Dense Dislocation Walls Research Articles

Behaviour of low and medium carbon free cutting steels during deformation to large strains

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.

Behaviour of low and medium carbon free cutting steels during deformation to large strains

Low and medium carbon free cutting steels were deformed by cold rolling to reductions of up to 98%. The resultant microstructures were observed and characterised using optical microscopy, SEM, and TEM. Deformation of the pearlite grains and manganese sulphide inclusions was quantified in terms of their relative plasticity (compared to that of the steel). The evolution of the ferrite microstructure in the steels was seen to be dependent on the volume fraction of pearlite present. The ferrite grains in the low carbon steels underwent structural subdivision characterised by the formation of dense dislocation walls and microbands. At intermediate rolling deformations much of the strain was accommodated inhomogeneously in narrow bands of shear (S bands). Strain in the pro-eutectoid ferrite of the medium carbon steel occurred in a more homogeneous manner owing to the constraints imposed by the pearlite. The manganese sulphides and pearlite in the steels also acted as fiducial markers of the surrounding ferrite flow. Plasticity of the sulphides was generally found to be less than the overall rolling strain. However, within certain narrow strain ranges, sulphide plasticities greater than that of the steel were measured.

New grain formation during warm deformation of ferritic stainless steel

Microstructural evolution accompanied by localization of plastic flow was studied in compression of a ferritic stainless steel with high stacking fault energy (SFE) at 873 K (≈0.5 Tm). The structure evolution is characterized by the formation of dense dislocation walls at low strains and subsequently of microbands and their clusters at moderate strains, followed by the evolution of fragmented structure inside the clusters of microbands at high strains. The misorientations of the fragmented boundaries and the fraction of high-angle grain boundaries increase substantially with increasing strain. Finally, further straining leads to the formation of new fine grains with high-angle boundaries, which become more equiaxed than the previous fragmented structure. The mechanisms operating during such structure changes are discussed in detail.

Dense dislocation walls and microbands aligned with slip planes—theoretical considerations

Some of the dislocation boundaries in cold deformed f.c.c. metals at low and intermediate strains lie on crystallographic slip planes and others have a macroscopic direction with respect to the sample axes (i.e. they are non-crystallographic). A model for the occurrence of the former type of dislocation boundaries is proposed. The model combines slip pattern analysis and dislocation theory. It is assumed (i) that the dislocations in the boundaries are generated by slip, (ii) that the deformation temperature is low enough to exclude dislocation climb and (iii) that the driving force for formation of boundaries is minimisation of the energy stored in the boundaries. Formation of crystallographic boundaries is predicted if two active slip systems in the same slip plane account for a large fraction of the total slip. For single crystals the agreement between predicted and experimentally observed crystallographic and non-crystallographic boundaries is excellent. For different grain orientation in poly crystalline aluminium specimens, the agreement between prediction and experiment is satisfactory in view of the complexity of polycrystal studies compared to studies of single crystals.

The Effect of Stress-State on the Large Strain Inelastic Deformation Behavior of 304L Stainless Steel

In metals, large strain inelastic deformation processes such as the formation of a preferred crystallographic orientation (crystallographic texture) and strain hardening processes such as the formation and evolution of dislocation substructures depend on stress-state. Much of the current large strain research has focused on texture. Crystallographic texture development and strainhardening processes each contribute to the overall material behavior, and a complete description of large strain inelastic material response should reflect both. An investigation of the large strain behavior of 304L stainless steel (SS 304L) subjected to compression, torsion, and sequences of compression followed by torsion and torsion followed by tension is reported. This paper focuses on the stress-state dependence of strain-hardening processes as well as the relative effect such processes have on the overall material behavior. To characterize these processes, transmission electron microscopy (TEM) as well as magnetization investigations were conducted at different strain levels and under different deformation modes. The γ → α′ martensitic transformation which occurs in this material was found to be related to both the strain level and stress state. Dislocation substructures in the form of Taylor lattices, dense dislocation walls, and microbands were also present. The ramifications of using a thin-walled tubular torsion specimen were also explored.

