Density Of Free Dislocations Research Articles

Influence of microstructure on slurry erosion resistance of medium carbon high silicon steels

The present study compares slurry erosion resistance of austempered and tempered medium carbon high silicon steels. A 0.62C-1.91Si-0.85Mn-0.23Cr steel was subjected to austempering at 300, 350 and 400 °C for 10 min to develop a microstructure comprising of bainite and retained austenite. With the increase in the austempering temperature, the mechanical stability of retained austenite decreased, and the size of bainitic ferrite laths increased. As a result, strain hardening ability and toughness of the steels decreased with austempering temperature. Another set of steel with the same composition was quenched and tempered at 300, 450 and 550 °C for 60 min. An increase in the tempering temperature led to an increase in the size and fraction of carbides and a decrease in the mean free path and dislocation density. Strain hardening ability and toughness of tempered steels was substantially small than austempered steels. Both the austempered and tempered steels were subjected to slurry erosion. Austempered steels exhibited higher slurry erosion resistance as compared to tempered steels. Slurry erosion resistance of austempered steels has been noted to depend on the mechanical stability of retained austenite and the size of bainitic ferrite laths. Due to the difference in the extent of deformation-induced transformation of retained austenite into martensite in the steels austempered at different temperatures, number of mobile dislocations generated (to accommodate the dilatation due to transformation) in bainitic ferrite laths differed. A large number of mobile dislocations in the specimens austempered at 300 °C blunted the propagating crack and delayed the material loss during erosion. The erosion resistance of tempered steels was observed to decrease with an increase in tempering temperature. A large fraction of coarse carbides led to the formation of many decohesion sites in steels tempered at 450 and 550 °C. Further, a small number of mobile dislocations present per unit volume in the steels tempered at high temperatures could not resist the propagation of cracks nucleated from the decohesion sites leading to a high erosion rate in these steels.

Strengthening Mechanisms in a Medium-Carbon Steel Subjected to Thermo-Mechanical Processing

Strengthening mechanisms were examined in a Fe-0.43C-1.60Si-0.01Mn-1.1Cr-0.95Mo-0.08V-0.05Nb steel exhibiting a yield strength (YS) of 1310 MPa after tempering and 1550 MPa after tempforming. The dislocation strengthening gave a major contribution to the overall YS of the steel in the tempered condition, whereas dispersion strengthening was a major contributor to the YS of the steel in the tempformed condition. High values of dislocation strengthening after tempering were attributed to dislocations composing the lath boundaries. A high density of free dislocations provided nearly the same dislocation strengthening after tempforming. Warm rolling after tempering led to alignment of intercrystallite boundaries along the rolling direction that decreased the interparticle spacing between M23C6 carbides located at these boundaries and thus increased the magnitude of dispersion strengthening. The boundary strengthening contributed to overall YS significantly due to small lath thickness after tempering and nanoscale spacing between lamellar boundaries after tempforming.

The electroplastic effect in coarse-grained and ultrafine-grained titanium

One of the well-known features of the external action of the electric current in the process of plastic deformation is the electroplastic effect manifesting in a decrease in flow stresses and an increase in plasticity (deformability). Understanding the nature of the electroplastic effect provides targeted regulation and application of the effect to improve the efficiency of metal working processes or to change the structure and properties of materials. The deformation behavior of commercially pure titanium under the impact of an electric current of critical density from 12 to 400 A/mm2 is considered. The electroplastic effect in coarse-grained (d = 50 μm) and ultrafine-grained (d = 500 nm) VT1-0 titanium has been studied under a combination of tensile deformation and applied current of various modes and regimes, including the single-pulse, multipulse and direct current modes. It is shown that a decrease in the grain size contributes not only to an increase in the strength characteristics, but also to a decrease in the electroplastic effect, the mechanism of which is closely related to the density of mobile dislocations. It has been shown that the manifestation of the electroplastic effect in titanium is controlled by the grain size, and a decrease in the grain size leads to its electroplastic degradation and finally to the complete disappearance in the amorphous state due to a decrease in the density of free dislocations.

Evolution of Structure and Properties of Nickel-Enriched NiTi Shape Memory Alloy Subjected to Bi-Axial Deformation.

