Subgrain Boundaries Research Articles

Nano gradient structuring at Ti-6Al-4V surface induced by ultrashort-pulse laser peening

We have successfully fabricated gradient micro/nano structuring on the surface top layers of Ti-6Al-4V alloy using a shock-peening technique facilitated by femtosecond and picosecond laser pulses, without the need for coatings or confinement. These micro/nano structures encompass ultrafine grains, extensive subgrain boundaries, hierarchical nanotwins, and complex dislocation morphologies. The ultrafine grains, ranging in size from hundreds of nanometers to a few micrometers, were predominantly located in the grain-refined regions, whereas the micro/nano dislocation structures were predominantly found in regions of severe plastic deformation. These observations suggest a promising avenue for achieving high-precision gradient structuring in the field of metallic surface engineering.

Stress relaxation aging of 7050 aluminum alloy under elastic and plastic loadings: A perspective from the geometrically necessary dislocations

Aerospace integral panels with large curvature often experience not only elastic loading but also plastic loading during creep age forming. However, the stress relaxation aging behaviors and mechanisms under loading stresses ranging from elastic to plastic regions, especially from the perspective of geometrically necessary dislocations (GNDs), have not been fully explored. The stress relaxation aging (SRA) behavior of 7050 aluminum alloy under elastic (200/250/300 MPa) and plastic loadings (350/400 MPa) are studied by uniaxial SRA, hardness and tensile tests, and corresponding mechanisms are clarified by Electron Back Scatter Diffraction (EBSD) tests and transmission electron microscopy (TEM) observations. Regardless of the loading stress, the stress relaxation of 7050 alloy shows a typical two-stage behavior (rapid stress reduction and stable stress relaxation rate). The second stage of stress relaxation under elastic loading shows an abnormal stress rebound, while the stress continuously decreases under plastic loading, and hardness and yield strength at 350 MPa are lower than that at 200 MPa during SRA. These results are attributed to strong dislocation pinning from small and dense precipitates at elastic loading and dislocations shearing large and sparse precipitates at plastic loading, respectively. Furthermore, both the stress reduction and its ratio increase with the increase of initial stress. GNDs contribute to the formation of low-angle grain boundaries (LAGBs) and accumulate at the LAGBs intersection to coordinate grain orientation for creep deformation. Compared to elastic loading, plastic loading promotes global growth of intragranular precipitates by elastic stress field and local heterogeneous nucleation and coarsening of precipitates at subgrain boundaries by more GNDs.

Making large-size fail-safe steel by deformation-assisted tempering process

Synergistically improving the strength and toughness of metallic materials is a central focus in the field of physical metallurgy. Yet, there is a noticeable lack of research in strengthening and toughening large-size metal components, whereas those components are extensively used in the modern industry. In this work, a deformation-assisted tempering (DAT) process was proposed to create a novel microstructure in 1.4 tons low-alloyed plain steel. After DAT treatment, the steel contains low dislocation density but high density of low-angle subgrain boundaries and dispersed spherical nano carbides. Such microstructure enables a much better combination of tensile strength and fracture toughness compared to the small-size quench and temper steels. The significant improvement in low-temperature impact toughness is due to the occurrence of delamination and subsequent large plastic deformation at the notch tip. The DAT process can provides a new strategy for the development of large-size fail-safe steel with excellent strength and fracture resistance.

Impact of multi-scale microstructural heterogeneities on the mechanical behavior of additively manufactured and post-processed Nb-based C103 alloy

Laser powder-bed fusion (LPBF) processed Nb-based alloy C103 (Nb-10Hf-1Ti wt.%) develops a complex, hierarchical microstructure comprising a fine-scale solidification cell structure, overlaid with a dense dislocation-network outlining the cell boundaries, within the primary grains. Additionally, sub-grain boundaries and a fine-scale dispersion of nano-sized hafnium oxide precipitates, possibly forming during solidification, decorate the solidification cell boundaries as well as exist within the cells. This complex hierarchical microstructure results in impressive tensile mechanical properties. Post-build stress-relieving annealing and hot isostatic pressing (HIP) largely annihilates the solidification cell structure and associated dislocation network, lowering the strength but with substantial recovery of tensile ductility. Nevertheless, the resulting microstructure offers higher strengths as compared to their wrought counterparts.

