Dislocation Cell Research Articles

Effect of carbon segregation at prior austenite grain boundary on hydrogen-related crack propagation behavior in 3Mn-0.2C martensitic steels

The present study aimed at strengthening prior austenite grain boundary (PAGB) cohesive energy using carbon segregation and investigated the effect of carbon segregation at PAGB on the microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steels. At the low hydrogen content (below 0.2 wt. ppm), the fracture initiation toughness (JIC) and tearing modulus (TR), corresponding to crack growth resistance, were significantly improved by carbon segregation. In contrast, JIC and TR did not change by carbon segregation at the high hydrogen content (above 0.5 wt. ppm). Considering the non-linear relationship between the toughness properties and the PAGB cohesive energy, the experimentally evaluated toughness properties (JIC and TR) and the GB cohesive energy previously calculated by first-principles calculations were semi-quantitatively consistent even at the high hydrogen content. The microstructure observation confirmed that the plastic deformation associated with crack propagation, such as the local ductile fracture of uncracked ligaments and the formation of dislocation cell structures / nano-voids, played an important role in the non-linear relationship between the toughness properties and PAGB cohesive energy.

Investigation of the effects of graphene on Al0.3CoCrFeNi high entropy alloy/3graphene composite materials based on different crystal structures

The influence of graphene layers and crystal structure types on the nanoindentation reinforcement mechanism of Al0.3CoCrFeNi high-entropy alloy/graphene (HEA/Gp) composite materials was studied using molecular dynamics (MD) simulations. The results indicate that the influence of graphene layers on composite materials is related to the position of the indenter. Before the indenter contacts the graphene layer, some dislocations are absorbed by the graphene, unable to form dislocation cells and dislocation locks to hinder the deformation of the matrix, resulting in the hardness of the composite material being lower than that of the HEA matrix. After the indentation contacts the graphene layer, dislocations can penetrate the graphene to form a high-density dislocation cell. Due to graphene's excellent deformation resistance, the composite material's load-bearing capacity is much greater than that of the HEA matrix. In addition, stress propagates along the grain boundaries in polycrystalline and twinned polycrystalline structures. Grain boundaries decrease the material strength, while twinned boundaries can refine grain size and enhance grain stability. Additionally, composite materials' elastic recovery increases first and then decreases with the increase in graphene layers and twinning. The hardness of the material shows an anomalous phenomenon about grain size following a Hall-Petch relationship. When the angle between graphene layers is 0, the stress distribution inside the material is most uniform.

Superior passivation capability for biomedical-grade CoCrMo alloys fabricated by laser powder bed fusion with nitrogen doping

Nitrogen, a non-metallic element, is widely abundant in nature and possesses numerous advantageous properties for metals and alloys, and supersaturated nitrogen content can be obtained through non-equilibrium rapid solidification. In this work, the microstructure and passivation properties of CoCrMo alloys prepared by the laser powder bed fusion (LPBF) technique under nitrogen and argon atmospheres were compared through multi-scale microstructural characterization combined with advanced electrochemical testing methods. Results show that the N content in LPBF N2-CoCrMo alloy (0.1839 wt%) demonstrates more than one order of magnitude higher than that of LPBF Ar-CoCrMo counterparts (0.0046 wt%). Despite comparable sub-microstructures in grain structure and dislocation cells, the LPBF N2-CoCrMo alloy demonstrates a higher corrosion potential of −0.34 VSCE and lower corrosion current density in comparison to the LPBF Ar-CoCrMo in a chloride-containing solution. Moreover, the passive film of LPBF N2-CoCrMo alloys shows a low defect density with a high-content and dense Cr2O3 layer therein. Our research calls attention to alloy design enhancement by modifying protective atmospheres during the additive manufacturing process.

