Grain Boundary Triple Junctions Research Articles

High temperature embrittlement of Inconel 625 alloy manufactured by laser powder bed fusion

The comprehensive mechanical behavior of Inconel 625 alloy fabricated by laser powder bed fusion (LPBF) and hot isostatic pressing (HIP) was studied. Tensile experiments were performed at six temperatures ranging from room temperature to 1000 °C to explore the effect of temperature on the mechanical properties, and the microstructure and fracture mechanism was investigated by combining electron backscatter diffraction, transmission electron microscopy, and atom probe tomography. In comparison with the properties of conventionally fabricated Inconel 625 alloys, the mechanical properties at room temperature are essentially the same. However, the plasticity of the Inconel 625 alloys fabricated by LPBF is drastically decreased with increasing temperature, especially when the temperature exceeds 700 °C due to the transformation from transgranular ductile fracture to intergranular fracture. Detailed characterization of cracks and grain boundaries (GBs) reveals significant segregation of non-metallic elements at GBs, which impedes the movement of GBs and decreases their strength, thus enhancing the intergranular embrittlement and resulting in the drastic reduction of ductility. Moreover, the irregular grain morphologies result in numerous GBs directional sharp changes and steps, as well as GBs triple junctions, in which the stress concentration is further enlarged, thus promoting the nucleation of voids and cracks and its associated high temperature embrittlement. The underlying mechanism of ductility deterioration of LPBF-fabricated Inconel 625 alloy at elevated temperatures would provide important insights into its performance at high temperatures.

Migration of grain boundary triple junctions in nanocrystalline metals initiated by accumulated dislocations

The evolution of grain boundary (GB) networks in polycrystalline metals depends not only on the movement of the constituent GBs but also on the migration of GB triple junctions (TJs), the latter would have strong influence on the mechanical properties of polycrystals. We investigate the migration mechanism of TJs in face-centered cubic metals using molecular dynamics simulations. The TJ migration is revealed to follow a typical stick-slip behaviour that involves TJ grooving and coordinated GB migration. Upon external shear loading, the TJ experiences a quick migration process, resulting in a grooved TJ configuration with curved GBs at the TJ. The accumulation of dislocations at the TJ facilitates the TJ grooving process, since it compensates the angle gap caused by the inconsistent shear-migration coupling factors between the GBs. After reaching a critical grooving depth, the TJ migration is stagnated, while the connecting GBs begin to migrate via disconnections emitted from the TJ. Based on the mechanism of accumulated dislocation-initiated TJ migration, we propose an energetic model that considers the extra energy barrier for the accumulation of dislocations, to quantify the effect of GB misorientation on TJ migration. These findings deepen our understanding of the physical origin of TJ migration and point out the importance of dislocation accumulation on coordinated GB motion in nanocrystalline metals.

Coexistence of Intermetallic Complexions and Bulk Particles in Grain Boundaries in the ZEK100 Alloy

Magnesium-based alloys are highly sought after in the industry due to their lightweight and reliable strength. However, the hexagonal crystal structure of magnesium results in the mechanical properties’ anisotropy. This anisotropy is effectively addressed by alloying magnesium with elements like zirconium, zinc, and rare earth metals (REM). The addition of these elements promotes rapid seed formation, yielding small grains with a uniform orientation distribution, thereby reducing anisotropy. Despite these benefits, the formation of intermetallic phases (IP) containing Zn, Zr, and REM within the microstructure can be a concern. Some of these IP phases can be exceedingly hard and brittle, thus weakening the material by providing easy pathways for crack propagation along grain boundaries (GBs). This issue becomes particularly significant if intermetallic phases form continuous layers along the entire GB between two neighboring GB triple junctions, a phenomenon known as complete GB wetting. To mitigate the risks associated with complete GB wetting and prevent the weakening of the alloy’s structure, understanding the potential occurrence of a GB wetting phase transition and how to control continuous GB layers of IP phases becomes crucial. In the investigation of a commercial magnesium alloy, ZEK100, the GB wetting phase transition (i.e., between complete and partial GB wetting) was successfully studied and confirmed. Notably, complete GB wetting was observed at temperatures near the liquidus point of the alloy. However, at lower temperatures, a coexistence of a nano-scaled precipitate film and bulk particles with nonzero contact angles within the same GB was observed. This insight into the wetting transition characteristics holds potential to expand the range of applications for the present alloy in the industry. By understanding and controlling GB wetting phenomena, the alloy’s mechanical properties and structural integrity can be enhanced, paving the way for its wider utilization in various industrial applications.

