Dislocation Cell Walls Research Articles

Influence of dislocation cells on hydrogen embrittlement in wrought and additively manufactured Inconel 718

Hydrogen embrittlement (HE) is a major issue for the mechanical integrity of high-strength alloys exposed to hydrogen-rich environments, with diffusion and trapping of hydrogen being critical phenomena. Here, the role of microstructure on hydrogen diffusion, trapping and embrittlement in additively manufactured (AM) and wrought Inconel 718 is compared, revealing the key role played by dislocation cells. Trapping behaviour in hydrogen-saturated alloys is analysed by thermal desorption spectroscopy and numerical simulations. A high density of hydrogen traps in cell walls, attributed to dense dislocations and Laves phases, are responsible for the local accumulation of hydrogen, causing significant loss in strength, and triggering cracking along dislocation cell walls. The influential role of dislocation cells alters fracture behaviour from intergranular in the wrought alloy to intragranular for the AM alloy, due to the large proportion of dislocation cells in AM alloys. In addition, the cellular network of dislocations accelerates hydrogen diffusion, enabling faster and deeper penetration of hydrogen in the AM alloy. These results indicate that the higher HE susceptibility of nickel superalloys is intrinsically associated with the interaction of hydrogen with dislocation walls.

Microstructure Evolution and Fretting Wear Mechanisms of Steels Undergoing Oscillatory Sliding Contact in Dry Atmosphere.

Variations in the microstructure and the dominant fretting wear mechanisms of carbon steel alloy in oscillatory sliding contact against stainless steel in a dry atmosphere were evaluated by various mechanical testing and microanalytical methods. These included scanning electron microscopy and energy dispersive spectrometry with corresponding elemental maps of the wear tracks, in conjunction with cross-sectional transmission electron microscopy of samples prepared by focused ion beam machining to assess subsurface and through-thickness changes in microstructure, all as a function of applied load and sliding time. Heavily dislocated layered microstructures were observed below the wear tracks to vary with both the load and sliding time. During the accumulation of fretting cycles, the subsurface microstructure evolved into stable dislocation cells with cell walls aligned parallel to the surface and the sliding direction. The thickness of the damaged subsurface region increased with the load, consistent with the depth distribution of the maximum shear stress. The primary surface oxide evolved as Fe2O3 and Fe3O4 with increasing sliding time, leading to the formation of a uniform oxide scale at the sliding surface. It is possible that the development of the dislocation cell structure in the subsurface also enhanced oxidation by pipe diffusion along dislocation cores. The results of this study reveal complex phase changes affecting the wear resistance of steels undergoing fretting wear, which involve a synergy between oxidative wear, crack initiation, and crack growth along dislocation cell walls due to the high strains accumulating under high loads and/or prolonged surface sliding.

Multiscale characterisation study on the effect of heat treatment on the microstructure of additively manufactured 316L stainless steel

Additively manufactured 316L components are known to exhibit complex three-dimensional microstructures on multiple length-scales. The effect of heat treatment on the stability of grains and dislocation cell structures within the microstructure has previously been investigated but there are appreciable disagreements surrounding the nature of the texture that is present in the specimens, and there is also limited published work on the compositional homogeneity of specimens, particularly on the atomic scale, after heat treatments. In this study, we investigate the effect of applying two different heat treatments to 316L components produced by laser powder bed fusion before characterising them using a combination of electron microscopy and atom probe tomography. The importance of using multiple characterisation techniques, which span from the nanometre to micrometre scale, in addition to carefully and accurately describing analysis methods when investigating the evolution of the microstructure of additively manufactured 316L steels is demonstrated. Our EBSD (Electron Back Scatter Diffraction) results show the presence of a strong texture in the build direction of the samples, and a reduction in morphological texture as a result of heat treatment. TEM (Transmission Electron Microscopy) results indicate a dissolution of the dislocation cell structure that forms during solidification. Atom probe tomography was used to investigate compositional homogeneity in the samples and indicated that there are regions enriched in Cr, Mn, Mo, and Ni in the As-Printed samples that are likely associated with the dislocation cell walls. The atom probe results also reveal the presence of impurities, such as Co, which were not detected in the feedstock powder.

