Angle Grain Boundaries Research Articles

Development and morphological analysis of the zone refining process for high purity germanium

A highly efficient purification process has been developed to enhance the purity of germanium (Ge) up to 13 N from an initial 4N purity. Hall effect measurements demonstrated that part of the zone refined Ge (ZRGe) bars exhibited p-type conductivity and high purity with a net charge carrier concentration of around 4–9 × 1010 cm−3. Noteworthy observations include a rise in carrier concentration (1011 to 1012 cm−3) at a very low zone refining (ZR) velocity of 1 mm/min, which is attributed to the diffusion of impurities from the container into the Ge melt. The purification efficiency was enhanced by optimizing the ZR velocity to around 4–5 mm/min. Purification was carried out at different process steps using graphite and quartz containers. Subsequent microstructural examinations, conducted in conjunction with wet-chemical etching of the sample unveiled the presence of various grains, grain boundaries (GBs), and low & high-density of dislocation clusters: These observations were made in samples that had undergone zone refining under various gas atmospheres including argon, nitrogen and hydrogen. Electron back scattered diffraction (EBSD) analysis reveals the characteristics of multiple grains and grain boundaries (GBs) of Ge refined in argon and nitrogen atmosphere, including low-angle GBs, high-angle GBs with coincide site lattice (CSL) Σ3, Σ9, Σ27a, and random angle GBs. In contrast, the EBSD results of Ge refined in hydrogen atmosphere shows larger grains with only a few low-angle grain boundaries. The materials refined in argon and nitrogen show a lower carrier lifetime (10–30 μs) owing to the presence of various high angle grains boundaries as well as low purity. Conversely, zone refining in hydrogen resulted in the carrier lifetime of around 50 μs in polycrystalline Ge wafers with larger grains.

Study on the Plastic Deformation Mechanism of Void‐Containing Twin‐Crystal Magnesium with Symmetric Tilt Grain Boundary Angles

AbstractA twin‐crystal magnesium (Mg) model with 9 different symmetric tilt grain boundary (STGB) angles (20°–80°) with preset nanohole defects is established by atomsk and lammps software. The mechanical behavior of grain boundary (GB) angles on twin‐crystal Mg with cavity defects and its cavity evolution are simulated by the molecular dynamics method with embedded atomic potential, and the plastic deformation mechanism is revealed. The results show that the yield stress decreases with the increase of the STGB Angle during the stretching process. When the GB Angle increases from 20° to 80°, the yield stress decreases from 2.28 to 1.42 Gpa. This is because the larger the STGB Angle is, the larger the Schmidt factor is, and the easier it is to start dislocation slip during the stretching process. On the other hand, the larger the Angle of STGB, the more the number of atomic voids at the interface, and the more the number of dislocation nucleation points. The larger the Angle of STGB, the lower the strength of twinning Mg but the better the plasticity to avoid fracture. The plastic deformation mechanism mainly includes the nucleation of Shockley incomplete dislocation at STGBs, dislocation slip generates stacking faults (SFs), GB migration, and base plane dislocations.

Mechanism of grain boundary angle on solidification cracking in directed energy deposition Hastelloy X superalloys

Solidification cracking occurs only when the grain boundary (GB) angle (θ) exceeds a critical value. This value, known as the critical cracked GB angle (θ*), can be predicted from the grain coalescence theory based on GB-angle-dependent GB energy. However, the calculated value (θc*) is always less than the measured value in experiments (θe*), which is also confirmed in our directed energy deposition Hastelloy X superalloys. In addition to GB energy, there are evidences showing that GB angle can affect cracking by changing dendrite spacings. We show by experiments and phase field simulations that, same as GB energy, the dendrite spacings at GBs increase with GB angle, but its effect on solidification cracking sensitivity (SCS) is opposite to GB energy. Depending on their relative contributions, three ranges can be identified. In the first range of θ<θc*, both dendrite spacings and GB energy have negligible effects on dendrite coalescence, compared to the case inside a grain. In the second range of θc*<θ<θe*, dendrite spacings counteract the effect of high GB energy on SCS. It is exactly this effect that induces the gap in θ* between theory and experiments. In the third range of θ>θe*, GB energy plays a dominant role and leads to severe solidification cracking. After including the effect of dendrite spacings on SCS, we predict θ*=15° in directed energy deposition Hastelloy X superalloys, close to the experimental value of 18°. These new findings provide new insights for suppressing cracking by controlling the dendrite spacings near GBs.

