-induced hardening–softening–hardening–softening behaviour occurs. For coarse-grained titanium, 〈a〉-type dislocation multiplication, twinning and a high pressure-induced α-to-ω phase transformation play major roles to accommodate deformation, leading to a significant strain hardening. As deformation proceeds, dynamic recrystallisation leads to a decrease in dislocation density, especially for 〈a〉-type dislocations, leading to a slight strain softening. The 〈c〉-component dislocation multiplication dominates the deformation when the grain size decreases to 100 nm and 〈c〉-component dislocation multiplication, grain refinement and the α-to-ω phase transformation contribute to the second strain hardening. The following strain softening is attributed to dynamic recovery.     

From dislocation junctions to forest hardening

The mechanisms of dislocation intersection and strain hardening in fcc crystals are examined with emphasis on the process of junction formation and destruction. Large-scale 3D simulations of dislocation dynamics were performed yielding access for the first time to statistically averaged quantities. These simulations provide a parameter-free estimate of the dislocation microstructure strength and of its scaling law. It is shown that forest hardening is dominated by short-range elastic processes and is insensitive to the detail of the dislocation core structure.     

Mechanism of strain hardening and dislocation-structure formation in metals subjected to severe plastic deformation

The equations of dislocation kinetics are used to theoretically analyze the mechanism of strain hardening and the formation of fragmented dislocation structures in metals at large plastic strains. A quantitative analysis of the available data on aluminum and an aluminum-magnesium alloy shows that strain hardening at large plastic strains and the formation of fragmented dislocation structures are related to the interaction and self-organization of geometrically necessary dislocations (GNDs). On the microscale, the source of the GNDs is a locally nonuniform plastic deformation induced by a dislocation-density gradient in dislocation-cell boundaries.     

Molecular dynamics investigation of plastic deformation mechanism in bulk nanotwinned copper with embedded cracks

The plastic deformation of bulk nanotwinned copper with embedded cracks under tension has been explored by using molecular dynamics simulations. Simulation results show that the cracks mainly act as dislocation sources during the plastic deformation and occasionally as sinks at later stage. The dislocation pile-up, accumulation and transformation at twin boundaries (TBs) control the plastic hardening and softening deformations. The TB dislocation pile-up zone is estimated to be 5.6–8 nm, which agrees well with previous experimental and simulation results. Furthermore, it is found that the flow stress vs. dislocation density at the hardening stage follows the Taylor-type relationship.     

The effect of size on dislocation cell formation and strain hardening in aluminium

The formation of dislocation cells has a significant impact on the strain hardening behaviour of metals. Dislocation cells can form in metals with a characteristic size defined by three-dimensional tangles of dislocations that serve as “walls” and less dense internal regions. It has been proposed that inhibiting the formation of dislocation cells could improve the strain hardening behaviour of metals such as Al. Here we employ in situ scanning electron microscope compression testing of pure Al single crystal pillars with physical dimensions larger, close to and smaller than the reported cell size in Al, respectively, to investigate the possible size effect on the formation of dislocation cell and the consequent change of mechanical properties. We observed that the formation of dislocation cells is inhibited as the pillar size decreases to a critical value and simultaneously both the strength and the strain hardening behaviour become strongly enhanced. This phenomenon is discussed in terms of the effect of dimensional restriction on the formation of dislocation cells. The reported mechanism could be applied in polycrystalline Al where the tunable physical dimension could be grain size instead of sample size, providing insight into Al alloy design.     

Contribution from dislocation to deformation resistance in polycrystals of copper-based solid solutions

The evolution of dislocation structure in solid solutions of Cu-Al and Cu-Mn systems with different grain sizes and at different test temperatures is studied by means of transmission electron diffraction microscopy. The scalar density of dislocations is measured and its relationship to the flow stress of alloys is determined. Changes in the contribution from dislocation hardening to deformation resistance upon variations in the contributions associated with changes in grain size, solid-solution hardening, and test temperature are analyzed.     

Computer simulation of the interaction between an edge dislocation and Cu precipitates in bcc iron

The molecular dynamics method was used to determine the energy of the interaction between a 1/2 [111] (110) edge dislocation and a Cu precipitate in bcc Fe. It was shown that three ranges of precipitate dimensions, characterized by different types of hardening, can be distinguished as a dislocation crosses over a precipitate. It was found that maximum hardening was due to the structural instability of the particles relative to the bcc—9R transformation, with this instability resulting from the stress fields produced by a dislocation.     

Microhardness studies on melt-grown single-phase mixed (NaK)Cl and solution-grown two-phase NaClKCl crystals

Microhardness studies were carried out on melt-grown (NaK)Cl crystals. The quenching strains and the difference in the ionic sizes of the cations constituting the mixed system introduced large numbers of defects, viz. dislocations, grain boundaries, etc. The etching experiments and supplementary X-ray studies clearly revealed that the Na⁺ or K⁺ ions are precipitated at the dislocation sites. These phase particles strongly interact with dislocations so as to obstruct the mobility of the latter contributing to the hardening mechanism. Results are compared with solution-grown two-phase mixed NaClKCl pure NaCl and pure KCl crystals.     