Influence of different cycle strain amplitudes on the microstructure and properties of 2024 aluminum alloy

The effect of varying strain amplitudes on the tensile properties and microstructure of aluminum alloy 2024 after a solution treatment (ST) of 495 °C for 1 h followed by cyclic strengthening at room temperature in the presence of various strain amplitudes is methodically investigated. Cyclic strain treatment substantially leads to the enhancement of the strength and plasticity of the alloy. Different strain amplitudes result in distinct strengthening mechanisms in the tested specimens. The specimens treated with low strain amplitudes (Δεt/2 = 0.1–0.4%) exhibit the least changes in the dislocation cell size and the dislocation density. The strengthening is mainly attributed to the precise adjustment of solute cluster precipitation. In specimens treated with high strain amplitudes (Δεt/2 = 0.5–1.1%), various strain amplitudes result in various dislocation configurations. The S-phase precipitation occurs selectively in the dislocation cell walls, forming solute cluster-composite dislocation loops within the dislocation cells, enhancing both the strength and plasticity of the alloy. The rational cyclic strain treatment not only yields the achievement of the precise control of precipitate phases, but also regulates the dislocation configurations. The obtained results reveal that the high-density cluster-composite dislocation loops are capable of resolving the conflict between increasing dislocation density and reducing dislocation pile-up, providing new insights into alloy strengthening.

Exceptional thermal stability of additively manufactured CoCrFeMnNi high-entropy alloy with cellular dislocation structures

CoCrFeMnNi high-entropy alloy (HEA) was additively manufactured (AM) by laser powder-bed fusion (L-PBF). The AM CoCrFeMnNi has prominent cellular dislocation structures with a small number of Mn-rich oxides. The thermal stability of the AM CoCrFeMnNi was investigated by isochronal annealing treatment at various temperatures from 400 to 1300 °C for 1 h. Microstructural analysis shows slow dislocation recovery, retarded recrystallization process, and precipitation of additional Cr-Mn based oxides during thermal annealing, resulting in exceptional thermal stability and retained high hardness at elevated temperatures. By thermodynamic calculations, a low stored energy of 1.31 MJ/m3 and a high activation energy of 353 kJ/mol for recrystallization were estimated for the AM CoCrFeMnNi. The exceptional thermal stability of the AM CoCrFeMnNi HEA is mechanistically attributed to the low crystallographic misorientations across the dislocation cell walls, sluggish atomic diffusion, and the pinning effects of the oxide nanoprecipitates.

Influence of Dislocation Substructure on Size-Dependent Strength of High-Purity Aluminum Single-Crystal Micropillars

In order to understand the influence of dislocation substructures on the size-dependent strength (smaller is stronger) of micron-sized metals, we have fabricated single-crystal cylindrical micropillars with various diameters approximately ranging from 1 to 10 μm, which were prepared on the surface of the fully annealed sample and the subsequently cold-rolled samples of high-purity aluminum (Al). The annealed micropillars exhibited a size dependence of the resolved shear stress required for slip. The shear stress (τi) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of τi/G=0.33(d/b)−0.63. The size-dependent strength was reduced by cold-rolling, resulting in lower power-law exponents (0.26~0.31) for the correlation in the cold-rolled specimens. The fine dislocation substructures introduced by the cold-rolling could be associated with the reduced size-dependent strength, which can be rationalized using the stochastic model of the dislocation source length in an assumption of homogenously distributed dislocations existing in the experimental micropillars. The inhomogeneous dislocation substructure with various dislocation cell sizes would contribute to a variation in the measured strength depending on the location, likely due to the probability of exiting the dislocation cell walls (local variation in dislocation density) in the micropillars fabricated on the cold-rolled Al samples.

Pre-deformation aging strengthening mechanism of Cu–15Ni–8Sn alloys prepared by laser additive manufacturing

The recently introduced concept of “additive manufacturing +" can be used to prepare parts with complex shapes and also high-performance alloys. In this paper, Cu–15Ni–8Sn alloys were prepared by laser powder bed fusion (L-PBF) and laser-directed energy deposition (L-DED), and successively solutionized, 80% cold-rolled, and aged at 400 °C. This unconventional process increased the yield strength of L-PBF and L-DED specimens to 1389 MPa and 1317 MPa, with a considerable elongation of 1.9% and 2.8%, respectively. It also greatly shortened the preparation process compared with other methods. The microstructural and mechanical changes of L-PBF and L-DED samples undergone the same pre-deformation aging treatment are consistent. It implies that the pre-deformation aging strengthening mechanism of two kinds of printed samples is basically the same. Pre-deformation changed the morphology and distribution of the spinodal structures and ordered phases, and promoted the nucleation of discontinuous precipitates (γ-D03 phase) that preferentially precipitated along elongated grain boundaries and shear bands. This weakened the strengthening effect of D022 and L12 ordered phases. Pre-deformation expedited the aging process, but this was accompanied by a drop in the aging strengthening effect and was more sensitive to the aging temperature. In addition, the stacking faults and L-C dislocations along dislocation cell walls coexisted with the ordered phases for a long time, which inhibited the nucleation of the γ phase and produced a good strengthening effect.

