Low Energy Dislocation Structures Research Articles

Surface Modification Affecting Fatigue Behaviour InPolycrystalline Copper- A Micro Probe Study.

Surface modification under some circumstances becomes an effective way to improve fatigue life. In tlus context practical application utilizes various procedures by realidng the unique role of surface conditions on fatigue crack initiation and propagation. The current study intents to illuminate fundamental aspects on at least three levels: first on the level of the remote external field being a critical variable. second on the micro-structural level including low energy dislocation structures and third on the level of geometrical changes. Beside others. shot peening is considered to be a viable option for surface modification which serves here as a typical process in the selected model material of pure copper. Mainly constant amplitude tension/compression fatigue was applied with stress ratio of R=-l. The strain amplitude for a given cyclic span controlled the driving force. For a possible resistance change a variation of surface modification was introduced which enabled a comparative study. Tests \\.ere conducted at ambient tempcrature using computerized closed loop electro/liydraulic system. Vismlization techniques included Scanning Electron Microscopy (SEM). Transmition Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). The study introduces quantitative methodology that provides infonnation on the local micro shear strain due to surface upset. This facilitated physical evaluation on the role of surface modification as related to the sequential events during thc early stages of fatigue darnage.

Low cycle fatigue behaviour of low carbon microalloyed steel: microstructural evolution and life assessment

In this paper the cyclic stress–strain response, low cycle fatigue (LCF) behaviour, and evolution of dislocation structures under LCF loading in the case of a low carbon microalloyed steel are discussed. The cyclic stress response revealed cyclic softening resulting from the propagation of Lüders bands. The experimental LCF life was compared with the life predicted using Tomkins' model and the modified universal slopes (MUS) equation. While the life predicted by Tomkins' model showed good correlation with the experimental results, the life predicted using the MUS equation grossly overestimated the life. Inclusion induced delaminations under cyclic loading were thought to be responsible for the overestimation by the MUS equation. Low energy dislocation structures, i.e. cells, were observed near the fracture surfaces. Interrupted tests revealed cell formation after 10 cycles at a total strain amplitude of 0·3%.

Polycrystal constraint and grain subdivision

Typically, intergranular constraint relations of various sorts are introduced to improve the accuracy of prediction of texture evolution and macroscale stress–strain behavior of metallic polycrystals within the context of simple polycrystal averaging schemes. This paper examines the capability of a 3-D polycrystal plasticity theory (Kocks, U.F., Kallend, J.S., Wank, H.-R., Rollett, A.D. and Wright, S.I. (1994), popLA, Preferred Orientation Package—Los Alamos. LANL LA-CC-89-18), based on the Taylor assumption of uniform deformation among grains, to predict texture evolution and stress–strain behavior for complex finite deformation loading paths of OFHC Cu. Compression, shear and sequences of deformation path are considered. It is shown that the evolution of texture is too rapid and that the intensity of peaks is more pronounced than for experimentally measured pole figures. Comparisons of both stress–strain behavior and texture evolution are made with experiments, with and without the inclusion of latent hardening effects. It is argued that grain subdivision processes accommodate intergranular kinematical constraints, leading to the notion of a generalized Taylor constraint that considers the distribution of subgrain orientations. The subdivision process is assumed to follow the experimentally observed refinement of low energy dislocation structures associated with geometrically necessary dislocations. A modification of the kinematical structure of crystal plasticity is proposed based on generation of geometrically necessary dislocations that accommodate a fraction of the plastic stretch and rotation at the scale of a grain.

The behaviour of incidental dislocation boundaries and geometrically necessary boundaries during warm tensile deformation of aluminium

It has been well established experimentally that, during plastic deformation of a polycrystal, individual grains are gradually subdivided into mutually misoriented regions separated by dislocation rotation boundaries. At small strains, slightly misoriented ordinary dislocation cells are created that are characterized by tangled dislocation boundaries. According to the theory of low-energy dislocation structures (LEDS), these boundaries result from the statistical mutual trapping of glide dislocations into low-energy configurations and are, therefore, termed incidental dislocation boundaries (IDBs). With increasing strain, planar dislocation walls called geometrically necessary boundaries (GNBs) develop gradually on the background of ordinary dislocation cells and, thus, divide the grain into cell blocks (CBs) that contain from one to several cells.

