Low Energy Dislocation Structures Research Articles

Local crack arrestability and deformation microstructure evolution of hydrogen-related fracture in martensitic steel

Intergranular cracks in the uncharged specimen were arrested at low-angle prior austenite grain boundary (PAGB) segments of several micrometers. In contrast, even small, low-angle PAGB segments with sub-micrometer sizes impeded the propagation of hydrogen-related intergranular crack. At the hydrogen-related quasi-cleavage crack tip, the crystallographic orientation changed abruptly, and deformation microstructures developed, including the formation of low-energy dislocation structures. A certain degree of crack growth resistance (intrinsic crack growth resistance) in the hydrogen-related fractures could be attributed to the intense localized plastic works involved in the arrest of intergranular cracks and the propagation of quasi-cleavage cracks.

Microstructural evolution of Al-Zn-Mg-Cu alloy during ultrasonic surface rolling process

This paper systematically investigates the microstructural evolution of Al-Zn-Mg-Cu alloy during ultrasonic surface rolling process (USRP). Al-Zn-Mg-Cu alloys with different gradient deformation degrees were prepared by USRP conducted with 1, 2, 6, 8, 10 and, 12 processing cycles, respectively, and then the deformation structures from the surface of samples with different deformation degrees were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The results indicated that with the increase of processing cycle, the microstructural evolution at different depths of surface deformation layer is basically the same. But the closer to the surface, the faster microstructure evolves. The basic microstructural evolution order of Al-Zn-Mg-Cu alloy during USRP is scattered dislocation, dislocation cells (DCs), elongated dense dislocation cell walls (DDWs), thick laminated structure and nanolaminated structure. According to results of crystallography and atomic level analysis, the microstructural evolution was described in detail. Scattered dislocation structure is formed by dislocation proliferation. According to the low energy dislocation structures (LEDS) principle, the scattered dislocation structure is transformed into DCs structure through dislocation movement. Under the action of large shear stress, DCs structure is elongated along the direction of shear stress to form elongated DDWs structure. The transformation of elongated DDWs into thick laminated structure and the continuous refinement of laminated structure are mainly achieved by the formation of the low angle grain boundaries (LAGBs) and the transformation from LAGBs to high angle grain boundaries (HAGBs). In some nanolaminated structure regions, the laminated structure is further refined by twin deformation. Our work provides a theoretical basis for the adjustment of the actual processing technology.

Effect of strain of cryorolling on structure and strength of nickel

The effect of isothermal rolling at liquid nitrogen temperature with reductions up to 90 % on the structural-mechanical behavior of coarse-grained nickel was evaluated by TEM, SEM-EBSD and X-ray analysis methods, and microhardness measurements. The highest hardening rate of the metal was found at the initial stage of rolling (up to about 50 %), after which it decreased slightly with further reductions. Detailed analyses of the evolution of dislocation density, grain size and grain boundary spectra have shown that the behavior found was attributed to an extremely low deformation temperature, resulted in a decreased rate of accumulation of crystalline defects, as well as the formation and rearrangement of dislocation structure. Due to the strong suppression of dynamic recovery and recrystallization, even when rolling to a reduction of 90 %, a relatively homogeneous coarse-fibered structure with a developed nanocellular substructure and less than 5 % fraction of nanoscale dynamically recrystallized grains was formed. It was concluded that the main structural strengthening factors in cryogenically rolled nickel was the formation of low-energy dislocation structures and their transformations into a well-developed substructure with nanosized crystallites separated predominantly by low-angle boundaries.

