Dislocation Substructure Evolution Research Articles

A 2xxx aluminum alloy crept at medium temperature: role of thermal activation on dislocation mechanisms

Fuselage skin of the future supersonic civil transport aircraft requires a material resistant to long-term creep at temperatures ranging from 100 to 130 °C. A candidate is the 2650 aluminum alloy which presents such properties at relatively high temperatures ( 130 °C =0.43 T melting ). From an accelerated creep test conducted at 150 °C under a 280 MPa load and subsequent observations of the dislocation microstructures by transmission electron microscopy, the role of thermal activation on dislocation mechanisms is analyzed. It appears that thermal activation favors cross-slip activity and allows dislocations to glide in non-close-packed planes, namely the {0 0 1} planes. This is the first time that evidence of primary {0 0 1} glide is reported. Associated to a strain-assisted decrease of the precipitate density, thermal activation softens the material and seems to contribute to the acceleration of the strain rate (tertiary stage) by facilitating bypassing of precipitates and the production of mobile dislocations. The different creep stages are explained in terms of individual dislocation mechanisms and not by referring to the evolution of dislocation substructures.

Alloying effects on dislocation substructure evolution of aluminum alloys

The constitutive response of aluminum alloys is controlled by the evolution of dislocation substructure including mobile and forest dislocation density, cell size distribution and morphology, and misorientation angle between neighboring cells. The present study focuses upon the small strain regime and compares the measured microstructural evolution of 3003, 5005, and 6022 aluminum alloys during deformation. Room temperature tensile deformation experiments were performed on industrially manufactured specimens of each alloy and the evolving microstructure was compared with the mechanical response. The dislocation structure evolution was characterized using transmission electron microscopy and orientation imaging of deformed specimens. It was observed that structural evolution is a function of lattice orientation and the character of neighboring grains. In general, the dislocation cell size and misorientation angle between dislocation cells evolves systematically with deformation at relatively small strain levels.

Mechanisms of globularization of Ti-6Al-4V during static heat treatment

The mechanisms controlling static globularization of Ti-6Al-4V after deformation and annealing at 900 °C and 955 °C were established. Microstructural observations suggested that the process of globularization can be divided into two stages. The first includes microstructural changes during deformation and the initial stages of static heat treatment; the second occurs during prolonged static annealing. The initial stage consists of segmentation of the lamellae via boundary splitting, whereas microstructural coarsening characterizes the latter stage. Thus, the process of static globularization is only moderately dependent on the formation and evolution of dislocation substructure; the additional driving force is provided by the reduction in interface energy. The duration of the initial stage of static globularization was calculated by estimating the time required for the completion of the boundary-splitting process. The calculations were in excellent agreement with microstructural observations and showed that the duration of the initial stage at 900 °C and 955 °C lasted approximately 10 hours and 1 hour, respectively.

Contribution of deformation-induced intragranular microstructure to the plastic anisotropy of metals

In metals, plastic deformation causes the development of dislocation patterns. These patterns depends on the material and on the strain mode. Strain path changes usually cause dramatic modifications of these substructures. In most metals, they have an important effect on the plastic anisotropy, even if the prestrain is only of the order of 10 %. The well-known Bauschinger and cross effects are often caused by these re-arrangement of the dislocation patterns. It has recently proved possible to simulate this for IF-steel in a multilevel model for the plastic deformation of polycrystals based on the Taylor theory. It essentially simulates the development of dislocation substructure and how it is affected by strain path changes. It calculates the stress required to maintain glide on each of the slip systems. It is found that from a very small plastic strain on, the critical resolved shear stress becomes different on each slip system. This removes the ambiguity problem of the Taylor-Bishop-Hill theory. In this paper, the model is used to discuss the effect of prestrain on the yield loci of IF-steels, as well as the effect on the r-values which describe the plastic anisotropy of sheet material. Comparisons with experimental data are given.

Evolution of dislocation substructure in vapour-deposited lead films on glass substrates

Systematic variation of anisotropic X-ray line broadening in vapour-deposited Pb films of various thicknesses on glass substrates is studied using dislocation model of strain anisotropy incorporated into a modified Williamson-Hall and modified Warren-Averbach procedure. For smaller film thicknesses in the range of about 100-200 nm, the dislocation network is less correlated and contains substantial screw component and twin or growth faults. The films of larger thicknesses (greater than 500 nm), on the other hand, consist mostly of edge dislocations with a more regular and correlated dislocation structure. The present study reveals the inadequacy of applicability of conventional Warren-Averbach analysis in regular and correlated dislocation structures.