Effect of the grain size on the rate of energy storage during the tensile deformation of an austenitic steel

The effect of the grain size on the energy storage process in a low carbon austenitic steel deformed in tension is studied. The energy conversion at each instant of the deformation process is characterized by the instantaneous rate of energy storage, de s de w , where e s is the stored energy and e w is the mechanical energy expended on the plastic deformation. It has been shown experimentally that, in the initial stage of plastic deformation in this austenitic steel, the dependence of the rate de s de w on e w exhibits a maximum. The location of the maximum depends on the grain size of the material. In fine-grained samples, the maximum appears at smaller strains. After reaching a certain degree of deformation, plots of de s de w vs. the strain for the samples of both groups are practically the same. These results are interpreted in terms of the microstructural evolution during deformation. It has been shown that the grain boundaries favour the formation and affect the evolution of low energy dislocation structures.

Low-energy dislocation cell structures in ionic cubic lattice crystals

Structure state diagrams for ionic crystals with NaCl-type lattices indicate the presence of at least two types of cellular dislocation structures not contributing to local lattice misorientation. In this work, generalized data are presented concerning these low-energy dislocation structures. The ways of their formation and properties are considered from the viewpoint of dislocation wall structure analysis, simulation of dislocation array formation dynamics, thermal stability of similar structures and work hardening.

Effects of low energy dislocation structures on crack tip shielding in ionic crystals

Dislocation structures near the tip of a crack and its effect on the local stress intensity factor in NaCl crystals are investigated by using the etch pit technique and the photoelastic method. In the early stage of slip activity, shielding-type dislocations are introduced from a crack tip and their arrangement indicates the presence of a dislocation-free zone. By using the photoelastic method, it is surely corroborated that those dislocations cause crack tip shielding to raise the value of fracture toughness in the early stage of dislocation generation. On the other hand, when slip activities are increased around the crack tip, dislocations of antishielding type as well as shielding type are introduced to produce slip bands consisting of high density dislocations and the dislocation-free zone disappears, suggesting that the increase in the activity of dislocations does not necessarily contribute to raising the value of fracture toughness in NaCl crystals. Neighboring dislocations of shielding and antishielding type mutually screen their long-range stress fields, so that their introduction indicates the formation of a low energy dislocation structure (LEDS). Thus, LEDS formation may have a marked influence on fracture behavior in NaCl crystals.

Self-organized dislocation structures (SODS) in fatigued metals

An attempt to introduce a thermodynamic classification of dislocation structures produced by cyclic deformation is made. A fundamental division is proposed for all types of dislocation structures into two classes: 1. those in equilibrium and 2. strongly non-equilibrium self-organized dislocation structures (“SODS”). It is shown that the concept of low energy dislocation structures (“LEDS”) introduced by Kuhlmann-Wilsdorf can be applied to dislocation structures of the first type, while the concept of SODS is useful in situations connected with the onset of fatigue instabilities (e.g., the formation of the ladder structure of persistent slip bands and dislocation rearrangements accompanying strain bursts during cyclic deformation, e.g., ramp loading). It is stressed that the issue of dislocation patterning in fatigued metals should be considered a part of the general problem of nonlinear dynamical systems functioning in far-from-equilibrium conditions. In this sense the classification proposed has common features between “physico-chemical” behavior of dipolar and multipolar dislocation populations in fatigued metallic alloys, on the one hand, and such essentially dipolar systems as ferrofluids, magnetic bubble materials, and phospholipid monolayer domains, on the other hand.