Hydrogen Embrittlement Mechanism of Ultrafine-grained Iron with Different Grain Sizes

To investigate the effect of grain sizes on hydrogen embrittlement of 4N-purity iron, miniature tensile tests were conducted after hydrogen charging for the ultrafine-grained specimens produced by high-pressure torsion and subsequent annealing. Hydrogen embrittlement indexes defined from reduction of area were increased with decreasing grain size, and shear-type fracture occurred with fine dimples on the fracture surface of the diagonally raptured tensile specimen with a smaller grain size. The formation and growth of microvoids at triple junctions of grain boundaries ahead of propagated cracks were responsible for such earlier shear-type fracture because necking between adjacent microvoids more likely and extensively occurred. In the specimens with larger grain sizes or without hydrogen charging, on the other hand, local coalescence and growth of microvoids were predominant due to longer distances between triple junctions, resulting in void coalescence-type fracture with coarser dimple patterns. Therefore, hydrogen atoms introduced by hydrogen charging are considered to enhance the formation of deformation-induced vacancies in ultrafine-grained iron, resulting in shear-type fracture with finer dimple patterns.

Grain Boundary Wetting by the Second Solid Phase: 20 Years of History

Grain boundaries (GBs) can be wetted by a second phase. This phase can be not only liquid (or melted), but it can also be solid. GB wetting can be incomplete (partial) or complete. In the case of incomplete (partial) wetting, the liquid forms in the GB droplets, and the second solid phase forms a chain of (usually lenticular) precipitates. Droplets or precipitates have a non-zero contact angle with the GB. In the case of complete GB wetting, the second phase (liquid or solid) forms in the GB continuous layers between matrix grains. These GB layers completely separate the matrix crystallites from each other. GB wetting by a second solid phase has some important differences from GB wetting by the melt phase. In the latter case, the contact angle always decreases with increasing temperature. If the wetting phase is solid, the contact angle can also increase with increasing temperature. Moreover, the transition from partial to complete wetting can be followed by the opposite transition from complete to partial GB wetting. The GB triple junctions are completely wetted in the broader temperature interval than GBs. Since Phase 2 is also solid, it contains GBs as well. This means that not only can Phase 2 wet the GBs in Phase 1, but the opposite can also occur when Phase 1 can wet the GBs in Phase 2. GB wetting by the second solid phase was observed in the Al-, Mg-, Co-, Ni-, Fe-, Cu-, Zr-, and Ti-based alloys as well as in multicomponent alloys, including high-entropy ones. It can seriously influence various properties of materials.

Transition between a nano-sized prismatic dislocation loop and vacancy cluster in α-iron: An atomic scale study

Plastic deformation and the presence of hydrogen enhance the concentration of vacancy-type defects in α-iron, wherein monovacancies can accumulate as a planar vacancy cluster (VC). The planar VC can then be nucleated into a corresponding prismatic dislocation loop (PDL), which can also be directly generated through cross-slip mechanisms of a screw dislocation. Inversely, a PDL can be converted to a corresponding VC. High densities of VCs and PDLs can significantly influence the mechanical properties of iron-based materials. Here, a quantitative energy barrier is established for the transition between nano-sized 1/2 〈111〉 PDLs and VCs using the nudged elastic band method for different applied strains. It was concluded that a cluster size of more than 19 vacancies can easily nucleate into a PDL if a VC with planar configuration is formed in α-iron. Further, the nano-sized VC to PDL transition is enhanced under compressive strain while the nano-sized PDL to VC transition is enhanced under tensile strain. This result indicates that nano-sized VCs generated by tensile deformation are converted to a PDL after unloading and diffusing away. Inversely, nano-sized PDLs that form by deformation can easily diffuse to high tensile stress regions near stress singularities, such as crack tips and triple-junctions of grain boundaries, and then accumulate as high-density VCs.