A polycrystal plasticity-cellular automaton integrated modeling method for continuous dynamic recrystallization and its application to AA2196 alloy

Continuous dynamic recrystallization usually dominates the microstructural evolution in hot working of aluminum alloys, in which the high-angle grain boundaries of new grains mainly originate from the gradual increase in subgrain misorientation angles. In this work, an integrated computational method is proposed to simulate continuous dynamic recrystallization process of aluminum alloys by coupling three-dimensional cellular automaton and visco-plastic self-consistent models. The stress response, dislocation accumulation and recovery, and evolution of crystal orientations are computed in the context of polycrystal plasticity; the formation and rotation of subgrains, followed by stored energy and curvature-driven boundary migration, are captured and visualized by cellular automaton. The non-octahedral slip mode {110}<110> is additionally introduced to capture the 〈001〉 texture during hot compression. A universal cell topology deformation method is adopted to achieve an effective track of grain morphology evolution during plastic deformation. The proposed simulation framework is validated through simulating the isothermal uniaxial compression process of AA2196 alloy under different temperatures and strain rates. The orientation dependence of CDRX during compression is numerically reproduced by correlating the subgrain formation and rotation process with the activation state of slip systems. The simulated macroscopic flow stress, 3D microstructure and inherent microstructural characteristics such as subgrain size, subgrain boundaries and textures are in good agreement with the experimental results. The proposed method provides an effective and efficient tool for multi-scale simulation of hot forming process of aluminum alloys.

Stress sensitivity and its influence on high-temperature creep behavior of a novel 2nd-generation low-cost nickel based single crystal superalloy

To meet the application requirements under complex service conditions for aero engine blades, the influence of stress-state on creep behavior of a self-designed Re-low Ni-based single crystal superalloy at 1038 °C over a wide stress range of 137–207 MPa was investigated. With applied stress decreasing from 172 to 137 MPa, the creep life rapidly extends by a factor of 2.41, showing strong stress dependence. Further, the creep mechanisms under different stresses are discussed based on microstructural characterization and threshold stress calculation results. With the increasing applied stress, interfacial dislocation networks exhibit sparse and irregular shapes. Particularly, in necking zone of the fractured specimen at 1038 °C/137 MPa, due to the long-duration accumulation of deformation storage energy, dislocation arrays within γ′ rafts gradually evolve into dislocation walls through recombination and annihilation of dislocations, then develop into subgrain boundaries. Finally, some recrystallization grains with large misorientations (>10°) form by subgrain-rotation. Moreover, based on the calculated threshold stress, dislocations climbing can play a major role in creep deformation throughout the steady-state stage under a stress range of 137–207 MPa. Nevertheless, with the arrival of tertiary stage, the increasing actual stress resulted from necking deformation can also promote dislocations to cut or bypass γ′ rafts under lower stresses (137 and 172 MPa), which is consistent with the final evolution of dislocation substructures. Overall, the findings from this work are helpful to understand the high-temperature creep mechanism of low-cost single crystal superalloys, so as to improve the creep resistance of materials.

Effect of TIG remelting on the microstructure, mechanical properties, and corrosion behavior of 5052 aluminum alloy joints in MIG welding

