Formation Of Dislocation Structure Research Articles

Formation of grains and dislocation structure of geometrically necessary boundaries

A continuum dislocation model of formation of grains whose boundaries have a non-vanishing thickness is proposed. For a single crystal deforming in simple shear the lamellar structure of grains with thin layers containing dislocations as the geometrically necessary boundaries turn out to be energetically preferable. The thickness and the energy of this type of grain boundary are computed as functions of the misorientation angle.

A dislocation kinetic model of the dislocation structure formation in a nanocrystalline material under intense shock wave propagation

A dislocation kinetic model of the formation and propagation of plastic shock waves in nanocrystalline materials (with a grain size of 1–100 nm) at pressures ranging from 1 to 50 GPa has been discussed theoretically. The model is based on a nonlinear equation of the reaction-diffusion type for the dislocation density, which includes the processes of multiplication, annihilation, and diffusion of dislocations with a strong absorption of the dislocations by nanograin boundaries. The solution of this equation is obtained in the form of a traveling dislocation density wave propagating with a constant velocity. The dependences of the dislocation density and dislocation front width on the nanograin size and pressure in the wave are determined. A comparison of the obtained dependences with the available results of the experiments and molecular dynamics simulations of shock-deformed nanocrystalline materials demonstrates their good quantitative agreement.

Study of isothermical sintering of finely divided and mechanically alloyed iron powders

The sintering kinetics has been studied and the sintering activation energies of finely divided powders and mechanically alloyed mixture of iron – 0,5 % C – 5,6 % ferrophosphorus have been calculated. Dislocation density calculated by the X-ray phase analysis and microscopic metallog- raphy increases up to 10 11 cm –2 ; and the dislocation ordering process in the finely divided powders and mechanically alloyed mixture is found when milling time increases. Based on the porosity values, which vary in time of isothermal sintering at 900–1100 °C of powders finely divided for different times, Ivensen equation coefficients, sintering activation energy, and structural defect concentration being in evidence in the beginning of isothermal sintering are calculated. The results show the formation of dislocation structure with high density of thermally unstable dislocations to activate sintering slightly in the course of milling; the structural defects created in forming the solid solution during mechanical alloying and hetero- diffusion affect the sintering activation much more.

Formation of Dislocation Structure in LiF Crystals Irradiated with Swift Heavy Ions under Oblique Incidence

The structural modi cations of LiF irradiated with swift heavy ions under oblique angles have been investigated using AFM, SEM, chemical etching, nanoindentation and optical absorption spectroscopy. LiF crystals were irradiated under incidence angles of 30 and 70 degrees with 2.2 GeV Au ( uence 5× 10 ions cm−2) and 150 MeV Kr ions ( uence 10 10 ions cm−2). Structural study on sample cross-sections shows that two damage regions, (1) nanostructured zone and (2) dislocation rich zone, which are typical for irradiations at normal incidence, appear also in samples irradiated under oblique angles. However in the latter case a more complex structure is formed that leads to stronger ion-induced hardening.

X-ray micro-beam characterization of lattice rotations and distortions due to an individual dislocation

Understanding and controlling the behaviour of dislocations is crucial for a wide range of applications, from nano-electronics and solar cells to structural engineering alloys. Quantitative X-ray diffraction measurements of the strain fields due to individual dislocations, particularly in the bulk, however, have thus far remained elusive. Here we report the first characterization of a single dislocation in a freestanding GaAs/In0.2Ga0.8As/GaAs membrane by synchrotron X-ray micro-beam Laue diffraction. Our experimental X-ray data agrees closely with textbook anisotropic elasticity solutions for dislocations, providing one of few experimental validations of this fundamental theory. On the basis of the experimental uncertainty in our measurements, we predict the X-ray beam size required for three-dimensional measurements of lattice strains and rotations due to individual dislocations in the material bulk. These findings have important implications for the in situ study of dislocation structure formation, self-organization and evolution in the bulk.