The evolution of pit morphology and growth kinetics in aluminum during atmospheric corrosion

Assessing the lifetimes of alloys in humid, corrosive environments requires growth kinetic information regarding individual instances of damage, e.g. pit growth rates. Corrosion rates measured at the continuum scale using mass change convolute the rate of pit nucleation and growth, providing limited information on local kinetics. The current study used in-situ X-ray computed tomography to measure growth rates of individual pits in aluminum over 100 h of exposure in a humid, chloride environment. While pits grew at relatively constant rates over the first hours after nucleation, significant growth-rate nonlinearities subsequently occurred. These were linked to both droplet spreading, which altered the cathode size, and changes in the mode of pit growth. Pit morphology appeared to influence the dominant growth mode and the duration of pit growth. Post-mortem serial sectioning revealed pits preferentially attacked grain-boundary triple junctions and dislocation boundaries.

Mechanisms of the preferential cleavage of Ti2AlNb-based alloy: The influence of grain size and local plasticity

An in-depth understanding on the preferential cleavage of the matrix B2 phase, with an ordered body-centered cubic (BCC) structure, is pivotal to improve the processability of Ti2AlNb-based alloys. This research employed in-situ micro tensile tests, multiscale microstructure characterizations, and crystal plasticity simulations to study the grain-size dependent cleavage of the alloy with a full B2 phase. The material was intentionally prepared with two grain sizes differing by order of magnitude, i.e., the millimeter-level coarse grain (CG) and the micron-level fine grain (FG). The experimental results showed that the cleavage of CGs always occurred on the {100} planes, even when the loading direction was much less favorable for the cleavage of these planes, whereas the cleavage of FGs took place on both {100} and {110} planes. The investigation demonstrated that the plastic deformation takes a critical role in selecting cleavage planes. Full-field crystal plasticity simulations demonstrated that, for the CGs, the plastic deformation associated with cleavage on {100} planes is less than that associated with cleavage on {110} planes, indicating a less plastic deformation-induced blunting effect with cleavage on {100} planes. For the FG material, however, the existence of plentiful grain boundaries (GBs) leads to significantly heterogeneous plastic deformation, local stress distribution deviating from the imposed global stress, and hot spots of stress and strain concentration at GB triple junctions. These local plasticity heterogeneities enable the possibility for cleavage on both the {100} and {110} planes.

Influence of V addition on the mechanical properties of FeCo alloys: a molecular dynamics study

Interatomic potentials for the Fe-Co and Fe-V binary systems have been developed based on an Fe interatomic potential with enhanced mechanical capabilities and merged to describe the Fe-Co-V ternary system based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. Emphasis has been placed on the correct prediction of FeCo elastic properties and antiphase boundary (APB) energies to ensure proper description of the mechanical behaviors. The potentials are then used to analyze the crack nucleation and propagation mechanisms in polycrystalline FeCo and FeCo-2V alloys under different ordering degrees at grain boundaries (GB) and within the grains. The results show that disordering inside grains has a larger impact on the ductility of the binary FeCo. The mobility of the antiphase domains (APD) inside the grains delays the ultimate tensile strength (UTS) and fracture point. The observed fracture procedure shows that triple junctions of GBs serve as the focal points of stresses concentration. The poor mobility at the triple GBs promotes crack nucleation and crack propagation, leading to an intergranular fracture in ordered and disordered binary alloys. The effects of adding 2 at. % V to the FeCo alloy have also been investigated via molecular dynamics (MD) and hybrid grand canonical Monte Carlo-MD (GCMC-MD) simulations. It was shown that V prefers to migrate to (a) the GBs in an ordered state, and (b) GBs and APBs in a disordered state. The ordered FeCo-2V displays a decreased UTS and loss of ductility at room temperature. Introducing V exclusively to the APBs greatly increases the mobility of APD and migration of atoms to GBs boosting the ductility of the alloy.

