Formation Of Dislocation Cell Structure Research Articles

TEM observations of variation of dislocation cell structures along the building direction in SLM-316L stainless steel

Ultra-fast cooling and cyclic thermal heating associated with the additive manufacturing by selective laser melting (SLM) often lead to the formation of multi-scale microstructures in manufactured metallic materials. It has been frequently reported that a dislocation cell structure develops within the grains in SLM-316L stainless steels (ss), which has important effects on their mechanical properties and thermal stability. However, the formation mechanism of the dislocation cell structure is still under debate. In this study, we used transmission electron microscopy (TEM) techniques to characterize the variation of dislocation structures along the building direction in a SLM-316L ss sample. It exhibits various dislocation structures in the very surface melt pools. The distribution of dislocations at shallower locations is relatively discrete. While the dislocation cell structures are completely formed at deeper locations, which are similar to the bulk interior of the sample. The thermal cycle is not the main contribution to the formation of dislocation cell structures, but plays the role in enhancing the annihilation of dislocations inside the cells and sharpening the cell boundaries.

Combining electric discharge surface alloying and high-frequency mechanic impact post-processing for increased corrosion fatigue life of as-welded transverse non-load-carrying attachments of the S355 steel

The combined effects of severe plastic deformation induced by high-frequency mechanical impact (HFMI) treatment and the electric discharge surface alloying (EDSA) with chromium on the corrosion fatigue life of low-carbon S355 steel transverse non-load-carrying specimens are studied in this paper. First, for various alternations of the combined HFMI + EDSA process, the surface alloyed layer integrity and depths were analyzed using scanning electron microscopy (SEM) accompanied with energy dispersive spectroscopy (EDS) and hardness measurements. The alloying and hardening depths were respectively observed to be 10–30 μm and 0.15–0.75 mm and the surface hardness increased up to 3.6–3.9 GPa. Polarization tests demonstrated higher corrosion resistance of the EDSA-processed samples in 3%NaCl. The near-surface layers of the HFMI + EDSA(Cr) + HFMI processed sample characterized by X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) contain the dislocation-cell structure formation and partially dissolved cementite. The fatigue tests in 3%NaCl confirmed that the HFMI + EDSA(Cr) + HFMI post-weld process increased corrosion fatigue life (1.77 × 106 cycles) of the S355 steel welded joint at the stress level of 260 MPa by 2 and 12 times in comparison with the HFMI-treated (0.9 × 106) and as-welded (0.14 × 106) samples, respectively.

Formation mechanism of dislocation network of cell structure in cyclically deformed near-[formula omitted] copper single crystals

As the initiation and propagation of fatigue crack is strongly correlated to the dislocations in cell structures of metals, investigating the formation of dislocation cell structures is essential for understanding the fatigue process. Although the cell boundary evolves from the dislocation network, the formation process of the dislocation network of fatigued copper single crystal is not completely realized. In this study, the dislocation structures in near-[1‾11] copper single crystals cyclically deformed at 293 K were investigated to analyze the formation mechanism of the dislocation network. The dislocation networks were characterized using a high-voltage scanning transmission electron microscope, where the virtual-scanning transmission electron microscope technique (virtual-STEM) was employed to conduct the Burgers vector analysis of the dislocations. The low-dislocation-density network was consistent with the three perfect screw dislocations (b = a/2 [1‾01], a/2 [1‾10], and a/2 [011‾]) in the (111) primary slip plane. For the high-dislocation-density region, the perfect and partial dislocations coexisted in the network. As the dislocation density increased, the perfect dislocations dissociated into Shockley partial dislocations (b = a/6 [1‾21‾], a/6 [112‾], and a/6 [2‾11]). This study provides a reference for future applications of the virtual-STEM technique in the field of dislocation analysis.