Cyclic deformation behavior and failure mechanism of S32205 duplex stainless steel under torsional fatigue loadings

Torsional low cycle fatigue tests of ferrite-austenite duplex stainless steel S32205 at various shear strain amplitudes were conducted. The relationship between the evolution of dislocation structures in each constitutive phase and cyclic stress response and cyclic stress-strain curves was thoroughly studied. The surface damage evolution and short cracks nucleation behavior were also discussed. Results show that the cyclic softening occurred at all the strain amplitudes from 0.3% to 1.2% is due to the redistribution and localization of plastic strain and the development of strain transfer between phases, while the formation of regular dislocation cells contributes to achievement of cyclic saturation when the strain amplitude reaches 0.7%. The cyclic stress-strain curves show two-regimes with different cyclic strain hardening rates. As the plastic strain amplitude is smaller than 0.32%, the ferrite and austenite jointly accommodate the plastic strain within material matrix with the coexistence of representative planar slip and incipient wavy slip dislocation configurations in both phases, leading to a low cyclic strain hardening rate. By contrast, the ferrite dominates the plastic deformation with the formation of well-developed low-energy dislocation structures as the plastic strain amplitude is over than 0.32%, resulting in a high cyclic strain hardening rate. Moreover, microstructure observations reveal that the pronounced persistent slip markings in ferrite grains and phase boundaries are the preferential sites for microcrack nucleation and growth.

Microstructure and martensitic transformation in 316L austenitic steel during multiaxial low cycle fatigue at room temperature

Austenitic stainless 316L steel in cold worked condition was subjected to torsional cyclic loading with equivalent strain amplitudes of 0.3% and 0.4% and multiaxial (in-phase and out-of-phase) cyclic loading with equivalent strain amplitude of 0.35% at room temperature. Cyclic plastic response has been measured and analyzed. Internal structure in the as-received state was compared with the deformation microstructures that developed during cyclic straining with various strain histories. In all three investigated cases the volumes with dislocation structures corresponding to cyclic strain localization (persistent slip bands, irregular wall and channel structures) were found. In addition, also islands of deformation induced α′-martensite were observed. Transmission electron microscopy, selected area electron diffraction and ferritscope measurements allowed to estimate the fraction and distribution of α′-martensite islands. The shape of fatigue hardening/softening curves has been discussed in relation with the localization of cyclic plastic strain in the low energy dislocation structures and its blockade by the islands of α′-martensite.

Investigation of grain subdivision at very low plastic strains in a magnesium alloy

In-situ tensile loading combined with electron backscatter diffraction (EBSD) measurements has been used to investigate the plastic deformation of a magnesium alloy. A novel EBSD mapping is presented, based on construction of maps showing the rotation axis component in the sample coordinate frame of the misorientation from each pixel to the average grain orientation in the deformed sample. Using this mapping it is shown that the pattern of grain subdivision, even at very low plastic strains, can be revealed simultaneously in a large number of grains. In addition, it is demonstrated how maps of the rotation axis corresponding to the misorientation between each pixel and the initial grain orientation provide complimentary information directly useful for crystal plasticity analysis. A detailed slip system analysis shows that the grain subdivision can be accounted for according to the low energy dislocation structures (LEDS) model of work-hardening by differences in the slip amplitudes within different parts of each grain.

Low-Energy Dislocation Structure (LEDS) character of dislocation boundaries aligned with slip planes in rolled aluminium

For the specific slip geometry of two sets of coplanar systems (a total of four systems) in fcc metals, the range of dislocation networks in boundaries aligned with one of the two active slip planes is predicted from the Frank equation for boundaries free of long-range elastic stresses. Detailed comparison with experimental data for eight dislocation boundaries in cold-rolled aluminium grains of the 45° ND rotated Cube orientation is conducted. It is concluded that the boundaries are Low-Energy Dislocation Structures, which are in good agreement with the Frank equation while also lowering the energy by dislocation reactions. Cross slip plays a role in the boundary formation process.

Deformation behavior and microstructure evolution of pure Cu subjected to electromagnetic bulging

In this paper, pure Cu sheet with thickness of 1mm was electromagnetically budged to form a conical shape workpiece. The deformation behavior and the microstructural evolution of the Cu sheet under electromagnetic bulging were systematically studied using strain analysis, optical microscopy (OM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). It is found that the strain distribution in the workpiece is quite un-uniform from the bottom to the top due to the inhomogeneous electromagnetic pressure generated by the spiral coil. The characterization of microstructure reveals that the bulge deformation of the Cu sheet is governed by dislocation multiple slip and cross slip, which cause finally the formation of cell structures. However, the size and the boundary width of cells are closely related to the plastic strain, i.e., the cell size and boundary width decrease with increasing strain. In addition, low misorientation angle of cells inside the grains increases with increasing strain. This structural evolution is discussed on the basis of the low energy dislocation structures (LEDS) theory.

