Boundary Microcracks Research Articles

An insight into the creep failure mechanism of sliver defect in the second-generation nickel-based single crystal superalloy CMSX-4

Sliver defects are common casting imperfections in the preparation of single crystal blades, significantly impacting the high-temperature mechanical performance of blades and potentially leading to blade failure. A detailed study was conducted on the microstructure of sliver in the second-generation nickel-based single crystal superalloy CMSX-4, focusing on its influence on creep performance at 980 °C/250 MPa. Results indicate a substantial impact of sliver on the creep performance of single crystal superalloys, with a 26.37 % reduction in creep rupture time for samples with sliver compared to those without. Fracture analysis reveals that sliver primarily contributes to performance deterioration by promoting the extension of grain boundary microcracks through grain boundary sliding during the creep process, accelerating sample fracture. Utilizing crystal plasticity theory, a successful fit was achieved for the creep rupture life of samples with sliver, allowing the prediction of fracture life for samples with different orientations of slivers. Predictive results suggest that, for CMSX-4 alloy, the orientation of the sliver should be restricted to within 8°.

Response of 11B enriched ZrB2 ultra-high temperature ceramic to neutron irradiation at elevated temperatures

ZrB2, an ultra-high temperature ceramic (UHTC) is being considered for use in fusion reactor first-wall structures, yet its response to irradiation remains poorly understood. This study employed scanning/transmission electron microscopy (S/TEM), synchrotron X-ray diffraction (XRD), finite element calculations, and thermal property measurements to thoroughly investigate the neutron-irradiation effects on 11B-enriched ZrB2. Neutron irradiations were conducted at 220 °C and 620 °C, with a neutron fluence of 2.2 × 1025 neutron/m2 (energy > 0.1 MeV), resulting in 3.9 dpa and 4200 appm He. The study revealed the unusual prevalence of prism loops and a > c anisotropic lattice swelling, likely linked to the low c/a ratio of ZrB2, leading to grain boundary microcracking. Reducing the grain sizes was effective in reducing intergranular cracking and macroscopic swelling. The observation of cavities in ZrB2 irradiated at 620 °C, as opposed to 220 °C, prompts questions about the temperature at which vacancies in ZrB2 become mobile, and the role of neutron absorption by 10B in elevating irradiation temperatures. Isotopic enrichment in 11B proves to be a viable strategy for mitigating helium production in transition-metal diborides, which is a critical consideration for nuclear applications. Irradiation-induced defects reduce the thermal diffusivity and conductivity of ZrB2 by a factor of 4–9, which has important implications for its role as a plasma-facing material in fusion reactors that drive high heat fluxes through first-wall materials. This comprehensive study lays the foundation for understanding ZrB2 behavior under neutron irradiation and highlights important phenomena to consider for various material applications.

Comprehensive investigation on linear friction welding a dissimilar material joint between Ti17(α+β) and Ti17(β): Microstructure evolution, failure mechanisms, with simultaneous optimization of tensile and fatigue properties

In order to advance the use of Ti17 (α+β)-Ti17(β) dissimilar material blisks produced with linear friction welding (LFW) for advanced aeroengines, detailed studies of failure mechanisms and performance enhancements of such joints were conducted. The changes in microstructure including phase transformations and dynamic recrystallization were investigated. Also, the tensile failure mechanism of the joint was investigated by in-situ tensile test using scanning electron microscopy, while the high-cycle fatigue failure mechanism was studied using quasi-in-situ electron back-scattering diffraction observations combined with transmission electron microscopy characterization. A post-weld annealing heat treatment (PWAHT) was applied for optimizing tensile and fatigue strength. Results shows that α dissolution and β continuous dynamic recrystallization (CDRX) are controlling microstructure changes during welding. Tensile loading failure is related to local dislocation slip concentration on the Ti17(β) side of the thermo-mechanically affected zone with limited α strengthening phase and low degree β CDRX; while fracture under high-cycle fatigue loading is related to equiaxed α/metastable β boundary microcracks on the Ti17 (α+β) side of the heat-affected zone, which was produced by unequal deformation between the two phases. Following PWAHT for 630 °C at 3 h, re-precipitation of secondary α occurred, which increased dislocation movement resistance in the weak areas, producing tensile and high-cycle fatigue strengths of 1196.4 MPa and 550.2 MPa, respectively, higher than those of BM-(β). In addition, an adequate plasticity of the joint with elongation of 8.7 % (82.1 % of Ti17(β)) was reached.