Selection of crack initiation locations of high cycle fatigue in high-Mn austenitic steel at 77 K

• Triple junction of grain boundaries is the preferential location for fatigue crack initiation. • Crack may initiate at twin boundaries under the shear stress. • Crack initiation of twin boundaries may be slowed down by stress relaxation caused by detwinning. In the present paper, a high-Mn austenitic steel with grain sizes of ∼27 μm was prepared to investigate the different Interactions between individual grain boundaries (IGBs), triple junctions of GBs (TJGBs), and Twin boundaries (TBs) with persistent slip bands (PSBs) at 77 K. TJGBs are the preferential locations for the fatigue crack initiation rather than IGBs and TBs. TBs have the best fatigue cracking resistance, when the PSBs on both sides of TBs are coplanar with TBs, Crack may initiate under the shear stress, but the shear stress acting on the crack propagation direction will be small, making TBs crack difficult to propagate. This process may be slowed down by stress relaxation caused by detwinning.

Disconnection-mediated Twin/Twin-junction migration in FCC metals

We present the results of novel, time-resolved, in situ HRTEM observations, molecular dynamics (MD) simulations, and disconnection theory that elucidate the mechanism by which the motion of grain boundaries (GBs) in polycrystalline materials are coupled through disconnection motion/reactions at/adjacent to GB triple junctions (TJs). We focus on TJs composed of a pair of coherent twin boundaries (CTBs) and a Σ9 GB in copper. As for all GBs, disconnection theory implies that multiple modes/local mechanisms for CTB migration are possible and that the mode selection is affected by the nature of the driving force for migration. While we observe (HRTEM and MD) CTB migration through the motion of pure steps driven by chemical potential jump, other experimental observations (and our simulations) show that stress-driven CTB migration occurs through the motion of disconnections with a non-zero Burgers vector; these are pure-step and twinning-partial CTB migration mechanisms. Our experimental observations and simulations demonstrate that the motion of a GB drags its delimiting TJ and may force the motion of the other GBs meeting at the TJ. Our experiments and simulations focus on two types of TJs composed of a pair of CTBs and a Σ9 GB; a 107° TJ readily migrates while a 70° TJ is immobile (experiment, simulation) in agreement with our disconnection theory even though the intrinsic mobilities of the constituent GBs do not depend on TJ-type. We also demonstrate that disconnections may be formed at TJs (chemical potential jump/stress driven) and at GB/free surface junctions (stress-driven).

Energy model of transformation of two-time decomposition mode of dislocation structure at triple junction: Ultra-fine grained materials

Ultrafine-grained metal materials exhibit strong mechanical properties, but often have poor plasticity. Since the volume fraction of the triple junctions of the grain boundaries in the material is very high, the dislocation at triple-junction of grain boundaries can play a key role in excellent performance of the material, such as high strength and good plasticity in many cases. In this paper, based on one-time decomposition mode of the dislocation pile-up at the triple junction of grain boundaries proposed by Fedorov et al. [Acta Materialia, 51 (2003)887–898], a new energy model is proposed for the two-time decomposition mode of the dislocation pile-up at the triple junction. With this model, we explore the effect of the critical conditions on the decomposition mode of the second dislocation and their transformation between two modes and get the criterion for the transformation of two modes, such as the magnitude of stress, the angle of triple junction and the distance of the decomposed dislocations moving. The transformation of the different decomposition modes of the second dislocation is related to the displacement distance of the dislocations decomposed from the first dislocation of the dislocation pile-up. The transformation criterion of the different decomposition mode of the second dislocation, the strengthening and weakening of the triple junction of the grain boundaries and the dislocation mechanism of superplasticity in plastic deformation processes under loading can be used to analyze semi-quantitatively.