Development and testing of microcompression for post irradiation characterization of ODS steels

For decades, the nuclear materials community has been interested in smaller tests for obvious benefits like in limited space or reduction in activity. In this work, we are presenting micropillar tests on ODS steel with a grain size of ∼450nm and finely distributed oxides as a part of an attempt to use this method in evaluating the change in mechanical properties of nuclear materials after irradiation. Systematic works combining microcompression tests and TEM observations showed that there is no significant size effect on the measured strengths and the dislocation structures of the ODS alloy down to a pillar size of 1μm. We used both circular and square pillars and discuss the effect of pillar geometry with the evaluated strength and errors. The measured strengths at half the pillar height were found to be in good agreement with the bulk strength measured from a miniature tensile test. The cross-sectional TEM observation of the deformed pillar showed that oxide particles act as strong internal constraining sources for dislocation movement and the formation of dislocation structure.

High temperature mechanical properties and surface fatigue behavior improving of steel alloy via laser shock peening

Laser shock peening was carried out to reveal the effects on ASTM: 410L 00Cr12 microstructures and fatigue resistance in the temperature range 25–600°C. The new conception of pinning effect was proposed to explain the improvements at the high temperature. Residual stress was measured by X-ray diffraction with sin2ψ method, a high temperature extensometer was utilized to measure the strain and control the strain signal. The grain and precipitated phase evolutionary process were observed by scanning electron microscopy. These results show that a deep layer of compressive residual stress is developed by laser shock peening, and ultimately the isothermal stress-controlled fatigue behavior is enhanced significantly. The formation of high density dislocation structure and the pinning effect at the high temperature, which induces a stronger surface, lower residual stress relaxation and more stable dislocation arrangement. The results have profound guiding significance for fatigue strengthening mechanism of components at the elevated temperature.

Kinetics for dislocation structure formation in contact area of squeezed crystalline solids

Kinetics for dislocation structure formation in contact area of squeezed crystalline solids

Microstructural degradation of Gr.91 steel during creep under low stress

Microstructural changes during creep at 600 °C under 70 MPa were investigated in the case of interrupted Gr.91 steel samples by taking into account the dislocation structure and Z-phase formation. The creep life monotonically increased with a decrease in the applied stress at each temperature considered in the study. However, the long-term creep life was shorter than that determined from the short-term creep data at 600 °C and 650 °C, meaning premature failure. The subgrain size gradually increased during creep up to 70,000 h, after which rapid subgrain coarsening occurred. Preferential recovery of the subgrain structure occurred around the prior-austenite grain boundary (PAGB) after 50,000 h and 70,000 h. After creep rupture, subgrain recovery was observed over the entire area of each sample. Z-phase formation was clearly visible for 30,000 h after creep. The number density of the MX particles gradually decreased after 30,000 h because of Z-phase formation. After creep rupture, the number density of the MX particles was almost the same as that of the Z-phase particles. During creep, the V content of the Z-phase gradually increased but the Nb content decreased. Changes in the chemical composition of the Z-phase occurred after a longer time in Gr.91 steel than in 12Cr steel.

Fatigue behavior and microstructure of an Al-Mg-Sc alloy at an elevated temperature

Al-Mg-Sc alloy polycrystals bearing Al3Sc particles with different sizes, i.e. 4, 6 and 11 nm in diameter, have been cyclically deformed at 423 K under constant plastic-strain amplitudes, and the microstructural evolution has been investigated in relation to the stress-strain response. Cyclic softening after initial hardening is found in specimens with small particles of 4 and 6 nm, but no cyclic softening takes place in specimens with larger particles of 11 nm. These features of cyclic deformation behavior are similar to the results previously obtained at room temperature. Transmission electron microscopy observations reveal that dislocations are uniformly distributed under all applied strain amplitudes in the specimens containing large particles of 11 nm, whereas slip bands are formed in the cyclically softened specimens bearing smaller particles. The cyclic softening is explained by a loss of particle strength through particle shearing within strongly strained slip bands. The 6 and 11 nm Al3Sc particles have a stronger retardation effect on the formation of fatigue-induced stable dislocation structure than 4 nm particles at 423 K.