Machine learning in Al2TiO5 flexible ceramics with microcrcaks strengthened for damage mode automatic identification

AbstractThe damage modes of Al2TiO5 flexible ceramics (AT) with microcracks strengthened were studied by machine leaning for damage mode identification. Results show that the optimal number of clusters increases from 3 to 4 with the increase of ZrO2 content. Meanwhile, the concentrated region of cluster with the same label gradually shifts to the direction of low amplitude with increase of ZrO2. In addition, the quantity of cluster is easy saturated since the AT grain boundary microcracks are strengthened by the formation of ZrTiO4. For achieving the automatic identification of different damage modes, support vector machine based on Kennard‐stone training set selection was used to distinguish the damage modes of other samples in the same sample set. Almost the same evolution trend of corresponding clusters in new samples and training samples proves high efficiencies of this method.

The influence of ZrO2 on the microstructure and mechanical properties of Al2TiO5 flexible ceramics

To obtain aluminum titanate (AT, Al2TiO5) ceramics with excellent mechanical properties, the ZrO2 was introduced into AT flexible ceramic to modify the microstructure of it. Results show that ZrO2 can significantly improve the mechanical properties of AT flexible ceramics. When the addition of ZrO2 is 8 wt% and 12 wt%, the optimal strain and strength increase by ~52% and ~ 46%, respectively. The statistical results show that ZrO2 can not only refine the AT grains but also improve the aspect ratio, which leads to more events of grain pull-out and transgranular fracture. Since the generated ZrTiO4 is pinned in the AT grain boundary microcracks, the phenomenon of branching and deflection of main crack will increase, rising the resistance of main crack propagation. It is expectable that the discovery of this work is benefitted to solve the brittle limitation of ceramics.

Investigation on the microcrcaks strengthening effect in the damage behavior of Al2TiO5 flexible ceramics based on the AE

In this study, the effect of microcrcaks strengthening on damage evolution of Al2TiO5 (AT) flexible ceramics was investigated by acoustic emission (AE). Results reveal that with increasing content of ZrO2, the strength of AT flexible ceramics improves gradually and the strain increases at first then decreases. When the content of ZrO2 is 12% and 8%, the strength and strain at maximum stress are increased by 154.92% and 38.27%, respectively. In addition, thanks to the “strengthening effect” of ZrTiO4 on grain boundary microcracks, the damage development thresholds (σmin and σonset), elastic modulus and fracture toughness of AT flexible ceramics modified with ZrO2 are significantly improved. Moreover, the visible transverse cracks and matrix shedding in ATZ-0 are the most, but the cumulative AE damage events in ATZ-8 and ATZ-4 are more than ATZ-0 at late stage, for which the fracture of ZrTiO4 glass phase and crack branching induced by ZrTiO4 should be responsible.

Pore structure and crack characteristics in high-temperature granite under water-cooling

The changes of porosity and microcracks of high-temperature granite under water-cooling have a great influence on the safety and efficiency of geothermal resources exploitation. To analyze the formation conditions of the pore structure and microcracks of granite specimens at different temperatures under water-cooling, the rock porosity was measured by NMR, and the rock microstructure was observed by PM and SEM. The results show that the closure of microcracks occurs at 150 °C and the grain boundary microcracks appear at 300 °C. And the porosity and pore size begin to increase, the number of pores remains stable, and intracrystalline microcracks appear at 450 °C. When T = 600 °C, macropores and trans-granular microcracks appear inside the rock. When 600 °C < T ≤ 750 °C, the increase rate of porosity decreases, the number of macropores increases, the number of micropores decreases, and the total number of pores increases exponentially. The porosity and the number of macropores increase rapidly, and the aperture of transgranular microcracks increases after T = 750 °C. The change of granite microstructure leads to the decrease of thermal conductivity, which decreases with the increasing temperature. During geothermal exploitation, the thermal conductivity of dry hot rock can be characterized by porosity, and the relationship is K=C + D × Ex.

