Grain-boundary Mechanism Research Articles

Localized corrosion of thermally aged Cu-containing Ni-13Cr-10Fe alloys: The role of Cu enrichment induced grain boundary sensitization

This study reports a new mechanism of grain boundary (GB) sensitization without precipitation of Cr-rich phases at GBs in Cu-containing Ni-Cr-Fe alloys. Copper segregation at GBs during annealing can deplete the boundaries of Cr due to the repulsive interaction between Cr and Cu. The depletion of passivating Cr along GBs can cause intergranular corrosion susceptibility, as evidenced by electrochemical testing and post-mortem morphological characterization. The implications of this new sensitization mechanism on the localized corrosion resistance of Cu-containing commercial alloys are discussed.

Mechanisms of grain boundary α precipitation in the metastable β-titanium Ti-5Al-5Mo-5V-3Cr

In this study, we investigated the early nucleation and growth of grain boundary α during the aging heat treatment of Ti-5553. Through a correlative approach using scanning electron microscopy, electron backscatter diffraction, transmission Kikuchi diffraction, transmission electron microscopy, and atom probe tomography, high-angle β-grain boundaries were investigated in detail to understand the evolution of the precipitation of grain boundary α through nucleation and subsequent growth. It is found that at grain boundaries there is a pronounced nucleation of α precipitates with similar orientations. Growth of these similarly orientated α precipitates, in the later stages, leads to their agglomeration and the formation of grain boundary α. This investigation provides insights into the formation of grain boundary α in its early stages of nucleation and growth.

In-situ EBSD tensile revealing the evolution mechanism of high angle grain boundaries in Al–Zn–Mg alloy profile with heterogeneous structures

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs.

The role of grain size in achieving excellent properties in structural materials

Advanced structural materials are expected to display significantly improved mechanical properties and this may be achieved, at least in part, by refining the grain size to the submicrometer or the nanocrystalline range. This report provides a detailed summary of the role of grain size in the mechanical properties of metals. The effect of grain size on the high temperature behavior and the development of superplasticity is illustrated using deformation mechanism maps and the development of exceptional strength through grain refinement hardening at low temperatures is also discussed. It is shown that the deformation mechanism of grain boundary sliding, as developed theoretically, can be used to effectively predict both the high and low temperature behavior of metals having different grain sizes. This analysis explains the increase in strain rate sensitivity in ultrafine-grained metals with low and moderate melting points and the ability to increase both the strength and ductility of these materials to thereby overcome the strength-ductility paradox. The recent development of hybrid materials is also reviewed and it is demonstrated that, although these hybrids have received only limited attention to date, they provide a potential for making significant advances in the production of new structural materials.

Understanding the coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by comparing with commercial equivalent coercivity magnets

The grain boundary diffusion (GBD) technology effectively utilizes heavy rare earth (HRE) elements to improve the coercivity of sintered Nd-Fe-B magnet. Significantly, clarifying its coercivity improvement mechanism is essential for further optimizing the process and performance. Through comparative analysis with commercial conventional magnets (CSH) processing equivalent coercivity, this study investigated the specific mechanism by which the GBD method enhances the coercivity of HRE elements in an efficient manner. To achieve the same coercivity (∼21.4 kOe), GBD magnets contained only 0.25 wt% Tb, whereas CSH magnets had 1.17 wt% Dy and 1.18 wt% Tb. The microstructure analysis revealed that the Tb element in the GBD magnet was concentrated in the outer layer of the grain, constituting 17.37 wt% of the total rare earth content. While, in CSH magnets, the HRE element in the main phase grains accounted for only 9.58 wt% of the total rare earth elements, including Dy (3.83 wt%) and Tb (5.75 wt%). The characteristic core-shell structural grains in GBD magnet also endowed it with an uneven magnetic domain structure and a more concentrated “abrupt” magnetization reversal. Based on the characterization results the nucleation positions of the two types of magnets were analyzed by fitting Hci/Ms and Ha/Ms. The results can serve as a guide for enhancing coercivity in grain boundary diffusion sintered Nd-Fe-B magnets, aiding in process and property optimization, and advancing the understanding of nucleation type mechanisms.

Breaks in Hall-Petch Relationship in Magnesium

Magnesium and its alloys display a non-usual relationship between flow stress and grain size at room temperature. Breaks in the Hall-Petch relationship have been reported in the literature. Inverse Hall-Petch behavior in which flow stress reduces with grain size decreasing has also been reported in pure magnesium and magnesium alloys with ultrafine and nanocrystalline structures. The present overview discusses these effects in terms of controlling deformation mechanisms. The distinct strength observed in pure magnesium and magnesium alloys with ultrafine grained structure is also discussed. It is shown that experimental data for fine and ultrafine grained magnesium alloys agree with a model suggested recently based on the mechanism of grain boundary sliding. It is also exhibited that the stability of the grain structure might control the strength of ultrafine grained samples.