Simulation of dynamical interaction between dislocations and dipolar loops

The article describes a model of interaction dynamics between a dislocation and dipolar dislocation loops. The interaction is essential for dipolar dislocation structure formation in early stages of a hardening process. For the description of the dislocation curve a direct parametric approach is employed whereas the loops are treated as rigid objects. The model equations are solved approximately by means of the finite-volume method. Physically interesting phenomena can be captured by the model provided the simulation covers long time periods. The strong interaction between the dislocation and the loops causes growing nonuniformity of distribution of discrete nodes along the dislocation curve. This effect is balanced by two proposed types of tangential redistribution of the discrete nodes. The redistribution is tested in simulations of loop clustering.

Correlation between the behavior of nanocrystalline HCP metals and the dislocation model for the minimum grain size obtainable by milling

Recently, a dislocation model that quantitatively relates the minimum grain size obtainable by milling, d min , to several physical parameters, such as the activation energy for self-diffusion and the stacking fault energy, in a nanocrystalline (nc) material was developed. The development of the model was based on the suggestion that the minimum average grain size, d min , is the result of a balance between the formation of dislocation structure and its recovery by thermal processes. The predictions of the above model were found to be in good agreement with experimental data and trends reported for nc-FCC and nc-BCC metals. In this paper, the validity of the model to the description of the behavior of nc-HCP metals for which data on the minimum grain size, d min , are available is examined. Also, the general applicability of the model to other severe plastic deformation (SPD) processes is discussed.

Deformation Modes in Stainless Steel During Laser Shock Peening

In order to study the deformation mechanisms during ultrahigh strain rate deformation of face centered cubic metals, laser shock peening of a 304L stainless steel is systematically investigated. Two deformation modes—microtwins and microbands—and their interrelationship during high strain rate deformation are discussed in detail. Transmission electron microscopy and selected area electron diffraction are employed to study the deformation modes. It is found that twinning takes place even when the shock pressure is much less than the critical twinning stress in stainless steels. Theoretical critical twinning stress is not the only criteria to decide the deformation modes of twinning or slip. The formation of twinning and slip can be affected by the factors such as loading profile, loading stress/strain rate, stacking fault energy, grain sizes, and cell substructures. Factors that influence twin-slip transition in shock loading are discussed. The formation of dislocation structure is compared with those predicted using 3D dislocation dynamic simulation.

Microstructure and Mechanical Properties of Ni-Free High Nitrogen Austenite Stainless Steels Fabricated by Mechanical Alloying Method

Particular attention is called to nickel-free or lower nickel austenitic stainless steels containing high nitrogen from the viewpoint of protections against earth resources and Ni allergy for human being. We have made high nitrogen steels (HNS) that were containing 0.7-2.0mass%N and 23Cr-2Mo by mechanically alloying method and the microstructure and mechanical properties have been examined. HNS containing 1.0-1.6mass%N have nearly full austenite phase and the mean grain diameter was about 2-3μm for every HNS. The strength and work hardening rate increased and the elongation decreased with increasing nitrogen content. Especially, HNS containing 1.0mass%N exhibited superior balance of strength-ductile properties, that is, yield strength of 1200MPa and fracture elongation of 36%. The yield point phenomena and ductile-brittle transition behavior can be seen in spite of austenitic stainless steels. The planar dislocation arrays were seen on the deformed structure. These results suggest that the formation of planar dislocation structure, high work hardening rate for high nitrogen steels and temperature dependence of mechanical properties might be caused by the formation of the Mo-N interstitial-substitutional atom couples.

