Change In Dislocation Density Research Articles

Changes in the structure and conducting properties of copper nanotubes as a result of bombardment with O3+ ions

The results of targeted modification of the structure and properties of copper nanotubes by accelerated O3+ ions at a fluence of 1 × 109 to 5 × 1011 cm−2 with an energy of 1.75 MeV/nucleon are reported. XHray diffraction methods have been used to study the dynamics of changes in the crystallite shape, dislocation density, and the orientation of copper nanotubes before and after irradiation. It has been shown that irradiation with accelerated ions has a significant effect on the change in texture coefficients and dislocation density. At a fluence of 1 × 1010 cm−2 or higher, the accumulation of oxygen in the nanotube structure is observed, which leads to the appearance of oxide compounds.

Quantitative Evaluation of Texture and Dislocations during Annealing after Hot Deformation in Austenitic Steel Using Neutron Diffraction

Microstructural change during hot compressive deformation at 700 oC followed by isothermal annealing for a Fe-32Ni austenitic alloy was monitored using in situ neutron diffraction. The evolution of deformation texture with 40% compression and its change to recrystallization texture during isothermal annealing were presented by inverse pole figures for the axial and radial directions. The change in dislocation density was tracked using the convolutional multiple whole profile fitting method. To obtain the fitting results with good accuracies, at least 60 s time-interval for slicing the event-mode recorded data was needed. The average dislocation density in 60 s after hot compression was determined to be 2.8 x 1014 m-2, and it decreased with increasing of annealing time.

Defect substructure change in 100-m differentially hardened rails in long-term operation

By methods of optical and transmission electron diffraction microscopy the evolution in structural phase states of surface layers head of differentially hardened rails of DT 350 category to the depth up to 10mm along the central axis after the passed tonnage of 691.8mln.tbrutto was studied. The formation of gradient substructure being expressed in regular change in scalar and excess density of dislocations, curvature-torsion amplitude of steel crystal lattice, degree of deformation transformation of lamellar pearlite structure was defected. The failure mechanisms of cementite plates were considered.

Rayleigh Surface wave based non linear ultrasound to assess effect of precipitation hardening during tempering in P92 steel

This study presents the generation and variation of second harmonic of Rayleigh surface wave due to the change in precipitation morphology during tempering of P92 steel. The specimens from P92 steel were normalized at 1075°C and then tempered in the temperature range of 745–835°C at a step of 30°C for 3hrs followed by air cooling. Non linear ultrasonic (NLU) parameter which is defined as the ratio of the amplitude of second harmonic to the square of the amplitude of the transmitted signal frequency was measured at each temperature and was correlated with the changes in precipitates number density, volume fraction and dislocation density. It was observed that the measured non linearity parameter is sensitive to the precipitate coherency strain. Similar trend in the variation of mechanical properties and NLU parameter with the tempering temperature suggests that this technique can be useful in monitoring material strengthening due to the precipitation hardening and precipitation-dislocations interactions.

Effect of the state of carbon on ductility in Fe-0.017mass%C ferritic steel

The influence of the state of carbon on ductility in Fe-0.017mass%C alloy was systematically investigated. Moreover, the changes in dislocation density, dislocation substructure, and dislocation character with tensile strain were evaluated using X-ray line profile analysis together with transmission electron microscopy. The solute carbon maintains uniform elongation while simultaneously increasing the ultimate tensile strength by increasing the work-hardening rate; however, post-uniform elongation is significantly impaired by the occurrence of dynamic strain aging (DSA). The coarse intergranular cementite leads to superior uniform elongation; however, post-uniform elongation deteriorates presumably owing to the void formation in the cementite/matrix interface or the fracture of cementite. Meanwhile, the fine transgranular cementite increases the yield strength, thus impairing the uniform elongation; however, it maintains post-uniform elongation. This is because small cementites less than about 1µm in size do not deteriorate the post-uniform elongation. The solute carbon prominently increases the dislocation density, particularly the edge dislocation density, with tensile strain owing to DSA, leading to the prohibition of dislocation cell structure formation.

Mesoscale modeling of dynamic recrystallization behavior, grain size evolution, dislocation density, processing map characteristic, and room temperature strength of Ti-6Al-4V alloy forged in the (α+β) region

Abstract This work focuses on a series of microstructural predictions regarding the dynamic recrystallization (DRX) behavior, change in grain size and dislocation density. Additionally, this study includes the prediction of the room-temperature-strength and the processing map characterization of Ti-6Al-4V alloy after forging in the (α+β) region. The experimental tests of the forgings reveal that the mechanism of microstructural conversion depends on the testing conditions. That is, superplastic behavior is exhibited at lower strain rates and higher temperatures. With an increasing strain rate and decreasing temperature, DRX occurs in a continuous manner. A set of constitutive equations that model the microstructural evolution, processing map characteristics and room temperature strengths is established by optimization of experimental data, followed by implementation into the DEFORM-3D software package. There is satisfactory agreement between the experimental and simulated results. Thus, a series of constitutive models (established in this work) is found to enable the reliable prediction of properties of Ti-6Al-4V alloy after forging in the (α+β) region.