The effect of a promising method of performing a thermomechanical treatment which provides the nanocrystalline structure formation in bulk NiTi shape memory alloy samples and a corresponding improvement to their properties was studied in the present work. The bi-axial severe plastic deformation of Ti-50.7at.%Ni alloy was carried out on the MaxStrain module of the Gleeble system at 350 and 330 °C with accumulated true strains of e = 6.6-9.5. The obtained structure and its mechanical and functional properties and martensitic transformations were studied using DSC, X-ray diffractometry, and TEM. A nanocrystalline structure with a grain/subgrain size of below 80 nm was formed in bulk nickel-enriched NiTi alloy after the MaxStrain deformation at 330 °C with e = 9.5. The application of MaxStrain leads to the formation of a nanocrystalline structure that is characterized by the appearance of a nano-sized grains and subgrains with equiaxed and elongated shapes and a high free dislocation density. After the MaxStrain deformation at 330 °C with e = 9.5 was performed, the completely nanocrystalline structure with the grain/subgrain size of below 80 nm was formed in bulk nickel-enriched NiTi alloy for the first time. The resulting structure provides a total recoverable strain of 12%, which exceeds the highest values that have been reported for bulk nickel-enriched NiTi samples.

On the microstructural evolution and failure mechanism in laser powder bed fusioned Ti–6Al–4V during low cycle fatigue at room and elevated temperatures

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

Yielding Behavior and Strengthening Mechanisms of a High Strength Ultrafine‐Grained Cr–Mn–Ni–N Stainless Steel

The microstructure of Cr–Mn–Ni–N stainless steel is regulated by a combination of cold deformation and subsequent martensite reversed annealing. The yield strength and tensile strength of the tested steel are greatly improved to 1220 and 1357 MPa, respectively, by conventional heavy cold rolling (66% thickness reduction) and a subsequent annealing process (750 °C for 5 min) relative to those before cold rolling (austenized state); at the same time, the total elongation keeps an appropriate value. The results show that the grain refinement strengthening acts as the primary factor for the strengthening of the tested steels while the precipitation behavior from the M23C6‐type carbides is not quite effective. Additionally, different yield phenomena and microstructural evolution for tested steels with 66% thickness reduction annealed at 750 °C with different holding times are systematically analyzed and discussed. Lüders‐like deformation features noticeably appear but become weakening with the longer annealing time. The change in initial free mobile dislocation density is the core cause of the preyield plateau phenomenon.

Heuristic Design of Advanced Martensitic Steels That Are Highly Resistant to Hydrogen Embrittlement by ε-Carbide

Many advanced steels are based on tempered martensitic microstructures. Their mechanical strength is characterized by fine sub-grain structures with a high density of free dislocations and metallic carbides and/or nitrides. However, the strength for practical use has been limited mostly to below 1400 MPa, owing to delayed fractures that are caused by hydrogen. A literature survey suggests that ε-carbide in the tempered martensite is effective for strengthening. A preliminary experimental survey of the hydrogen absorption and hydrogen embrittlement of a tempered martensitic steel with ε-carbide precipitates suggested that the proper use of carbides in steels can promote a high resistance to hydrogen embrittlement. Based on the surveys, martensitic steels that are highly resistant to hydrogen embrittlement and that have high strength and toughness are proposed. The heuristic design of the steels includes alloying elements necessary to stabilize the ε-carbide and procedures to introduce inoculants for the controlled nucleation of ε-carbide.

Effect of high magnetic field on the recovery of tempered martensite

Influence of a 12 T high magnetic field on the recovery of tempered martensite in the chromium-containing steel was investigated. The results showed that a high magnetic field caused the percentage of low angle boundaries θ between 5° ~ 8° to increase by about 8%, while correspondingly decreased the percentage of high angle boundaries (>15°) by 9%. With the application of the magnetic field, the strain energy increased due to the higher density of free dislocations in the martensite, which leads to the retardation of recovery and little change in its lath shape due to the inhibition of the transition from low to high angle boundaries to maintain lower energy in the matrix.