Fatigue deformation-induced response in a superduplex stainless steel

The cyclic deformation behavior of SAF 2507 superduplex stainless steel (SDSS) was studied under constant plastic-strain amplitudes. The cyclic hardening/softening curves show initial hardening, followed by softening and, finally, saturation behavior. Two regimes can be differentiated in the cyclic stress-strain curve (CSSC) of SDSS. The transition point at which the cyclic strain-hardening rate changes is identified to be ɛ p/2=7 × 10−3. Transmission electron microscopy (TEM) results on dislocation structures suggested that there is a close relationship between the CSSC, hardening/softening curves, and the dislocation substructure evolution. In the low-plastic-strain-amplitude regime of the CSSC, the dislocation activity in the austenite grains is found to be higher than that in the ferrite grains. At higher plastic strain amplitudes, low-energy dislocation structures are found in the ferrite grains, while clusters and bundles of dislocations can be observed in the austenite grains. Strain localization due to formation of these structures resulted in a decrease in the cyclic strain-hardening rate within the high-plastic-strain-amplitude regime. Dislocation substructure evolution is also used to explain the shape of the hardening/softening curve.

Simulation of substructural strengthening in hot flat rolling

During the last decade, the mechanism of substructure strengthening has been a research focus. In recent years, some physical models based on three internal state variables: dislocation density, subgrain size and misorientation across subgrain boundaries, have been proposed. To compute the material strength by coupling these physical models with the finite element method (FEM) requires the prediction of the evolution of the above three variables. In this paper, the status of modelling the dislocation substructure evolution by FEM is first reviewed. Problems and difficulties in the simulation are investigated. Various physical models have been incorporated into a commercial FEM program FORGE2 ®. Good agreement is found between the predicted and experimental measured subgrain size. The computed dislocation density and misorientation also show reasonably agreement with the experimental observations. Finally, the material strength is computed by adopting different strengthening models. Further work on the prediction of microstructure for aluminium alloys is discussed.

Power-law and exponential creep in class M materials: discrepancies in experimental observations and implications for creep modeling

This paper discusses our current understanding of the processes thought to be dominant in the exponential creep regime as well as the implications for creep modeling relating to both power-law and exponential creep regions. The significance and implications of creep controlled by vacancy diffusion along dislocation cores are discussed. It is pointed out that creep substructures, other than subgrains, have been reported in the literature, and a bifurcation diagram is presented to demonstrate how this evolution can occur from an initially homogeneous dislocation substructure. The use of nonlinear dislocation dynamics in creep modeling is advocated to rationalize the observed diversity in the creep substructures. It is demonstrated that the dislocation substructure evolution models can be coupled with a viscoplastic model through the volume fractions of the ‘hard’ and ‘soft’ phases. This coupling is shown to lead to the stress-subgrain size relationship in a simple and a natural way.

Cyclic plasticity of nickel at low plastic strain amplitude: hysteresis loop shape analysis

The cyclic plasticity of nickel was studied by accomplishing room temperature fully-reversed fatigue experiments at constant plastic strain amplitudes of 1.0×10 −4 and 2.5×10 −4 on single crystal and polycrystal nickel with 290 μm grain size. The cyclic plasticity behavior within a hysteresis loop was analyzed by measuring shape characteristics of the loop. Parameters that were evaluated include the loop shape parameter, friction stress, back stress, and second derivative of stress with respect to total strain. Results indicate that these parameters correlate well with the development of dislocation substructures. In addition, at low plastic strain amplitude, polycrystal and single crystal hysteresis loops exhibit a distinct constriction. The constriction is more pronounced in single crystals cycled at the lower plastic strain amplitude.