In this work, the effect of tungsten inert gas (TIG) remelting on the microstructure, mechanical properties, and corrosion resistance of 5052 aluminum alloy joints produced by metal inert gas (MIG) welding is investigated. The results indicate that after TIG remelting of MIG welds, the proportion of dendrites increases in the remelted weld zone (WZ), porosity decreases by 76.9%, and the Fe elements in the second phase become coarser and more segregated. In contrast, the Mg elements form a solid solution with a decreased degree of segregation. In addition, the average grain size of the remelted structure decreases, while the proportion of sub-grain boundaries (SGBs) and the kernel average misorientation (KAM) values increase. Moreover, the heat-affected zone (HAZ) widens, and the grains become coarser. An increase of 4.5% in tensile strength and 20.0% in elongation was observed in the TIG-remelted joints after TIG remelting. Electrochemical and immersion tests on the TIG-remelted microstructure showed reduced corrosion resistance. Electrochemical tests mainly exhibited galvanic corrosion, resulting in the formation of pits. The types of corrosions observed in the immersion test differ from those in the electrochemical test, primarily exhibiting exfoliation corrosion and pitting corrosion. A higher geometric dislocation density (ρGND) in the structure increases the susceptibility to corrosion.

Resolving localized geometrically necessary dislocation densities in Al-Mg polycrystal via in situ EBSD

The distribution of geometrically necessary dislocation (GND) densities is critical to understanding the heterogeneous plastic deformation at intragranular scales in polycrystals. In this work, we performed an in situ electron backscatter diffraction (EBSD) measurement during the tensile test on a polycrystalline Al-Mg alloy. The EBSD patterns were processed through the integrated digital image correlation algorithm to enhance angular resolution. Based on the Nye dislocation density tensor, GND densities of 18 dislocation types in fcc crystals were resolved and mapped at macroscopic strains ranging from 5% to 16%. The evolution of GND distribution showed that GNDs were generated near grain boundaries at small strains and later localized at grain interiors along subgrain boundaries and slip bands at large strains. The inhomogeneous increase in GND densities of different dislocation types, representing dislocation substructures, along the subgrain boundaries was disclosed. Meanwhile, high GND densities were observed along the slip bands when a double noncoplanar slip system was activated inside the grains. The obstructed movement of dislocations by Lomer junctions explained the GND storage in different {111} slip planes. The existence of Lomer junctions was proven by Burgers circuit analysis with high-resolution transmission electron microscopy, which explained the increase in [101] screw dislocation density according to the dislocation reaction.

An Internal-State-Variable-Based Continuous Dynamic Recrystallization Model for Thermally Deformed TC18 Alloy.

Continuous dynamic recrystallization (CDRX) is widely acknowledged to occur during hot forming and plays a significant role in microstructure development in alloys with moderate to high stacking fault energy. In this work, the flow stress and CDRX behaviors of the TC18 alloy subjected to hot deformation across a wide range of processing conditions are studied. It is observed that deformation leads to the formation of new low-angle grain boundaries (LAGBs). Subgrains rotate by absorbing dislocations, resulting in an increase in LAGB misorientation and the transition of some LAGBs into high-angle grain boundaries (HAGBs). The HAGBs migrate within the material, assimilating the (sub)grain boundaries. Subsequently, an internal state variable (ISV)-based CDRX model is developed, incorporating parameters such as the dislocation density, adiabatic temperature rise, subgrain rotation, LAGB area, HAGB area, and LAGB misorientation angle distribution. The values of the correlation coefficient (R), relative average absolute error (RAAE), and root-mean-square error (RMSE) between the anticipated true stress and measured stress are 0.989, 6.69%, and 4.78 MPa, respectively. The predicted outcomes demonstrate good agreement with experimental findings. The evolving trends of the subgrain boundary area under various conditions are quantitatively analyzed by assessing the changes in dynamic recovery (DRV)-eliminated dislocations and misorientation angles. Moreover, the ISV-based model accurately predicts the decreases in grain and crystallite sizes with higher strain rates and lower temperatures. The projected outcomes also indicate a transition from a stable and coarse-grained microstructure to a continuously recrystallized substructure.