Enhancing corrosion resistance of additively manufactured 316L stainless steel by fabricating pillar arrays

By using the electrochemical etching technique, pillar arrays with a size of ∼50–200 nm are fabricated on the surface of 316L manufactured by laser powder bed fusion. The nanopillars morphology is induced by the selective anodic dissolution of dislocation cell walls that are decorated with high local concentrations of Cr/Fe/Ni solute atoms. The appearance of pillar arrays extends the passivation range and increases the pitting potential of 316L in HNO3 solution, while the corrosion current density decreases in NaNO3 solution. The enhanced corrosion resistance was attributed to the coeffect of elimination of corrosion-sensitive features and hydrophobicity prompted by electrochemical etching.

Enhancing hydrogen embrittlement resistance of medium entropy alloys by forming dislocation cell walls

Hydrogen embrittlement is a complex phenomenon resulting in severe deterioration of ductility or load-bearing capacity of structural metals due to the presence of hydrogen atoms. In order to discover hydrogen-tolerant metals, in this work, we introduce two novel face-centered-cubic phase medium entropy alloys (MEAs) (Cr12Ni12MnxFe76−x, x = 24 and 16). Interestingly, The Cr12Ni12Mn24Fe52 alloy showed high resistance to hydrogen embrittlement at ambient temperature, while exhibiting an increase in strength and ductility. Through experimental assessment, we demonstrate that the stacking fault energy and the resultant dislocation structures enable our MEA to achieve hydrogen-compatible characteristics.

Microstructure and Solute Segregation around the Melt-Pool Boundary of Orientation-Controlled 316L Austenitic Stainless Steel Produced by Laser Powder Bed Fusion.

For this article, we studied the microstructure and solute segregation seen around the melt pool boundary of orientation-controlled 316L austenitic stainless steel produced by laser powder bed fusion, using transmission electron microscopy and energy-dispersive x-ray spectroscopy. We found that the solidification cellular microstructures could be visualized with the aid of solute segregation (Cr and Mo) during solidification. Mn-Si-O inclusions (10-15 nm in diameter) were distributed along the lamellar boundaries, as well as in the dislocation cell walls. It is believed that the grain growth of the inclusions can be effectively suppressed by rapid quenching during the laser powder-bed fusion process. A thin region without cellular microstructures was observed at the melt-pool boundary. The cellular spacing widened near the bottom of the melt-pool boundary, owing to the decrease in the cooling rate. Atomic-structure analysis at the lamellar boundary by high-resolution transmission electron microscopy revealed a local interfacial structure, which is complementary to the results of electron back-scatter diffraction.

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.

Microstructural evolution during short heat treatment of constrained groove pressed Cu‐5%Zn alloy

Severe plastic deformation (SPD) is a widely used technique to obtain superior material properties specially mechanical properties. Constrained groove pressing (CGP) is found to be the most attractive SPD technique for the deformation of sheets and plates. However, this technique results in microstructural inhomogeneity during processing. The microstructural inhomogeneity can be alleviated by employing a thermal cycle, which assists in controlled recovery and recrystallisation. The current work focuses on, the microstructure evolution of one pass as-deform CGP sample followed by a short heat-treated (SHT). A correlative imaging technique of transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM) showed the presence of dislocation cell structure in an as-deformed condition. The short heat treatment resulted in the transformation of the dislocation cell wall to high-angle boundaries, with a further increase in heat-treatment time resulting in grain growth.