Effect of Si on the dislocation state within martensite of ultra-high strength hot-rolled medium Mn steel with good ductility

Medium Mn steels are promising candidates for the third-generation advanced high-strength steels due to their good combination of ultimate tensile strength (UTS) and total elongation after fracture (TEL). Most investigations of medium Mn steels focused on the volume fraction of retained austenite (RA) and stability of RA; however, the strengthening phase martensite was paid little attention to. The present study systematically investigated the effect of Si on the deformation behavior of martensite in 0.2C–6.8Mn-(0, 2, 4)Si (wt. %) hot-rolled medium Mn steels without any heat treatment. The UTS and TEL of the 4Si hot-rolled medium Mn steel are 2163 MPa and 11.7%, respectively, which is rare in the existing hot-rolled medium Mn steels. All three hot-rolled steels had practically equal fraction and mechanical stability of RA, so that the influence of Si on the evolution of martensite fine structure and the nature of the ultra-high UTS and good ductility during deformation could be revealed. By combining the X-ray and neutron diffraction techniques, it was found that the addition of Si increased the dislocation density, especially screw dislocations, and decreased the crystallite size within martensite. The ultra-high UTS originates from the enhanced strain hardening rate caused by the Si alloying, high dislocation density and possibly increased number of martensite lath boundaries and dislocation cell formation. The good ductility mainly stems from the formation of dislocation cell structure due to the addition of Si causing dislocations to be mostly of screw type. The dislocation cells can also improve the strain hardening rate. This work opens a new pathway towards tailoring the dislocation state within martensite for substantially enhanced the mechanical properties of hot-rolled medium Mn steels.

Effect of Microstructure on the Mechanical Response of Hydrogen-Charged Pure Iron

In this paper, we investigate how two different microstructures in pure iron affect the dislocation mobility in hydrogen-charged and non-charged samples by conducting stress-relaxation tests. The effective activation volume of the pure iron for both types of microstructures (cold-rolled and annealed samples) has been determined for both H-charged and uncharged material. Information about the dislocation structures formed during stress relaxation is provided by conducting TEM analysis. We employ a self-consistent kinetic Monte-Carlo (SCkMC) model of the ½ [111] screw dislocation in Fe to investigate how hydrogen affects the mobility and behavior of the dominant mobile dislocation in Fe at different stresses and H concentrations. The results from our simulations show the following: (i) at low stresses the deviation from the primary slip plane in the presence of H is lower than the deviation in the uncharged Fe. The deviation angle decreases with increasing H concentration; (ii) at higher shear stresses, the higher probability for kink-pair formation in the secondary (110) planes in the presence of H, leads to an enhanced deviation from the primary slip plane, which increases with increasing H concentration. We use the results of stress-relaxation tests and SCkMC simulations to propose an explanation for the formation of dislocation cell structures in pure and hydrogen charged Fe in the cold-rolled and annealed samples.

Cyclic Extrusion Compression Process for Achieving Ultrafine-Grained 5052 Aluminum Alloy with Eminent Strength and Wear Resistance

Previous studies have yet to show a consistent effect of severe plastic deformation (SPD) processing on the wear behavior of different metals and alloys. To fill this scientific gap, this study investigated the effect of the cyclic extrusion compression (CEC) process, as one of the prominent SPD techniques, on the wear behavior of AA5052. In addition, the microstructure evolution and mechanical properties of the sample before and after the process were experimentally examined and studied. It was found that the yield and ultimate tensile strength of the AA5052 improved significantly after the first pass, while the elongation-to-failure decreased considerably. Further, the subsequent passes mildly changed the trend of increasing strength and reducing elongation-to-failure. SEM morphology indicated that the ductile mode of the initial annealed alloy changed to a combination of ductile and brittle failure modes, in which the level of the brittle failure mode increased with the addition of passes. TEM observations showed that the grain refinement during the CEC process included the formation of dislocation cell structures, subgrain boundaries, and low-angle grain boundaries, with the subgrain boundaries initially evolving into low-angle grain boundaries and, eventually, due to the imposition of additional plastic strain, into high-angle grain boundaries. Furthermore, the CEC process and its increased number of passes led to a significant improvement in wear resistance due to the enhanced tensile strength achieved through grain refinement. In this regard, the wear mechanism of the initial alloy was a combination of adhesion and delamination, with the plastic deformation bands changing to plowing bands with decreased adhesive wear during the process. Eventually, oxidization was found to be a mechanism contributing to wear under all conditions.