Structure and mechanical properties of a high-carbon steel subjected to severe deformation

The structure and mechanical properties of a high-carbon eutectic steel subjected to the cold plastic deformation by hydrostatic extrusion in a wide range of true strain have been studied. Using scanning and transmission electron microscopy, it has been shown that the formation of cellular, fragmented, and submicrocrystalline structures occurs in the ferritic constituent of the pearlite structure of the steel upon extrusion. This is a consequence of the occurrence of dynamic recovery and continuous dynamic and post-dynamic recrystallization, which cause a decrease in the density of free dislocations at the true strain of more than 1.62. The partial dissolution of the carbide phase is also observed. It has been found that, at a true strain of up to 0.81, the strength properties of the investigated steel are determined mainly by subgrain, dislocation, and precipitation mechanisms of the strengthening; in the deformation range of 0.81–1.62, the role of the grainboundary strengthening increases. At strains above 1.62, grain-boundary strengthening is a prevailing mechanism in the formation of the level of strength properties of the extruded U8A steel. The ultimate tensile strength and yield stress over the entire strain range only uniquely correlate with the density of highangle boundaries; the dependences of the strength characteristics on other structural parameters are not monotonic.

The effect of crystal-plastic deformation on isotope and trace element distribution in zircon: Combined BSE, CL, EBSD, FEG-EMPA and NanoSIMS study

Plastically-deformed zircon grains from granulite-facies (Ivrea-Verbano Zone, Southern Alps, Italy) and amphibolite-facies (Tauern Window, Eastern Alps, Austria) shear zones have been investigated. The main focus was on the effects of crystal-plastic deformation on the distribution of trace elements and their isotopes. Zircon grains reveal crystal-plastic deformation patterns in form of (I) gradual bending of the lattice (high density of free dislocations), (II) highly-deformed margins revealing a combination of low- and high-angle boundaries and gradual bending of the lattice, and (III) a low-angle boundary (LAB) network. Chemical and isotopic maps and profiles show that trace elements in zircon are re-distributed in all plastically-deformed domains. Changes in trace element composition along LABs are sometimes revealed by brighter CL and darker BSE signal. LABs and domains of high free-dislocation density have depletion in U, Y, Yb and enrichment in Ce, La and Nd, while Ti and P are either enriched, depleted or remain unaffected, and Hf demonstrates stability. Y and Yb are decreased in concentration across LABs, and have oscillating concentration in domains of high free-dislocation density. Our observations confirm that crystal-plastically deformed domains in zircon act as effective pathways for trace cations.The Pb isotopic system is disturbed by crystal-plastic deformation microstructures, as indicated by relative 207Pb/206Pb ages showing significant discordance in plastically-deformed zircon domains. In deformed domains, a positive correlation between dislocation density and the degree of isotopic age distortion is observed. Fractured or porous domains and domains with high density of (sub)grain boundaries are enriched in common Pb from the matrix, and thus show significantly older relative 207Pb/206Pb ages than pristine domains. In contrast, Pb loss occurs in domains with high free-dislocation density at a distance of 5–10μm from the grain boundary, caused by out-diffusion of radiogenic Pb.This study adds to a growing database on the consequences of crystal-plastic deformation for trace elements and isotopic systems in zircon. We provide additional evidence that such deformation cannot be neglected and may have important implications for zircon geochronology and geochemistry.

Mineralogical Characteristics of Garnet in Garnet Mica Schist and Its Tectonic Significance in the Tongbai Section of the Shangdan Fault Zone

Studies on the metamorphism and deformation conditions of rocks in the suture zone are good ways to discuss the orogenic process and mechanism. The microstructure and ultramicrostructure features of minerals are true embodiment of formation environment of the orogenic belt. Based on the study of microstructure, ultramicrostructure deformation characteristic and compositional zonation of garnets in garnets-mica schists in the Tongbai mountain, east section of the Shangdan fault zone, the results show that the three types of garnets have suffered various states of plastic deformation. The dynamic recrystallization of garnets is due to the subgrain growth and boundary migration. The dislocations are mainly free dislocation and dislocation walls. The free dislocation density ρ = 6.14 × 108/cm2, and dislocation movement are mainly slips; slip planes are 1/2 {110} and {001}. Garnet microprobe analysis shows that it belongs to almandine, and reflects it has undergone epidote-amphibolite to amphibolite facies metamorphism. Compositional zonation of garnet shows that the rocks had experienced progressive metamorphism. First metamorphic environment was continuous temperature-pressure and in mid-term there were two non-synchronous transient cooling and decompression processes, and it finally underwent decompression and warming process of the thermal relaxation environment. The formation condition of garnet-mica schist is estimated: T = 562°C - 617°C, and P = 0.77 - 1.02 GPa. The differential stress is 0.511 GPa and strain rate is 4.97536 × 10-10 m/s. After systemic analysis, a conclusion is drawn that the plastic deformation mode, deformation mechanism and formation environment of garnets are closely related to the formation and development of the Shangdan fault zone. It truly reflects that the Shangdan fault zone, as the suture zone of Yangtze and north China plate, has been subjected to early medium-grade metamorphism. With the continuous compression after the collision, the left-lateral shearing happened and caused the formation of high density dislocations and subgrains of the garnets; finally the plastic deformation happened and the bulging recrystallization formed in the period of heat relaxation had a relative low stress.