Investigation of microstructure and thermal expansion behaviour of a functionally graded YSZ/IN718 composite produced by laser-powder bed fusion

Yttria-stabilized zirconia (YSZ) has been extensively used as a thermal barrier coating (TBC) for high-temperature alloys. In this research, the microstructural attributes and thermal expansion behavior of an additively manufactured functionally graded composite (YSZ/IN718 FGC) were investigated. First, a powder mixture of IN718 and YSZ (ranging from 1 wt% to 4 wt%) was achieved via low-energy ball-milling at 100 rpm. Differential scanning calorimetry (DSC) analysis depicted the melting behaviour of the powder mixtures of different YSZ content. The endothermic peaks in the DSC curve, at ∼1360 ℃, suggested that the energy needed to melt the powder mixture increases with increasing YSZ content. These varying powder compositions were used to fabricate FGC samples using L-PBF. The study investigated the effects of laser power variation along the YSZ gradient, successfully mitigating the fusion defects that had been initially observed in FGC samples. Microscopic analyses, comprising scanning electron microscopy (SEM) and transmission electron microscopy (TEM), depicted the elemental segregation of Nb and Mo at the solidification subgrain boundaries (SSBs). Furthermore, the electron backscattered diffraction (EBSD) results demonstrated the refinement of grains and a reduction in textural intensity, particularly along the YSZ gradient. Energy dispersive spectroscopy (EDS) and electron probe micro analysis (EPMA) were used to validate the uniform dispersion of YSZ particles throughout the composite matrix, with the EPMA line scan affirming a predominantly linear YSZ content variation. Lastly, the coefficients of thermal expansion (CTEs) were evaluated, both parallel and perpendicular to the build direction. A decrease of approximately ∼6 % in CTE was noted along the building or gradation direction. Notably, perpendicular to the building direction, the CTE values for specimens containing 4 wt% and 2 wt% YSZ witnessed significant reductions of ∼32 % and ∼12 %, respectively. This research significantly contributes to the understanding of the microstructural intricacies and thermal behavior of YSZ/IN718 FGC produced by L-PBF process.

Effect of subgrain microstructure on the mechanical properties of Invar 36 specimens prepared by laser powder bed fusion

Invar 36 samples were fabricated via laser powder bed fusion (LPBF) at various energy densities. In this work, a parameter range for the preparation of full-density Invar 36 samples was determined. The microstructural evolution and tensile properties of the materials were studied in the as-LPBFed state, and the role of subgrain structures on the fracture mechanism was investigated. According to the results of this analysis, as the energy density increased, the density first increased and then decreased. The maximum density (99.94 %) reached 65 J/mm3, and the simple density reached full density at 40–90 J/mm3. Due to constitutional supercooling, the microstructure of the alloy is composed of cellular subgrains and columnar subgrains. Columnar subgrains grow perpendicular to the boundary of the molten pool to the center of the molten pool, while cellular subgrains primarily exist in the remelting area of the molten pool. In the full density range, the ultimate tensile strength of the samples reaches 437–484 MPa, and differences in the morphology, distribution, and micronicomechanical properties of the subgrains are responsible for the differences in macroscopic properties. At 40, 65, and 90 J/mm3, the elastic moduli of the columnar subgrains are approximately 155.1 GPa, 161 GPa, and 162.4 GPa, respectively; the elastic moduli of the cellular subgrains are approximately 136.2 GPa, 140.2 GPa, and 139.9 GPa, respectively, which means that in the elastic deformation stage, the columnar subgrains are more resistant to elastic deformation; in the plastic deformation stage, different subgrains have different effects on crack propagation; the columnar subgrains grow perpendicularly to the melt pool boundary, which has a hindering effect on the extension of microcracks; and the cellular subgrains have a homogeneous grain size so that the microcracks can be easily extended along the cellular subgrain boundaries.

