Dislocation Substructure Research Articles

Evaluation of change in dislocation substructure during tensile deformation in 1200 aluminum by using In-situ XRD measurement

Evaluation of change in dislocation substructure during tensile deformation in 1200 aluminum by using In-situ XRD measurement

Effect of high-current pulsed electron beam treatment on defect substructure of the high-entropy alloy of Co – Cr – Fe – Mn – Ni system

High-current pulsed electron beam surface treatment is a method of materials modifying, which improves the mechanical properties of metal materials. Due to high-speed heating, evaporation, recrystallization, as well as plastic deformation, dislocations with high density are formed in the surface and, as a result, an increase in indicators of various physical and mechanical properties, such as hardness, wear resistance, etc., is observed. Since currently high-entropy alloys are a relatively new class of materials, the effect of pulsed electron beam treatment on the dislocation substructure has not yet been established. In this work, a non-equiatomic high–entropy alloy of the Co – Cr – Fe – Mn – Ni system, made using a wire-arc additive manufacturing, was subjected to surface treatment using a high-current pulsed electron beam with an energy density of 30 J/cm2. By the method of studying thin foils using transmission electron microscopy, it was found that the treatment does not affect the chemical composition of the alloy, but leads to serious changes in the dislocation substructure. A nonmonotonic change in the scalar density of dislocations was revealed, reaching a maximum value of 5.5·1010 cm–2 at a distance of 25 µm from the irradiation surface. It is shown that an undirected cellular dislocation substructure with cell sizes from 400 nm to 600 nm is formed at this distance from the surface. With further distance from the surface at a distance of up to 45 µm, the dislocation substructure changes from cellular to cellular-mesh. At a distance of 120 – 130 µm, the effect of a high-current pulsed electron beam is not observed – the substructure corresponds to the substructure of the initial alloy with a chaotic distribution of dislocations.

Plastic deformation induced microstructure transition in nano-fibrous Al-Si eutectics

Indentation plastic zone of Al-20 wt% Si laser rapid solidified (LRS) alloy with nanoscale, fibrous, fully eutectic microstructure is characterized using a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM) and scanning/transmission electron microscopy (S/TEM). In contrast to cracking around the indents in the as-cast Al-Si alloy, the LRS nano-fibrous eutectic microstructure demonstrated higher strength, strain-hardening, and uniformly distributed plasticity around indents with no cracks. A transition in morphology from long nano-fibrous Si to short nano-fibrous Si is observed underneath the indent in LRS eutectics. The mechanism for this morphological evolution with increasing plastic strain is postulated to involve three stages of the deformation process: First, the formation of closely-spaced dislocation arrays in the nano-channels of Al matrix confined by Si fibers. Second, fiber segmentation when the stress field of the dislocation array exceeds the critical stress for fracture in Si nano-fibers. Third, recovery of dislocation substructures and sub-grain formation in Al leading to a rotation of fractured Si segments. The final microstructure consisted of short Si nano-fibers along triple junctions of the subgrains in Al. The implication of this unusual microstructure evolution with increasing plastic strain on the plastic co-deformation in high-strength LRS nano-eutectic microstructures was discussed in this report.

Substructure-sensitive crystal plasticity with material-invariant parameters

Even though crystal plasticity models have been available for decades, the quantification of material parameters is still a matter of debate. Polycrystalline experimental results can normally be reproduced by multiple sets of parameters, raising concerns about the best parameterization to predict the grain-level response. This work presents a novel physics-based crystal plasticity model based on mesoscale dislocation substructures, which are used to characterize material parameters independently. We employ a unique set of parameters with known uncertainty to reproduce the mechanical response of FCC single- and poly-crystals. We demonstrate that mesoscale parameters are material-invariant and can be used to model FCC metals with similar dislocation substructures such as for Cu, Ni and Al. Furthermore, the model is validated by comparing to experimental single- and poly-crystalline stress–strain curves and mesoscale dislocation substructure images. This novel modeling approach is intrinsically designed to predict the response of materials with similar dislocation substructures without the need of single crystal experimental data for calibration.