Minor element doping effects on microstructure and mechanical properties of a non-equiatomic FeNiCoCr high-entropy alloy

Through the addition of minor alloying elements, we have designed and prepared a new non-equiatomic six-membered Fe41Ni20Co20Cr10Al5V4 (at. %) high-entropy alloys (HEAs), with the aim of further improving HEA performance. The effect of annealing temperature on HEA microstructure and mechanical properties was systematically studied by using a variety of characterization methods. The results show that HEA Fe41Ni20Co20Cr10Al5V4 is a single-phase face-centered cubic (FCC) solid solution structure. The yield strength HEA Fe41Ni20Co20Cr10Al5V4 doped with minor element doped is 15% higher than that of the base alloy Fe40Ni20Co20Cr20 with a similar grain size while retaining good plasticity (elongation >50%). This can be attributed to the increased lattice distortion induced by the doping elements (i.e., Al and V). In addition, the strengthening mechanisms of Fe41Ni20Co20Cr10Al5V4 HEAs were analyzed. HEA strengthening mechanisms mainly include solid-solution strengthening, grain-boundary strengthening, and dislocation strengthening. Among them, in partially recrystallized HEA, fine-sized dislocation cells formed by the intertwining of high-density dislocations significantly increase material strength. Due to the excellent plasticity (∼ 61%) inherent in HEA as-homogenized, further research can be carried to improve its strength with minimal compromise to plasticity in order to achieve a better balance between strength and ductility.

The formation mechanism of the nano-equiaxed (α+β) duplex phases structure during aging process after high pressure torsion deformation

The aim of this study was to investigate the effects of the nano equiaxed (α+β) duplex phases (NEDP) structure on the mechanical properties of metastable β type titanium alloy. To achieve this, the Ti-3.5Al-5Mo-4 V (Ti-B20) alloy was subjected to high pressure torsion (HPT) deformation and aging treatment. The results demonstrated that the NEDP (α+β) structure exhibited high thermal stability even after prolonged aging. Through systematic transmission electron microscopy (TEM) characterization, it was confirmed that the formation of the NEDP structure was attributed to the generation of crystal defects such as dislocation cells and dislocation pile-ups during HPT deformation. In addition, it was found that the orientation relationship between the α and β phases was [21̅1̅0]α//[111]β after prolonged aging, and the interlocked of coherent interface between the α and β phases allowed the NEDP (α+β) structure to maintain its thermal stability during long-term aging. Therefore, this paper provides valuable guidance for the fabrication of nano-equiaxed dual-phase structures.

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.

Microstructure evolution and surface strengthening behavior of 300 M ultrahigh strength steel under engineered surface treatments

In this work, the most efficient and common engineered surface treatments identified as shot peening (SP), laser shock peening (LSP) and ultrasonic surface rolling process (USRP) were utilized to impact the 300 M ultrahigh strength steel. The microstructure evolution and surface strengthening behavior were carefully characterized and compared for the 300 M steel with various treatments. Besides, the underlying mechanisms for difference in grain refinement and surface mechanical properties by the arranged surface modification are revealed. The results indicate that a gradient structure with nano-grains at the top surface appears on the SP, LSP and USRP treated samples. The process of repeated impacts of SP in different directions leads to activation of multiple slip systems, which thus causes severe refined grains and dense dislocation activities associated with dislocation tangles, dislocation walls and blocking. A great variety of strikes per unit area by USRP results in higher plastic deformation compared to SP where numerous dislocation cells and deformation bands were generated. In contrast to SP and USRP, fewer slip systems are activated in LSP due to the short duration time which thus leads to the thinnest refined grain layer and more planar dislocation structures. The combination of low plastic work and lack of refined grains leads to lowest full width at half maximum (FWHM) in case of LSP samples whereas high FWHM comes from both fine grain size and dislocations in SP. The grain refinement and plastic work contributes to an increase of microhardness and compressive residual stress for the SP, LSP and USRP samples. To be clear, SP leads to a higher magnitude of hardness and compressive residual stress on the topmost surface while LSP causes a larger depth due to that shock wave in LSP influences material to much greater depth. Besides, the decomposition and redistribution of carbides are also noticed on the surface layer after the surface treatments with very high strain rate where numerous dislocations accumulate at the border of the carbides and slip trace is also observed within the carbides.