Influence of grain size in the near-micrometre regime on the deformation microstructure in aluminium

The effect of grain size on deformation microstructure formation in the near-micrometre grain size regime has been studied using samples of aluminium prepared using a spark plasma sintering technique. Samples in a fully recrystallized grain condition with average grain sizes ranging from 5.2 to 0.8μm have been prepared using this technique. Examination in the transmission electron microscope of these samples after compression at room temperature to approximately 20% reduction reveals that grains larger than 7μm are subdivided by cell block boundaries similar to those observed in coarse-grained samples, with a similar dependency on the crystallographic orientation of the grains. With decreasing grain size down to approx. 1μm there is a gradual transition from cell block structures to cell structures. At even smaller grain sizes of down to approx. 0.5μm the dominant features are dislocation bundles and random dislocations, although at a larger compressive strain of 30% dislocation rotation boundaries may also be found in the interior of grains of this size. A standard 〈110〉 fibre texture is found for all grain sizes, with a decreasing sharpness with decreasing grain size. The structural transitions with decreasing grain size are discussed based on the general principles of grain subdivision by deformation-induced dislocation boundaries and of low-energy dislocation structures as applied to the not hitherto explored near-micrometre grain size regime.

Dislocations, boundaries and slip systems in cube grains of rolled aluminium

The relationship between the dislocation content of boundaries and the active slip systems is explored by characterisation of Burgers vectors, dislocation lines and relative densities in 11 boundaries in near-cube grains in 10% rolled aluminium. To provide a good basis for comparison, all the boundaries investigated lie in the longitudinal plane. Practically all dislocations have screw character, with Burgers vectors corresponding to the slip systems predicted active. The dislocations arrange in a regular grid and the boundaries are most likely low-energy dislocation structures.

Grain refinement process of pure iron target under hypervelocity impact

The structure of ultrafine grain is formed at the crater bottom of pure iron target under hypervelocity impact. The microstructures of different layers at the crater bottom were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The cross-section observation was performed to reveal the grain refinement process driven by plastic deformation. Firstly, low energy dislocation structures (LEDS) such as dense dislocation walls (DDWs) and dislocation tangles (DTs) refine the original grains and form intersecting lamellar structures. With increasing strain, DDWs and DTs transform into subboundaries with small misorientations to separate lamellar structure to cells. Subboundaries are converted to high misorientation grain boundaries, so ultrafine grains are formed. The formation of ultrafine grains was discussed in the dynamic recrystallization process due to the large strain and strain rate induced by spherical shock wave.

Insights beside assessments into small volume cyclic response of elastic plastic systems

In contrast to monotonic load studies, the engagement with a cyclic time dependent processes of small volume material response has been restrained. Crack tolerance in thin films almost by definition is restricted. Nevertheless, in terms of structural stability the fatigue resistance remains a critical concern regardless the scale. The cyclic displacement amplitudes can be enhanced by remote mechanical stress/strain or thermal origins resulted in similar outcome of cumulative irreversible micro-plasticity damage. Thus, the cyclic life in nano applications is mainly dominated by the crack nucleation controlled process. The current study is centered on the fatigue crack initiation stage as a first order design pillar and therefore require additional experimental input. Activities in elastic-plastic polycrystalline and single crystal systems in various crystal structures are described. Beside findings regarding the fatigue cracks initiation life, the role of compressive residual stress on the cyclic response has been also analyzed. The fatigue crack initiation criteria have been associated to other constitutive assessments that are related mainly to the crack stability equation elements. In addition, completely different insights are introduced into the concept of the critical film thickness, indicating the susceptibility to the crack formation. As addressed in heteroepitaxial layers, specific analogous characterization exists either in dislocation activities or by triggering crack formation. The physical aspects of threading dislocations growth and glide behavior and local decohesion ramifications are highly dependent on the layer thickness. Experimentally based, under strain control conditions at ambient temperatures the aforementioned fatigue crack initiation stage in small volume segments, obeyed a cumulative damage mechanism. It was also established that the low energy dislocation structures and the initiation life were dramatically dependent on the strain amplitude range. The role of compressive residual stress in prolonging the fatigue initiation life was also consistent. Even in silicon based materials, dislocation activities prevailed. It became apparent that categorical generalization in nano-silicon thin film fatigue behavior is beyond the susceptibility to the native oxide or environmental interaction. Issues like film processing methodology, film/substrate interfaces, residual stresses and micro-cracking feasibility occurrence as wall as critical sites still require further design insights.