Mechanical properties and microstructural characteristics of additively manufactured C103 niobium alloy

C103 is a refractory-based niobium alloy for high temperature applications in aerospace. This paper aims to evaluate the compatibility of C103 alloy as a choice of material for laser-based additive manufacturing (AM) technologies. Crack free AM specimens with relative densities of greater than 99% were successfully fabricated via laser powder bed fusion (L-PBF) method with horizontal and vertical build orientations. Static strength levels of the stress-relieved specimens exceeded that of their wrought counterparts. Higher yield and ultimate tensile strength of AM specimens were attributed to high dislocation density and the formation of dislocation cell structures, which was revealed by transmission electron microscopy. Loss of ductility in AM specimens was partially due to high density of dislocations (i.e., shorter mean free path for dislocations), while the presence of defects may also have compromised the total elongation to failure. Electron backscattered diffraction analysis revealed formation of sharp 001||BD rod texture in AM samples, which resulted in anisotropic mechanical behavior. Uniaxial tensile and compressive deformation led to the development of (001) <110> parallel to tensile direction and <111> parallel to build direction, respectively.

Cell structure formation in a two-dimensional density-based dislocation dynamics model

Cellular patterns formed by self-organization of dislocations are a most conspicuous feature of dislocation microstructure evolution during plastic deformation. To elucidate the physical mechanisms underlying dislocation cell structure formation, we use a minimal model for the evolution of dislocation densities under load. By considering only two slip systems in a plane strain setting, we arrive at a model which is amenable to analytical stability analysis and numerical simulation. We use this model to establish analytical stability criteria for cell structures to emerge, to investigate the dynamics of the patterning process and establish the mechanism of pattern wavelength selection. This analysis demonstrates an intimate relationship between hardening and cell structure formation, which appears as an almost inevitable corollary to dislocation dominated strain hardening. Specific mechanisms such as cross slip, by contrast, turn out to be incidental to the formation of cellular patterns.

On the low-cycle fatigue response of CoCrNiFeMn high entropy alloy with ultra-fine grain structure

High Entropy Alloys (HEAs) are a new class of multi-component alloys with excellent tensile strength-ductility combination. Their yield strength levels, however, are still low as compared to other high strength materials. Here, Equal Channel Angular Pressing (ECAP) was employed to improve the yield strength of the most well-known HEA, CoCrFeMnNi, through microstructural refinement. The cyclic response of both coarse and ultrafine grained CoCrFeMnNi was investigated during strain-controlled low cycle fatigue tests under fully reversed push-pull loading at room temperature. The microstructural evolution during cyclic loading was compared to the microstructure under quasi-static monotonic loading. Very high yield strength levels around 1 GPa were obtained after ECAP. In addition, ECAP samples demonstrated a superior fatigue life at relatively low strain amplitudes while coarse grained samples exhibited a better fatigue life at the highest strain amplitude considered. X-ray diffraction, electron backscattered diffraction and transmission electron microscopy were performed to reveal underlying mechanisms for the superior fatigue life and the overall transient behavior upon cycling. The fatigue life and hardening behavior were governed by the refined grain size, high density dislocation walls, and the annihilation of existing dislocations resulting in the formation of cell structures in the ECAP samples. The lower fatigue life of the ECAP samples at the highest strain amplitude is attributed to the higher stress amplitudes and cyclic softening due to accelerated dislocation annihilation. Finally, the formation of dislocation cell structures and high-density dislocation walls simultaneously is rationalized by the effect of applied stress on the partial dislocation separation.

Dynamic recrystallization-induced temperature insensitivity of yield stress in single-crystal Al1.2CrFeCoNi micropillars