Investigation of short-term creep properties of a coarse-grained Inconel 718 fabricated by directed energy deposition compared to traditional Inconel 718

Short-term creep performance of a coarse-grained Inconel 718 alloy fabricated by Directed energy deposition (DED) technique has been investigated at 650 °C in comparison to a conventional wrought material. In order to study the effect of heat treatment and sample orientation on the creep behaviour, as-deposited specimens oriented parallel and perpendicular to the building direction have been treated by double aging (DA), solution annealing + double aging (SA) and homogenization + solution annealing + double aging (HSA) conditions. Microstructural changes along with their impacts on high temperature mechanical properties have been examined with respect to grain size, crystallographic orientation, precipitates, and dislocation substructure. Results show that, superior creep strength and creep ductility of DED-manufactured DA specimen can be achieved compared to wrought specimen. The process-induced dislocation substructure and the irregular columnar grains with the random network of grain boundaries primarily contribute to the superior creep behaviour. However, when the unique columnar grain features formed during DED process are replaced by recrystallized, equiaxed grain structure in the HSA case, the worst creep performance is observed due to the coarse grains with regular-shaped grain boundaries facilitating the crack propagation. In addition, anisotropy of creep performance with respect to sample orientation observed for the heat-treated DED specimens can be significantly alleviated by a higher temperature of solution treatment.

Influence of the Temperature-Strain Parameters on the Structure Evolution and Carbide Transformations of Cr-Ni-Ti Stainless Steel

Influence of strain accumulation schedule during hot rolling, under the thermomechanical control process (TMCP) on the structure evolution and carbide transformations of Cr-Ni-Ti austenitic stainless steel, is studied. The cellular, fragmented dislocation substructure and dynamically recrystallized (DRX) structure are observed in the steel with different strain accumulation schedules. It was found that the strain accumulation schedule, especially fractionality, affects the work-hardening and softening behavior quite significantly. The role of the strain accumulation schedule on the fragmented substructure and DRX structure evolution as well as carbide transformations and the relationship between the microstructure changes due to TMCP and the mechanical properties of studied steel, involving the recent ideas of the physics of large plastic strains, are considered.

A parallel discrete dislocation dynamics/kinetic Monte Carlo method to study non-conservative plastic processes

Non-conservative processes play a fundamental role in plasticity and are behind important macroscopic phenomena such as creep, dynamic strain aging, loop raft formation, etc. In the most general case, vacancy-induced dislocation climb is the operating unit mechanism. While dislocation/vacancy interactions have been modeled in the literature using a variety of methods, the approaches developed rely on continuum descriptions of both the vacancy population and its fluxes. However, there are numerous situations in physics where point defect populations display heterogeneous concentrations and/or non-smooth kinetics. Here, a kinetic Monte Carlo (kMC) approach for modeling vacancy transport in response to arbitrary stress fields is used. Vacancies are treated as point particles and are coupled to the dislocation substructure representing a deformed material via an advection term defined by the local stress gradients. The stress fields and the dislocation substructure are evolved using a discrete dislocation dynamics (DDD) module. To extend the coupled model to the treatment of large systems, we have implemented it in the massively-parallel DDD code ParaDiS. To avoid numerical incompatibilities associated with merging deterministic (DDD) and stochastic (kMC) integration algorithms, we cast the entire elasto-plastic-diffusive problem within a single stochastic framework, taking advantage of a parallel kMC algorithm to evolve the system as a single event-driven process. The large-scale implementation enables the study of the evolution of a variety of dislocation-defect scenarios governed by non-conservative transport kinetics. After carrying out an exhaustive numerical and computational analysis of our parallel algorithm, we show results that emphasize situations where inhomogeneous vacancy dynamics are of relevance, and compare discrete kinetics to continuum solutions for several cases.

The effect of microstructure evolution on the ratchetting-fatigue interaction of carbide-free bainite rail steels under different heat-treatment conditions

Low-cycle fatigue experiments emphasizing the ratchetting are performed on the carbide-free bainitic (CFB) rail steel under different heat-treatment conditions. The effects of microstructure and retained austenite (RA) on the ratchetting-fatigue interaction are investigated through macroscopic and microscopic characterizations. The results indicate that the CFB rail steel exhibits a non-saturated and strain amplitude-dependent cyclic softening feature and shows different ratchetting behaviors and failure modes at various stress levels. Ratchetting promotes the development of stable dislocation substructure and the transformation of blocky RA. The fatigue resistance of 1380 grade CFB steel under the online controlled cooling conditions is better than that of 1280 grade CFB steel under the air-cooling condition.