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. For the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

Molecular dynamics simulations of interaction of cascade damage with self-interstitial atom-loaded grain boundaries in tungsten

In this study, we conducted systematic molecular dynamics simulations to investigate the radiation resistance of grain boundaries (GBs) in tungsten with/without self-interstitial atoms (SIAs) loaded on them. The simulation results show that the presence of SIAs can extend the width of the Σ3<110>{112} twin GB compared to the SIA-free GB. When cascade damage occurs near Σ3, the presence of SIAs at Σ3 enhances radiation resistance. Specifically, it inhibits producing residual defects, particularly vacancies and prevents their aggregating into clusters. The radiation-resistant performance is most excellent when the SIA concentration at Σ3 is approximately 5 %. The influence of temperature and primary knock-on atom energy on radiation damage is also discussed. However, SIA-loaded high angle GBs and low angle GBs do not significantly enhance their radiation-resistant performance compared to their SIA-free counterparts. This study provides valuable insights into the radiation damage behaviors near GBs and contributes to understanding their radiation-resistant mechanisms.

How are microstructural defect interactions linked to simultaneous intergranular and transgranular fracture modes in polycrystalline systems?

The major objective of this investigation is to fundamentally understand and predict how intergranular (IG) and transgranular (TG) fracture modes nucleate and propagate in f.c.c. polycrystalline systems due to defects, such as total and partial dislocation densities and grain boundary (GB) structures and misorientations. A dislocation density crystalline plasticity (DCP) formulation based on the evolution and interaction of total and partial dislocation densities was integrated with a recently developed fracture approach to investigate the fracture nucleation and propagation of simultaneous multiple fracture events, including both IG and TG fracture events. The aggregate grains and GB orientations and morphologies are based on EBSD measurements that are representative of polycrystalline copper aggregates with a broad range of random high angle and low angle GBs. The predictions indicate that dislocation density pileups induce IG fracture due to interrelated stress, slip, and total and partial dislocation density accumulations and interactions for both high angle GBs (HAGBs) and low angle GBs (LAGBs). TG fracture nucleation and propagation occurred due to normal stress accumulations, which exceeds the fracture stress, along cleavage planes. Furthermore, it is shown how IG cracks transition from the GB plane to the cleavage planes as cracks nucleate and propagate. Accumulated plastic zones due to different slip system activities can impede and blunt crack propagation and fronts. These plastic zones form due to high Lomer and Shockley partial dislocation densities. These predictions, which are consistent with experimental observations, provide a fundamental understanding of how simultaneous failure modes initiate and propagate for physically representative microstructures.

Analytical energy formalism and kinetic effects of grain boundaries: A case study of graphene

Grain boundaries (GBs), inherent in polycrystalline materials, manifest a diverse array of features that substantially affect material properties. However, the incomplete knowledge of the relevance between structures and energetics of GBs impedes the understanding of their effects. Here, taking graphene as an example, we propose an analytical energy formula for GBs in grain-boundary angle space. Our study reveals that any given GB can be characterized by a geometric combination of symmetric GBs, adhering to the principle of uniformly distributing their dislocation cores along straight trajectories. The formation probability of GBs, as predicted by our theoretical derivation, aligns well with both high-throughput calculations and experimental statistics. Furthermore, we unveil the elusive kinetic effects on GBs by contrasting experimental statistics with energy-dependent thermodynamic effects. This study not only presents a robust model to describe energetically favorable GBs in graphene, offering insight into the formation of GBs in two-dimensional materials, but also reveals the kinetic effects of GBs in material synthesizing process.

Study on Austenite Grain Growth Behavior of GCr15 Bearing Steel

The effects of austenitizing temperature and holding time on the austenite grain growth behavior of GCr15 bearing steel are reflected in this paper. SEM and EBSD deeply reveal the effect of austenite grain size on the misorientation angle of grain boundary. The results show that both the holding time and the austenitizing temperature will make the austenite grain size grow to a certain extent. However, the change of temperature will cause a greater change in grain size. The grain size will also have different growth trends in different austenitizing temperature ranges. The grain size of Gcr15 test steel is combined with the Sellars model to derive its growth kinetics model.