LEDS in ultra-high strain-rate deformation

Regimes involving very high strains (e 900%) and high strain rates (e 10 4 to 10 7 s −1 ) provide intriguing and often systematic examples of low-energy dislocation structures (IEDS). In this broad study, microstructure evolution in plane-wave shock-loaded Ni and Cu is compared with microstructures observed in starting Cu and To shaped charge liner cones, and recovered jet fragments and slugs. These microstructures, in turn, are compared with corresponding end-point microstructures in a Ta explosively formed penetrator (EFP) utilizing TEM techniques. In addition, microstructures associated with a hypervelocity impact crater in Cu are compared with the Cu shaped charge microstructures. Dynamically induced grain refinement (subgrain formation) and recovery play varying and important roles in the evolution and features of observed LEDS, and are a manifestation of adiabatic heating due to high strain and strain-rate effects ( (e) (e)). LEDS, which are characteristic of high strain and ultra-high strain-rate deformation, cause an inversion in hardness and flow stress profiles since as these microstructures decrease in size, hardness will decrease. By contrast when low-temperature LEDS decrease in size hardness increases

A crystallographic study of dislocation cell arrangement in aluminium deformed at an elevated temperature

The arrangement of dislocation cells in a commercial aluminium alloy 1145 subjected to uniaxial tensile deformation at 150 °C to a true strain of 0.1 has been investigated using transmission electron microscopy and convergent beam electron diffraction. Both misorientations between neighbouring dislocation cells and misorientation gradients across the grain as well as dislocation boundary orientations have been analysed. As an example, illustrating general qualitative features of the deformation microstructure as a whole, results of a detailed crystallographic analysis of a typical cell arrangement are presented. The results obtained in the study are compared to those available for similar deformation conditions at room temperature and the validity of some assumptions from the theory of low-energy dislocation structures is discussed. It is concluded that the observed cell structure has a complex hierarchical character and neither misorientation vector distributions nor dislocation boundary orientations correspond to the simple checker-board arrangement of cells predicted by the above theory.

Low-energy dislocation structures in cyclically deformed Ni 3Al single crystals

Low-energy dislocation structures in Ni 3Al single crystals (oriented for single slip) fatigued at room temperature were studied using transmission electron microscopy. In general, dislocation structures in fatigued Ni 3Al were composed of primary superdislocations, edge superdislocation bundles (mutually trapped edge superdislocations), primary edge superdislocation dipoles (line vector ∥ [1 21] ), and faulted superdislocation dipoles (line vector ∥ [1 10] or ∥ [01 1] ). No wall or cell structure was observed. Paired primary superdislocations, as they glided in the (111) slip plane during cyclic deformation, were energetically unstable because of the fluctuation of the separation between two superpartial dislocations ( b = ± 1 2 [ 101]) . The fluctuation led to an increase in the line energy of the primary superdislocations, and resulted in the onset of several dislocation reactions. Since the resulting dislocations of these reactions were all sessile, their line energy remained unchanged during cyclic deformation. Accordingly, they were considered to be energetically more stable than primary superdislocations.

Substructure evolution of copper polycrystals under different testing conditions: conventional strain control and ramp loading

Substructure evolution has been studied by transmission electron microscopy in cyclically deformed copper polycrystals during conventional strain control testing and ramp loading. In both cases, and very similar to the substructure evolution previously reported for copper single crystals, dislocation structures develop from high and medium energy dislocation structures into low energy dislocation structures with increasing cumulative plastic strain. The two clearest differences between the substructure evolution of copper polycrystals conventionally tested and those ramp loaded are found to be the levels of homogeneity, from grain to grain, and the density of secondary dislocations within the developing loop patches. During ramp loading, the very small increment of the load per cycle promotes a much more homogeneous substructure, from grain to grain, higher densities of secondary dislocations and, therefore, lower energy loop patches than those observed in conventionally tested specimens. At stress amplitudes high enough to produce strain localization during ramp loading, substructure heterogeneities occur, which are explained by an orientation effect in which those grains with multislip behavior promote rapid evolution of low energy dislocation structures, i.e. persistent slip bands.