High-entropy alloys, a new class of metallic materials, exhibit excellent mechanical properties at high temperatures. In spite of the worldwide interest, the underlying mechanisms for temperature dependence of mechanical properties of these alloys remain poorly understood. Here, we systemically investigate the mechanical behaviors and properties of Al1.2CrFeCoNi (comprising a body-centered cubic phase) and Al0.3CrFeCoNi (comprising a face-centered cubic phase) single-crystal micropillars with three orientations ([100], [110], and [111]) at temperatures varying from 300 to 675 K by using in situ compression of micropillars inside a scanning electron microscope. The results show that the yield stresses of Al1.2CrFeCoNi micropillars are insensitive to temperature changes, and their flow stresses and work hardening rates increase slightly with increasing temperature from 300 to 550 K, which differs from the typical temperature dependence of yield/flow stresses in metals and alloys. In contrast, Al0.3CrFeCoNi micropillars exhibit typical thermal softening. Furthermore, it is found that the Al1.2CrFeCoNi micropillars exhibit a transition from homogenous deformation to localized deformation at a critical temperature, while the Al0.3CrFeCoNi micropillars always maintain a well-distributed and fine slip deformation. Detailed transmission electron microscopy analyses reveal that dynamic recrystallization (involving dislocation tangles, and formation of dislocation cell structures and sub-grains) plays a key role in the observed temperature insensitivity of the yield stress and increasing flow stress (and work hardening rate) with increasing temperature in the Al1.2CrFeCoNi micropillars, and that thermally activated dislocation slip leads to thermal softening of the Al0.3CrFeCoNi micropillars. The differences in deformation modes and temperature dependence of the mechanical properties between Al1.2CrFeCoNi and Al0.3CrFeCoNi essentially originate from the differences in dislocation activities and slip systems since the two alloys adopt different phases. Our findings provide key insights in the temperature dependence of mechanical properties and deformation behaviors of high-entropy alloys with body-centered cubic and face-centered cubic phases.

Dislocation-induced breakthrough of strength and ductility trade-off in a non-equiatomic high-entropy alloy

In conventional metallic materials, strength and ductility are mutually exclusive, referred to as strength-ductility trade-off. Here, we demonstrate an approach to improve the strength and ductility simultaneously by introducing micro-banding and the accumulation of a high density of dislocations in single-phase high-entropy alloys (HEAs). We prepare two compositions (Cr10Mn50Fe20Co10Ni10 and Cr10Mn10Fe60Co10Ni10) with distinctive different stacking fault energies (SFEs) as experimental materials. The strength and ductility of the Cr10Mn50Fe20Co10Ni10 HEA are improved concurrently by grain refinement from 347.5 ± 216.1 µm to 18.3 ± 9.3 µm. The ultimate tensile strength increases from 543 ± 4 MPa to 621 ± 8 MPa and the elongation to failure enhances from 43±2% to 55±1%. To reveal the underlying deformation mechanisms responsible for such a strength-ductility synergy, the microstructural evolution upon loading is investigated by electron microscopy techniques. The dominant deformation mechanism observed for the Cr10Mn50Fe20Co10Ni10 HEA is the activation of micro-bands, which act both as dislocation sources and dislocation barriers, eventually, leading to the formation of dislocation cell structures. By decreasing grain size, much finer dislocation cell structures develop, which are responsible for the improvement in work hardening rate at higher strains (>7%) and thus for the increase in both strength and ductility. In order to drive guidelines for designing advanced HEAs by tailoring their SFE and grain size, we compute the SFEs of Cr10MnxFe70–xCo10Ni10 (10 ≤ x ≤ 60) based on first principles calculations. Based on these results the overall changes on deformation mechanism can be explained by the influence of Mn on the SFE.

Discrete dislocation plasticity analysis of the high-temperature cyclic response of composites

Discrete dislocation plasticity (DDP) analysis of the high-temperature cyclic deformation of two-phase composites comprising a plastic matrix and elastic precipitates is presented. Deformation of the matrix is by climb-assisted glide of dislocations while the precipitates deform by a combination of bulk elasticity and stress-driven interfacial diffusion. The DDP calculations predict a cyclically softening response due to the formation of dislocation cell structures within the matrix. The dislocation cell sizes decrease with decreasing size of the unit cell (or equivalently matrix channels) and this results in an increased cyclic softening rate in composites with smaller unit cells. Interfacial diffusion also enhances the formation of dislocation cell structures and thereby promotes cyclic softening. These results are consistent with predictions of the creep behaviour that indicate that the increase in the creep rate (i.e. tertiary creep) is also associated with the formation of dislocation cell structures within the matrix.