Microstructural instabilities: dislocation substructures with pronounced mechanical effects

Microstructure evolution is largely dominated by the internal stress fields that appear upon the appearance of inhomogeneous structures in a material. The hardening behaviour of metals physically originates from such a complex microstructure evolution. As deformation proceeds, statistically homogeneous distributions of dislocations in grains become unstable, which constitutes the driving force for the development of a pronounced dislocation substructure. The dislocation structure already appears at early stages of deformation due to the statistical trapping of dislocations. Cell walls contain dislocation dipoles and multipoles with high dislocation densities and enclose cell-interior regions with a considerably smaller dislocation density. The presence and evolution of such a dislocation arrangement in the material influence the mechanical response of the material and is commonly associated with the transient hardening after strain path changes. This contribution introduces a micromechanical continuum model of the dislocation cell structure based on the physics of the dislocation interactions. The approximation of the internal stress field in such a microstructure and the impact on the macroscopic mechanical response are the main items investigated here.     

A study of the photoinduced conformational mobility of spin-labeled regenerated rhodopsin by ESR spectroscopy

Saturation transfer EPR spectroscopy is used to study the photoinduced conformational changes of spin-labeled (Cys140 and Cys316) rhodopsin in a photoreceptor membrane. Illumination of rhodopsin with “yellow” light (λ > 450 nm), converting it from the dark form into metarhodopsin II, a physiologically active form, is accompanied by an increase in the mobility of the cytoplasmic loop (Cys140) and eighth alpha-helix (Cys316), whereas the subsequent illumination with “blue” light (λ < 445 nm), which converts metarhodopsin II into a mixture of inactive photoregenerated products, results in a decrease in the mobility. The restoration of the specific features of the photoinduced changes in the conformational mobility of the cytoplasmic loops of rhodopsin regenerated in the dark is demonstrated.     

晶体塑性有限元方法研究辐照对多晶铜力学性能的影响

将辐照硬化理论与晶体塑性理论结合, 运用ABAQUS有限元分析软件模拟辐照后多晶铜的拉伸过程。分析辐照效应对材料屈服强度、硬化过程、晶体变形等力学性能的影响, 研究位错密度的演化及空间分布规律。数值模拟表明: 辐照效应提高多晶铜的屈服应力, 影响不同阶段的硬化和软化现象; 辐照剂量增大导致位错密度增殖总体变缓, 空间不均匀度增大; 晶体的塑性变形及晶体转动也受到辐照的影响, 在相同的应变条件下, 辐照剂量越大, 晶体塑性变形程度越小, 塑性变形分布不均匀度变大, 同时晶体转动程度及转动角离散度增大。     

Mathematical model of successive plastic deformations in three mutually perpendicular directions

A mathematical model is formulated for successive plastic deformations in three mutually perpendicular directions when dislocation annihilation is negligible. In some slip systems, the motion of the dislocations changes sign when the deformation axis changes. This results in markedly slower strain hardening of the crystal. Comparison of the theoretical and experimental hardening curves shows that they agree qualitatively at small deformations. The sharp divergence of the curves at large deformations results from neglect of dislocation annihilation in the calculations. Tomsk State Architectural and Construction Academy. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 80–86, September, 1997.     

Effect of the degree of long-range atomic order on hardening characteristics and dislocation structure of single crystals of the alloy Ni3Fe with the [111] orientation of the strain axis

Transmission electron microscopy has been used to investigate the evolution of the dislocation structure of single crystals of the alloy Ni₃Fe with the [111] orientation for different degrees of long-range atomic order. A relation has been established among the staged nature of the hardening curve, the characteristics of strain hardening, and the quantitative parameters of the dislocation structure.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 62–69, February, 1992.     

Modelling the yield point phenomena during deformation at elevated temperatures: case study on Inconel 600

The discontinuous yield behaviour (DYB) of Inconel 600 was studied during hot compression tests at temperatures in range of 850–1150°C and strain rates of 0.001–1?s^?1. The yield point phenomena were observed in the temperature range of 850–1000°C and strain rates of 0.001–0.1 s^?1. The DYB was modelled by considering the evolution of dislocation density at the early stages of yielding. The opposite effects of dislocation multiplication, dislocation interaction (work hardening) and dynamic recovery (DRV) were considered. It was shown that the dislocation multiplication and DRV result in flow softening, while the dislocation interaction leads to work hardening. The model was established in a way to consider the effects of various microstructural evolutions on the σ(ε) function. The discontinuous flow curves were fitted by the developed model with acceptable precision. The variations of material constants with temperature and strain rate were found physically meaningful. The dislocation multiplication parameter was determined at various temperatures and strain rates. It was concluded that the rate of dislocation multiplication increases as temperature rises or strain rate declines. Accelerated dislocation multiplication leads to less drop in yield stress between the upper and lower yield points.     

A crossover from hardening to softening in nanocrystalline Ni by annealing and rolling

Two similar crossover behaviors from hardening to softening were revealed in both annealed and rolled nanocrystalline (NC) Ni by nanoindentation tests, which is totally different from that in coarse-grained samples. X-ray diffraction and transmission electron microscopy results show that the dislocation density continuously decreases with the increment of annealing temperature, whereas it first increases and then decreases with the increased rolling strain. The change of rate sensitivity in annealed NC Ni is different from that of the rolled sample. The crossover from hardening to softening by annealing is attributed to grain-boundary relaxation, while dislocation accumulation and annihilation is responsible for the crossover behavior in rolled NC Ni.