Cyclic Deformation of Advanced High-Strength Steels: Mechanical Behavior and Microstructural Analysis

The fatigue properties of multiphase steels are an important consideration in the automotive industry. The different microstructural phases present in these steels can influence the strain life and cyclic stabilized strength of the material due to the way in which these phases accommodate the applied cyclic strain. Fully reversed strain-controlled low-cycle fatigue tests have been used to determine the mechanical fatigue performance of a dual-phase (DP) 590 and transformation-induced plasticity (TRIP) 780 steel, with transmission electron microscopy (TEM) used to examine the deformed microstructures. It is shown that the higher strain life and cyclic stabilized strength of the TRIP steel can be attributed to an increased yield strength. Despite the presence of significant levels of retained austenite in the TRIP steel, both steels exhibited similar cyclic softening behavior at a range of strain amplitudes due to comparable ferrite volume fractions and yielding characteristics. Both steels formed low-energy dislocation structures in the ferrite during cyclic straining.

The morphology and crystallography of lath martensite in alloy steels

The morphology and crystallography of lath martensite in two Mn-containing interstitial free steels and a maraging steel were examined in detail by a combination of transmission electron microscopy, electron backscatter diffraction in a scanning electron microscope and optical microscopy. Indirect determination of the orientation relationship between the lath martensite and the austenite was made by analysis of misorientation distributions, and a similar orientation relationship was found for the three alloys in accordance with previous observations in low-carbon steels. Furthermore, the formation of six variants in a given packet and the preferential arrangement into blocks of low-misorientation variant pairs demonstrate a universality of morphology and crystallography of lath martensite. The presence of six variants in a packet can be accounted for by the minimization of the total shape strain introduced during the transformation. The lath boundaries developed within the volume of a certain variant show alternating misorientations and a mixed tilt and twist character characterizing these structures as low-energy dislocation structures.

Dislocation structure and deformation in iron processed by equal-channel-angular pressing

The evolution of dislocation structure in pure Fe during equal-channel-angular pressing (ECAP) is investigated. Also, the effect of the formation of this dislocation structure on deformation and fracture behavior is examined. The results show that intensive dislocation cell blocks are present after one pass and even more after subsequent pressings. The low-energy dislocation structures (LEDS) may have changed into the high-energy dislocation structures (HEDS) in the final several pressings. The high-density array of dislocations plays a significant role in strengthening. The HEDS may cause the materials to lose work-hardening ability and show a cleavage morphology of the fracture surface. A proper subsequent annealing treatment will lead to the evolution of HEDS to LEDS while maintaining little grain growth. This change in the nature of dislocation structures allows ultrafine-grained materials to achieve an excellent combination of high strength and high ductility.