Microstructural evolution of adiabatic shear bands in pure copper during impact at high strain rates

A systematic study was conducted on copper of commercial purity (99.9% purity) to elucidate the sequence of events that leads to the evolution of adiabatic shear bands and the structure of the evolved shear bands after strain localization during deformation at high strain rates and large strains. A direct impact Hopkinson Pressure Bar was used to deform different specimens at increasing impact momentum and strain rate followed by microstructural characterization using metallographic techniques and transmission electron microscopy. It was observed that sequential occurrence of emergence of dislocations from grain and twin boundaries, formation of dislocation cell structures and substructures with varying cell sizes and cell walls, dislocation-nucleation controlled softening and extensive micro twinning characterized the structure of the evolved adiabatic shear bands as a function of impact momentum and strain rate. The dislocation cell structures and substructures were typically made up of high-density dislocation walls surrounding low-density dislocation cell interiors. The microhardness distribution within the evolved shear bands increased up to a peak value at a critical impact momentum and strain rate (≥ 45kgm/s and ≥6827s−1). Above this threshold, microhardness of the regions within the evolved shear bands decreased because of the occurrence of softening. However, the first specimen that exhibited softening had high-density dislocation cell walls surrounding dislocation-free cell interiors with no observed recrystallized grains. Despite the onset of softening both within and outside the shear bands, the regions within the shear bands were always harder than the regions outside the shear bands. The shear bands that exhibited significant softening were clad with vast distribution of microtwins in addition to evolved refined grains and sub-grains. It is discussed that thermally activated dislocation processes as a result of the rise in temperature during impact, dynamic recovery and dynamic recrystallization do not necessarily result in strain localization in the impacted copper specimens because their effects are observed in the structure of the evolved shear bands at a latter stage when strain localization had already occurred.

Effect of the state of carbon on ductility in Fe-0.017mass%C ferritic steel

The influence of the state of carbon on ductility in Fe-0.017mass%C alloy was systematically investigated. Moreover, the changes in dislocation density, dislocation substructure, and dislocation character with tensile strain were evaluated using X-ray line profile analysis together with transmission electron microscopy. The solute carbon maintains uniform elongation while simultaneously increasing the ultimate tensile strength by increasing the work-hardening rate; however, post-uniform elongation is significantly impaired by the occurrence of dynamic strain aging (DSA). The coarse intergranular cementite leads to superior uniform elongation; however, post-uniform elongation deteriorates presumably owing to the void formation in the cementite/matrix interface or the fracture of cementite. Meanwhile, the fine transgranular cementite increases the yield strength, thus impairing the uniform elongation; however, it maintains post-uniform elongation. This is because small cementites less than about 1µm in size do not deteriorate the post-uniform elongation. The solute carbon prominently increases the dislocation density, particularly the edge dislocation density, with tensile strain owing to DSA, leading to the prohibition of dislocation cell structure formation.

Concurrent precipitation and associated texture evolution in AA 6082 alloy during high pressure torsion (HPT) processing

The effect of severe plastic straining by HPT processing on the mechanical properties and the microstructure evolution of a commercial AA 6082 alloy was studied. The HPT processing was carried out by torsion of 0.5 rotation per minute under 2GPa load, for up to 5 turns. The material was solutionized before HPT. Mechanical homogeneity is achieved after 3 turns with a saturation hardness of ~ 165HV and further increased to ~ 175HV after 5 turns, while the alloy showed ~ 68HV before HPT straining. The increase in hardness during HPT has been attributed to concurrent precipitation during HPT. The development of an ultrafine grained (UFG) microstructure was analyzed by investigating samples prepared from both the centre and the edge of the HPT discs after 1, 3 and 5 turns using TEM. In case of 1 turn, subgrain formation occurred at the edges of the disc while at the centre dislocation cell structure formation was observed. Subgrains are formed at the centre and the edge after 3 HPT turns with an average diameter of ~ 300nm that further decreased to ~ 200nm after 5 HPT turns. Texture evolution after 1 turn consists of formation of A*2, C, A and A- components. However, with increasing straining, these components remain stable and no new component arises. This has been attributed to continuous dynamic recrystallisation as the dominating mechanism of grain refinement that can occur in this alloy under the given severe plastic straining.

Quantifying dislocation microstructure evolution and cyclic hardening in fatigued face-centered cubic single crystals

Discrete dislocation dynamics simulations were performed to investigate the dislocation microstructure evolution and cyclic hardening during the early stages of fatigue loading in nickel single crystals. The effects of the crystal size and initial dislocation densities on both the mechanical response and the evolution of dislocation microstructure were quantified. Crystals having an initial dislocation density of 1012 m−2 and diameter less than 2.0μm do not show any dislocation density multiplication or cyclic hardening. In contrast, crystals having the same initial dislocation density and diameters larger than 2.0μm show a significant dislocation density accumulation in the form of dislocation cell-like structures, even after only a few number of loading cycles. This dislocation density accumulation was also accompanied by considerable cyclic hardening. The dislocation cell size and its wall thickness increase with increasing crystal size. With increasing dislocation density the critical crystal size, at which dislocation cell-structures form, decreases. The information theoretic entropy is utilized as a metric to quantify the extent of dislocation patterning and the formation and evolution of dislocation cell structures over time. Cross-slip was found to play a dominant role in the dislocation cell-structure formation. Further insights on the mechanisms contributing to the observed behavior are presented and discussed.