Microstructure evolution, crack initiation and early growth of high-strength martensitic steels subjected to fatigue loading

Fatigue loading induced microstructure evolution featured by Fine Granular Area (FGA) in high-strength martensitic steels have close ties to the fatigue crack initiation and early growth, which are worthy of being investigated. On this issue, we conducted Very High Cycle Fatigue (VHCF), High Cycle Fatigue (HCF) and Low Cycle Fatigue (LCF) tests on three different types of specimens. Combined with post-mortem/quasi in-situ Scanning Electron Microscope (SEM) and Electron Back-Scattered Diffraction (EBSD) observations, as well as further Transmission Kikuchi Diffraction (TKD) and Transmission Electron Microscope (TEM) analyses, we captured two different types of microstructure evolution behaviors during the fatigue loading. One is the “dynamic precipitation”, a kind of phase transformation from martensitic matrix (BCC) to Nb- and Mo-rich small carbides (MC, M7C3) accelerated by repeated slight plastic deformation, whose formation does not rely on the stress concentration effect and also has no ties to the plastic strain localization and fatigue crack initiation. Another is the “local grain refinement” around those stress singularities (crack tip, or interior inclusion) generating substantial amounts of fine sub-grains in VHCF, HCF and LCF regimes, which can then promote fatigue crack initiation and early growth along sub-grain boundaries, fine/coarse grain boundaries and martensitic packet boundaries.

The synergistic effect of gradient nanostructure and residual stress induced by expansion deformation on the tension-tension fatigue performance of Ti6Al4V holes

A large number of titanium alloy structural components in aircraft are joined and assembled through holes. The complex service conditions pose high requirements for the fatigue performance of the holes. The study aims to use expansion deformation to induced gradient nanostructures and residual stress enhance the fatigue performance of Ti6Al4V holes. By studying the evolution process of the microstructure in titanium alloys, the formation mechanism of gradient nanostructures has been obtained, and their impact on various stages of fatigue fracture is analyzed. The results showed that the split sleeve cold expansion (CE) process caused severe plastic deformation on the surface of the hole, resulting in a large amount of LAB production and a transformation of the main texture components. Meanwhile, the gradient nanostructures are formed on the hole surface, which include nanocrystalline layer, superfine lath grain layer, lath grain layer and severe deformation layer. During the expansion deformation process, the slip of dislocations leads to the formation of high-density dislocation structures, which evolve into subgrain boundaries and then undergo dynamic recrystallization through subgrain rotation to achieve grain refinement. What's more, the degree of grain refinement, depth of gradient nanostructure, and residual stress increase as the expansion deformation intensifies. The depth of gradient nanostructure is 300 μm, and the maximum residual stress is −419 MPa, when the deformation is 5 %. The combined effect of gradient nanostructures and high residual compressive stress induced by expansion deformation has increased the total fatigue life and crack propagation life of CE-5 specimen by 2.9 and 3.4 times compared to NCE specimen. This work provides an effective method and theoretical analysis for improving the fatigue life of titanium alloy holes.

Study on the mechanism of rapid densification and structural evolution of C/C composites by cooling pyrolysis

A large thermal gradient about 190 ℃/cm was established in the thickness direction of preform and the cylindrical carbon/carbon composites with radial gradient pyrocarbon (PyC) distribution were rapidly prepared via the cooling pyrolysis technology in the high temperature surface based on the large thermal gradient chemical vapor infiltration (CPTHTS-LTGCVI). The decrease of initial temperature and the increase of cooling rate both contribute to the opening of closed pore and pore expansion, which are conducive to improving the densification degree. Moderate initial temperature (about 1130℃) and cooling rate (10 ℃/5 h) are optimal to rapidly preparing C/C composites with good density uniformity. For the whole cylindrical C/C composites, the faster the cooling rate, the higher the density and density uniformity, the fewer the super macropores. As the cooling rate is 10 ℃/2.5 h, the density of sample can reach 1.57 g/cm3 within 50 h, and the carbon yield in the first stage and accumulation can reach 84 % and 50 %, respectively. From the inner wall (IW) to the outer wall (OW), there are three successive regions about RL, RL+SL and SL PyC with gradually weakened texture, which is related to the insufficient pyrolysis caused by the decreased temperature and increased concentration, and the aromatization caused by the extended gas retention time. The edge dislocations and non-coherent sub-grain boundaries of PyC may be related to the distorted aromatic carbon planes containing five-membered rings. The edge dislocations may be the main reason of texture reduction and structural evolution of PyC.