Effect of carbon-fluorine additive to flux on the structure, defective substructure and fracture surface of electric arc surfacing of low-carbon wire

By the methods of transmission and scanning electron microscopy the effect of a carbon-fluorine containing additive in a flux of a silicomanganese slag with a low content of manganese oxide on the structural-phase state, defect substructure and fracture surface of an electric arc surfacing of low-alloy steel is investigated. A quantitative analysis of the parameters of structure, phase composition, dislocation substructure of the surfacing with the additive into a flux and without it is made. It is shown that the deposited metal with the additive into a flux has a layer surface structure. It is detected that a predominant fracture mechanism is a ductile fracture with a formation of the pit structure of the fracture. The areas of a quasibrittle fracture are much more rarely detected. The main structural constituent of the deposited metal are the α-phase grains. The second structural constituent of the surfacing is the pearlite grains of lamellar morphology whose relative content in the surfacing with the carbon-fluorine containing additive to a flux is three-fold higher than in the surfacing to a flux without it. In most cases the α-phase grains are fragmentary. The particles of iron carbide of a round shape of 20–80 nm in size are revealed in the α-phase grains of the deposited metal with the carbon-fluorine containing additive to a flux. The metal of the surfacing with the additive to a flux is characterized by the higher values of scalar and excess dislocation density and the amplitude of curvature-torsion of a crystal lattice.

Gradients of Structure, Phase Composition, and Dislocation Substructure of Rails under the Ultra Long-Term Operation

In this paper, the structure, phase composition, dislocation substructure, and carbon redistribution in differentially quenched 100-meter rails after the ultra long-term operation (passed gross tonnage since 2013 — 1770 mln. tons) are studied using modern materials science and engineering approaches. The regularities of main parameters variations, such as changes in a relative variety of different morphological structures, scalar and excessive dislocation density, volume fractions of cementite, and redistribution of carbon atoms, are identified at the rail head cross section along the central axis and fillet rounding radius at the distances of 0, 2, 10 mm from the surface. It is suggested that the decrease of volume fraction of carbide phase in head surface layer can be related to the decarburization and appearance of carbon atoms at the steel structure defects. It is noted that transformations along the central axis proceed slower in relation to changes of the rounding radius of the fillet.

On the similitude relation for dislocation wall thickness under cyclic deformation

Dislocation substructures have been extensively characterized to explain the origin of strain hardening. Regions of high dislocation densities (walls) constrain the glide of mobile dislocations in regions of lower dislocation density (channels). Transmission electron microscopy (TEM) has shown that the distance in between walls is inversely proportional to flow stress, which is often referred as to the similitude principle. However, it still remains unclear whether a similitude scaling law exists for the dislocation wall thickness or wall fraction. The understanding of such a scaling law is instrumental to support substructure-based crystal plasticity, validate dislocation dynamics models, and explain substructure formation processes. Hence, this work surveys TEM images from various FCC metallic materials under cyclic deformation at various temperatures to assess the existence of the similitude principle for dislocation walls. The results demonstrate that the wall thickness does follow similitude, but the proportionality constant depends on the temperature for some structures.

The evolution of microstructure and mechanical properties of Zn-0.8Mg-0.2Sr alloy prepared by casting and extrusion

Zinc-based alloys containing elements well-tolerated by the organism (Mg, Ca, Sr) are considered as perspective biodegradable materials for an application like medical devices such as fixations of fractured bones or even stents. In the presented paper we characterize the relations between microstructure and mechanical properties of extruded Zn-0.8Mg-0.2 Sr alloy (wt%) depending on various parameters like extrusion temperature (150–300 °C) and ratio (11 or 25). Typical analysis including SEM with EBSD and mechanical tests indicate a strong dependence of obtained data on both extrusion temperature and ratio. Relatively wide range of elongation to fracture (2–22%) and anisotropy in compression yield strengths regarding loading direction (50–150 MPa) are explained by the huge effect of grain size, material texture and also the existence of dislocation substructures in materials extruded at elevated temperatures. Based on obtained results, appropriate extrusion conditions (200 °C, extrusion ratio 25) are suggested to reach the combination of superior mechanical properties 244 MPa, 324 MPa and 22% for tensile yield strength, ultimate tensile strength and elongation to fracture, respectively.

