Cellular Dislocation Structure Research Articles

Plastic deformation behavior and strengthening mechanism of SLM 316L reinforced by micro-TiC particles

In this work, the integrated study on the microstructure evolution, plastic deformation behavior, and strengthening mechanism of Selective Laser Melting (SLM) 316L austenitic stainless steel (ASS) reinforced with micro-TiC particles was investigated. The results indicated that there were lots of small angle grain boundaries in SLM 316L ASS, and the high dislocation density and component segregation of the cellular dislocation structure resulted in a good combination of high strength and good elongation in performance. The tensile strength, yield strength, and elongation of SLM 316L ASS were 682 MPa, 571 MPa, and 49.9%, respectively. The addition of TiC reinforced particles induced microstructure evolution, including grain refinement, reduction of epitaxial columnar dendrites, reduction of low angle grain boundary (LAGB) ratio and increase of Σ3 (60°/<111>) twin boundaries. Through the effects of grain refinement strengthening, Orowan strengthening, and dislocation strengthening, SLM 316L-2TiC showed a higher tensile strength of 1021 MPa and higher yield strength of 809 MPa, while maintaining an elongation of 30.8% due to the suppression of plasticity by twinning induced plasticity (TWIP) effect. Although nano-reinforced particles were commonly used to improve mechanical properties, in our work, we chose to add micrometer-sized TiC particles, and the strength of the SLM 316L-2TiC metallic matrix composites (MMCs) exceeded the previous reports of adding nanoparticles. The reason was that the small contact angle between TiC and 316L provides good wettability between the particles and the melt, and the selection of laser processing parameters ensures good bonding between TiC particles and the substrate.

Superior gradient heterostructured alloys fabricated by laser powder bed fusion via annealing and ultrasonic nanocrystal surface modification

Developing metal additive manufactured (AM) parts with excellent mechanical properties broaden the utilization of AM parts in various industrial applications. Recently, gradient structures have received significant attention owing to their excellent mechanical properties. In this study, we propose a new strategy to obtain a superior gradient structure using annealing and ultrasonic nanocrystal surface modification (UNSM) on laser powder bed fusion (LPBF) fabricated 316 L stainless steel. The post-LPBF annealing treatment disturbed the cellular dislocation structure, which led to the optimized materials for gradient structure. The resulting gradient structure after the UNSM treatment has a thicker gradient layer with a significant strain partitioning between the domains than the LPBF followed by UNSM without annealing. As a result, the present LPBF-annealing-UNSM alloys show superior strength-ductility synergy compared to the LPBF-UNSM samples.

Heat treatment induced microstructural evolution and strength enhancement of Al–Cr–Fe–Ni–V high-entropy alloy fabricated by laser powder bed fusion

Herein, different heat treatment protocols are adopted to further improve the mechanical properties of the Al–Cr–Fe–Ni–V high-entropy alloy (HEA) fabricated by laser powder bed fusion (LPBF) additive manufacturing (AM). The initial LPBF-processed HEA rendered a combination of high strength (ultimate tensile strength (UTS) of 810.93 MPa) and ductility (fracture elongation (FE) of 35.43%), which could be attributed to the hierarchical microstructure with fine grains, cellular dislocation structure, and L12 nano-precipitation. When the annealing temperature is increased from 573 K to 1473 K, the cellular dislocation structure is disappeared, and both L12 and B2 phases are progressively precipitated in the FCC matrix. Simultaneously, the tensile strength dramatically increased at the expense of ductility reduced and LPBF-processed HEA demonstrated a high UTS of 1192.84 MPa and FE of 15.32% after annealing at 1173 K. The results reveal that precipitation strengthening plays a significant role in improving the mechanical properties of LPBF-processed HEAs during annealing. For instance, L12 and B2 precipitates hindered the dislocation movement and, in turn, enhanced the pinning force of boundary migration. Moreover, the highly coherent and low misfit FCC-L12 interfaces minimized the strain accumulation due to the dislocation shear and, thereby, prevented the crack initiation.

Role of cellular structure on deformation twinning and hetero-deformation induced strengthening of laser powder-bed fusion processed CuSn alloy

The sub-grain cellular dislocation structure has been reported to be the primary reason for the enhanced mechanical properties in laser powder-bed fusion (LPBF) parts. In the current work, the contribution of the cellular dislocation structure to the yield strength of LPBF processed CuSn alloy is estimated to be ~45%. In addition, this work shows that the cellular dislocation structure significantly controls the deformation behavior of LPBF processed CuSn alloy by suppressing the formation of deformation twinning. Post-LPBF heat treatment with fully recrystallized microstructures devoid of cellular dislocation structure showed pronounced twinning activity. The reduced homogeneous slip length due to the fine dislocation cell structure ~600 nm and increased stacking fault energy due to the cellular Sn segregation significantly increased the activation energy for the nucleation and propagation of the partial dislocations and suppressed the deformation twinning in the as-built samples. Furthermore, the present work shows that cellular dislocation structure contributes significantly to the hetero-deformation induced strengthening, much higher than the heterogeneous grain structure in the LPBF samples.