Dense dislocation walls and deformation banding in commercial purity aluminium

Dense dislocation walls and deformation banding in commercial purity aluminium

Large strain deformation structures in aluminium crystals with rolling texture orientations

Four single crystal orientations of high purity aluminium have been deformed in channel die compression up to strains of ∼ 1 to correlate the dislocation substructures, in single and polycrystals with the slip system distribution. Three orientations are close to the stable rolling texture components of fcc metals: (110)[ 112], (112)[11 1] and (112)[ 174] and one is very unstable (121)[ 311] . The substructures are characterized on the longitudinal section over a wide range of scales by optical microscopy. TEM and SEM with EBSD. Low energy dislocation matrix structures composed of cells, cell blocks, dense dislocation walls and first generation microbands are observed in all orientations in agreement with the microstructures of rolled polycrystals. The S (213)[ 142] and C (112)[11 1] orientations also develop narrow bands of localized glide associated with relatively high local misorientations. The S orientation exhibits characteristic S-shaped band structures of first generation microbands sheared on {111} planes whereas the C orientation forms non-crystallographic shear bands. These two orientations can be considered stable in terms of average texture but unstable in terms of microstructure.

Microstructural Evolution in Nickel during Rolling from Intermediate to Large Strains

High-purity nickel (99. 99 pct) with a grain size of 80 to 100 µm was deformed by cold-rolling from 37 to 98 pct reductions (von Mises effective strains ofevm = 0. 5 to 4. 5). The deformation microstructures and texture at five strain levels were observed and characterized using transmission electron microscopy (TEM) and neutron diffraction. The microstructures evolved within a framework common to medium and high stacking fault energy fee polycrystals. This framework consists of structural subdivision by higher angle boundaries (geometrically necessary boundaries) at one volume scale and at a smaller volume scale by lower angle cell boundaries (incidental boundaries) for all strain levels. We have characterized the dislocation boundaries, including dense dislocation walls (DDWs), microbands (MBs), and lamellar boundaries (LBs) in terms of crystallographic and macroscopic orientations, morphology, and frequency of occurrence. The microstructural evolution is discussed with special emphasis on factors that contribute to the transition from structures characteristic of small and medium strain microstructures to those characteristic of large strain microstructures.

Microstructural evolution of pure iron during hot rolling

AbstractThe microstructural evolution of commercially pure iron has been investigated via hot rolling. Results from optical microscopy, transmission electron microscopy, and microdiffraction show that, at the initial stage of deformation, equiaxed subgrains form at the original grain boundaries. Long layered substructures composed of parallel dense dislocation walls form within grains, and in some cases microbands are also observed. With the increase of strain, the dense dislocation walls persist and their misorientations increase. At the same time, new subboundaries with smaller misorientations form between the dense dislocation walls, and eventually fully equiaxed subgrains develop throughout the specimen. The development of the microstructures and the formation mechanism of micro bands are discussed.MST/1740

Flow stress anisotropy caused by geometrically necessary boundaries

The microstructural anisotropy of deformed metal is related to the formation of geometrically necessary boundaries such as dense dislocation walls and microbands. These boundaries have a macroscopic orientation with respect to the sample axes and they can resist slip due to a high concentration of dislocations. A model has been proposed for this microstructural anisotropy based on the assumptions that (i) the average slip plane is at an angle of 45° to the direction of the applied stress and that (ii) a strengthening parameter is the mean distance in the slip plane between the geometrically necessary boundaries. For different macroscopic arrangements of such boundaries, the model predictions are in good qualitative and quantitative agreement with experiments.

Microstructural evolution in rolled aluminium

The evolution of the microstructure of cold-rolled pure aluminium (99.996%) was studied by transmission electron microscopy (TEM), extending a previous study that was limited to 30% strain (ε = 0.36). The present work concentrates on rolling strains from 50% (ε = 0.69) to 90% (ε = 2.3). The observations are explained through the governing principle that, in the course of polyslip, grains break up into volume elements within each of which fewer slip systems operate simultaneously than required by the Taylor model. The boundaries between the volume elements accommodate the lattice misorientations arising from the correspondingly different glide. They are therefore geometrically necessary boundaries and in TEM they appear variously as dense dislocation walls, different types of microbands and subgrain boundaries. The morphology and the spatial arrangement of the geometrically necessary boundaries were characterized and it is discussed how these boundaries form with increasing strain as an integral part of the microstructural evolution.

Cell and band structures in cold rolled polycrystalline copper

AbstractThe effect of plastic strain on the deformation microstructure has been investigated in polycrystalline copper rolled at room temperature to 5, 10, 20, and 30% reduction in thickness (equivalent strain 0·06–0·42). Results from transmission electron microscopy (TEM) observations show that dense dislocation walls (DDWs) and cells develop during the initial stages of cold rolling. Grains having a high density of DDWs are described as high wall density (HWD) structures, and grains having a low density of DDWs are described as low wall density (LWD) structures. These structures are characterised by cell size, misorientation across the cell walls, and the crystallographic orientation of the grains in which they appear. The DDWs in the HWD structures have special characteristics, extending along several cells and having a misorientation across them greater than that across ordinary cell boundaries at the same strain. The DDWs appear to have a macroscopically determined orientation. Analysis of their crys...