Overview no. 96 evolution of f.c.c. deformation structures in polyslip

The microstructural evolution during polyslip in f.c.c. metals in investigated by the examples of Al, Ni, NiCo alloys and an AlMg alloy, deformed at room temperature either by rolling or by torsion. The principles governing this evolution appears to be the following: (a) There are differences in the number and selection of simultaneously acting slip systems among neighboring volume elements of individual grains. In any one volume element (called a cell block), the number of slip systems falls short of that required for homogeneous (Taylor) deformation, but groups of neighboring cell blocks fulfil the Taylor criterion collectively. (b) The dislocations are trapped into low-energy dislocation structures in which neigboring dislocations mutually screen their stresses. The microstructural evolution at small strains progresses by the subdivision of grains into cell blocks delineated by dislocation boundaries. These boundaries accomodate the lattice misorientations, which result from glide on different slip system combinations in neighbouring cell blocks. The cell blocks are subdivided into ordinary cells and both cell blocks and cells shrink with increasing strain. All observations appear to be in good accord with the theoretical interpretation. However, some problems remain to be solved quantitatively.

Cold deformation microstructures

AbstractThe evolution of the cold deformation microstructure is described for medium to high stacking fault energy, single phase fcc metals. Macroscopic strain accommodation for polycrystalline metals is considered, and it is suggested that grains subdivide during deformation on a smaller and smaller scale, and that each volume element is characterised by an individual combination of slip systems. A number of microstructural observations (especially of aluminium, nickel, and copper) are described, and dislocation arrangements are discussed on the basis of the general principle that they are low energy dislocation structures. It is shown that the microstructural evolution is quite similar in polycrystalline metals and in single crystals deforming by multislip, and ways in which metallurgical parameters such as stacking fault energy and grain size can affect the microstructure are examined. The general principle of grain subdivision during cold deformation is discussed with reference to the microstructural ...

Cold deformation microstructures

The evolution of the cold deformation microstructure is described for medium to high stacking fault energy, single phase fcc metals. Macroscopic strain accommodation for polycrystalline metals is considered, and it is suggested that grains subdivide during deformation on a smaller and smaller scale, and that each volume element is characterised by an individual combination of slip systems. A number of microstructural observations (especially of aluminium, nickel, and copper) are described, and dislocation arrangements are discussed on the basis of the general principle that they are low energy dislocation structures. It is shown that the microstructural evolution is quite similar in polycrystalline metals and in single crystals deforming by multislip, and ways in which metallurgical parameters such as stacking fault energy and grain size can affect the microstructure are examined. The general principle of grain subdivision during cold deformation is discussed with reference to the microstructural observations.MST/1290

Strain hardening and substructural evolution in NiCo solid solutions at large strains

Torsion deformation was used to investigate dislocation substructure evolution at large strains in high purity nickel and NiCo solid solutions. Observations of small strain dislocation structures formed in stage III revealed that the laminar dislocation structure observed after large strains in stage IV develops from short paired dislocation sheets within the tangled dislocations of an equiaxed cell wall. The development of these short paired dislocation sheets into long microbonds occurs gradually by a multiple-slip process in accordance with the principles of low energy dislocation structures and without the occurrence of a shear instability. The plane of these sheets and /or microbands does not correspond to a {111} slip plane. As these microbands form, a misorientation between the interior of the paired sheets and the surrounding matrix develops and increases with increasing strain.