Phase-field simulation of sintering process of ceramic composite fuel

Due to the limitation of existing experimental techniques, it is difficult to observe the evolution of microstructure in the sintering process in real time, resulting in a lack of in-depth understanding of the sintering mechanism of two-phase composite fuels. Therefore, it is greatly important to carry out theoretical simulation studies in the sintering process of composite fuels. In this work, a phase-field model of the two-phase sintering process of ceramic composite fuel is established, and the sintering process of UN-U&lt;sub&gt;3&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt; composite fuel is simulated by using this method. The simulation results show that during the formation of sintering neck, the surface deformation of the grains with higher surface energy is significant. The size of the final equilibrium dihedral angle formed by the two-phase double grains depends on the ratio of the grain boundary energy to the surface energy of the two phases. The phenomenon of large grains swallowing small grains does not occur between the two unequal double grains. Subsequently, the pore shrinkage and the properties of the trident grain boundary among the two-phase three grains are investigated in the sintering process. It is found that the angle of the trident grain boundary formed by the two-phase three grains deviates from 120°. The high-energy barrier at the grain boundary hinders the diffusion of the pore vacancies along the grain boundary, resulting in a slow shrinkage rate of the pore vacancies at the trident grain boundary. In addition, the simulation results of the microstructure evolution of two-phase polycrystalline sintered tissue with different volume fraction ratios show that the grain boundary diffusion plays a major role in the two-phase sintering process. The grain growth of the phase with a higher volume fraction is dominant, and there exists a hindrance to the migration of grain boundaries between two-phase grains. The phenomenon of grain migration exists between grains of the same phase.

Revealing the room temperature superplasticity in bulk recrystallized molybdenum

Body-centered cubic refractory metallic materials exhibit excellent high-temperature strength, but often suffer from brittle intergranular fracture due to the recrystallization-induced enrichment of trace elements at grain boundaries (GBs). Here, we report a fully-recrystallized pure molybdenum (Mo) material with room temperature (RT) superplasticity, fabricated by a facile method of powder metallurgy, Y-type hot rolling and annealing. By engineering the ultralow concentration of O at GBs, the inherent GB brittleness of Mo can be largely eliminated, which, in conjunction with high fractions of soft texture and low angle GBs, enables a significant development of ordered dislocation networks and the effective dislocation transmission across low angle GBs. Synergy of these factors greatly suppress the brittle intergranular fracture of Mo, contributing to an enhanced deformability of 108.7% at RT. These findings should have general implication for fabricating a broad class of refractory metals and alloys toward harsh applications.

On the co-nucleation of adjoining twin pairs at grain boundaries in hexagonal close-packed materials

This work addresses the formation of adjoining twin pairs (ATPs) at grain boundaries (GBs) in hexagonal close-packed (hcp) metals from the perspective of the co-nucleation (CN) of pairs of deformation twins. A continuum defect mechanics model is proposed to investigate the energetic feasibility of the CN of ATPs resulting from the dissociation of GB dislocations. Application of the model to a set of GBs in Mg reveals that CN is largely preferred over the nucleation of a single twin variant for low misorientation angle GBs, which is consistent with previous experimental analyses. The CN events are subsequently analyzed considering the GB character, and the twin system alignment (m′) and Schmid factors to obtain better insights into the conditions governing the formation of ATPs via CN. The likelihood of CN even at low m′ values reveals that CN events could be responsible for the formation of ATP pairs in these scenarios.