Anisotropic cellular structure and texture microstructure of 316L stainless steel fabricated by selective laser melting via rotation scanning strategy

In this work, the effects of solidification and remelting under chessboard 67° rotation scanning strategy on the microstructure and properties of selective laser melted (SLMed) 316L stainless steel (316L SS) were studied, including dendrite microstructure, element distribution, grain orientation, dislocation structure, microhardness and corrosion resistance. We put forward a complete foundation relationship among texture, cellular structure and mechanical property influenced by anisotropy. Main results are summarized as follows: The 〈101〉 texture formation is the result of dendrite epitaxial growth. The cellular dislocation structure highly overlapped with dendrite significantly changed in remelting zone. Hence the high ratio of 〈101〉 texture signifies the anisotropy distribution of epitaxially growth dendrite which released the internal stress. The dislocation structure has a contribution to strength similar work hardening strengthening. Meanwhile, the 316L SS obtains Orowan strengthening via the nano oxide pinning effect in dislocation wall. Thus, the dislocation density and oxide distance obvious changed in different directions of cellular dislocation structure, resulting in the cellular dendrite regions performed higher microhardness than the columnar dendrite regions. The drastic change of dendrite increased the crack sensitivity of SLMed 316L SS, and the junction of different epitaxially growth direction dendrites formed high energy grain boundary.

Delayed deformation-induced martensite transformation and enhanced cryogenic tensile properties in laser additive manufactured 316L austenitic stainless steel

Abstract The cellular dislocation structure is an important microstructure aspect in fusion-based laser metal additive manufactured (MAM) parts. Its role in increasing strength and ductility at room temperature (298 K) is a known phenomenon. In this work, we have shown that the cellular dislocation structure not only improves the strength but also dramatically increases the mechanical stability of the austenite and retards the deformation-induced martensite transformation (DIMT) of austenite at cryogenic temperature (77 K). The delayed and slow rate of DIMT led to an enhanced strength-ductility synergy in the direct energy deposited and selective laser melted 316 L stainless steel samples at 77 K compared to the wrought processed samples. The current study establishes the capability of MAM technologies in controlling the DIMT behavior and realizing materials with high strength and ductility combination.

Effect of Cellular Dislocation Structure on the Strength of Additively Manufactured 316L Stainless Steel

Additively manufactured (AM) 316L stainless steel (SS) often contains cellular dislocation structure which is a distinct microstructural feature compared with those fabricated traditionally, like casting and forging. The role of this unique cellular dislocation structure on the mechanical properties of the AM 316L SS needs to be determined to guide its further performance improvement. In this study, the effect of cellular dislocation structure on the strength of AM 316L SS was investigated via micro-mechanical compression test. Single crystalline micro-pillars were firstly prepared from both the as-built and annealed AM 316L SS bulk specimens, with and without cellular dislocation structure relatively. The results show a significant increase of the yield strength of the micro-pillars with the cellular dislocation structure. The micro-pillars containing cellular dislocation structure with different sizes and morphologies have been studied to evaluate the effect of cellular dislocation structure on the strength of AM 316L SS.

Strain gradient induced grain refinement far below the size limit in a low carbon hypoeutectoid steel (0.19 wt% C) via pipe inner surface grinding treatment

A low carbon hypoeutectoid steel (0.19 wt% C) with proeutectoid ferrite and pearlite dual-components was subjected to surface plastic deformation via pipe inner surface grinding (PISG) at room temperature. The deformation microstructures for each component were systematically characterized along depth, and the patterns of structural evolution toward nanometer regime as well as the governing parameters were addressed. Proeutectoid ferrite grains were refined down to 17 nm, and the pattern covering a length scale of 4–5 orders of magnitude from micron- to nanometer-scale follows: formation of cellular dislocation structure (CDS), elongated dislocation structure (EDS), ultrafine lamellar structure (UFL) and finally the nanolaminated structure (NL). The pearlite experiences the deformation and refinement, and finally the transforming the ultrafine pearlite (UFP) into nanolaminated pearlite (NLP) with the ferrite lamellae as thin as 20 nm. Refinement for both UFL (UFP) and NL (NLP) can be realized via forming novel extended boundaries within ferrite lamellae. A critical lattice curvature of ∼2.8° is required for forming such extended boundary, corresponding to a minimum strain gradient of 0.25 μm−1 for a 100 nm-thick lamella. Refinement below size limit (expressed by lamellar thickness dT in nm) is correlated with the strain gradient (χ, in μm−1) by: dT=12.5χ. Refinement contributions from strain gradient caused by PISG processing and material heterogeneity were discussed.