Effect of grain size and ramp loading on the low-amplitude cyclic stress-strain curve of polycrystalline copper: Llanes, L. and Laird, C. Mater. Sci. Eng. A Sept. 1990 128A, (2), L9–L12

The cyclic stress–strain behavior of copper with different grain sizes/textures has been studied. The results show that at high plastic strain amplitudes, secondary hardening occurs after the primary hardening stage in small-grained specimens; and cyclic softening occurs after the primary hardening stage in large-grained specimens. At low plastic strain amplitudes, the effect of grain size/texture is small, saturation occurs after the primary hardening stage in all specimens. The secondary hardening found in small-grained specimens is associated with the formation of equiaxed cells. In a small-grained specimen, it contains more twin and grain boundaries and twin steps than a large-grained specimen; these boundaries and twin steps in particular promote secondary slip activity, therefore enhance the formation of equiaxed cells. The cyclic softening found in large-grained specimens is mainly associated with the condensation of loop patches into dense dislocation walls.

Geometrically necessary, incidental and subgrain boundaries

During the deformation of polycrystals, the grains break up into domains within which the selection of operative slip systems differs. The domains then subdivide into “cell blocks”. Locally, each group of cell blocks comes near to fulfilling the Taylor criterion when taken collectively, but the number of active glide systems in any one cell block is fewer than predicted. The boundaries between cell blocks and/or domains accommodate the lattice misorientations which result from glide on the different slip system combinations. They are therefore named “geometrically necessary bpundaries”. They, like all boundaries capable of accommodating variable lattice misorientations, are composed of dislocations. Microscopically, such boundaries appear as “dense dislocation walls” and “microbands”. Geometrically necessary boundaries are distinguished from ordinary dislocation cell boundaries by the absence of a change of glide systems across the latter. In materials deforming with a cell structure, ordinary dislocation cell boundaries as well as traditional “deformation bands” arise from the mutual trapping of dislocations into low-energy configurations. Such cell boundaries or walls are therefore named “incidental dislocation boundaries”. The misorientation across incidental boundaries is typically much smaller than for geometrically necessary boundaries. A further distinction is their respective on the flow stress. The average spacing of dislocation cell walls is inverse proportional to the flow stress whereas geometrically necessary boundaries obey the Hall-Petch relationship. Since they tend to occur more frequently the incidental boundaries typically control the flow stress. At increasing strain the angles between dislocation cells increase and different slip system combinations can operate in neighbouring cells. Cell walls are then no longer incidental boundaries but geometrically necessary boundaries. Such boundaries are termed “subgrain boundaries”.

Microstructural evolution in nickel during rolling and torsion

Microstructural evolution in high purity nickel was investigated by comparing the dislocation substructures formed following torsion deformation with those following rolling. Results from transmission electron microscopy observations and measurements of microdiffraction show a common evolutionary path in rolling and torsion from (i) dislocation tangles to (ii) equiaxed cells bounded by dense dislocation walls (DDWs) which form cell blocks, then to (iii) microbands which, together with DDWs, bound smaller and smaller cell blocks, and, finally, to (iv) subgrains. During this evolution, grains are continually subdivided by the formation of new cell blocks. New cell blocks arise through the formation of new DDWs between cell blocks or of first generation microbands by the subdivision of a single DDW Since the dislocation content of a microband is principally derived from that of the parent DDW, microbands can take various forms, such as a string of small pancake shaped cells in highly recovered DDWs, short double walls in discontinuous DDWs, or long paired dislocation sheets from continuous DDWs which can be very straight if formed close to {111} planes. At moderate strains and subsequent to microband formation, localised shear and shear offsets were observed in some first generation microbands. A few second generation microbands, in which localised shear is part of the formation process, were also observed. Localised shear was much more prevalent in rolling than in torsion. These results provide evidence for the theory that grains are divided into regions in which slightly different slip systems operate. Collectively, these regions provide evidence for strain accommodation, although in individual regions the number of slip systems is less than that required by the Taylor criterion.MST/1333

Microstructural evolution in nickel during rolling and torsion

AbstractMicrostructural evolution in high purity nickel was investigated by comparing the dislocation substructures formed following torsion deformation with those following rolling. Results from transmission electron microscopy observations and measurements of microdiffraction show a common evolutionary path in rolling and torsion from (i) dislocation tangles to (ii) equiaxed cells bounded by dense dislocation walls (DDWs) which form cell blocks, then to (iii) microbands which, together with DDWs, bound smaller and smaller cell blocks, and, finally, to (iv) subgrains. During this evolution, grains are continually subdivided by the formation of new cell blocks. New cell blocks arise through the formation of new DDWs between cell blocks or of first generation microbands by the subdivision of a single DDW Since the dislocation content of a microband is principally derived from that of the parent DDW, microbands can take various forms, such as a string of small pancake shaped cells in highly recovered DDWs, ...