Cyclic deformation and microstructural behaviour of reduced activation ferritic–martensitic steels

The present paper presents results about cyclic behaviour and the evolution of the dislocation structure of reduced activation ferritic–martensitic steels and commercial martensitic steels AISI 410 and 420. The variation of the free dislocation density within subgrains and subgrain size was mainly analysed during the cyclic softening of EUROFER 97 steel. From the analysis of the flow stress components, the friction and back stresses, and the information of the evolution of the dislocation structure, it could be concluded that the softening of tempered martensitic steels at 20°C is produced by the contribution of the friction stress and aided later by the back stress.

Effect of multiaxial deformation Max-strain on the structure and properties of Ti-Ni alloy

The severe plastic deformation (SPD) forming ultrafine-grained (nanocrystalline or nanosubgrained) structure is one of the most effective ways to improve the functional properties of Ti-Ni-based shape memory alloys [1, 2]. In the present work, the SPD of near-equiatomic Ti-Ni alloy was carried out using the multi-axial deformation module Max-strain, which is a part of the physical simulation system "Gleeble 3500". The deformation was performed at a constant temperature of 400°C with speed of 0.5 mm/s in six passes without interpass pauses. The accumulated true strain was about 3. As a result, a mixed ultrafine-grained/subgrained structure with grain/subgrain sizes from 50 to 300 nm and a high density of free dislocations formed. The resulting structure is close to a nanoscale region and provides a significant advantage in the basic functional property – completely recoverable strain – as compared with a conventional recrystallized structure: 7% versus 2%.

Quantification of the effect of early microstructural degradation during creep of 9Cr–1Mo–NbV steels at 600 °C

A microstructure-based model for the prediction of the creep resistance at 600°C of 9%Cr steels was applied to literature data, in order to clarify the reasons for the drop in creep strength observed for certain heats of P91 steels. The analysis demonstrated that the observed sigmoidal behaviour of the time to rupture vs. applied stress graph can be explained by the progressive and early dissolution of the fine MX particles which, in the lath interior, interact with dislocations obstructing their mobility. The same phenomenon, whose effects in part eclipse those of the recovery of the substructure, i.e. the lath/subgrain growth and the reduction in free dislocation density, explains the abrupt decrease in the room temperature hardness as measured after rupture with different test durations.

Calcite dissolution kinetics in view of Gibbs free energy, dislocation density, and pCO2

The dissolution of calcite in a full range of saturation conditions is investigated to explore the kinetic effect of Gibbs free energy, dislocation density, and the presence of atmospheric pCO2. Experiments are carried out in a mixed-flow reactor at room temperature (25°C) in both closed and open (to air) settings, and calcite samples are prepared by fragmentation and milling to generate different defect densities. Experimental observations show a highly nonlinear dependence of the dissolution rates on the Gibbs free energy; however, the kinetics does not seem to be affected by the samples' dislocation density, nor the presence of atmospheric pCO2 at any saturation condition. Fitting the conventional transition state model (TST) to the observed rate - free energy relationship indicates that, though the TST rate equation is sufficient to describe the dissolution kinetics near and far from equilibrium, it clearly overestimates the dissolution rate when the system sits in between. These results suggest that: (i) the classic TST model may not be sufficient to depict the relation between dissolution rate and Gibbs free energy once solution saturation falls from its extrema. (ii) The steps associated with increased crystal defects may be overwhelmed by those regenerated at corners and edges of calcite particles through layer-by-layer dissolution along the cleavage directions. (iii) The presence of CO2 in ambient environments bears little importance to calcite dissolution possibly due to the slow response of aqueous HCO3− to pCO2 change at low CO2 partial pressure conditions.