Dy6Fe13Cu grain boundary restructured Nd-Fe-B permanent magnets

Pursuing high coercivity has long been a focus of the whole Nd-Fe-B community to enable its widespread application in emerging sectors. In the present work, the controlling mechanism of high-coercivity Dy6Fe13Cu grain boundary restructured (GBR) magnets is investigated. Compared to the starting magnet with a coercivity value of 17.2 kOe, the coercivity of the restructured magnet with 3 wt% Dy6Fe13Cu (equals to 1.65 wt% Dy) increases to 22.8 kOe, with the coercivity increment level of 3.39 kOe per unit weight percentage of Dy addition. Results show that low-melting-point Dy6Fe13Cu additive contributes to the formation of continuous intergranular layer isolating ferromagnetic 2:14:1 grains and promotes the infiltration of Dy into the (Nd, Dy)2Fe14B magnetically hardening shell. Both effects enable the more efficient utilization of scarce Dy to enhance the coercivity of bulk Nd-Fe-B magnets. It delights future work for fabricating high-coercivity magnets via the GBR approach.

Clarifying the dominated coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by Pr-based alloys

Low-melting point Pr-based alloys have been confirmed to be cost-effective grain boundary diffusion sources for enhancing the coercivity of Nd-Fe-B magnets. Various coercivity enhancement mechanisms have been reported for these alloys so far, but the dominated one has not been clarified. Here, the sintered Nd-Fe-B magnet was treated by Pr75Al25 alloy grain boundary diffusion at different temperatures. A detailed investigation on the diffusion kinetics of the elements and the microstructure evaluation have been carried out. After 800 °C diffusion, the intrinsic coercivity of the magnet increased from 1070 kA/m to 1348 kA/m without significant reduction of remanence. The increased rare earth-rich phase continuously distributed along the grain boundaries has great contribution to the magnetic decoupling, leading to high coercivity. In comparison, after 900 °C diffusion, although Pr and Al diffused more sufficiently in the magnet, a drastic lattice diffusion occurred and less grain boundary layer was formed, resulting in insufficient coercivity enhancement. The results thus indicate that, for Pr-Al diffusion, the formation of RE-rich layer is more important than that of (Nd,Pr)2Fe14B shell in order to enhance the coercivity. Therefore, different from that for heavy rare earth diffusion, the diffusion treatment for Pr-based sources should be carefully optimized to form continuous grain boundary phase and suppress the excessive lattice diffusion.

M23C6-assisted formation mechanism of grain boundary cellular structure and its role on the brittle fracture in an austenitic Fe-Mn-Al-C lightweight alloy

The austenitic Fe-Mn-Al-C lightweight alloy is limited using its full-hardened state due to its brittleness after long-term aging treatment. This study investigates the formation mechanism of grain boundary precipitates and their effect on crack initiation and propagation. The alloy showed typical age-hardening behavior but exhibited intergranular fracture after a critical aging time. TEM study revealed that the amount of Mn segregation increased and Al decreased at the grain boundary at the early aging stage. M23C6 is first formed at the grain boundary, and α-ferrite is formed at the coherent M23C6/γ interface. Finally, the grain boundary cellular structure of α-ferrite+κ-carbide is formed by the continuous formation of α + κ and growing into their incoherent interfaces with γ. As this cellular structure decorates the grain boundaries, the fracture type changed from ductile to intergranular brittle fracture. The grain boundary cracks initiated at the coarse grain boundary κ-carbide and propagated through the {100} planes of α and κ. The lowest theoretical cohesive strengths in the {100} planes of both phases are suggested as the origin of the crack initiation and propagation.