Effect of vacuum diffusion bonding on the mechanical and conductive properties of bonded bulk copper single crystals

Recrystallization-free atomic-level (AL) bonding between bulk copper single crystals was achieved by combining chemical mechanical polishing (CMP) and vacuum diffusion bonding. Under various bonding conditions, the bonded copper specimens can be classified into three categories: AL-bonded, HH-bonded, and L-bonded specimens. AL-bonded copper single crystals were obtained by performing bonding under 20 MPa at 600 °C for 60 min, achieving the mechanical and electrical properties that are closest to those of the parent copper single crystal. The AL-bonded interface is characterized by an asymmetrically tilted sub-grain boundary with a 2° misorientation and consists of two sets of edge dislocations with mutually perpendicular Burgers vectors. The formation of the AL-bonded interface depends on two critical factors: the creation of ultra-flat bonded surfaces and performing diffusion bonding within the elastic deformation range. Achieving an ultra-flat surface that eliminates residual stress allows for bonding under relatively low pressure. With a moderate bonding temperature and holding time, it is possible to simultaneously avoid the formation of voids and recrystallizations at the bonding interface, thereby achieving a high-quality AL-bonded interface between bulk copper single crystals.

Plastic deformation behavior of Mg–8Li dual phase alloy during multi-directional compression

The Mg–8Li dual phase alloy was deformed by multi-directional compression with different passes. Microstructure evolution of deformed Mg–8Li alloy was analyzed by electron backscatter diffraction. X-ray diffraction profile was used to calculate the dislocation density in α-Mg and β-Li phase to reveal the quantitively change trend under different compression passes. The mechanical properties were assessed through tensile and compressive tests. The Mg–8Li alloy exhibited optimal mechanical properties when compressed with 12 passes (sample 12MDC), with a compression yield strength of 183.1 MPa and a tensile yield strength of 124.2 MPa. The improved mechanical properties in sample 12MDC were attributed to an increase in {10 1‾ 2} twin and {101‾ 3}-{101‾ 2} double twin boundaries, along with the presence of sub-grain boundaries and a reduced grain size in the α-Mg phase. Additionally, the inconsistent results between microhardness and dislocation density calculation manifests the uneven distribution of dislocations in β-Li phase. Broken α-Mg phase brings more phase interface to relieve the strain in β-Li phase, which results to a decreased dislocation density in sample 12MDC (1.19 × 1011/m2). Results show that weaken tension-compression yield asymmetry and more even distribution of dislocation in both phases could be achieved when the compression passes increases to 12.

Portevin-Le Chatelier behavior in AlMgScZr alloys: Effects of Al3(Sc,Zr) dispersoid distribution and grain structure

Plastic instability caused by the Portevin-Le Chatelier (PLC) effect remains a challenge for Al–Mg alloy sheets during shape-forming processes, as it causes unsightly stretcher-strain markings on the surface of final products. In this study, the PLC behavior in annealed AlMg and AlMgScZr alloy sheets with varying Al3(Sc,Zr) dispersoid distributions and grain structures were investigated by combining tensile tests, microstructural characterization, and DIC analysis. The results show that the presence of Al3(Sc,Zr) dispersoids can inhibit the PLC effect, which is manifested by a decrease in serration amplitude, PLC band velocity, and an increase in critical strain. The inhibition effect is shown to be highly related to the distribution of Al3(Sc,Zr) dispersoids and resultant grain structures. The alloy with the densest dispersoid distribution and finest subgrain structure showed the slightest PLC effect. The reduction in serration amplitude and PLC band velocity is ascribed to the augmented resistance to local strain softening, caused by inhibition of dislocation motion by Al3(Sc,Zr) dispersoids and subgrain boundaries. In addition, a high density of Al3(Sc,Zr) dispersoids can strongly trap vacancies, which is responsible for the increase in critical strain.