On the Cyclic Softening Mechanisms of Reduced Activity Ferritic/Martensitic Steels

AbstractLow‐cycle fatigue tests were performed in three different ferrite/martensite steels, i.e., the European RAFM steel EUROFER 97 and the commercials AISI 410 and AISI 420, at room temperature (RT) and at 550°C. After the first few cycles, a cyclic softening that continues up to failure is observed for all these steels. The cyclic softening exhibited by AISI 420 is less pronounced than for the other two steels. The comparison between the mechanical responses of the materials was based on the study of the flow stress components, i.e., the friction and the back stresses, and their correlation with the microstructure evolution. In most cases, the strong cyclic softening observed is produced by the decreasing stress values exhibited by both stress components. However, at RT, for AISI 420, the back stress does not present variation during cycling. The decrease of the free dislocation density inside the subgrains and the growth of the mean subgrain size represent the main microstructural evolution.

Microstructural characterization of EUROFER 97 during low-cycle fatigue

The quenched and tempered reduced-activation ferritic/martensitic steel EUROFER 97 is one of the candidates for structural components of Generation IV nuclear power plants. The cyclic behaviour of this steel during isothermal plastic strain-controlled tests was investigated at room temperature and at 550°C. Under low-cycle fatigue test this steel shows, after the first few cycles, a pronounced cyclic softening accompanied by microstructural changes such as the decrease of the free dislocation density inside the subgrain. The rate of softening increases with temperature being very pronounced at temperatures above 500°C. The evolution of the flow stress during cycling was studied by analyzing the so-called “back” and “friction” stresses obtained from the hysteresis loops measured along the entire test. From the analysis of the hysteresis loops and corroborated by electron microscopy observations, it can be concluded that the strong cyclic softening observed is produced by the decrease exhibited by the friction stress. The Taylor coefficient was calculated measuring the evolution of the free dislocation density.

Influence of strain rate on P92 microstructural stability during fatigue tests at high temperature

9-12%Cr creep-resistant ferritic-martensitic steels are candidates for structural components of Generation IV nuclear power plants. However, they are sensitive to softening during fatigue and creep-fatigue loading. To better understand softening mechanisms in ASTM Grade 92, fatigue tests were carried out at 823 K at various strain amplitudes. Two different values of the strain rate (2 10−3 s−1 and 10−5 s−1) were used for one strain amplitude. The softening behavior is mainly due to microstructural evolution. Examination of fractured specimens (hardness tests, TEM) shows an influence of strain rate on both increase in subgrain size and decrease in free dislocation density during cycling. Study of the evolution of isotropic, kinematic and viscous contributions to stress during fatigue tests shows a decrease in the kinematic contribution during cycling. A simplified mean field polycrystalline model based on subgrain growth is proposed in order to account for this strain rate effect. Potential impact on further creep resistance behavior is discussed.

Novel UV devices on high-quality AlGaN using grooved underlying layer

A grooved Al 0.25Ga 0.75N underlying layer on an AlN-coated sapphire substrate was used to grow crack free and low dislocation density Al 0.25Ga 0.75N to successfully realize high-performance UV A light emitters. A light-emitting diode grown on a grooved AlGaN underlying layer exhibited an output power of 12 mW at a DC current of 50 mA for a peak emission wavelength of 345 nm with an external quantum efficiency of 6.7%, which is the highest to date in this wavelength region. We also fabricated UV A laser diodes with an emission wavelength of 356 nm at a pulsed injection current of 414 mA.

The effect of pressure on transformation temperatures and some physical parameters of Fe–32Mn–6Si–3Cr shape memory alloy

An experimental investigation of the effect of pressure on shape memory behavior of Fe–32%Mn–6%Si–3%Cr alloy was undertaken. There are significant differences in the Gibbs free energy, dislocation density and transformation temperatures of the alloy due to applied pressure.