Restoration of plasticity of cold-deformed aluminum alloy by short-term irradiation with accelerated Ar+ ions

Based on results of structural analysis and mechanical tests of initially cold-rolled 1-mm-thick sheets made of 1441 aluminum alloy, the efficiency of radiation annealing was proved, which was used instead of standard furnace annealing. It has been established that exposure of cold-deformed metastable alloy 1441 of the Al-Li-Cu-Mg system to beams of accelerated Ar+ ions (E = 20–40 keV) initiates the process of explosive rebuilding of the initial cellular dislocation structure in them with the formation of subgrains (at fluences of ~1015–1016 cm−2; a corresponding irradiation time from ~1 to 10 s; T ≤ 130°С), as well as recrystallization and grain growth processes (at fluences of 1.75·1016–2·1017 cm−2). In the course of irradiation, a reduction in size and partial dissolution of Al8Fe2Si and Al3Zr coarse intermetallic particles formed during crystallization and the formation of θ′ (Al2Cu) and S1 (Al2LiMg) strengthening phases also take place.It was established that the ion irradiation-induced fast processes occur in the whole volume of 1-mm-thick samples far beyond (by factor ~104) an Ar+ projected range at temperatures significantly below (by 100–200°С) the temperature of intermediate recrystallization annealing used in cold-rolling practice.The non-thermal nature of the fast-flowing initiating processes under irradiation has been shown. It was shown that of fast flowing radiation annealing, which may be qualified as a giant long-range effect under ion irradiation, is comparable with standard furnace annealing.

Mikrostrukturni procesi koji se odvijaju tokom puzanja legure AA2024-T3 zavarene trenjem sa mešanjem

The poor weldability of AA2024 aluminum alloy limits its use for industrial applications. Being a non-fusion welding process, Friction Stir Welding (FSW) seems to be a promising solution for welding this alloy. FSW was applied in the current study in order to butt weld AA2024-T3 aluminum alloy plates and to study the creep behavior of the weld. Creep tests were conducted at 250 0C and 315 0C both on the parent material and on the friction stir welded specimens. A comprehensive Transmission Electron Microscopy (TEM) study together with High Resolution Scanning Electron Microscopy (HRSEM) study and Energy Dispersive X-ray Spectroscopy (EDS) analysis were conducted in order to investigate the microstructural processes. The parent material seems to contain two kinds of Curich precipitates - coarse precipitates having the size of a few microns each and uniformly dispersed fine nanosized precipitates. However, this microstructure was found to be unstable at the temperature range of 250-315 0C, secondary precipitation was found to take place, this secondary precipitation is responsible for grain boundary decoration and the appearance of secondary rod-shaped precipitates and for some degree of coarsening of the nanosized precipitates inside the grains. TEM study yielded that the material undergoes dynamic recrystallization (DRX) during creep as well as during the FSW process. Various stages of the development of dislocation networks into a cellular dislocation structure and finally into dislocation free recrystallized grains were recorded. The friction stir welded material, which has already recrystallized during welding, undergoes DRX during creep so that ultra-fine grains are being created concurrently. Precipitation processes at the friction stir welded material occur as well during creep. The instability of the microstructure during creep and exposure to high temperature plays an important role in the analysis of the creep results. The influence of the above microstructure changes occurring during creep on the creep behavior will be referred and discussed.

Evolution of cellular dislocation structures and defects in additively manufactured austenitic stainless steel under ion irradiation

The evolution of irradiation-induced defects in additively manufactured (AM) austenitic stainless steel was investigated in situ by 1 MeV Kr ion irradiation at 450 and 600 °C in a transmission electron microscope. Cellular dislocation structure in AM steel act as sink/trap sites for the irradiation-induced defects, resulting in the lower density and smaller dislocation loops in AM steel than conventional forged (CF) steel at 450 °C. The higher stacking fault energy and local stress concentration induced by cellular dislocation structure in AM steel promotes the unfaulting process and the formation of network dislocation at 600 °C.