Coupled quantitative modeling of microstructural evolution and plastic flow during continuous dynamic recrystallization

Continuous dynamic recrystallization (cDRX) dominates microstructural evolution during the hot working of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. However, in reality, a lack of quantitative and visual modeling of the process hinders its widespread application in the hot working process. In this study, using a recently developed multilevel cellular automaton (MCA) that integrates the newly established cell switching rules and topology deformation technique, a novel mesoscale MCA-cDRX model was constructed to investigate the evolution of both microstructures and macroscopic mechanical response in the hot working of AA7075 aluminum alloy. By considering the evolution of dislocation density and the orientation angle of the local cells as the primary clues, the plastic flow, recrystallization kinetics, features of subgrain size and high-angle grain boundaries, and influence of initial matrix characteristics on the cDRX mechanism were analyzed. The model predictions are consistent with the experimental data. Quantitative analysis confirms that the incubation time for the initiation of subgrain formation is significantly short. The fine-grain matrix and high initial volume fraction of low-angle grain boundaries can significantly accelerate the progress of cDRX owing to a stronger accumulation of dislocations in the dislocation cell walls through the climb and cross-slip mechanisms in the deformed aluminum alloy. The subgrain size is dependent on the Zener-Hollomon parameter. The developed simulation framework offers an effective means to allow the visualization of the cDRX.

Deformation faulting and dislocation-cell refinement in a selective laser melted 316L stainless steel

The selective laser melted (SLM) 316L stainless steel (316L SS) has shown superior tensile ductility and doubled yield strength compared to its wrought counterpart. The significantly improved yield strength has been attributed to the unique cellular substructures featured by Cr/Mo-segregation and trapped dislocations. The excellent ductility of SLMed 316L SS has been mainly understood from the pronounced deformation twinning, which, however, is just one of the dominant plastic deformation mechanisms in 316L SS. Here instead, we report two other fundamental deformation micro-mechanisms, i.e., deformation faulting and dislocation cell refinement, in the SLM 316L SS. Our results showed that deformation faulting and dislocation cell refinement characterize the whole tensile deformation significantly except for deformation twinning. These newly found mechanisms synergistically dominated the deformation behavior at low strain levels while deformation faulting and twinning play a crucial role at medium and high strain levels. The pre-existing stacking faults (SFs) in cellular substructure is mainly responsible for the significant deformation faulting. At the early deformation stage, these pre-existing SFs provide faulting nuclei for deformation faulting, leading to wide SFs. These wide SFs in different cellular interiors penetrate the cellular boundaries and overlap as the strain increases, resulting in long stacking fault ribbons (SFRs). We attributed the formation of DCs to the measured medium stacking fault energy (SFE) of ∼18 mJ/m2. Upon straining, equiaxed dislocation cells (DCs) were generated by new dislocation cell walls (DCWs) inside the cellular substructure, and refined by either the dissociation of fully developed DCWs or the continuously formed new DCWs. Together with deformation twinning, these two fundamental deformation mechanisms jointly led to a steady strain hardening rate during tension and thus a superior tensile ductility of the SLM 316L SS with high yield strength. These findings provide new insights into the excellent strength-ductility combination of SLMed 316L SS and the experimental basis for crystal plasticity modeling and simulation, as well.

Effect of stress-relief heat treatments on the microstructure and mechanical response of additively manufactured IN625 thin-walled elements

Metallic structures fabricated with laser powder-bed fusion (LPBF) often have elemental segregations that are far from equilibrium and high levels of residual stresses due to the localized melting and rapid solidification that occur during printing. These unique characteristics result in mechanical property debits that make it difficult to employ such printed parts in critical applications. However, post-processing heat treatments can minimize the negative effects of residual stresses and reduce segregation. In this study, we measure the effect of two stress-relief heat treatments, 870°C for 1 h (industry-recommended) and 800°C for 1 h (potential alternative), on the microstructure and room temperature, monotonic quasi-static mechanical response of LPBF-processed thin-walled Inconel 625 T-elements. Electron backscatter diffraction analysis revealed an overall columnar microstructure with a strong <001> texture that is closely aligned with the build direction. The well-developed dislocation cell walls in both stress-relieved cases were found to be decorated with two types of blocky precipitates, namely, (Nb, N)-rich MX and (Nb, Mo, Si, N)-rich -M6X with scant evidence of the deleterious δ phase. The slightly larger size and higher volume fraction of precipitates in the 870°C case was found to have a small effect on the mechanical response of the T-elements under the current loading conditions and the difference in mechanical response of the two heat treatments was only significant at the very later stages of deformation. These findings make 800°C for 1 h a viable stress-relief alternative for our IN625 samples under the loading conditions studied here.