The effects of grain boundary structure and bulk Cr content on grain boundary degradation behavior of Ni-Cr binary alloys in high temperature CO2

The effects of grain boundary (GB) structure and bulk Cr content (from 15 wt.% to 30 wt.%) on GB oxidation behavior of Ni-Cr binary alloys were investigated in 650°C/1atm CO2 gas. The degradation behaviors of GBs, which varies greatly (including preferential intergranular oxidation, general intergranular oxidation and GB passivation), is dictated by the available Cr source which depends on the bulk Cr content and the Cr diffusion flux to the reaction front. The Cr diffusivity along GB is dependent on GB structure and decreases in the order of random high angle GB (RHAB), low angle GB (LAGB), incoherent twin boundary (ITB) and coherent twin boundary (CTB). Interestingly, other coincident site lattice boundary (CSLB (Σ>3)) shows a Cr diffusivity between ITB and CTB. More importantly, the threshold bulk Cr content for achieving GB passivation is also closely related to GB structure. To be specific, LAGB completely passivates when the bulk Cr content is between 20 wt.% and 25 wt.%, while ITB completely passivates at a bulk Cr content of 25 wt.% and CSLB (Σ>3) completely passivates when the bulk Cr content is between 25 wt.% and 30 wt.%. In contrast, CTB is subject to general intergranular oxidation while RHAB shows passivation regardless of the bulk Cr content.

Micro-mechanical behavior of lamellar structured Ti6Al4V alloy upon compression via experimental and crystal plasticity study

The micro-mechanical behavior of lamellar structured Ti6Al4V alloy to a small compressional strain of 4.5% was studied. Strain and stress intends to localize along grain boundaries (GBs), triple points (TPs) as well as grain interiors (GIs). The maximum strain and stress is found at some GB-TPs with soft and grain clustering mostly owing to their strong strain incompatibility. Their heterogeneous distribution is highly related to individual crystallographic orientation, their size, shape and grain-grain interactions. The local dislocation behavior forms ledges along with Geometrically necessary dislocations (GNDs) to accomodate strain incompatibility. EBSD and crystal plasticity (CP) simulation disclosed some long range meso-bands (about 45° to loading direction) which crosse multi-grains and some mini-bands within grain interiors (GIs) reflecting a concentrated strain behavior and ductile nature of the Ti6Al4V material. Prismatic slip dominates Ti6Al4V lattice (mainly from surface dislocation activation) determined by its low critical resolved shear stress (CRSS) which is away from some findings of literature. Dislocation is found commenced from some high angle GBs (HAGBs) and extened into GI which can pass through GBs of both low (LA) and high angles given their slip plane coherency. The substructures (α-lamellae) separated by LAGBs within α-colonies, have no significant effect to dislocation movement. Fatigue failure can be inferred from the early stage micro-mechanical behavior through accumulated plastic strain and strong strain incompatability. Fractography reaveals fatigue failure from surface, sub-surface, shearing, cleavage and intergranular modes are presumed highly related to surface defects, compressional residual stress gradient, easy prismatic slip and soft and hard GBs. Multiple crack origins from such combinations are deemed for reduced fatigue life of shot peened samples regardless of the surface compressional residual stress being generated.

Enhanced carbon diffusion efficiency and work hardening effect of AISI 9310 steel via pre-laser shock peening

By using pre-laser shock peening (pre-LSP), low temperature carburizing (LC) is successfully applied in AISI 9310 steel to obtain a carburized layer that is approximately 34 μm thickness, which is thicker than the carburized layer achieved by LC alone of about 10 μm. Pre-LSP enhances diffusion mechanism including two factors: structural and kinetic. Structurally, LSP induces density dislocation such as dislocation walls (DWs) that evolve into grain boundaries (GBs), thereby providing paths for accelerating carbon atom diffusion. This transformation results in an increase in Kernel average misorientation (KAM), high angle GBs (HAGBs), and deformed grains. Kinetically, LSP lowers activation energy, making it easier for carbon to diffuse in BCC ferrite. Furthermore, the LSP-LCed sample exhibits a work hardening depth of 400 μm, which is significantly higher than the 60 μm work hardening depth of LCed sample. The main composition of carburized layer is hard Fe3C phase, which exists mostly along α-Fe GBs. The compound work hardening effect observed in the LSP-LCed sample is attributed to plastic deformation induced by pre-LSP and the formation of hard Fe3C phase via LC. This study presents a new method for strengthening contact component with interfacial damage requirements, such as wear and contact fatigue resistance.