The Dominant Influence of Plastic Deformation Induced Residual Stress on the Barkhausen Effect Signal in Martensitic Steels

The paper presents the results of investigation of the influence of plastic deformation on the magnetic properties of martensitic steel (P91 grade). The properties of the hysteresis loops as well as of the Barkhausen effect (BE) signal are analysed for both tensile and compressive loading up to varepsilon = 10% of plastic deformation. The choice of the steel and of the deformation range is unique, since for such combination one can expect high residual stresses (both compressive and tensile) in the material that does not exhibit saturation of the BE intensity as a function of elastic stress. The obtained relationships show that for the low level of deformation the dislocation density changes may play a dominant role, yet for higher deformation level the residual stress becomes a dominant factor. It leads to the strong decrease of the BE signal for tensile deformation and an increase for the case of compression. It agrees well with the assumption that the tensile plastic deformation results in the compressive stresses appearance in the soft (magnetically active) sub-regions of the material whereas for the compression one can expect a residual stress of a tensile nature in those areas. Both deformation modes result in the increase of coercivity of the samples, yet the increase observed for the tensile deformation is significantly higher since both the residual compressive stress and increase of dislocation density have a strong effect on the material coercivity. The change of the hysteresis loops steepness agrees well with the notion of the dominant role of residual stresses too.

Innovative analysis of Luders band behaviour in X80 pipeline steel

In this study, the influence of Luders bands, which are frequently observed after the pipe coating process, is investigated. Stratified tensile test specimens, machined from different locations across the thickness of an X80 line pipe, were tested. The Luders bands were observed by using a digital image correlation (DIC) in the specimen machined from halfway across the pipe wall thickness. The specimens from the selected locations related to the Luders bands were studied and compared by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and standard mechanical testing. The results display that the microstructure, lattice constant, dislocation density and Schmid factors change when the Luders bands pass through the tensile test specimen, which brings about the difference in mechanical properties. The study of the influence of Luders bands on microstructures and mechanical properties enables better understanding of the strain-ageing mechanism.

In situ electron backscatter diffraction study of twinning in commercially pure titanium during tension-compression deformation and annealing

Tension-compression forward and reverse loading followed by annealing at 773K for 1h was carried out on commercially pure titanium using an in situ heating stage with tensile testing facility in scanning electron microscope fitted with electron backscatter diffraction setup. Twins of four different variants were observed during tension in load control mode in a relatively large grain within the area of inspection. Morphology of the twins changed during subsequent deformation stages and un-indexed points within the twins decreased or increased which may be due to changes in dislocation density during unloading and compression loading stages. Reloading in tension caused barrelling of the twins. Some of the twins disappeared and new twins formed during annealing. Although all the variants show similar Schmid factor for extension twinning, there is variation in lateral thickening of different twin variants within the same grain which can be explained from variation in elastic modulus. It is also observed that twin intersections can play an important role in nucleation and growth of new twins.

Microstructure evolution and constitutive equations for the high-temperature deformation of 5Cr21Mn9Ni4N heat-resistant steel

The high-temperature deformation behavior of 5Cr21Mn9Ni4N heat-resistant steel was studied using a Gleeble-1500D thermal simulation test machine for high-temperature compression tests. The deformation temperatures ranged from 1273 K to 1393 K, and the strain rates ranged from 0.1 s−1–10 s−1. The height reductions were 20%, 40%, and 60%. The flow stress of 5Cr21Mn9Ni4N increased with increasing strain rate, whereas it decreased with increasing temperature. The high-temperature deformation of 5Cr21Mn9Ni4N began from the strain-hardening stage to the steady-state deformation stage. The characteristics of the stress–strain curves were determined through the interaction of work hardening, dynamic recovery, and dynamic recrystallization. The relationship between microstructure and processing parameters was analyzed. A set of unified viscoplastic constitutive equations based on changes in dislocation density, volume fraction of dynamic recrystallization, and grain size was established to predict deformation behavior and microstructure during a high-temperature working process. The average relative error between the calculated and experimental flow stress was 4.8%. This finding indicated that the constitutive equations could be used to predict the flow behavior of 5Cr21Mn9Ni4N accurately during high-temperature deformation.