Two-stage temper embrittlement of amorphous FeBSi alloys and structural changes examined by SR-small angle X-ray scattering

Transmission electron microscopy (TEM) examinations on as-received, cold worked, as well as cold worked and creep tested phosphorus-alloyed oxygen-free copper (Cu-OFP) have been carried out to study the role of the cell structure. The cell size decreased linearly with increasing plastic deformation in tension. The flow stress in the tests could also be correlated to the cell size. The observed relation between the flow stress and the cell size was in excellent agreement with previously published results. The dense dislocation walls that appeared after cold work in tension is likely to be the main reason for the dramatic increase in creep strength. The dense dislocation walls act as barriers against dislocation motion and their presence also reduces the recovery rate due to an unbalanced dislocation content.

Deformation structures in lightly rolled pure aluminium

The dislocation structure of lightly rolled aluminium is free of substantial long-range stresses and thus is a low-energy dislocation structure (LEDS). It consists of ordinary cell walls, dense dislocation walls (DDWs) and microbands (MBs) which are stretched out along DDWs and are composed of small pancake-shaped cells. In one particular sample studied, MBs were found in the orientation of shear bands, although they are not observed macroscopically. Since the DDWs and MBs appear together as if forming one general feature they have been dubbed DDW-MBs. The structure can be explain on two hypotheses: (i) All volume elements enclosed by DDWs, including MBs, are blocks of dislocation cells sharing the same combination of active glide systems which, however, are fewer in number than would be needed to satisfy the Taylor condition fully, for the reason that the rate of work hardening increases with increasing number of simultaneously activated glide systems. (ii) MBs are formed later than the DDWs on which they are situated. They complement the deformation due to the earlier cell blocks towards a better approximation of the Taylor condition. It is considered that MBs assume the orientation of shear bands when the strain in them leads to geometrical softening.

Partial molar strain field of hydrogen in α-Fe

The resistance to hydrogen embrittlement (HE) of CrMnFeCoNi high-entropy alloy (HEA) at both room and cryogenic temperatures was examined through tensile experiments on specimens hydrogenated via cathodic electrochemical charging method. Two representative steels, i.e. 316L stainless steel (SS) and X80 pipeline steel (PS), were chosen for comparison due to their similar main constituent elements to CrMnFeCoNi HEA. Results show that the hydrogen pre-charged CrMnFeCoNi HEA has the smallest loss of ductility among the three materials at room temperature, while displays no reduction of elongation at 77 K, compared with the uncharged one. Fracture surfaces at both room and cryogenic temperatures of hydrogen pre-charged CrMnFeCoNi HEA are mainly composed of dimples, indicating ductile fractures, while brittle characteristics occur in pre-charged 316L SS and X80 PS. Typical deformation microstructure of the hydrogen pre-charged CrMnFeCoNi HEA at room temperature is tangled dislocations instead of highly dense dislocation walls (HDDWs) found in the pre-charged 316L SS. At 77 K, more deformation twins are formed in the both materials. Reasons for a higher resistance to HE of CrMnFeCoNi HEA at room temperature are attributed to the formation of less hydrogen trapping sites, thus a lower degree of hydrogen enrichment than 316L SS. While at 77 K, the atomic hydrogen is not able to promptly accumulate near these trapping sites due to its slow diffusion rate, which leads to strong HE resistance.

Microstructural developments in type 316L stainless steel during low-cycle fatigue at 350–550°C and their effects on cyclic strength and life

AbstractThe low-cycle-fatigue behaviour and the related dislocation substructures of type 316L stainless steel in both the solution annealed and aged states have been investigated. Fully reversed, strain controlled continuous fatigue tests were performed on the material over strain ranges of ±0·25, ±0·3125, ±0·375, and ± 0·5% at a constant strain rate of 10−3 s−1 and at temperatures of 350, 450, and 550°C. It is shown that cyclic strengthening increases with temperature and short-term aging. This is attributed to the interaction between dislocations and carbide precipitates, which grow during fatigue as well as during aging. A model which predicts fatigue-associated growth is developed and shows reasonable agreement with experimental results. The results also demonstrate that the dislocation structures observed in the annealed specimens at all three temperatures are characterized by cells. Aging produces smaller and better-defined cells at 350°C, but causes thick and dense dislocation walls (as opposed to...