Atomic insights into the effect of rapid heating pretreatment on the mechanical stability of ⟨100⟩ symmetrical tilt GBs in nanocrystalline materials

Nanograined materials possess ultrahigh strength, while their processing and technological applications are constrained by inherent thermal and mechanical instability. Existing experiments show that the stability of Cu nanograins can be enhanced by performing a rapid heating pretreatment that reduces the grain boundary (GB) energy by changing the GB structure. The variation in the GB structure inevitably affects the migration mechanism of GBs. However, the effect of the pretreatment-induced variation in migration mechanisms on stability remains unclear. Here, the shear deformation of a series of ⟨100⟩ symmetrical tilt GBs after rapid heating pretreatment is systematically investigated by molecular dynamics simulations. The simulations of unheated GBs are also included for comparison. Our results show that the rapid heating pretreatment does not improve the mechanical stability of GBs with tilt angles larger than 36.87° but rather enhances the mechanical stability of those with tilt angles less than 36.87° by the transformation of migration behavior from the normal ⟨110⟩ mode to (i) a ⟨100⟩ mode; (ii) an inhomogeneous mixed one that reconciles the ⟨110⟩ and ⟨100⟩ modes; and (iii) an inhomogeneous ⟨110⟩ mode. The former leads to an increase in the critical shear stress that is required to initiate the migration, whereas the latter two result in a decrease in the migration distance. The variation in the GB migration mechanism is attributed to the change in the GB structure from an ordered kite structure to a disordered one. The research gives an atomic insight into the stabilizing mechanism of nanocrystalline materials with rapid heating pretreatment.

Investigation of the Abnormally Large Grains of a Super304H Heat-Resistant Steel Tube over Long-Term, High-Temperature Service

Abstract Abnormally large grains (ALGs) were detected in the outer walls of Super304H superheater steel tubes of an ultra-supercritical unit in long-term service. In this study, the microstructure and mechanical properties of steel tube samples were investigated to reveal the formation mechanism of the ALGs. The results showed that the room-temperature and high-temperature strength and plasticity of the coarse-grain zone were much lower than those of the fine-grain zone. Inhomogeneous residual strain distribution in the outer wall led to high distortion energy differences in the as-supplied steel tube. The low-distortion austenite grains annex the neighboring high-distortion grains via the strain-induced grain boundary migration mechanism. Moreover, the two-dimensional step mechanism of the faceted grain boundaries also causes the ALGs to form.

Simplified crystal grain boundary engineering of solid electrolyte-infused LiNi0.5Mn1.5O4 cathodes for high cycling stability lithium-ion batteries

Grain boundary has been a key challenge for crystal structure stability of cathode material during cycle process, especially for LiNi0.5Mn1.5O4 (LNMO) for its high working voltage. In previous reports, crystal grain boundary engineering has been proposed as a potential method to improve the stability of grain boundary, but the complex operation and high cost limitted its practical application. Herein, based on previous reports, we proposed a simplified crystal grain boundary engineering under guidance of corrosion simulation test. This method can targeted protect grain boundary by infusing solid-state-electrolyte assisted with molten salt. Meanwhile, part of F− ions from solid-state-electrolyte was doped into the bulk of LNMO during anneal process, which can expand the lattice parameter. The modified samples exhibit superior electrochemical property, especially under harsh environment, such as, at 55 °C the modified sample (LNMO-CG-3%) delivers a discharge capacity of 133 mA h/g under 2C, much larger than that of LNMO (89.5 mA h/g). Furthermore, the action mechanism of crystal grain boundary engineering on improving cycling stability is investigated from aspects of crystal structure stability and charge-discharge reaction process. This paper provides new insights for surface modification, if engineering, it may be a big step forward for the commercialization of LNMO as cathode material in LIBs.

Revealing shear-coupled migration mechanism of a mixed tilt-twist grain boundary at atomic scale

Shear-coupled grain boundary (GB) migration greatly influences the plasticity and creep resistance of nanocrystalline materials. However, the atomistic mechanisms underlying the shear-coupled migration of general mixed tilt-twist GBs (MGBs) remain largely elusive to date. Here, using in-situ high-resolution transmission electron microscopy and molecular dynamics simulations, we uncover the atomic-scale migration behavior of a typical MGB, i.e., 〈001〉{200}/〈01¯1〉{1¯11} GB, during the room-temperature shear deformation of Au nano-bicrystals. Two distinct migration patterns showing the opposite signs of shear-coupling factor were observed and further revealed to be mediated by the motion of GB disconnections with different crystallographic parameters and exhibit different lattice correspondence relations, i.e., 〈001〉{020}-to-〈01¯1〉{200} and 〈001〉{020}-to-〈01¯1〉{111}. Simulation results confirm that the two distinct migration patterns could be activated under different stress/strain states. Moreover, excess GB sliding and GB plane reorientation were found to accommodate the GB migration in both experiments and simulations, likely due to the necessity of establishing a point-to-point lattice correspondence during GB migration. These findings provide atomic-scale experimental evidence on the disconnection-mediated migration of MGBs and elaborate on the hitherto unreported complex shear response of MGBs, which have valuable implications for optimizing the ductility of metallic nanocrystals through controlling GB migration.