Microstructure evolution and strengthening mechanism of pulsed current diffusion bonding TiAl/Ti2AlNb joints with in-situ rapid hot deformation

With the development of aero-engine toward high thrust-to-weight ratio, improving the strength of TiAl/Ti2AlNb joint has attracted considerable attention, as it is the key to manufacturing TiAl-Ti2AlNb composite turbine that combines high temperature resistance and weight reduction. However, the TiAl/Ti2AlNb diffusion bonded joint that is currently receiving the most attention is limited in strength by brittle interface structure and interfacial residual stresses, which cannot meet application requirements. In this study, a novel strategy of employing strong pulsed current to promote diffusion bonding of TiAl/Ti2AlNb joints and in-situ rapid hot deformation treatment was applied to improve this situation. This result shows that TiAl and Ti2AlNb alloys can be efficiently joined by pulsed current diffusion bonding (PCDB) with parameters of 950 ℃/10 min/10 MPa, the shear strength of the joint was 258.3 MPa. The subsequent in-situ rapid hot deformation treatment significantly strengthened the TiAl/Ti2AlNb joint, resulting in a shear strength of 443 MPa, which was 171.1 % of the pre-deformation strength. The formation of nanograined microstructure and precipitates acicular O phases after hot deformation-induced phase transformation of the diffusion bonding layers (DBLs) were the main strengthening mechanisms for the deformed joint. The large number of subgrain boundaries and the high dislocation density formed in the DBLs further increase the strength in this region. This study verified an effective of pulsed current diffusion bonding TiAl/Ti2AlNb joint with in-situ rapid hot deformation strengthening, providing a new strategy for obtaining high-strength dissimilar Ti-Al intermetallics joints.

The Role of Dislocation Type in the Thermal Stability of Cellular Structures in Additively Manufactured Austenitic Stainless Steel.

The ultrafine cellular structure promotes the extraordinary mechanical performance of metals manufactured by laser powder-bed-fusion (L-PBF). An in-depth understanding of the mechanisms governing the thermal stability of such structures is crucial for designing reliable L-PBF components for high-temperature applications. Here, characterizations and 3D discrete dislocation dynamics simulations are performed to comprehensively understand the evolution of cellular structures in 316L stainless steel during annealing. The dominance of screw-type dislocation dipoles in the dislocation cells is reported. However, the majority of dislocations in sub-grain boundaries (SGBs) are geometrically necessary dislocations (GNDs) with varying types. The disparity in dislocation types can be attributed to the variation in local stacking fault energy (SFE) arising from chemical heterogeneity. The presence of screw-type dislocations facilitates the unpinning of dislocations from dislocation cells/SGBs, resulting in a high dislocation mobility. In contrast, the migration of SGBs with dominating edge-type GNDs requires collaborative motion of dislocations, leading to a sluggish migration rate and an enhanced thermal stability. This work emphasizes the significant role of dislocation type in the thermal stability of cellular structures. Furthermore, it sheds light on how to locally tune dislocation structures with desired dislocation types by adjusting local chemistry-dependent SFE and heat treatment.

On the role of geometrically necessary dislocations in void formation and growth in response to shock loading conditions in wrought and additively manufactured Ta

This study investigates the role of geometrically necessary dislocations (GNDs) and microstructure on void nucleation and growth in wrought and additively manufactured (AM) tantalum subjected to high-strain rate loading. Multi-modal 3D data was collected using TriBeam tomography to calculate GND densities and their spatial relationship to voids. A microstructural comparison between the wrought and AM samples identified distinct void shapes and locations, with intragranular voids and more spherical voids frequently observed in the AM dataset. Results indicate that voids preferentially form at both high-angle grain boundaries and low-angle subgrain boundaries, the latter of which are frequently observed in the AM material. Through a radial distribution analysis of all voids in the datasets, significant GND localization to near-void-surface regions was observed in both samples. 3D crystal plasticity simulations were employed to extend the experimental observations, revealing higher void growth rates in [111] oriented grains when compared to [001] grains. The simulations also suggest that GNDs can be generated as part of the void growth process, with more GND accumulation for growth in a [111] grain than a [001] grain. These findings provide valuable insights into the links between nanoscale void nucleation, mesoscale void growth, and microstructural effects in dynamically loaded tantalum.