A comparison of conventional and additively manufactured 316L under thermomechanical fatigue

The present study compares the thermomechanical fatigue (TMF) behaviour of 316L stainless steel manufactured conventionally by hot rolling and additively by laser powder bed fusion (L-PBF). Machined cylindrical specimens were tested under strain-controlled in-phase and out-of-phase TMF loading at a temperature range of 550–750 °C and total mechanical strain amplitudes of εamech = 0.2–0.6 %. While the conventional 316L significantly outperforms the L-PBF 316L under in-phase TMF, their lifetimes are comparable under out-of-phase TMF. Under in-phase TMF, creep damage in the form of intergranular crack networks occurs which is significantly more pronounced for the L-PBF 316L due to the higher amount of interfaces. Under out-of-phase TMF, the damage is mostly due to stress-assisted oxide cracking and the crack propagation is fatigue-dominated. TEM inspection revealed that L-PBF 316L exhibits a cellular dislocation structure in the initial state, which rearranges only slightly during cycling. For conventional 316L similar dislocation cells form during TMF cycling indicating that they represent a stable dislocation arrangement in 316L under TMF loading. This is evidenced by the rather stable cyclic stress response of the L-PBF material when compared to the conventional material.

Mechanical properties of base metal and heat-affected zone in friction-stir-welded AA6061-T6 at ultra-low temperature of 20 K

This study investigates the mechanical properties of a friction-stir-welded (FSW) AA6061-T6 aluminum alloy at ultra-low temperature (ULT) of 20 K. In-situ neutron diffraction and orientation imaging microscopy were employed to compare the tensile deformation behavior of the base metal (BM) and heat-affected zone (HAZ) in the FSW aluminum plate. The results demonstrate that compared to room-temperature (RT), ULT induces a significant improvement in tensile strength and ductility in both the BM and HAZ. The enhanced mechanical properties in BM at ULT result from a more homogeneous deformation than occurs at RT. On the other hand, HAZ at ULT exhibits an even lower yield strength than at RT, but the strain hardening rate (SHR) is the most significant among the alloys, leading to a tensile strength of 346 MPa and the highest ductility of 46.8%. The lowest yield strength corresponds to the lowest-hardness zones in HAZ, caused by dissolved/coarsened precipitates during the FSW process. The highest SHR in HAZ at ULT is attributed to the hardening of the {111} grain families and finer dislocation cell structures. The dislocation densities of both the BM and HAZ increased considerably during tensile deformation at ULT, which accounts for the improvement in tensile strength. Revealing the mechanism behind the remarkable improvement in the mechanical properties of FSW AA6061-T6 at ULT suggests its applicability for cryogenic applications.

Effect of power density on microstructure characterization and fatigue behavior of laser shock peened Rene-80 Ni-based superalloy

The microstructure plays a crucial role in determining the mechanical properties and performance of materials, particularly in high-strength alloys such as Rene-80. This study focuses on the microstructure characterization of Rene-80 nickel-based superalloy after undergoing Laser Shock Peening (LSP), an advanced surface treatment technique imparting beneficial residual stresses and improving fatigue life. The primary objective of this research is to investigate how varying power densities during the LSP process affect the microstructure of the Rene-80 superalloy. The power density in LSP is a critical parameter that influences the intensity of shock waves imparted onto the material’s surface, consequently impacting the microstructure. Through advanced microscopy techniques and analysis, the study explores the resulting microstructural changes, including surface roughness, dislocation density, and the presence of defects like dislocations and dislocation cells. These alterations are vital indicators of the material’s mechanical properties, such as tensile and fatigue strength. LSP samples were characterized using various techniques, including field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) analysis. Additionally, the mechanical performance of the samples was assessed through low-cycle fatigue tests and tensile tests, both conducted before and after LSP treatment. The optimal power density for Laser Shock Peening is determined to be 2.5 HEL. At this power density, the shock wave generated does not create a molten layer on the surface but accumulates defects, resulting in maximum compressive residual stress at the surface. LSP induces intense plastic deformation in the sample, distorting γ’ precipitates. Higher power densities lead to the formation of cross-linked sets of slip bands, highlighting defect slip as the main mechanism of plastic deformation in the LSP process. Understanding these effects is essential for optimizing the LSP process parameters to achieve desired mechanical properties and enhance the performance and longevity of components made from Rene-80 nickel-based superalloy in high-stress applications. It was found that the fatigue life of the samples increased by 500% and optimizing the effect of power density on the LSP process led to an improvement in the fatigue life of the Rene 80 samples.