A preliminary investigation of dislocation cell structure formation in metals using continuum dislocation dynamics

A continuum dislocation dynamics model capable of capturing the cellular arrangements of dislocations in deformed crystals is presented. A small strain formulation of the model is given, followed by sample results of stress-strain behaviour, dislocation density evolution, dislocation cell pattern, lattice rotation, and geometrically necessary dislocation density and strain energy density distributions. An important finding of the current work is that dislocations form patterns under all circumstances due to their long range interactions. It is found, however, that the famous cell structure pattern forms when cross slip is activated. It is also found that cells are 3D sub-regions surrounded by dislocations walls in all directions, and they form, disappear, and reappear as a result of the motion of cell walls and formation of new walls by cross slip. It is further found that the average cell size is connected with the applied resolved shear stress according to the similitude principle observed in related experiments. The importance of these results is briefly discussed in the context of recrystallization.

Computational modelling of mesoscale dislocation patterning and plastic deformation of single crystals

We present a continuum dislocation dynamics model that predicts the formation of dislocation cell structure in single crystals at low strains. The model features a set of kinetic equations of the curl type that govern the space and time evolution of the dislocation density in the crystal. These kinetic equations are coupled to stress equilibrium and deformation kinematics using the eigenstrain approach. A custom finite element method has been developed to solve the coupled system of equations of dislocation kinetics and crystal mechanics. The results show that, in general, dislocations self-organize in patterns under their mutual interactions. However, the famous dislocation cell structure has been found to form only when cross slip is implemented in the model. Cross slip is also found to lower the yield point, increase the hardening rate, and sustain an increase in the dislocation density over the hardening regime. Analysis of the cell structure evolution reveals that the average cell size decreases with the applied stress, which is consistent with the similitude principle.

Dynamic strain ageing, deformation, and fracture behavior of modified 9Cr–1Mo steel

Tensile behavior of modified 9Cr–1Mo steel was studied over the temperature range from room temperature (RT) to 600 °C at three strain rates 10−5, 10−4 and 10−3 s−1. Serrated plastic flow was observed from 250 to 350 °C at strain rate of 10−4 s−1, signifying occurrence of dynamic strain ageing (DSA). Characteristic features of DSA such as plateau/peak in yield and tensile strength, minima in ductility, negative strain rate sensitivity, and peak in internal friction were also observed. Activation energy for DSA was found equivalent to that for diffusion of nitrogen, hence nitrogen was considered to cause DSA. Dislocation substructure in the region of DSA revealed dislocation debris, kinks, bowing of dislocations and high density of dislocations. Irrespective of temperature from 200 to 450 °C there was formation of dislocation cell structure; however, cell size in the region of DSA was smallest. Fractographic analysis showed rosette type fracture at RT resulting from longitudinal splitting. It was associated with decohesion of interface of carbide particles and prior austenite grain boundaries. Non uniform and shallow dimples were observed in fibrous zones in the region of DSA. Contours of dual shear lip zone were observed at elevated temperatures from 200 to 450 °C.

Impact of instrumental constraints and imperfections on the dislocation structure in micron-sized Cu compression pillars

In situ micro-Laue diffraction was used to study the plasticity in three 7 μm, initially identical, single-crystalline Cu pillars during compression. Movements of the Laue spot as well as Laue spot streaking were analyzed to obtain real-time insights into the storage of excess dislocations and the possible formation of dislocation cell structures. The results reveal that instrumental constraints lead to dislocation storage at the sample base and top, but will not affect the storage of excess dislocations in the sample center in case of an ideal alignment. In contrast, misaligned samples show early yielding due to the activation of an unpredicted slip system, storage of excess dislocations also in the sample center and, at a later stage, the formation of a complex dislocation substructure.