X-Ray Microbeam Laue Pattern Studies of the Spreading of Orientation in OFHC Copper at Large Strains

This paper assesses, via X-ray microbeam diffraction, the effects of development of dislocation substructure on the distribution of sub-grain misorientations in annealed OFHC copper deformed to large strains for compression, for shear, and for sequences of compression followed by shear. Polychromatic synchrotron x-radiation was used to study samples from four strain histories: virgin specimens, 50% effective strain in compression, 100% effective strain in torsion, and 50% compressive strain followed by 50% torsion. A very narrow beam illuminated an approximately 15 μm diameter column through the sample, and the microstructure of the specimens was mapped by translating the sample along two orthogonal axes perpendicular to the beam by increments of 10 μm. The beam diameter was considerably smaller than the average grain size in the virgin material. Both the degree of substructure formation and the nature of the distributed microstructure were quantified from the resulting Laue diffraction patterns. The polychromatic diffraction patterns of the polycrystalline samples consisted of well-defined streaks, and the azimuthal angular width of the streaks increased with plastic strain in a manner consistent with the scaling of the misorientation distribution of high angle boundaries for sub-grains reported recently using electron microscopy techniques limited to thin foils or thin surface layers. A lattice spin correction is introduced based on this scaling law in a simple extended Taylor scheme of polycrystal plasticity to achieve a retardation of texture development that is consistent with experimental results.

Development of Grain Interior Strain Localizations during Plane Strain Deformation of a Deep Drawing Quality Sheet Steel.

Development of dislocation substructures was characterized in an aluminum killed deep drawing quality steel at four different plane strain deformations. At and above 20% reduction, the most significant substructural feature was micro bands (MBs). MBs appeared as paired dislocation walls of 0.2–0.4μm thickness and were always at an angle of approximately 37° with rolling direction (RD). As the traces of the MBs were more than 5° of {110} and {112}, closed packed planes of the bcc system, —they were termed as first generation1) or non-crystallographic using the convention16) commonly used in fcc metals. Other than the pre-deformation high angle boundaries, MBs were the only feature with large enough misorientations necessary for optical visibility. At least for the range of strain and strain path used in the present study, the first generation MBs can be considered as the so-called grain interior strain localizations. Relative presence and effectiveness of MBs were quantified in different microtexture components from the MB spacings along TD (λ) and the average misorientation across MBs (θMB) and these appear to determine the stored energies of different microtexture components.

Change characteristics of static mechanical property parameters and dislocation structures of 45 # medium carbon structural steel during fatigue failure process

A series of experiments at both macroscopic and microscopic levels were carried out to investigate the variation in static mechanical property parameters ( yield stress σ 0.2, elastic modulus E, reduction of area ϕ, elongation δ, strain hardening exponent n, ultimate strength σ b) of 45 # medium carbon structural steel during fatigue damage process. The possible reasons for the variation of the static mechanical property parameters are qualitatively discussed through analyzing the evolution of dislocation substructures in the steel during cycling. A composite parameter of the static mechanical property called static toughness is proposed for characterizing comprehensively the variation of the static mechanical property parameters, and a quantitative relationship between the variation of static toughness and the dissipation of cyclic plastic strain energy is established.

221 転位微視組織の発展を考慮した結晶塑性モデルとその応用

This paper proposes constitutive model for dislocation substructure evolution within the framework of crystal plasticity where cell formation and dislocation pile-ups at the cell boundary are considered. A concept of imaginary cell size representing chracteristic length scale of the evolved substructures are introduced. Recovery model based on activation energy of cross slip are also developed. The model is applied to several loading conditions.

Multiscale Polycrystal Plasticity

Polycrystal plasticity models are commonly developed with a narrow focus on the grain as the fundamental unit of crystallographic orientation and anisotropic behavior. However, deformation and strengthening mechanisms occur simultaneously at multiple length scales and may lead to bulk deformation behavior of metals that is substantially different from that predicted by simple forms of polycrystal plasticity. The development of dislocation substructure occurs at subgrain scales while, at the same time, geometrically necessary dislocation boundaries (GNBs) are generated that extend over several grain diameters. A framework is presented here for the efficient treatment of multiple, simultaneously evolving strengthening mechanisms. The theory focuses on a macroscale hardening surface representation of the strengthening due to GNB formation. Crystallographic shear flow resistance is determined via a mapping procedure of the macroscale hardening surface to the length scale of grains. Predicted stress-strain curves based on the hardening surface formulation are compared to experimental data and polycrystal plasticity predictions for OFHC Cu. It is demonstrated that the hardening surface model of GNB strengthening mechanisms can provide improved predictive capability of nonproportional loading behavior of Cu compared to conventional slip system hardening laws commonly used in polycrystal plasticity applications.