Strengthening a non-equiatomic quaternary high-entropy alloy via concurrent gradient structures of nanotwin and dislocation cell

One main drawback of FCC high-entropy alloys (HEAs) is their insufficient room temperature yield strength. In this work, concurrent gradient structures of nanotwins and dislocation cells have been introduced into a non-equiatomic quaternary Fe40Ni20Co20Cr20 (at. %) high-entropy alloy by ultrasonic shot peening to meet this challenge. The gradient alloy exhibits a significantly enhanced yield strength, i.e., from the original 204 MPa to 559 MPa, combining with a well-retained ductility of 29 % and a good work hardening ability. Strengthening mechanisms at different depths (5, 30, 200, 400 μm) are also carefully discussed. The current work presents a strategy for hardening FCC HEAs significantly while maintaining the single solid solution microstructure.

Strong-Yet-Ductile Eutectic Alloys Employing Cocoon-Like Nanometer-Sized Dislocation Cells.

Eutectic alloys (EAs) with superior fluidity are known to be the easiest to cast into high-quality ingots, making them the alloys of choice for making large-sized structural parts. However, conventional EAs (CEAs) have never reached strength-ductility combinations on par with the best in other alloy categories. Via thermomechanical processing of cast Ni-32.88wt%Fe-9.53wt%Al CEAs, a cocoon-like nano-meshed (as fine as 26 nm) network of dislocations (CNN-D) is produced via recovery annealing, through the rearrangement of cold-work-accumulated dislocations anchored by dense pre-existing nanoprecipitates. In lieu of traditional plasticity mechanisms, such as TWIP and TRIP, the CNN-D is particularly effective in eutectic lamellae with alternating phases, as it instigates nanometer-spaced planar slip bands that not only dynamically refine the microstructure but also transmit from the FCC (face-centered-cubic) layers into the otherwise brittle B2 layers. These additional mechanisms for strengthening and strain hardening sustain stable tensile flow, resulting in a striking elevation of both strength and ductility to outrank not only all previous CEAs, but also the state of the art-additively manufactured eutectic high-entropy alloys. The CNN-D thus adds a novel microstructural strategy for performance enhancement, especially for compositionally complex alloys that increasingly make use of nanoprecipitates or local chemical order.

Dislocation cells tailored precipitation enhancing mechanical properties of a high Gd content Mg alloy

High Gd content Mg alloys are expected to overcome the poor heat resistance of magnesium (Mg) alloys. However, the low ductility limits their applications. Nano-precipitates is a viable method to enhance the strength without seriously plummeting ductility. In this study, ultrafine and densely arranged dislocation cells, with an average size of 27 nm, were obtained in Mg-11.5Gd-2Y-0.5Zr-0.5Ag-0.5Nd alloy after hot rolling. During the aging process, the dislocation cells facilitated the precipitation of fine and high-density β'. In the peak-aged (225 °C, 15 h) alloy, the volume fraction of β′ could reach 20.3 %, and the average length and thickness of β′ are 10 and 5.9 nm, respectively. The alloy with such a microstructure exhibited a yield strength (YS) of 403 MPa, and an EL of 8.3 % at room temperature. YS of the alloy was still 357 MPa, 310 MPa, and 206 MPa at 200 °C, 250 °C, and 300 °C, respectively. The high YS is attributed to grain boundary, dislocation, solid solution, and precipitation strengthening, among which precipitation strengthening is dominant. Moreover, the nano-β′ could be sheared, thereby coordinating the plastic deformation. Therefore, the present alloy achieved high YS while maintaining good ductility.