Deformation kinetics of coarse-grained and ultrafine-grained commercially pure Ti

The elevated temperature deformation kinetics of ultrafine-grained (UFG) Ti of commercial purity (grade 2) produced by equal channel angular pressing (ECAP) at elevated temperature was measured and compared to that of coarse-grained (CG) Ti. Like CG Ti, the UFG Ti exhibits dynamic strain aging effects related to transition between jerky glide of dislocations without solute cloud and viscous glide with cloud. In the jerky region the yield stress of UFG Ti is distinctly enhanced compared to CG Ti. The steady state deformation resistances of CG and UFG Ti are not significantly different. The jerky → viscous transition results in a relative maximum of deformation resistance as function of strain, which is explained by the opposing effects of cloud formation and coarsening of dislocation structure. The high content of grain boundaries and dislocations in the initial state of UFG Ti stabilizes the material against break-away of dislocations from their clouds.

Magnetic behaviour of low-carbon steel in parallel and perpendicular directions to tensile deformation

Degradation of a low-carbon steel with plastic tension has been investigated after unloading by magnetic methods (hysteresis and Barkhausen noise (BN) emission). The measurements were done with magnetization in parallel as well as in perpendicular direction to the previous elongation. The dislocation structure formation was checked by transmission electron microscopy (TEM). Evolution of magnetic parameters with applied strain was examined and applicability of magnetic methods for non-destructive testing (NDT) is discussed. It was concluded that the change of the two-peak profile of BN envelope and differential permeability (leading to hysteresis loop bulging) in the parallel direction to the one-peak profile in the perpendicular direction is due to the deformation-driven accumulation of residual stresses.

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.

Residual Stresses, Thermomechanical Behavior and Interfaces in the Weld Joint of Ni‐based Superalloys

The paper relates aspects of interfaces, residual stresses and dislocations with joint behaviour in the Ni–based single crystals. It is focused on the understanding the fundamental driving forces and response of the materials which leads to the formation of dislocation structure during local melting and solidification. Oscillations in the dislocation structure are observed at both macro and micro scales. The underlying mechanism of these processes is essential to real world applications.

Misfit Dislocations in SiGe/Si Heterostructures: Nucleation - Propagation - Multiplication

We present experimental data on the effect of low-temperature buffer layers on the dislocation structure formation in SiGe/Si strained-layer heterostructures under thermal annealing. Specific subjects include mechanisms of misfit dislocation nucleation, propagation and multiplication as well as the role of intrinsic point defects in these processes. Samples with lowtemperature Si (400°C) and SiGe (250°C) buffer layers were grown by MBE. In general, the processes of MD generation occur similarly in the heterostructures studied independently of the alloy composition (Ge content: 0.15, 0.30) and kind of buffer layer. Intrinsic point defects related to the low-temperature epitaxial growth influence mainly the rate of misfit dislocation nucleation.

Structure of metal materials under bulk doping

The method of bulk doping of metallic materials is effected by super deep penetration of dispersed particles as a consequence of dynamic interaction with a barrier. Such a treatment has considerable potential, since it'increases the number of physic-mechanical and service properties of metals and alloys. The mechanism underlying the interaction of solid dispersed particle, with a metal matrix has been investigated by several authors. However, the problem of structural changes taking place in the matrix near particles and the trajectories of their motion has yet to be adequately studied. It would provide the possibility of understanding the mechanism of mass transfer of particles into metals with different compositions and structures. The process of hardening of the materials with different types of crystalline lattices by mean of explosive doping is considered as multifaceted. The similarities of, and the differences between, the mechanisms underlying the hardening of materials having body-centered (bcc) and face-centered cubic (fee) lattices are discussed. The role of long - range stress-fields in the formation of dislocation structure of metals subjected to explosive doping has been investigated. The intricate nature of the spatial configuration of stress fields near retarded particles and the role of these fields in hardening the treated metallic materials has been demonstrated.