Deformation Microstructure of a Copper Single Crystal after Loading by Spherically Converging Shock Waves

A layer by layer study of the structure of a 34-mm ball of a copper single crystal after loading by spherically converging shock waves has been performed using transmission electron microscopy. The deformation microstructure along directions \(\left\langle {100} \right\rangle \) and \(\left\langle {110} \right\rangle \) has been studied. It has been revealed that the character of the deformation microstructure substantially depends on both the direction of the shock-wave propagation and on the depth of the layer location in the sample. In the near-surface layers of the ball that are located perpendicular the \(\left\langle {100} \right\rangle \) direction, a well-pronounced cellular dislocation structure is present; in the layers located perpendicular to the \(\left\langle {110} \right\rangle \) direction, no formation of the cellular structure occurs; there is only a high density of homogeneously distributed dislocations. Regardless of the single-crystal orientation, microbands, microtwins, banded structures, and recrystallized grains are detected along with the dislocations. Dislocation vacancy loops are observed in all layers of the ball.

Advanced processing of EBSD data to distinguish the complex microstructure evolution of a Cu-Ni-Si alloy induced by fatigue

The present work aims at identifying the evolution of the microstructure of a Cu-Ni-Si (Corson alloy) alloy after low cycle fatigue by means of advanced electron microscopy analysis techniques (SEM-ECCI, Orientation Imaging Microscopy by means of SEM-EBSD and TEM). Deep attention is paid on the material subjected to high cyclic strains where a transient cellular dislocation structure transformed into precipitate free bands. It is shown that the formation of dislocation cells leads to a high GROD and high GOSaera value and its standard deviation. However, the presence of precipitate free bands results in the low GOSarea level and its low standard deviation.

Anisotropy of plasticity and structural transformations under uniaxial tension of iron crystallites

Molecular dynamics simulation has been performed to investigate the onset and evolution of plasticity in nanosized bcc iron crystallites with different crystallographic orientation under uniaxial tension. Calculations have shown that the onset of plasticity in the iron crystallite is associated with the nucleation of either dislocations or twins. These defects nucleate and grow as a result of structural transformations that locally change the lattice type. In stretching along the [112¯] direction, plastic deformation begins mainly due to the formation and growth of a single twin, at the front of which the bcc → fcc → bcc transformation occurs. Dislocations nucleate in the interior of the twin as the twin grows. In stretching along the [11¯0] direction, plasticity results from the formation of a large number of twins in the crystallite. Regions with fcc and hcp lattice structure arise at the front of these twins. With tensile strain increase, the contribution of twinning to the crystallite accommodation decreases because twins transform into dislocations. The onset of plasticity in the sample stretched along the [1 1 1] direction has a pronounced dislocation character. New dislocations appear at existing dislocations through the formation of regions with fcc lattice. A large part of the dislocations escape to the free surfaces during stretching, leaving the crystallite with a cellular dislocation structure and a large number of vacancies. Tension both in the elastic region and above the yield point causes local bcc → fcc → bcc transformations in the crystallite. They always occur in regions with increased stresses. The formation of single fcc atoms in the elastic stretching region is of thermal fluctuation nature, and their average lifetime is one atomic vibration period. The lifetime of the fcc atoms at the twin front is determined by the front propagation rate and is about 5 atomic vibration periods.

Microstructure Stability During Creep of Friction Stir Welded AA2024-T3 Alloy

The poor weldability of the AA2024 aluminum alloy limits its use in industrial applications. Because friction stir welding (FSW) is a non-fusion welding process, it seems to be a promising solution for welding this alloy. In the current study, FSW was applied to butt weld AA2024-T3 aluminum alloy plates. Creep tests were conducted at 250 and at 315 °C on both the parent material and the friction stir welded specimens. The microstructures of the welded and non-welded AA2024-T3 specimens before and after the creep tests were studied and compared. A comprehensive transmission electron microscopy study together with a high-resolution scanning electron microscopy study and energy-dispersive x-ray spectroscopy analysis was conducted to investigate the microstructure stability. The parent material seems to contain two kinds of Cu-rich precipitates—coarse precipitates of a few microns each and uniformly dispersed fine nanosized precipitates. Unlike the parent material, the crept specimens were found to contain the two kinds of precipitates mentioned above together with platelet-like precipitates. In addition, extensive decoration of the grain boundaries with precipitates was clearly observed in the crept specimens. Controlled aging experiments for up to 280 h at the relevant temperatures were conducted on both the parent material and the welded specimens in order to isolate the contribution of exposure to high temperatures to the microstructure changes. TEM study showed the development of dislocation networks into a cellular dislocation structure in the case of the parent metal. Changes in the dislocation structure as a function of the creep strain and the FSW process were recorded. A detailed creep data analysis was conducted, taking into account the instability of the microstructure.