Synergistic improvement of strength and ductility via doping cerium into PH13–8Mo stainless steel by laser powder bed fusion

In this study, the PH13–8Mo stainless steel parts doped without and with cerium (Ce) were fabricated via laser powder bed fusion followed by post-heat treatment, and systematically compared in terms of microstructure, phase constituent, and tensile properties. The comparative results show that doping Ce-modified grains with the equiaxed morphology and finer size, increased the mechanical stability of austenite, and enhanced the sphericity of oxide inclusion in the resultant PH13–8Mo. Additionally, the coherency between the newly-formed CeAlO3 inclusion and matrix was effectively improved after doping Ce, as compared to the original aluminum oxide inclusion without doping Ce. The resultant PH13–8Mo parts doped with Ce yielded an ultimate tensile strength of 1446 ± 20 MPa with a fracture elongation of 16.0% ± 1.5%, for the first time meeting the AMS 5629E H1000 standard for the PH13–8Mo made by additive manufacturing (AM). The enhanced strength results from the strengthening effects of nanoscale precipitation (inclusion) and grain refining. Meanwhile, the ductilizing mechanism can be attributed to the enhanced inclusion sphericity and ameliorative coherency between the inclusion and matrix, and improved misorientation angle of grain boundary owing to the modification by Ce, which efficiently reduced the stress concentration and enhanced cracking resistance during deformation. Therefore, doping rare earth elements presents a promising pathway to synergistically improve the strength and ductility of stainless steels by AM.

Role of grain boundary character on Bi segregation-induced embrittlement in ultrahigh-purity copper

Bismuth (Bi), as an impurity element in copper and copper-based alloys, usually has a strong tendency of grain boundary (GB) segregation, which depends on the GB characters. However, the influence of such a segregation on the properties of ultrahigh-purity copper has been rarely reported and the exact structural arrangements of Bi atoms at different GBs remain largely unclear. In this study, we investigated the influence of trace amounts of Bi (50-300 wt ppm) on the ductility of an ultrahigh-purity copper (99.99999%) in the range of room temperature to 900 °C. The tensile results show that the addition of Bi seriously damages the ductility of the ultrahigh-purity copper at temperatures of 450–900 °C, which is due to the GB segregation of Bi. On this basis, such a segregation behavior at different types of GBs, including high and low angle GBs (HAGBs/LAGBs), and twin boundaries (TBs), via the scanning electron microscope-electron backscattered diffraction (SEM-EBSD) and aberration-corrected scanning transmission electron microscope (AC-STEM) investigations, combined with the first-principles calculations were systematically studied. The atomistic characterizations demonstrate an anisotropic Bi segregation, where severe enrichment of Bi atoms typically occurs at the HAGBs, while the absence of Bi adsorption prevails at LAGBs or TBs. In particular, the segregated Bi at random HAGBs exhibited the directional bilayer adsorption, while the special symmetrical Σ7 HAGB presented a unique Bi-rich cluster superstructure. Our findings provide a comprehensive experimental and computational understanding on the atomic-scale segregation of impurities in metallic materials.

Attributes of intergranular corrosion in AA6061-AA7075 double sided friction stir weld

The present study aims to observe the distinguishable progression of intergranular corrosion (IGC) in dissimilar AA6061-AA7075 double-sided friction stir welds (DSFSW) with and without employing secondary heating. Transverse weld specimen sectioned along the weld line, to separate advancing and retreating side, are electrochemically corroded. Secondary heating helps in impeding the most accelerated corrosion rate dominated over the AA7075 region by approximately 50%. The double stirred region remains least susceptible to corrosion due to the dissolution and break down of precipitates. The degree of homogenization during stirring of dissimilar alloy controls the degree of material depletion during corrosion, making intercalated structure unfavourable in terms of corrosion. IGC is found more aggressive in AA7075 structurally intercalated with AA6061 in stir zone. The coarse grain, wider precipitate free zones and continuity of the precipitates along grain boundary (GB) are interpreted as most influential factors. Additional/secondary heating resulted into retardation of IGC within the AA7075 but resulted continuous IGC along the GBs of AA6061. Low angle GBs appears more resistive against IGC as they allow the formation of lean precipitates on contrary to high angle GBs. Secondary heating boosts breakdown and dissolution of corrosion sensitive secondary phase particles reducing the intergranular corrosion tendency.