Effect of recrystallization annealing on microstructure and tensile properties of Inconel 617B alloy cold rolled pipes

The effects of recrystallization annealing on the microstructure evolution of Inconel 617B alloy pipes and its tensile behavior at 750 °C were investigated. As the annealing temperature increases, the grain size and grain growth rate increase dramatically due to the dissolution of the precipitated particles and the enhanced diffusion of solute atoms. Oil-quenched (OQ) samples generated evenly dispersed Cr-rich carbides with stacking faults (SFs) and Lomer-Cottrell (L-C) locks near the carbides, whereas water-quenched (WQ) samples had comparatively clean grain boundaries and matrix. The interaction of Cr-rich phases and dislocations in the oil-quenched sample resulted in the strengthening of the alloy. Besides, because the precipitation of the Cr-rich phase led to the reduction of localized stacking fault energy (SFE), the alloy could coordinate the deformation by forming nano-deformation twins, which significantly improved the tensile ductility and allowed the alloy to have a larger work hardening index (n). At 1180 °C, holding 4 min, then oil quenched annealing conditions, the sample achieved the excellent strength-elongation combination, i.e., the ultimate tensile strength (UTS) of 445 MPa and the elongation (EI) of 47.8% under tensile loading at 750 °C, since the appropriate grain size cooperated with the precipitated phase. The stress concentration caused by the Cr-rich phase in the grain leads to cavitation nucleation. However, the cavitation nucleation at the triple junction of grain boundaries caused by the precipitation phase and grain boundary slips in the samples with other annealing conditions was still the foremost responsible for the fracture.

Precipitation Evolution in the Austenitic Heat-Resistant Steel HR3C upon Creep at 700 °C and 750 °C.

HR3C (25Cr-20Ni-Nb-N) is a key material used in heat exchangers in supercritical power plants. Its creep properties and microstructural evolution has been extensively studied at or below 650 °C. The precipitation evolution in HR3C steel after creep rupture at elevated temperatures of 700 °C and 750 °C with a stress range of 70~180 MPa is characterized in this paper. The threshold strength at 700 °C and 750 °C were determined by extrapolation method to be 57.1 MPa and 37.5 MPa, respectively. A corresponding microstructure investigation indicated that the main precipitates precipitated during creep exposure are Z-phase (NbCrN), M23C6, and σ phase. The dense Z-phase precipitated dispersively in the austenite matrix along dislocation lines, and remained stable (both size and fraction) during long-term creep exposure. M23C6 preferentially precipitated at grain boundaries, and coarsening was observed in all creep specimens with some continuous precipitation of granular M23C6 in the matrix. The brittle σ phase formed during a relatively long-term creep, whose size and fraction increased significantly at high temperature. Moreover, the σ phases, grown and connected to form a large “island” at triple junctions of grain boundaries, appear to serve as nucleation sites for high stress concentration and creep cavities, weakening the grain boundary strength and increasing the sensitivity to intergranular fracture.

Comparison of edge cracking and tensile cracking in in-situ deformation at 150 °C of Mg-2Zn-1.5Mn alloy sheet

The Mg-2Zn-1.5Mn alloy (ZM21) sheets were hot-rolled in a single pass at 150 °C with different reductions through on-line heating rolling. Microstructures and edge cracking behavior of the rolled sheets were investigated. The in-situ tensile tests at 150 °C were also carried out, the crack initiation and propagation were compared with the edge cracking behavior of ZM21 sheets prepared by on-line heating rolling. The results reveal that the edge cracks are most likely to originate in the rolling direction and normal direction (RD-ND) plane due to the secondary tensile stress along RD. Edge cracking becomes more severe with an increasing reduction. The edge cracks mainly initiate and propagate in the fine recrystallized grain areas and the junction of recrystallized grains and sub-grains with hard orientation. The in-situ tensile test indicates that micro-cracks mainly initiate at the triple junction of grain boundaries where grains have hard orientation with low basal Schmid factor (SF). Meanwhile, those cracks are more likely to propagate along the grain boundaries with maximum difference in basal Schmid factor. Besides, the crack initiation and propagation during the in-situ tensile deformation at 150 °C are found not to be associated with the recrystallization.