Fracture strength and failure mechanism of graphene-containing grain boundaries and pores

Grain boundaries and pores commonly manifest in graphene sheets during experimental preparation. Additionally, pores have been intentionally incorporated into graphene to fulfill specific functions for various applications. However, how does the simultaneous presence of pores and grain boundaries impact the mechanical properties of graphene? This paper establishes uniaxial tension models of single-layer graphene-containing pores and three types of experimentally observed. The effect of interaction between pores and grain boundaries on the fracture strength of graphene was studied respectively for three types of grain boundaries by employing molecular dynamics simulations and considering factors such as pore size, the distance between pores and grain boundaries, and loading angle. A competitive mechanism between the intrinsic strength of pristine graphene with grain boundaries (referred to as pristine GGBs), which varies with the loading angle and the fracture strength of graphene sheets with pores that changes with the size of the pores, governs the fracture strength and failure modes of GGBs with pores. When the former exceeds the latter, the fracture strength of GGBs with pores primarily depends on the size of the pores, and fractures occur at the edges of the pores. Conversely, when the former is lower, the fracture strength of GGBs with pores relies on the loading angle and the distance between pores and grain boundaries, leading to grain boundary rupture. If the two strengths are comparable, the failure modes are influenced by the distance between pores and grain boundaries as well as the loading angle. The findings further elucidate the impact of coexisting grain boundaries and pores on the fracture behavior of graphene, providing valuable guidance for the precise design of graphene-based devices in the future.

Formation mechanism of DKDP surface in single point diamond fly-cutting process and the resulting degradation of laser-induced damage performance.

Laser damage performance of DKDP (KD2xH2(1-x)PO4) crystal is largely determined by the surface microstructures generated in the manufacturing process, more specifically, single point diamond fly-cutting process. However, because of the lack of knowledge about the formation mechanism and damage performance of the microstructures, laser induced damage of DKDP crystal remains a key issue limiting the output energy of the high power laser systems. In this paper, the influence of fly-cutting parameters on the generation of DKDP surface and the underlying material deformation mechanism have been investigated. Except for cracks, two kinds of new microstructures, namely micro grains and ripples, have been found on the processed DKDP surfaces. GIXRD, nano-indentation and nano-scratch test results prove that the micro grains are generated by the slip motion of the crystal, while the simulation results show that the cracks are induced by the tensile stress formed behind the cutting edge. Moreover, the formation of micro grains can facilitate the plastic chip flow through the mechanism of grain boundary sliding, which will further lead to a periodic fluctuation of the chip separation point and the formation of micro ripples. Finally, laser damage test results demonstrate that cracks will degrade the damage performance of DKDP surface significantly, while the formation of micro grains and micro ripples has little impact. The results of this study can deepen the understanding of the formation mechanism of the DKDP surface during the cutting process and provide guidance to improve the laser-induced damage performance of the crystal.

Effect of constrained inter-granular regions on the inverse Hall-petch phenomena

The paper makes an effort to lay out a critical assessment of grain size and its role in limiting strength gains in stable nanocrystalline (NC) metals. In particular, this study uses copper-tantalum binary alloys, in which the predominate mechanisms of grain boundary sliding and rotation are shut down, to decipher the breakdown of classical Hall-Petch behavior and its underlining mechanisms. Through varying grain sizes and tantalum concentrations, the results show Cu–Ta's strength exceeds the traditional strength limits anticipated when strictly applying smaller grain size translates to greater strength than that from the Hall-Petch relationship for NC Cu. Within this work, we also highlight the consistent linear behavior holding up to the point of losing full crystallinity given this alloy's ability to constrain failures occurring within other non-stabilized NC systems.

Achieving high heat-resistant property in dilute Mg-0.2Ce wrought alloy by retarding recrystallization process

Conventional heat-resistant Mg alloys always involve a high concentration of rare-earth solutes to produce amounts of thermostable second phases. In this work, however, the high heat resistance is achieved in a dilute Mg-0.2Ce (wt%) alloy, exhibiting a high ultimate tensile strength of ∼ 181 MPa at 250 ℃, which is superior to those of benchmark Mg alloys including ZK60, AZ80 and other rare-earth element containing Mg alloys. The high-temperature strength of present Mg alloy can be attributed to the suppressed recrystallization process due to the minor Ce addition, since the softening mechanism of grain boundary sliding at high-temperature can be effectively suppressed in the elongated un-recrystallized grains and the texture hardening is always effective in the deformed regions. In addition, the present Mg alloy exhibits excellent thermal stability during the long-term heating on account of the sluggish static recrystallized process. The high temperature strength decreases gradually with the annealing time, which can yet exceed 145 MPa after annealing at 300 ℃ for 6 h. Generally, the present work would provide important insight into developing novel low-alloyed and heat-resistant Mg wrought alloys.