Time dependent ductility improvement of stainless steel SS 316 using stress relaxation

Plastic deformation of materials experience stress relaxation with time when stopped intermittently. The relaxation alters the ductility and the strain hardening behaviour during subsequent deformation due to the change in mobile dislocation density and internal stress. The prior studies on ductility improvement due to stress relaxation have focussed on multiple relaxation in the uniform deformation zone. The effect of single relaxation on ductility improvement is not reported so far. In the present work, single relaxation tests are performed using stainless steel 316 material. The effect of strain, time and strain rate on the ductility improvement is presented. An increase in ductility is consistently observed in all the cases of stress relaxation experiment. Possible explanation on the underlying mechanisms for ductility improvement based on the present understanding is discussed. A power law and logarithmic relation of stress vs time are modified in the present work to account for these changes.

Cellular Automaton Modeling of Dynamic Recrystallisation Microstructure Evolution for 316LN Stainless Steel

Based on the theoretical model and physical mechanism of dynamic recrystallization (DRX) in metal materials, the dislocation density change, nucleation and grain growth model during the process of DRX are taken into account. And according to the nucleation driven by dislocation and grain growth kinetic, transformation rules are made. A modeling methodology coupling fundamental metallurgical principles based on amended nucleation rate with the cellular automaton (CA) technique is here derived to simulate the 316LN.The two-dimensional CA model uses quadrilateral element and periodic boundary condition and Von-Neumann neighbor type. The influence of strain, strain rate and deformation temperature on dynamic recrystallization volume fraction and average grain size are analyzed on the basis of established CA model.

A Physically Based Dynamic Recrystallization Model Considering Orientation Effects for a Nitrogen Alloyed Ultralow Carbon Stainless Steel during Hot Forging

The nitrogen alloyed ultralow carbon stainless steel is a good candidate material for primary loop pipes of AP1000 nuclear power plant. These pipes are manufactured by hot forging, during which dynamic recrystallization acts as the most important microstructural evolution mechanism. A physically based model was proposed to describe and predict the microstructural evolution in the hot forging process of those pipes. In this model, the coupled effects of dislocation density change, dynamic recovery, dynamic recrystallization and grain orientation function were considered. Besides, physically based simulation experiments were conducted on a Gleeble-3500 thermo-mechanical simulator, and the specimens after deformation were observed by optical metallography (OM) and electron back-scattered diffraction (EBSD) method. The results confirm that dynamic recrystallization is easy to occur with increasing deformation temperature or strain rate. The grains become much finer after full dynamic recrystallization. The model shows a good agreement with experimental results obtained by OM and EBSD in terms of stress-strain curves, grain size, and recrystallization kinetics. Besides, this model obtains an acceptable accuracy and a wide applying scope for engineering calculation.

Effects of aging time and temperature of Fe-1wt.%Cu on magnetic Barkhausen noise and FORC

Magnetic Barkhausen noise (MBN), hysteresis measurements, first order reversal curves (FORC), Vickers microhardness, and Transmission Electron Microscopy (TEM) analyses were performed on Fe-1wt.%Cu (Fe-Cu) samples isothermally aged at 700°C for 0.5 – 25 hours to obtain samples with different sized Cu precipitates and dislocation structures. Fe-Cu is used to simulate the thermal and irradiation-induced defects in copper-containing nuclear reactor materials such as cooling system pipes and pressure vessel materials. The sample series showed an initial increase followed by a decrease in hardness and coercivity with aging time, which is explained by Cu precipitates formation and growth as observed by TEM measurements. Further, the MBN envelope showed a continuous decrease in its magnitude and the appearance of a second peak with aging. Also, FORC diagrams showed multiple peaks whose intensity and location changed for different aging time. The changes in FORC diagrams are attributed to combined changes of the magnetic behavior due to Cu precipitate characteristics and dislocation structure. A second series of samples aged at 850°C, which is above the solid solution temperature of Fe-Cu, was studied to isolate the effects of dislocations. These samples showed a continuous decrease in MBN amplitude with aging time although the coercivity and hardness did not change significantly. The decrease of MBN amplitude and the appearance of the second MBN envelope peak are attributed to the changes in dislocation density and structure. This study shows that the effect of dislocations on MBN and FORC of Fe-Cu materials can vary significantly and should be considered in interpreting magnetic signatures.

The effect of reinforcement percentages on properties of copper matrix composites reinforced with TiC particles

In this research, copper matrix composites reinforced with different amounts of titanium carbide particles were produced by mechanical milling and in-situ formation of reinforcements. Morphology and size of milled powders were inspected by scanning electron microscopy (SEM) several times during milling process. Changes in lattice parameter, crystallite size, lattice strain, dislocation density and Gibbs free energy changes (due to increasing in dislocation densities and grain boundaries) in different samples (with different TiC particles contents) were studied by X-Ray Diffraction technique with Cu-kα radiation and using Nelson–Riley method and Williamson–Hall equation. Microstructure of samples after sintering was investigated by FESEM. Finally, densitometry, hardness, determination of electrical resistance and pin on disk wear test were performed and effect of reinforcement percentages on the physical and mechanical properties of composites was studied. Results show incredible improvement in mechanical properties with increasing in TiC value, even though, electrical conductivity dropped off considerably.