The influence of different additives on microstructure and mechanical properties of aluminum titanate ceramics

The aluminum titanate (AT: Al2TiO5) ceramics with different additives, including SiO2, MgO, Fe2O3 and their mixture, were prepared by reaction sintering at 1500 °C in air for 2 h. The influence of additives on the microstructure and flexibility of AT ceramics is studied. Research illustrates that additives, MgO, Fe2O3 and their mixture, can be dissolved in AT ceramics to promote the formation of elongated grains and grain boundary microcracks. The elongated grains can interlock together and restrain crack propagation while microcracks can provide gaps for grains to move, leading to obvious flexibility of sample. Especially, the AT ceramics doped with MgO and Fe2O3 (ATFM) show the optimum flexibility with a deflection about 0.55 mm and strength up to 13.85 MPa. While the additive of SiO2 can reduce the size of grains and microcracks, resulting in high mechanical strength but poor flexibility. The successful fabrication of such AT flexible ceramics will provide potential opportunity for anti-vibration and refractory application.

Correlation among grain boundary character, carbide precipitation and deformation in Alloy 690

ABSTRACTThe correlation among grain boundary character, carbide precipitation and deformation in the grain boundary engineering (GBE) treated Alloy 690 samples with and without pre-deformation aged at 715oC for 15 h was analysed by scanning electron microscopy and electron backscatter diffraction. The fraction of low Σ coincidence site lattice (CSL) grain boundary was enhanced by GBE treatment. The fraction of Σ3 grain boundary decreased, and most of Σ9 and Σ27 grain boundaries disappeared in the deformed GBE samples. After aging treatment, bigger carbide precipitated at coherent Σ3 grain boundary, however, most of plate-like carbide precipitated at incoherent Σ3 grain boundary disappeared in the pre-deformed GBE samples. The larger carbide precipitated on the random grain boundary in the 5% pre-deformed sample, while smaller carbide can be observed in the 15% pre-deformed sample. During the in situ tensile test of the aged GBE samples, grain boundary carbide migrated with the grain boundary migration. The slip bands go across Σ3 grain boundary directly, but cannot go across other grain boundaries. The high density of carbide plate precipitated near incoherent Σ3 and Σ9 grain boundaries can resist the evolution of slip bands. Compared to the Σ3 and Σ9 grain boundaries, Σ27 and random grain boundaries are more easily to form microcrack during deformation. The initiation of grain boundary microcrack not only related to the character of grain boundary but also related to the character of nearby grain boundaries. The phase interface of carbide and matrix is another region to initiate the microcrack.

Performance and Durability Enhancement of Tungsten Carbide Micro-Drills by Ti/ta-C Coatings

This paper presents results of the application of Ti/ta-C films to micro drilling operation for machining. Tetrahedral amorphous carbon (ta-C) films were successfully deposited on WC-Co substrates by a filtered cathodic vacuum arc (FCVA) system. The mechanical and flexion properties of Ti/ta-C films were systematically investigated. The experimental results show that the Ti/ta-C coated micro drills have the excellent microhardness, adhesion and flexion properties and represent the optimal coatings for micro drilling applications. The role of the Ti –sublayer on a sintered powder tungsten carbide substrate is not only limited by the adhesion improvement, but it is mainly used to neutralise the grain boundary microcracks on a surface. The results of drilling tests carried out on PCB boards showed that the durability and drilling efficiency of tools coated by Ti/ta-C films are significantly higher than that of uncoated ones.