Boosting thermoelectric performance of BayCo4Sb12 by interlinking large-aspect-ratio silver nanowires at the triple junction of grain boundaries

Decoupling electrons and phonons transport has been a major challenge for enhancing thermoelectric (TE) performance. Incorporation of guest nano-inclusions into host phases has been proven effective for enhanced phonon scattering but induces additional electron scattering, resulting in limited effect in the improvement of TE performance. Herein, one-dimensional high-conducting silver nanowires (AgNWs) with a large aspect ratio have been uniformly incorporated into Ba0.2Co4Sb12 matrix. The AgNWs become Y shaped at the triple junctions of the grain boundaries, which create more hetero-interfaces for tuning the TE properties. The semi-coherent hetero-interfaces minimized the electron scattering, and the large-aspect-ratio AgNWs facilitate their interlinks, thus remarkably promoting the carrier mobility due to the reduction of boundary potential barrier. Meanwhile, the AgNWs could form nano-meso-micro-scale defects on random cross-sectional planes due to random penetration angles, bringing in full-wavelength phonon scattering. In a combined effect, a peak zT of 1.24 was achieved, enhanced by ∼43%. The corresponding TE module exhibited a high conversion efficiency of ∼8%, among the highest ever reported. This work demonstrated an effective strategy for enhancing TE performance by incorporating one-dimensional large aspect ratio conducting nanowires into the host phases with simultaneous reduction of thermal conductivity and enhancement of electron mobility.

Unveiling the role of glassy nanodomains in strength and plasticity of crystal–glass nanocomposites via atomistic simulation

Crystalline metals and alloys are usually ductile owing to lattice dislocations and various slip systems, while bulk metallic glasses show ultrahigh yield strength with very limited plasticity. Combining the crystalline and glassy phases in one alloy has recently been shown to be promising for achieving both ultrahigh strength and good deformability. Yet, it is challenging to capture the dynamic dislocation behavior through the deformation process and elucidate the role of glassy domains on the excellent mechanical performance of the nanocomposites. Here, we unveil and visualize the atomic-scale interactions among dislocations, glassy nanodomains, and crystal–glass interfaces in a specially designed configuration via molecular dynamics simulation. The glassy nanodomains occupying the triple junctions of grain boundaries are found to optimize the dynamic partitioning of shear strains between the two phases, thus manipulating the production of both dislocations in the crystalline matrix and shear transformation zones in the glassy nanodomains. The crystal–glass interfaces where strain concentration can occur function as both dislocation sources and sinks for plasticity, which in turn alter the strain distributions in the two phases. Systematic observations further suggest that the glassy nanodomains can dynamically tune the dislocation content and configuration in the crystalline matrix throughout the deformation. The unveiled mechanisms thus open a pathway for the development of novel ultrahigh-strength and ductile materials by tuning dislocation behavior in the crystalline matrix via glassy nanodomains.