Theory of work-hardening applied to stages III and IV

Stage IV has become the accepted name for that work-hardening stage within which large plastic strains can occur at a very low, virtually constant work-hardening rate, as exemplified by cold rolling and wire drawing. By contrast, in the preceding stage III, the work-hardening rate decreases sharply with strain, whereas in the still earlier stage II, the work-hardening rate is also almost constant but has a high value. The classical paper by Langford and Cohen on drawn iron wire is now recognized as one of the earliest studies of stage IV. Already in 1970, a detailed theoretical analysis of that work based on the mesh length theory was presented[2] which has stood the test of time, although in it the Langford and Cohen experiments were considered to represent stage II on account of the operation of similitude and the almost constant work-hardening rate. The present paper re-examines the 1970 theoretical interpretation in terms of stage IV behavior, which necessitates reinterpretation of stage III. Included in the present interpretation are more recent insights regarding dislocation behavior in so-called LEDS, low-energy dislocation structures. It is concluded that stages II and IV differ, because in stage II, cross slip is insignificant, while in stage IV, it is unlimited. Accordingly, cross slip is gradually established in the course of stage III. However, similitude appears to operate in all three stages. By extension of the argument regarding stages III and IV, it is seen that stages V and VI could follow, including similitude, through the establishment of climb.

The effect of low energy dislocation structures on crack growth onset in brittle crystals

Discontinuous crack growth in semi-brittle crystals is a challenging phenomenon. What are the low energy dislocation structures that accompany stably growing cracks? It has been postulated that shielding dislocations and dislocation-free zones are a result of an equilibrium of forces at the crack tip. In some materials, such as iron, these arguments are used to demonstrate the marginal stability with respect to brittle or ductile fracture. Some recent work on Fe3%Si has shown that applied stress intensities K I approaching 100 MPa m 1 2 . The establishment and re-establishment of under gaseous hydrogen on the {001} cleavage plane. Initial growth starts near a K I of 20 MPa m 1 2 . The establishment and re-establishment of equilibrium with the crack tip dislocation slip band(s) is central to the issues involved in crack stability. For example, is it dislocation shielding by primary emitted dislocations, dislocation cutting and/or shielding as produced by secondary sources, residual stress fields in the wake, the lack of hydrogen or all of the above which leads to crack arrest? To assist in sorting out these complex issues, both crack initiation and arrest in brittle single crystals of Fe3%Si as well as polycrystalline high strength, low alloy steel have been accomplished. Both fracture surface morphology and slip band configurations with and without a prior overload were compared by Nomarski and scanning electron microscopy contrast. Evaluation of dislocation morphologies utilizes backscattered scanning electron microscopy and transmission electron microscopy techniques. The results are rationalized and interpreted in terms of equilibrium solutions for dislocation arrays at a crack tip. This utilizes both superdislocation and array approaches in a computer simulation program incorporating anisotropic elasticity. Equilibrium is simulated using a quasi-static approach. Reports on the relative contribution of dislocation arrays and environmental effects to crack stability are made with the caveat that such simulations are extremely sensitive to the input data set.

Determining subsurface stress distributions in tribological samples from dislocation cell sizes in low energy dislocation structures

The stress distribution that caused any observable low energy dislocation structure can be inferred from the scale thereof. For dislocation cells, the empirical formula τ ≈ 10 Gb/ L, with τ the resolved shear stress, L the average dislocation cell diameter, G the shear modulus and b the Burgers vector, is especially useful in this regard. As an example, it was applied to the subsurface dislocation cell size distribution underneath the wear scar on a copper foil 0.1 mm thick that had been glued onto a steel rod and then tested in a crossed-rods apparatus. The result confirms the theoretical expectation that the minimum required thickness, when foils are used to simulate the tribological behavior of bulk material, is at least twice, and more safely three times, the average contact spot diameter.

Low energy dislocation structures produced by cyclic softening

Cyclic softening occurs when a prestrained metal is subsequently fatigued at strain amplitudes sufficient to produce microslip in the inherited dislocation structures. The dependence of this threshold on the nature and magnitude of the prestrain is reviewed along with the changes which occur in the cyclic mechanical properties and the dislocation structures. It is shown that cyclic softening is a recovery process induced by cyclic strain and that it leads to low energy dislocation structures. Speculation on the relation of cyclically softened structures to the Dickson wall model is offered. New results on the dislocation structures produced in high-low amplitude block tests suggest softened structures are self-organizing non-equilibrium structures, and the connection of these with low energy structures is indicated.