Contribution of dislocation substructures developed during cold rolling to the formation of rolling textures in Al–Mg alloys

Abstract In order to investigate the mechanism of the development of rolling textures in Al –Mg alloys, hot bands of high-purity Al, Al-3 wt. %Mg and Al-5 wt. %Mg alloys were solution-treated at 450 °C and cold-rolled by varying rolling reductions up to 97%. Rolling textures of these specimens were investigated with the orientation distribution function analysis. Comparisons of the results of the texture analysis with those of the hardness measurement and transmission electron microscopy observations revealed that, in all specimens, the texture development occurred mainly at the later half of work hardening, i. e., at rolling reductions above 80 %, where dislocation substructures were well developed. In pure Al, in which well defined, elongated cell structures were developed, {112}<111> and {123}<634> components developed remarkably, whereas in the Al-5% Mg alloys, in which only dense dislocation tangles and ill defined small cells were developed, the development of {112}<111> and {123}<634> components was strongly suppressed. All these observations thus suggested that the development of rolling textures was strongly affected by the development of dislocation substructures, which occurred concurrently during rolling, and that the formation of elongated cell structures was more favorable for the development of rolling textures. In cold rolling, elongated cell structures may be the final form of dislocation substructures, providing both mechanical stability and orientation stability.

Structural-phase state and defective sub-structure of low-carbon steel welds

The structural-phase state, defective substructure and fracture surface of welds made of low-carbon alloy steel obtained with and without a carbon-containing additive have been investigated by scanning and transmission electron microscopy. A quantitative analysis of the parameters of the structure and dislocation substructure of the weld metal is carried out, and the contributions of the scalar and excess dislocation density to the strength of welds are estimated. It is shown that high values of scalar and excess dislocation density in a weld formed without a carbon-containing additive in the flux can lead to material embrittlement.

Toward superior fatigue and corrosion fatigue crack initiation resistance of Sanicro 28 pipe super austenitic stainless steel

The present work deals with the fatigue and corrosion-fatigue crack initiation in cold-pilgered Sanicro 28 super austenitic stainless steel. Toward this end, the stress-controlled symmetric reverse bending fatigue tests were conducted in air and mixed acid aqueous solution under the stress amplitude of 200–570 MPa with the stress ratio of −1 and frequencies of 1 and 15 Hz at ambient temperature. The fatigue study was supplemented with electrochemical experiments and advanced microstructural analyses to explore the damage characteristics in detail. The planar character of dislocation substructure owing to the low stacking fault energy of the material, and formation of parallel slip bands led to a more homogenous distribution of the imposed strain which retarded the progress of damage accumulation. According to the X-ray photoelectron spectrometer results, the formation of the insoluble and stable MoO3 oxide increased the rate of repassivation. This sticky oxide resisted against breakdown rendered by the attack of aggressive chloride ions. The corrosion fatigue behavior of the experimented alloy under the various test frequencies was discussed relying on the synergistic effects of mechanical and environmental damages.

Physical nature of rail surface hardening during long-term operation

A comparative quantitative analysis of the physical mechanisms of hardening of rails surface layers after extremely long­term operation has been performed. The method is based on previously established patterns of formation of structural-phase states and mechanical properties of differentially hardened long­length rails produced by JSC “EVRAZ ZSMK” at a depth of up to 10 mm in the cutting of rails along the central axis and cutting out after the missed tonnage of 1411 million tons. The calculations took into account the volume fractions and characteristics of a particular type of substructure. Increase in microhardness and hardness of the surface layers of the rails subjected to ultra­long operation on the experimental ring of the Russian Railways is multifactorial and is determined by superposition of a number of physical mechanisms. The contributions are estimated due to friction of the matrix lattice, internal phase boundaries, dislocation substructure, presence of carbide particles, internal stress fields, solid hardening, and pearlitic component of the steel structure. Regardless of the analysis direction (along the central axis of the head or along the axis of symmetry of the chip), strength of the rails metal depends on the distance to the surface: it increases as it approaches the top of the head. The most significant physical mechanisms have been established, which provide high strength properties of the metal of the rail head subjected to extremely long­term operation. In the subsurface layer (located at a depth of 2 – 10 mm) of the rail head, the most significant physical mechanisms are dislocation mechanism, due to the interaction of moving dislocations with stationary dislocations (dislocations of the “forest”); in the surface layer of the rail head, substructural mechanism, due to the interaction of dislocations with small­angle boundaries of fragments and subgrains of a nanometer–sized polygon. A comparison with the quantitative values of the rail hardening mechanisms after the missed tonnage of 691.8 million tons was carried out. It is shown that an increase in the missed tonnage in the range of 691.8 – 1411 million tons leads to a significant (1.5 – 2.0 times) increase in strength.