Disconnection-Mediated Twin/Twin-Junction Migration in FCC Metals

We present the results of novel, time-resolved, in situ HRTEM observations, molecular dynamics (MD) simulations, and disconnection theory that elucidate the mechanism by which the motion of grain boundaries (GBs) in polycrystalline materials are coupled through disconnection motion/reactions at/adjacent to GB triple junctions (TJs). We focus on TJs composed of a pair of coherent twin boundaries (CTBs) and a {\Sigma}9 GB. As for all GBs, disconnection theory implies that multiple modes/local mechanisms for CTB migration are possible and that the mode selection is affected by the nature of the driving force for migration. While we observe (HRTEM and MD) CTB migration through the motion of pure steps driven by chemical potential jump, other experimental observations (and our simulations) show that stress-driven CTB migration occurs through the motion of disconnections with a non-zero Burgers vector; these are pure-step and twinning-partial CTB migration mechanisms. Our experimental observations and simulations demonstrate that the motion of a GB drags its delimiting TJ and may force the motion of the other GBs meeting at the TJ. Our experiments and simulations focus on two types of TJs composed of a pair of CTBs and a {\Sigma}9 GB; a 107{\deg} TJ readily migrates while a 70{\deg} TJ is immobile (experiment, simulation) in agreement with our disconnection theory even though the intrinsic mobilities of the constituent GBs do not depend on TJ-type. We also demonstrate that disconnections may be formed at TJs (chemical potential jump/stress driven) and at GB/free surface junctions (stress-driven).

Insight into low-temperature sintering of samarium-doped ceria mixed with scavenging lithium

The refractory nature of oxide-ion conducting Sm-doped CeO2 (SDC) has hindered the development of SDC devices. Lithium has been found to be an excellent sintering aid that enables sintering at temperatures as low as 900 °C and also increases grain boundary conductivity. Even though Li–Si oxides melt only above the sintering temperature in reality, it is generally believed that these benefits are given by virtue of a formation of liquid phases with silicon contaminants. In this study, the key element for low-temperature sintering was revealed to be aluminum, which is spontaneously introduced into SDC by a covert but intense chemical reaction between the mixed lithium and the alumina crucible. The results show that 1 wt% lithium addition is responsible for the unintended introduction of 32 mol% aluminum. Computational thermodynamic calculations for a Li–Si–Al oxide system clarified that aluminum introduction is essential to reduce the melting point below the sintering temperature. A nano-scale analysis clearly indicates the segregation of aluminum at SDC grain-boundary triple junctions, like LiAlO2, while silicon appears to migrate towards the SDC-SDC grain boundaries as amorphous SiO2. This suggests that the decomposition of the Li–Si–Al liquid oxide filling the grain boundaries is associated with the reflow of silicon and the loss of lithium as sintering proceeds. It was shown that zirconium is also introduced into SDC due to ball-mill processing, and that it potentially forms Li–Si–Zr oxides which remain in their solid state during sintering. The results of this study highlight the importance of taking the impact of unintended aluminum introduction into consideration for a variety of ceramics mixed with even minute quantities of lithium.

Creep damage characteristics and evolution of HR3C austenitic steel during long term creep

HR3C austenitic heat resistant steel has been gradually applied as tubes to manufacture components of superheaters and reheaters in ultra-supercritical (USC) power plants. Creep tests were conducted at 650 °C under the applied stresses in the range of 150 MPa–250 MPa. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were employed to investigate the creep damage characteristics and evolution. The results show that intergranular microcracks and dense creep cavities along grain boundaries are the major features of creep damage. Different states of creep damage characteristic distribution are related to the applied stress and creep duration at high temperature. The wedge-cracking is prone to take place at the triple junctions of grain boundaries, and creep cavities are nucleated preferentially at M23C6 precipitates along grain boundaries. The nucleation and growth of creep cavities and wedge-cracks have a crystallographic orientation relationship with the neighboring austenite grains. The adjacent grains have a large difference of Schmid factor, and one of them is close to <1 1 1>//the stress axis orientation, which has greater resistance to deformation. The Orientation Deviation (KAM) obtained from EBSD analysis implies that the creep damage is obviously affected by the stress level. Microcracks under high stress are mainly attributed to the local deformation, and dense creep cavities under low stress are dominated by time-dependent diffusion. Under the intermediate stress level, local deformation and time-dependent diffusion act together, leading to the most serious damage.