Effect of Warm Rolling on Micro-deformation Behavior and Mechanical Properties of Columnar-grained Fe-6. 5 mass%Si Alloy

AbstractMicro-deformation behavior and mechanical properties of columnar-grained Fe-6. 5 mass%Si alloy before and after warm rolling were investigated by means of micro-indentation and three-point bending tests. The results show that the columnar-grained Fe-6. 5 mass%Si alloy before warm rolling presents sink-in mode of micro-indentation, while pile-up mode with a number of arc-shaped deformation bands exists in the warm-rolled alloy. Compared with that of the alloy before warm rolling, the maximum bending fracture stress and maximum bending fracture deflection of the warm-rolled alloy are increased by 96% and 50%, respectively. The different micro-deformation behavior and mechanical properties of the columnar-grained Fe-6. 5 mass%Si alloy are ascribed to the changes of dislocation density, dislocation configuration and long-range order degree, which significantly improve the room temperature plasticity of the alloy after warm rolling.

Heterogeneous plastic deformation and Bauschinger effect in ultrafine-grained metals: atomistic simulations

The effect of the dislocation density on yield strength and subsequent plastic deformation of ultrafine-grained metals was investigated in large-scale atomistic simulations. Polycrystalline models were constructed and uniaxial tension and compression were applied to elucidate the heterogeneous plastic deformation and the Bauschinger effect. The initial yield becomes heterogeneous as the dislocation density decreases owing to a wide range of Schmid factors of activated slip systems in each grain. A different mechanism of the Bauschinger effect was proposed, where the Bauschinger effect of ultrafine-grained metals is caused by the change in dislocation density in the process of forward and backward loadings.

Effect of light Si doping on the properties of GaN

An obvious increase in electron mobility and yellow luminescence (YL) band intensity was found in light Si doping GaN. For a series of GaN samples with different doping concentration, the dislocation density is almost the same. It is inferred that the abrupt increase in mobility and YL intensity does not originate from the change of dislocation density. The mobility behavior is attributed to the screening of scattering by dislocation and increase of ionized impurity scattering with the increase of Si doping concentration. At lower doping level, the screening of dislocation scattering is dominant, which results in the increase in carrier mobility. At higher doping level, the increase in ionized impurity scattering leads to the decrease in carrier mobility. Higher mobility causes longer diffusion length of nonequilibrium carrier. More dislocations will participate in the recombination process which induces stronger YL intensity in light Si doping GaN.

An insight into microstructural evolution during plastic deformation in AA6061 alloy after friction welding with alumina-YSZ composite

The microstructural studies of friction welding help in understanding microstructural changes occurred during friction welding. The effect of plastic deformation on microstructural changes in AA6061 alloys is the subject of the considerable practical interest. The development of this subject is remarkable, but a more detailed study could lead us to have a better understanding of the phenomenon. In the present study, Optical Microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM), microhardness and X-ray diffraction (XRD) were used to study the effect of plastic deformation on the grain structure and dislocation density of AA6061 alloy when there was a joint, with the ceramic. The effect of rotation speed and the degree of deformation appeared to be higher on the AA6061 alloy than on the ceramic part. Results showed different deformation mechanisms at different rotational speeds and confirmed unambiguously the change in grain size, microhardness, crystalline grain and dislocation density as a result of changing the distances from the interface.

Structure, texture, and properties of superconductive electrolytic niobium coatings on nickel and copper substrates

Structure, texture, chemical purity, and other characteristics of superconductive niobium coatings prepared via electrochemical deposition onto nickel and copper substrates in (LiF + NaF + KF)eut + 5 wt % K2NbF7 molten salt are studied. It is shown that, when depositing the coatings onto nickel, the intermediate layer of Ni3Nb and NiNb intermetallides is formed at the substrate-coating interface, which inhibits the diffusion of nickel in niobium. The nickel concentration in niobium coatings above this layer is (2–7) × 10−2 wt %. In coatings deposited onto the copper substrate the copper content does not exceed 1.5 × 10−4 wt %, and from the viewpoint of the amount of impurities, niobium is considered as a high purity metal. The dynamics of structure modifications in niobium coatings and changes in microdeformations and dislocation density in the deposited metal are explored depending on the current density at the cathode.