Partitioning-related microstructure evolution and mechanical behavior in a δ-quenching and partitioning steel

The splitting of the quenching and partitioning of quenching - tempering & partitioning (Q-T&P) process permits us to investigate the as-quenched and partitioned states of the novel δ-quenching & partitioning (δ-QP) steel, separately. Both quenched and partitioned steels consisted of ferrite and martensite/austenite islands. After tempering at 350 °C, C and Mn partitioned into the austenite during partitioning. Besides, the films comprised of martensitic-based dislocation sub-structures were formed, and these were enriched with all solutes including Al. The deformation behavior of δ-QP steels was significantly adjusted by partitioning treatment. The fracture strain was improved by 38% after partitioning step. The higher tensile strength was attributed to the strengthened ferrite and more efficient transformation induced plasticity (TRIP) effect. The unpinned dislocations slipping in ferrite of partitioned steel induced an obvious recovery of work hardening rate. Meanwhile, the improved deformation uniformity and martensite ductility contributed to the higher ductility. Based on such an analysis of microstructure-property relationship, it is conducive to develop high-grade δ-QP steels and give some guidelines on the optimal design of low-density alloy system.

Effect of V Content and Heat Input on HAZ Softening of Deep-Sea Pipeline Steel.

In this paper, the welding thermal cycle process of deep-sea pipeline steel was investigated by welding thermal simulation. The microstructure evolution, crystallology and second-phase precipitation behavior of the soft zone of the heat-affected zone (HAZ) were characterized and analyzed by combining scanning electron microscopy, electron back-scattered diffraction, transmission electron microscopy and hardness testing. The results show that HAZ softening appeared in the fine-grained zone with a peak temperature of 900–1000 °C for deep-sea pipeline steel, the base metal microstructure of which was the polygonal ferrite and acicular ferrite. Using V microalloying and low welding heat input could effectively decrease the softening of the HAZ fine-grained region, which was achieved by reducing the effective grain size, increasing the proportion of the dislocation substructures, and precipitating the nanoscale second-phase particles.

Influence of crystallographic orientation on the recrystallization of pure tantalum through microstructure-based estimation of the stored energy

Plastic deformation and static recrystallization of pure tantalum are studied using Electron BackScatter Diffraction (EBSD) technique. Recrystallization kinetics are discussed in light of stored energy estimation from EBSD data acquired on deformed microstructures. Characterization of deformed state reveals the influence of crystallographic orientation on the plastic deformation. Dislocation substructures are formed in the γ-fiber grains and almost no substructures are observed in the θ-fiber grains. Subsequent recrystallization directly inherits this orientation dependence of deformed state. Nucleation is promoted in the γ-fiber grains because of substructure development whereas nucleation is more sluggish to occur in the θ-fiber grains. Thus, a microstructure composed mostly of γ-fiber grains recrystallizes faster than a microstructure with a less strong texture, despite a strain nearly three times lower. At the polycrystalline scale (step size of 1.20 μm in the present case), recrystallization kinetics are better described with stored energy estimated through substructures than through dislocation density. At the substructure scale (step size of 90 nm), Geometrically Necessary Dislocation (GND) density seems to be correctly estimated; still recrystallization kinetics cannot be accounted for by this density because it does not account for the substructure formation.

Эволюция дислокационной структуры в слабоустойчивых состояниях при деформации твердых растворов сплавов

The results of a study of the evolution of the dislocation structure of polycrystals of homogeneous solid solutions in low-stability states in Cu-Mn-based alloys with a FCC crystal lattice are presented. It is found that each stage is characterized by its own deformation carrier in the form of a certain dislocation substructure (DSS), the volume fraction of which has a maximum at this stage. At the same time, at the considered stage, the carrier of deformation of the previous stage gradually disappears as the degree of deformation (ε) increases, however, a carrier of deformation of the subsequent stage appears. In a certain range of values of ε, deformation carriers characteristic of the previous, considered and subsequent stages simultaneously coexist. It is shown that transitions from some types of DSS to other types occur in some intervals of values of the degree of deformation ε. It is discovered that each stage of deformation has its own DSS - carriers of deformation. The transition from stage to stage is accompanied by the formation of new deformation carriers, which is a characteristic feature of weakly stable states of the system. Naturally, the described picture is observed because the study is carried out at such a grain size, which suggests the possibility of implementing a low-stability structural-phase state of the alloy, since the grain size acts in this case as a kind of control parameter.