Molecular Dynamics Study on the Effects of Substrate Grain Boundaries on the Adsorption State of Graphene: Implications for Nanoscale Lubrication

Graphene on polycrystalline surfaces, such as stainless steel, exhibits frictional strengthening properties during nanofriction. The adsorption state of graphene is directly related to the nanofriction behavior, and graphene’s adsorption state is affected by substrate grain boundaries. However, the mechanism of the influence of the substrate grain boundaries on the adsorption state of graphene is not clear. Herein, the adsorption states of graphene on different stainless-steel grain boundary surfaces are analyzed, and the mechanism of the substrate grain boundaries regulating the surface graphene bulge is revealed. The notion that graphene bulges are related to the type of substrate grain boundaries has been confirmed. Moreover, we demonstrate that the difference in the crystal surfaces on both sides of the grain boundaries and the gradient of adsorption energy are the key factors regulating the graphene bulge. Graphene bulging can only be caused by the existence of crystal surface differences in the grain boundary structure; therefore, the tilt grain boundaries are unable to cause graphene bulging. In addition, the adsorption energy distribution within the grain boundary will also influence graphene bulging if there is a grain surface difference in the grain boundary structure. When the direction of the adsorption energy gradient is perpendicular to the grain boundary, such a grain boundary structure favors the appearance of graphene bulges. In contrast, graphene bulging is suppressed when the direction of the adsorption energy gradient is parallel to the grain boundary. It provides a theoretical basis for explaining graphene’s nanofriction on polycrystalline substrates such as stainless steel. It is important for the use of graphene for the nano-lubrication of stainless-steel components in microelectromechanical systems.

Formation mechanism of mirror twin grain boundaries in molecular beam epitaxy grown monolayer WSe2–MoSe2 lateral heterojunctions

Mirror twin grain boundary (MTB) defects, being a special type of high-symmetry one-dimensional (1D) defects in two-dimensional atomically thin transition metal dichalcogenides (TMDCs), have received considerable interest due to their unique structures and intriguing 1D properties. However, formation and distribution of MTBs in hybrid TMDC materials such as heterojunction remain scarcely studied. Herein, we investigate the spatial distribution, lattice registry and formation mechanism of MTBs in molecular beam epitaxy grown monolayer WSe2–MoSe2 lateral heterojunctions using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM). MTBs manifest a much higher density in MoSe2 than in WSe2 domains with a few of them spanning coherently across the domain interface. Compositionally, a Mo-dominant rather than W-dominant configuration was observed in those MTBs located in WSe2 domains and its origin can be attributed to the preferable Mo substitution to W along the MTBs occurring at the later MoSe2 growth period. This proposed mechanism is supported by ab-initio density functional theory calculations and substitution dynamics captured by in-situ ADF-STEM. The present study deepens our understanding of MTBs in heterostructured TMDCs, which may also serve as an excellent platform for the exploration of intriguing 1D physics.

Study on the crack nucleation mechanism of Ti-2Al-2.5Zr alloy in low cycle fatigue: Quasi in-situ experiments and crystal plasticity simulation

The fatigue crack nucleation mechanism of Ti-2Al-2.5Zr alloy was investigated using the coupling method of quasi in-situ tests with crystal plasticity (CP) simulation. The constitutive parameters in the CP model were validated by digital image correlation based on scanning electron microscope (SEM-DIC). It was found that the transgranular and intergranular cracks nucleated inside grains and at grain boundaries (GBs), respectively, due to slip intrusions/extrusions along prismatic planes with highest SFpris. values, while quasi transgranular cracks were induced by the adjacent microstructure or cracks. The angle Ω between the sample surface and the slip direction of slip planes where the transgranular cracks nucleated was within the range of 20–55°. Three factors for predicting intergranular cracks were identified, and the slip shearing mechanism of GBs was proposed. The CP results revealed that a significant plastic strain difference was generated at GBs where intergranular cracks nucleated. The band averaging method showed better prediction ability than the grain averaging method, and a unified fatigue indicator parameter (FIP) combining the accumulated plastic strain, the strain incompatibility parameter (SIP), and the abovementioned crack prediction factors, was proposed, which improved the unified prediction ability for both transgranular and intergranular cracks.