Investigation of strengthening mechanisms in an additively manufactured Haynes 230 alloy

There are abundant studies on additive manufacturing of Ni alloys, such as Inconel 718. However, the studies on the microstructure and the deformation behavior of solid solution strengthened Ni alloys, such as Haynes 230, are limited. Here we compare the microstructure and the mechanical behavior of as-printed and stress-relieved Haynes 230 Ni alloys prepared by laser powder bed fusion. Transmission electron microscopy analysis revealed M23C6 carbides decorated at the dislocation cell walls in the as-printed material transformed to well-aligned arrays of M6C nanoprecipitates separated by an array spacing of 500 nm after stress-relief (heat treatment). The nanoprecipitates in stress-relieved alloy have formed numerous aligned orientation relationships with the matrix despite their large lattice mismatch. Post-tension microscopy analyses demonstrated high-density dislocation networks in the as-printed samples, whereas high-density stacking faults and nanoscale deformation twins were observed in the stress-relieved counterparts. This study provides insights for understanding the influence of nanoprecipitates on the deformation mechanisms of additively manufactured Ni alloys.

Dislocation cell walls with high dislocation density as effective hydrogen traps in Armco iron

AbstractThe effect of cold rolling (CR) deformation on the hydrogen permeation and trapping behavior of pure iron was systematically investigated. Increased deformation resulted in a decreased effective hydrogen diffusion coefficient, an extended effective lattice interstitial hydrogen diffusion path length, and increased hydrogen trap densities. However, the proportion of irreversible hydrogen traps to all traps was constant irrespective of the deformation level in the range of 30%–70% CR and increased when the deformation reached 80%. This is because the dislocation cell wall is composed of a central layer with high trap‐binding capacity and an outer layer with low trap‐binding capacity. More importantly, once the dislocation density in the center layer of the dislocation cell walls reached a relatively high value due to the severe plastic deformation, these regions become effective irreversible hydrogen traps.

Effect of dislocation cell walls on hydrogen adsorption, hydrogen trapping and hydrogen embrittlement resistance

This study investigated the effects of dislocation patterns introduced by cold rolling deformation on the hydrogen absorption properties, hydrogen trap behaviors and hydrogen embrittlement (HE) resistance for Armco iron under electrochemical charging. The surface hydrogen coverage first increases and then decreases with increasing deformation, and reaches a maximum at 50% deformation. Dislocation cell walls appear when the deformation is greater than 50%. Furthermore, the HE resistance is improved when the deformation reaches 80%. Because the dislocation cell walls can be used as effective hydrogen traps when the dislocation density reaches a sufficiently high value.

Damage assessment in Q690 high strength structural steel using nonlinear Lamb waves

The nonlinear ultrasonic technique has been used to assess the plastic damage in High strength Q690 steel. The Q690 steel was stretched to different levels of plastic strain so as to obtain the specimen with different degrees of plastic damage. When strain was increased from 1% to 9%, the nonlinear parameter shows a general upward trend, and in the subsequent strain process, the nonlinear response drop off instead of going up. Meanwhile, metallographic examination indicates that, within the strain 9%, the growth of the void as well as the variation of dislocation structure and density are the main damage characteristic in Q690 steel. Many dislocation structures such as the dislocation cell and dislocation wall have formed in the tensile specimen, which is contribute to raise of the nonlinear effect. The results indicate that the nonlinear ultrasonic technique can be utilized to characterize the plastic damage in Q690 structural steel at the early stage.

Dislocation structures and the role of grain boundaries in cyclically deformed Ni micropillars

Transmission electron microscopy and finite element-based dislocation simulations were combined to study the development of dislocation microstructures after cyclic deformation of single crystal and bicrystal Ni micropillars oriented for multi-slip. A direct correlation between large accumulation of plastic strain and the presence of dislocation cell walls in the single crystal micropillars was observed, while the presence of the grain boundary hampered the formation of wall-like structures in agreement with a smaller accumulated plastic strain. Automated crystallographic orientation and nanostrain mapping using transmission electron microscopy revealed the presence of lattice heterogeneities associated to the cell walls including long range elastic strain fields. By combining the nanostrain mapping with an inverse modelling approach, information about dislocation density, line orientation and Burgers vector direction was derived, which is not accessible otherwise in such dense dislocation structures. Simulations showed that the image forces associated with the grain boundary in this specific bicrystal configuration have only a minor influence on dislocation behavior. Thus, the reduced occurrence of “mature” cell walls in the bicrystal can be attributed to the available volume, which is too small to accommodate cell structures.