Microscopic characterization of microcrack development in marble after cyclic treatment with high temperature

Crack density of rocks is greatly affected by high temperature treatment and the induced thermal damage influences the strength and deformation characteristics of the rock. A good understanding of thermal cracking behavior is useful for geological evaluation of engineering structures associated with high temperature problems. This study investigates the characteristics of thermally-induced microcracks in a fine-grained dolomitic marble with different degrees of thermal damage using an optical microscope. Different degrees of thermal damage were first generated by treating the rock specimen with different heating and cooling cycles. Optical microscopy was then used to characterize the microcrack type and statistically examine the width, length, and anisotropy of thermally-induced microcracks. The results reveal that most of the generated microcracks induced by cyclic high temperature treatment are grain boundary microcracks. The width and length of microcracks significantly increases with an increasing number of heating and cooling cycles. It is also found that both grain boundary microcracks and intra-grain microcracks do not show predominant direction after thermal treatment. Finally, a quantitative relation is established to correlate the mechanical behavior of rocks (i.e., strength and modulus) with the crack density. The proposed relation is useful in understanding how the microstructure affects the properties of rocks after treatment with high temperature.

Boundary micro-cracking in metastable Fe45Mn35Co10Cr10 high-entropy alloys

Mechanically-induced martensitic transformation can be a double-edged sword: depending on composition and processing history it can either lead to various beneficial mechanical effects (e.g. transformation-induced plasticity, transformation-toughening), or induce local brittleness and damage nucleation. While several corresponding guidelines are presented in steels research, controlling microstructure metastability has not drawn sufficient attention in the quick-emerging field of high-entropy alloys. In the present work, we investigated the damage mechanisms of a mechanically metastable Fe45Mn35Co10Cr10 high-entropy alloy under uniaxial tensile loading. Our integrated in-situ scanning electron microscopy/electron backscatter diffraction experiments revealed a two-fold effect of the highly localized strain, induced by asynchronously transformed martensite, leading to boundary damage nucleation and dissimilarly oriented martensitic variant formation. The latter suppresses slip transfer between adjacent grains, further expediting the growth of the nucleated damage incidents. Based on these experimental observations and corresponding theoretical calculations, we discuss the underlying mechanisms and propose a sequence of micro-events that create the observed phenomena.

Nano-scale microstructure damage by neutron irradiations in a novel Boron-11 enriched TiB2 ultra-high temperature ceramic

Ultra-high temperature transition-metal ceramics are potential candidates for fusion reactor structural/plasma-facing components. We reveal the irradiation damage microstructural phenomena in Boron-11 enriched titanium diboride (TiB2) using mixed-spectrum neutron irradiations, combined with state-of-art characterization using transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Irradiations were performed using High Flux Isotope Reactor at ∼220 and 620 °C up to 2.4 × 1025 n.m−2 (E > 0.1 MeV). Total dose including contribution from residual Boron-10 (10B) transmutation recoils, was ∼4.2 displacements per atom. TiB2 is susceptible to irradiation damage in terms of dislocation loop formation, cavities and anisotropic lattice parameter swelling induced micro-cracking. At both 220 and 620 °C, TEM revealed dislocation loops on basal and prism planes, with nearly two orders of magnitude higher number density of prism-plane loops. HRTEM, electron diffraction and relrod imaging revealed additional defects on {101¯0} prism planes, identified as faulted dislocation loops. High defect cluster density on prism planes explains anisotropic a-lattice parameter swelling of TiB2 reported in literature which caused grain boundary micro-cracking, the extent of which decreased with increasing irradiation temperature. Dominance of irradiation-induced defect clusters on prism planes in TiB2 is different than typical hexagonal ceramics where dislocation loops predominantly form on basal planes causing c-lattice parameter swelling, thereby revealing a potential role of c/a ratio on defect formation/aggregation. Helium generation and temperature rise from residual 10B transmutation caused matrix and grain boundary cavities for the irradiation at 620 °C. The study additionally signifies isotopic enrichment as a viable approach to produce transition-metal diborides for potential nuclear structural applications.