Macroscopic behaviour versus dislocation substructures development under cyclic shear tests on the aluminium–3004 alloy

Cyclic shear tests of various amplitudes have been carried out on aluminium–3004 alloys. The macroscopic behaviour shows the occurrence of either cyclic softening or cyclic hardening, depending on the initial state of the alloy (recrystallised, recovered or extra-hardened). This macroscopic behaviour is discussed with the help of both isotropic and kinematic concepts and in connection with the texture, the development of dislocation substructures and its evolution upon straining.

Modeling effects of dislocation substructure in polycrystal elastoviscoplasticity

Microheterogeneity resulting from dislocation substructure evolution is introduced into polycrystal elastoviscoplasticity through a second rank tensorial internal state variable. This evolving tensorial variable operates at the scale of the grain to capture the effects of subgrain scale dislocation substructures. The microheterogeneity resulting from dislocation substructure evolution is mapped to each single crystal slip system as kinematic hardening and also affects the higher length scale of intergranular constraints in a self-consistent manner. The anisotropy from the microheterogeneity internal state variable improves the trends of correlation between polycrystalline calculation macroscale responses (stress-strain curve, texture, and axial stresses developed during fixed-end torsion) and the experimental results. Elastic moduli under finite strains were experimentally measured to corroborate numerical predictions. Calculations with the microheterogeneity internal state variable compared better with the elastic moduli measurements than did the Taylor model.

Thermodynamics of substructure transformations under plastic deformation of metals and alloys

The paper is devoted to behavior of the energy stored in deformed metals and alloys. The study is based on the well-known theoretical approaches and on the electron microscopy data obtained by the authors during investigation of the deformed Cu-Al and Cu-Mn f.c.c. alloys. Influence of the solid solution hardening value on the mechanisms of dislocation substructure evolution was studied. The accumulated energy for the substructure being formed at plastic deformation was evaluated. Ways of improvement of such evaluations were considered.

Nucleation of persistent slip bands in Cu single crystals under stress controlled cycling

The study of cyclic stress strain response of Cu-single crystals has been the subject of numerous investigations during the past decade. It was pointed out that the cyclic loading under certain conditions leads to strain localization zones of highly deformable material layers called Persistent Slip Bands (PSB). These PSB`s have been connected with the evolution of a characteristic dislocation substructure (ladder-like structure) and the formation of extrusion/intrusion slip markings on the specimen surface. Under the most frequently applied plastic-strain controlled cycling, a saturation plateau stress in the CSS-curve was observed. The resolved shear stress of this plateau was reported to be 28 MPa for single crystals of Cu oriented for single slip when cycled at room temperature. Kettunen reported results on Cu single crystals cycled under stress control with the desired stress amplitude reached in the first half-cycle. He found that PSB`s formed already at a shear stress amplitude as low as 17 MPa. Recent experiments with Cu-polycrystals showed that an increase in the length of the stress loading ramp resulted in an increase of the nucleating stress for PSB`s. For the case of direct loading a nucleation shear stress was calculated for values between 16 and 21 MPa dependingmore » on the constraint factor used to convert the stress values. Based on this knowledge the authors designed experiments on single crystal Cu specimens oriented for single slip under various loading ramp length (1--1,000 cycles) to study the nucleation conditions of PSB`s and the related saturation behavior.« less

An Analysis of a Set of Creep-Strain and Time-to-Creep Fracture Data for a SCR-1Mo-0.2V Steel

In the present paper, a remarkably extensive set of creep and creep fracture data for a 9Cr-1Mo-0.2V (P91 type) stainless steel published by Sklenicka and collaborators /1/ are re-analysed in an attempt to account for an unusual stress and temperature dependence of both the creep strain rate and the time to creep fracture. The analysis suggests strongly that (i) the creep strain rate is not recovery controlled, (ii) neither is the creep strain rate controlled by dislocation climb around carbide particles provided no interaction of dislocations with carbide particles takes place and finally (iii) the Rosler -Arzt model assuming an attractive interaction of dislocations with dispersed particles and thermally activated detachment of the dislocations from the particles fails to account for the creep data of interest. It is suggested that the carbide particles play an indirect role affecting the development of dislocation substructure in the course of creep and, especially, the effect of stress and temperature on its parameters. This is in accord with the original data interpretation by Sklenicka and his collaborators /1/. Everything that holds for the creep strain rate essentially holds for the time to creep fracture as well. In fact, the time to creep fracture is most probably controlled by the same process and/or mechanism as creep in the 9Cr-1Mo-0.2V (P91 type) steel, as originally suggested by Sklenicka et al. /1/.

Strain induced directional coarsening in Ni based superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.