Atomistic study of nano-cutting bicrystal cubic silicon carbide

To solve problems of low machining efficiency and surface/subsurface damage of silicon carbide (SiC), mechanisms of material dislodging and subsurface destruction of bicrystal SiC under nano-cutting were studied by using three-dimensional molecular dynamics (MD) simulation method. Effects of grain boundary (GB) tilt angle and cutting speed on stress, hydrostatic pressure, cutting force, temperature, and subsurface damage depth were analyzed. Results show that GB slope angle has important influence on the spread of strain, hydrostatic stress, and temperature, especially on subsurface damage depth. Specifically, with the increase in GB slope angle, subsurface damage layer decreases, and machined surface morphology improves. The machinability of small GB angle is better than the large GB angle, and the model with grain boundary angle of 4.24° has the best machinability. It is also found that with the increase in cutting velocity, the temperature increases, whereas cutting force and friction coefficient are reduced, which indicates that the increase in cutting velocity can effectively improve surface machined morphology. The research results provide a theoretical reference for future experimental research.

The effect of neighboring grain orientation on dislocation-grain interaction in Ti-5553 alloy

The accumulated dislocations are widely reported to be easily transmitted at low angle grain boundaries (GBs), while the associated mechanism at high angle GB needs further investigation. In this paper, the mechanisms associated with the interaction between GBs and 〈111〉 dislocations are investigated in Ti-5553 alloy with body-centered cubic structure. Slip traces transmitted into the neighboring grain or blocked by GBs are observed on the surface of the sample via secondary electron imaging. The corresponding slip system is determined with the combined use of EBSD-detected grain orientation and slip traces. The experimental results indicate that most slips can transfer at low angle GBs, while some slips can also transmit into the neighbor at high angle GBs. The misorientation angle of GBs is invalid to evaluate the mechanism induced by the interaction of dislocations with GBs. Crystallographic alignment-based analysis shows that slip transfer happens at GBs with favorable parameters of high geometric alignment ( m ′) and low residual Burgers vector magnitude (|∆ b |). The general relation of GB misorientation angle with m ′ and |∆ b | is established. The results indicate that low angle GBs always possess high m ′ and low ∆ b , while only some high angle GBs meet the requirement of large alignment parameters. This difference contributes to their preference of slip transfer. Finally, the orientations of best neighboring grains that are favorable for slip transfer are calculated, which is in agreement with the experimentally detected examples. • Dislocations can transmit at some high-angle grain boundaries with special orientations. • General effect of grain boundary misorientation angle on slip transmission is studied. • The best orientations of neighboring grains favorable for slip transmission are established for low- and high-angle GBs.

Temperature and ΔK dependence of grain boundary effect on fatigue crack propagation in a two-dimensional polycrystalline Ni-base superalloy

Effects of grain boundaries (GBs) on fatigue crack propagation (FCP) in a cast Ni-base superalloy were experimentally investigated using “two-dimensional” polycrystalline specimens extracted from a directionally solidified material. FCP tests were conducted at room temperature (RT), 700 °C and 900 °C under the stress intensity factor range, ΔK, increasing conditions, and distinctive FCP behavior both in grain interiors and around the GBs was observed in-situ. At RT, the crack propagated in the shearing mode along slip planes. At 700 °C, the cracking mode transitioned from the opening mode to the shearing mode with the increasing ΔK. At 900 °C, only the opening mode cracking occurred. The shearing cracks at RT and in the high ΔK region at 700 °C were retarded near the GBs, whereas the opening mode crack in the low ΔK region at 700 °C was not affected by the GBs. The retardation was attributed to a transition from the shearing mode to the opening mode induced by the slip plane incompatibility at the GBs, and the degree of the retardation showed a good correlation with the size of the shearing crack plane. At 900 °C, the crack propagated along high angle GBs with higher FCP rates than those in grain interiors. The acceleration was greater at the GBs with larger misorientation, which was attributed to higher GB energy. In high ΔK regions, secondary cracks were nucleated along high angle GBs ahead of main crack tips possibly because of external stress normal to the GBs and/or the strain incompatibility at the GBs.