Preparation of aluminum titanate flexible ceramic by solid-phase sintering and its mechanical behavior

Ceramics are known as a very brittle material. If this brittleness could be overcome, they can be applied in more fields. Itacolumite is regarded as a highly flexible rock for many narrow gaps at the grain boundaries in microstructure that allow a slight displacement of the particles when stressed. In this study, a ceramic mimicking the cracked microstructure of flexible sandstone (itacolumite) was successfully fabricated by sintering aluminum titanate (Al2TiO5: AT) powders. The phase compositions, apparent porosity, firing shrinkage and microstructure as well as mechanical behavior are examined. The influence of sintering temperature and holding time on the flexibility and microstructures of AT ceramics is studied. The results of fied-emission scanning electron microscopy and three-point bending test showed that the fired products benefited a microstructure with many grain boundary microcracks and showed a non-linear mechanical behavior due to its high thermal expansion anisotropy, which led to its flexibility like itacolumite. The flexibility of AT ceramics can be improved gradually with increasing sintering temperature and holding time. The optimal values for sintering temperature and holding time were determined as 1550 °C and 12 h. Three types of AT samples were chosen as reference: NF, F, and VF. NF sample had brittle behavior featuring a wide linear elastic zone, as typical ceramic materials. But F and VF sample had nonlinear ductile behavior that led to a higher flexibility due to grain boundary microcracks network and the interlocking of grains. The VF sample was more flexible than the F sample because of the greater average microcracks width.

Influence of grain size heterogeneity on strength and microcracking behavior of crystalline rocks

AbstractThis study numerically investigates the influence of material heterogeneity on the strength and deformation behavior and the associated microcracking process of a felsic crystalline rock using a grain‐based modeling approach in two‐dimensional Particle Flow Code. By using a heterogeneity index defined in this study, the heterogeneity induced by variation of grain size distribution can be explicitly incorporated into the numerical specimen models quantitatively. Under compressive loading, the peak strength and the elastic modulus are found to increase as the numerical model gradually changes from heterogeneous to homogeneous, i.e., a decrease of heterogeneity index. Meanwhile, the number of grain boundary tensile cracks gradually decreases and the number of intragrain cracks increases at the moment of failure. However, the total number of generated microcracks seems not to be significantly influenced by heterogeneity. The orientation of grain boundary microcracks is mainly controlled by the geometry of assembled grain structure of the numerical specimen model, while the orientation of intragrain microcracks is to a large degree influenced by the confinement. In addition, the development of intragrain cracks (both tensile and shear) is much more favored in quartz than in other minerals. Under direct tensile loading, heterogeneity is found to have no significant influence on the simulated stress‐strain responses and rock strength. Only grain boundary tensile cracks are generated when the numerical models are loaded in direct tension, and the position of generated macroscopic fracture developed upon failure of the specimen is largely affected by heterogeneity.

The use of the fracture initiation parameter F1 to predict microcrack nucleation at grain boundaries in Mg-2%Gd alloy

Intergranular deformation anisotropy and grain boundary (GB) microcrack nucleation were studied in Mg-2%Gd alloys by in-situ microscopy during tensile tests. A predictive fracture initiation parameter (fip) F1,which correlates with dislocation activity, twinning deformation and the geometrical orientation of GB, has been modified to evaluate the probability of the microcrack nucleation at GB. The statistical results revealed that the F1 factor which has been used to evaluate microcrack initiation in TiAl alloys can also well predict the nucleation of microcracks with this following law, i.e. GBs with larger F1 factor are more likely to cause microcrack nucleation.

Mechanistic coupling of dislocation and shear transformation zone plasticity in crystalline-amorphous nanolaminates

The deformation behavior of crystalline-amorphous nanolaminates is explored through molecular dynamics simulations using nanolaminate models that contain columnar nanograins in the crystalline layers to more closely resemble experimentally accessible nanolaminate structures. Quantitative analysis of the plastic strain distribution among competing mechanisms and their coupling at the nanoscale is accomplished through the implementation of continuum deformation metrics. The transfer of plastic strain between shear transformation zone (STZ) and dislocation plasticity initially transpires through the emission of dislocations from STZ activity impinging on the amorphous-crystalline interface (ACI). The addition of grain boundaries biases this process to regions near the boundaries at low strains, which reduces the activation barrier for the onset of dislocation plasticity. With increasing strain, dislocations are absorbed into the amorphous layers via slip transfer across the ACI, in turn triggering the activation of new STZs. Cooperative slip transfer between dislocations and STZs suppresses grain boundary microcracking collectively with large-scale shear localization, and provides an explanation for the enhanced toughness of crystalline-amorphous nanolaminates.