The orientation dependence of critical shear stresses in Al0.3CoCrFeNi high-entropy alloy single crystals

It is established that the critical shear stresses under tensile strain of Al0.3CoCrFeNi high-entropy alloy single crystals are independent of crystal orientation. It is shown that the development of the planar dislocation structure in Al0.3CoCrFeNi single crystals at T = 296 K leads to a decrease in the ratio between the strain-hardening coefficient and shear modulus relative to that the value for Cu single crystals, where the cellular dislocation structure is developed at the similar staking fault energy.

Transition of Dislocation Glide to Shear Transformation in Shocked Tantalum

ABSTRACTA TEM study of pure tantalum and tantalum-tungsten alloys explosively shocked at a peak pressure of 30 GPa (strain rate: ∼1 x 104 sec-1) is presented. While no ω (hexagonal) phase was found in shock-recovered pure Ta and Ta-5W that contain mainly a low-energy cellular dislocation structure, shock-induced ω phase was found to form in Ta-10W that contains evenly distributed dislocations with a stored dislocation density higher than 1 x 1012 cm-2. The TEM results clearly reveal that shock-induced α (bcc) → ω (hexagonal) shear transformation occurs when dynamic recovery reactions which lead the formation low-energy cellular dislocation structure become largely suppressed in Ta-10W shocked under dynamic (i.e., high strain-rate and high-pressure) conditions. A novel dislocation-based mechanism is proposed to rationalize the transition of dislocation glide to twinning and/or shear transformation in shock-deformed tantalum. Twinning and/or shear transformation take place as an alternative deformation mechanism to accommodate high-strain-rate straining when the shear stress required for dislocation multiplication exceeds the threshold shear stresses for twinning and/or shear transformation.

Damage to thick steel sheet in cyclic plastic flexure

The laminar development of damage in thick 10Г2С1 steel sheet during cold plastic deformation by cyclic flexure is investigated. The steel is subjected to pure flexure in a symmetric cycle, with strain of amplitude 5.5%. The variation in steel strength indicates the damage kinetics in the plastic deformation. In the region of reversible damage, the rate of change in the strength increases to a maximum and then declines. In the region of irreversible damage there is no further change in strength of steel before failure occurs. An upper limit is proposed for the damage of the steel: 20–30% of Nf, where Nf is the number of cycles to failure in cyclic extension–compression. In structural terms, the upper limit of reversible damage is assumed to correspond to the transformation of cellular dislocation structure to band structure.

Effect of powerful pulsed and continuous ion beams on the Al- Cu-Mg alloy structure

The paper considers the results of an electron microscopy study of the VD1 alloy of the Al-Cu-Mg system after cold working and subsequent irradiation with a powerful pulsed ion beam (70% C+ + 30% H+, E = 180 keV) in the pulsed-periodic mode (t = 80 ns, f = 0.1 Hz, j = 200 A/cm2, F = 1-1014 cm-2) and under the conditions of the generation of only one pulse (t = 180 ns, j = 100 A/cm2, F = 2-1015 cm-2). It is established that this irradiation noticeably affects the microstructure of the cold-worked 3 mm thick sheets of VD1 alloy. The initial cellular dislocation structure transforms into a subgrain one. The intensity of structural transformations in the alloy increases with ion current density of a pulse. A similar transformation of a dislocation structure over the entire thickness of the sample is observed under irradiation with continuous Ar+ ion beams (E = 20-40 keV) with not high fluences (1015-1016 cm-2).

THE STUDY OF DAMAGEABILITY OF PLATE STEEL UNDER CYCLIC PLASTIC BENDING

The authors have investigated the layer-by-layer development of damage of 10G2S1 plate steel during cold plastic deformation by cyclic bending. Deformation cyclic bending was performed according to the scheme of pure bending at symmetric cycle with amplitude of deformation of 5.5 %. The character of changes of the strength properties of steel shows the kinetics of damage in the process of plastic deformation. It was established that in the area of reversible damage the intensity of hardening increases, reaching a maximum value and then decreases. In the area of reversible damage the mechanical properties of steel doesn’t change until destruction. Criterion upper bound of damage is at (0,2 – 0,3)Nр , where Nр is the number of cycles to failure under cyclic tension – compression. The transformation process of the cellular dislocation structure in the passband is selected as a structural criterion of reversible damage.