Microstructural analysis of cold-drawn medium-carbon low-alloy martensite: effect of deformation and tempering temperature

Microstructure of cold-drawn (≈ 0.9 effective deformation) medium-carbon steel containing Cr and Mo, was investigated in samples tempered at 923 K (tempering at high temperature [HTT]) and 723 K (tempering at intermediate temperature [ITT]) using transmission electron microscopy, X-ray diffraction and microhardness. The study revealed distinct features in lath martensite. HTT samples showed reduced dislocation density and larger precipitates compared to ITT samples. After cold drawing, ITT samples exhibited lamellar dislocation cells and cell blocks, while HTT specimens primarily showed cell block structures. Analysis of precipitates revealed changes in orientation after drawing, as well as alterations in both inter- and intragranular precipitate density. ITT samples also displayed higher post-tempering hardness. This analysis provides quantitative insights into the microstructural evolution and mechanical behaviour after tempering and cold-drawing, which can help optimise processes for achieving ultrafine-grained structures.

Effect of Cyclic Ice Plug Deformation on Microstructure and Mechanical Behaviors of Nuclear-Grade Low-Carbon Tubular Steel.

This study subjected nuclear-grade 20# pipeline steel to cyclic freeze-thaw ice plugging tests, simulating the plastic deformation experienced by pipes during ice plug removal procedures. Subsequently, the dislocation morphology and mechanical properties of the specimens post cyclic ice plugging were examined. The cyclic ice plugging process led to an increase in the dislocation density within the specimens. After 20 and 40 cycles of ice plugging, the internal dislocation structures evolved from individual dislocation lines and dislocation tangles to high-density dislocation walls and dislocation cells. These high-density dislocation walls and cells hindered dislocation motion, giving rise to strain hardening phenomena, thereby resulting in increased strength and hardness of the specimens with an increasing number of ice plugging cycles. In addition, a large stress field was generated around the dislocation buildup, which reduced the pipe material's plastic toughness. The findings elucidate the effects of cyclic ice plugging on the microstructure and properties of nuclear-grade 20# pipeline steel, aiming to provide a theoretical basis for the safe and stable application of ice plugging technology in nuclear piping systems.

Effect of isothermal tempering time and temperature on microstructure and hardness of 3Cr5Mo2SiVN hot-work die steel

In this work, the effect of isothermal tempering time and temperature on microstructure and hardness of 3Cr5Mo2SiVN hot-work die steel was studied. The results indicated that with the increase of isothermal tempering time and temperature, the hardness significantly decreased, which corresponded to the microstructural recovery including the transformation of high angle grain boundaries (HAGBs), the precipitation and coarsening of precipitates, the annihilation and rearrangement of dislocations, the formation and coarsening of sub-grains, and the coarsening and coalescence of laths. The driving force of grain boundary transformation was provided by the dislocation- and heat-induced boundary migration. The increased density of HAGBs was induced by the coalescence of low-angle lath boundaries and low-angle sub-boundaries that continuously absorbed dislocations. In addition, the formation and coarsening of sub-grains were the results of dislocation rearrangement with dislocation cells acting as the core. Moreover, the V-rich M(C, N), Fe-rich M3C, (Fe, Cr)-rich M7C3 and M23C6 formed by the desolvation of alloying elements exhibited an obvious coarsening, decreasing the pinning effect of dislocations and then resulting in the hardness loss. This study offered a systematically revelation into the softening mechanism of hot-work die steel.