Advancing towards constitutive equations for the metal industry via the LEDS theory

A prime objective in the development of crystal dislocation theory has been, and at any rate should be, constitutive equations for practical use in the metal forming industry. Protracted controversies regarding workhardening theory have frustrated this goal for the past seven decades. They are fueled by the paradox that plastic deformation is a prime example for the second law of thermodynamics in converting mechanical work into heat with good efficiency, even while in seeming opposition to the second law it typically raises the internal energy of the deformed material. The low-energy dislocation structures (LEDS) theory resolves this difficulty by showing that, as always in inanimate nature, so also plastic deformation proceeds close to minimum free energy. Indeed recent evidence based on deformation band structures proves that plastic deformation typically proceeds very close to minimum energy among the accessible configurations. While plastic strain raises the flow stress, in ductile crystalline materials mostly through generating dislocation structures, but also through twins, kink bands, microcracks and others, Newton’s third law, i.e., force equilibrium, is always stringently obeyed. Therefore, deformation dislocation structures are in thermal equilibrium as long as the stress that generated them remains in place. Based on this concept of free energy minimization, the LEDS theory has long since explained, at least semiquantitatively, all significant aspects of metal strength and deformation, as well as the effects of heat treatments. The LEDS theory is the special case, namely, as pertaining to dislocation structures, of the more general low-energy structures (LEDS) theory that governs all types of deformation independent of the deformation mechanism, and that operates in all types of materials, including plastics.

Advancing towards constitutive equations for the metal industry via the LEDS theory

A prime objective in the development of crystal dislocation theory has been, and at any rate should be, constitutive equations for practical use in the metal forming industry. Protracted controversies regarding workhardening theory have frustrated this goal for the past seven decades. The are fueled by the paradox that plastic deformation is a prime example for the second law of thermodynamics in converting mechanical work into heat with good efficiency, even while in seeming opposition to the second law it typically raises the internal energy of the deformed material. The low-energy dislocation structures (LEDS) theory resolves this difficulty by showing that, as always in inanimate nature, so also plastic deformation proceeds close to minimum free energy. Indeed recent evidence based on deformation band structures proves that plastic deformation typically proceeds very close to minimum energy among the accessible configurations. White plastic strain raises the flow stress, in ductile crystalline materials mostly through generating dislocation structures, but also through twins, kink bands, microcracks and others, Newton’s third law, i.e., force equilibrium, is always stringently obeyed. Therefore, deformation dislocation structures are in thermal equilibrium as long as the stress that generated them remains in place. Based on this concept of free energy minimization, the LEDS theory has long since explained, at least semiquantitatively, all significant aspects of metal strength and deformation, as well as the effects of heat treatments. The LEDS theory is the special case, namely, as pertaining to dislocation structures, of the more general low-energy structures (LEDS) theory that governs all types of deformation independent of the deformation mechanism, and that operates in all types of materials, including plastics.

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.

Why do dislocations assemble into interfaces in epitaxy as well as in crystal plasticity? To minimize free energy

Dislocations commonly form planar arrays that minimize the free interfacial energy between relatively mismatched crystal volumes. In epitaxy and phase transformations, the causative misfit is that between differences in lattice structure and/or orientations of different phases. In deformed homogeneous crystalline materials, the planar dislocation arrays are grain and mosaic block boundaries that accommodate relative misorientations within the same crystal structure. Thus, overwhelmingly, planar dislocation arrays have a basically common origin, namely minimization of interfacial energies. Consequently, they are all subject to the low-energy dislocation structures (LEDS) hypothesis. While the specific applications of the underlying general theory are well advanced in terms of epitaxy, phase, and grain boundaries, in connection with plastic deformation that very basis is widely overlooked, if not denied. The present article aims to (a) document the fact that, while being formed, dislocation structures due to plastic deformation are in thermodynamical equilibrium, (b) firmly establish the outlined connection between planar dislocation arrays of all types, and, thereby, (c) establish the kinship between epitaxy and plastic deformation of crystalline materials.

The fatigue limits of copper single crystals cyclically deformed at constant plastic strain amplitudes

Based on systematic surface observations, experimental measurements on the fatigue limits of differently oriented copper single crystals cyclically deformed at constant plastic strain amplitudes are summarized, and an orientation dependence of the fatigue limit is sketched. It was found that the fatigue limits of crystals depend mainly upon the low-strain-amplitude dislocation structures, which are characteristic of the particular crystal orientations. When multiple-slip deformation is the main mode of deformation and ultimately produces stable lowenergy dislocation structures such as labyrinth and cell structures, the fatigue limit can be improved notably.