The microstructure evolution and strengthening mechanism of fine-grained Al–Mn alloy with nanoscale precipitates during cryorolling

In this work, the fine-grained Al–Mn alloys with nanoscale precipitates was designed, and the texture evolution and strengthening mechanism for fine-grained Al–Mn alloy with nanoscale precipitates during cryorolling were investigated by EBSD and TEM analysis. The results show that the dislocation tangles gradually evolved into dislocation cells, and then into subgrains/ultrafine grains with boosting rolling reduction. The initial grains and precipitates can be significantly refined by cryorolling. Furthermore, the dominating P and M textures in the solution-treated Al–Mn alloy gradually converted to α-fiber and β-fiber textures during cryorolling. The formation of α-fiber and β-fiber textures is beneficial for improving the strength of the Al–Mn alloy. An excellent strength-ductility synchronization can be achieved in the Al–Mn alloy cryorolled at a rolling reduction of 50 %. The strengthening mechanism of Al–Mn alloy subjected to cryorolling is associated with texture strengthening, dislocation strengthening, grain refinement strengthening and precipitation strengthening.

Effect of hydrogen on microstructure evolution and properties of high-strain pipeline steel under cyclic strain loading

The microstructure evolution and the fatigue damage mechanism of hydrogen-charged high-strain pipeline steel during the cyclic strain loading is studied. Hydrogen promotes the formation of dislocation walls and cells, and causes the lattice distortion iron atoms to move to the grain boundary, resulting in grain refinement. The fatigue life is reduced to more than 40 % when the strain amplitude reaches 0.5 % after hydrogen-charged. The main reasons for the reduce are the decrease of the number and the change of propagation mode of secondary cracks, and the increase of cleavage fracture and the decrease of deflection angle in the main cracks.

Revealing the exceptional cryogenic strength-ductility synergy of a solid solution 6063 alloy by in-situ EBSD experiments

A solid solution 6063 aluminium alloy features an exceptional combination of strength and ductility at 77 K. Here, the deformation mechanisms responsible for superior strength-ductility synergy and excellent strain hardening capacity at a cryogenic temperature of the alloy were comparatively investigated by in-situ electron backscatter diffraction (EBSD) observations coupled with transmission electron microscopy (TEM) characterization and fracture morphologies at both 298 and 77 K. It is found that kernel average misorientation (KAM) mappings and quantified KAM in degree suggest a higher proportion of geometrically necessary dislocations (GNDs) at 77 K. The existence of orientation scatter partitions at 77 K implies the activation of multiple slip systems, which is consistent with the results of potential slip systems calculated by Taylor axes. Furthermore, dislocation tangles characterized by brief and curved dislocation cells and abundant small dimples have been observed at 77 K. This temperature-mediated activation of dislocations facilitates the increased dislocations, thus enhancing the strain hardening capacity and ductility of the alloy. This research enriches cryogenic deformation theory and provides valuable insights into the design of high-performance aluminium alloys that are suitable for cryogenic applications.

Effects of Strain Rate on the GND Characteristics of Deformed Polycrystalline Pure Copper

Geometrically necessary dislocations (GNDs) play a pivotal role in polycrystalline plastic deformation, with their characteristics notably affected by strain rate and other factors, but the underlying mechanisms are not well understood yet. We investigate GND characteristics in pure copper polycrystals subjected to tensile deformation at varying strain rates (0.001 s−1, 800 s−1, 1500 s−1, 2500 s−1). EBSD analysis reveals a non-linear increase in global GND density with the strain rate rising, and a similar trend is also observed for local GND densities near the grain boundaries and that in the grain interiors. Furthermore, GND density decreases from the grain boundaries towards the grain interiors and this decline slows down at high strain rates. The origin of these trends is revealed by the connections between the GND characteristics and the behaviors of relevant microstructural components. The increase in grain boundary misorientations at higher strain rates promotes the increase of GND density near the grain boundaries. The denser distribution of dislocation cells, observed previously at high strain rates, is presumed to increase the GND density in the grain interiors and may also contribute to the slower decline in GND density near the grain boundaries. Additionally, grain refinement by higher strain rates also promotes the increase in total GND density. Further, the non-linear variation with respect to the strain rate, as well as the saturation at high strain rates, for grain boundary misorientations and grain sizes align well with the non-linear trend of GND density, consolidating the intimate connections between the characteristics of GNDs and the behaviors of these microstructure components.