An Integrated Creep-Fatigue Theory for Material Damage Modeling

This paper presents an integrated creep-fatigue (ICF) theory to describe the non-linear creep-fatigue interaction during thermomechanical loading. The ICF theory recognizes the damage evolution as a holistic process consisting of nucleation and propagation of surface or subsurface cracks in coalescence with internally distributed damage, leading to final fracture. In a polycrystalline material under combined cyclic and dwell loading, crack nucleation and propagation occurs by fatigue or oxidation mechanisms, whereas internally distributed damage often occurs in the form of grain boundary cavities or microcracks due to creep or dwell effects, particularly at high temperatures. Based on the above mechanism, a damage evolution equation is mathematically derived, and the generality of the above physical mechanisms warrants the applicability of the ICF theory over a wide range of stresses and temperatures. This paper uses Mar-M 509, a cobalt base superalloy, as an example to illustrate how the ICF theory describes creep and low cycle fatigue (LCF).

Dynamic Compressive Fracture of Ceramic Polymer Layered Composites

Dynamic compressive fracture evaluation is an essential characterization technique for advanced monolithic structural ceramics like alumina. Thus, for high strain rate induced damage tolerant applications, it is needed to take care of the characteristically brittle microstructure of the alumina ceramics. Hence, a smarter design concept has been involved based on the idea that cracks can either be arrested or deflected if a weak interface or interphase can be introduced. Thus, ceramic polymer layered composites (CPLC) were fabricated from high (e.g., 97%) density alumina disks pressureless sintered from sub-micron (d50-0.6□m) alumina powder. The dynamic fragmentation of the CPLC samples at a reasonably high e.g., 900.s-1 strain rate has been studied with the real time, high-speed, in-situ video images, obtained during their failure in SHPB tests. A new failure mechanism has been proposed based on these data and FESEM evidences of grain boundary microcrack, inter/intra-granular shear bands and micro-fracture/cleavage formation.

Comparison between Pile-Up Singularities and Stress Fields Induced by Thin Slip Bands. Application to the Prediction of Grain Boundary Microcrack Nucleation

Slip localization is widely observed in metallic polycrystals after tensile deformation, cyclic deformation or pre-irradiation followed by tensile deformation. Such strong deformation localized in thin slip bands induces local stress concentrations in the quasi-elastic matrix around, at the intersections between slip bands (SBs) and grain boundaries (GBs) where microcrack initiation is often observed. Since the work of Stroh, such stress fields have been mostly modeled using the dislocation pile-up theory which leads to stress singularities similar to the LEFM ones. The Griffith criterion has then been widely applied, leading usually to strong underestimations of the macroscopic stress to GB crack initiation. In fact, slip band thickness is finite: 20nm-1000nm depending on material, temperature and loading conditions. Then, many slip planes are plastically activated through the thickness, and not only one single atomic plane. To evaluate more realistic stress fields, numerous crystalline finite element (FE) computations have been carried out using microstructure inputs (slip band aspect ratio, crystal and GB orientation...). A strong influence of slip band thickness close to the slip band corner has been highlighted, which is not accounted for by the pile-up theory. But far away, the thickness has a negligible effect and the predicted stress fields are close to the one predicted by the pile-up theory. Closed-form expressions are deduced from the numerous FE computation results allowing a straightforward prediction of GB stress fields. Slip band plasticity parameters, such as length and thickness, as well as crystal orientation, GB plane and remote stress are taken into account. The dependence with respect to the various parameters can be understood in the framework of matching expansions usually applied to cracks with V notches of finite thickness. As the exponent of the GB stress close-field is only about one-half of the pile-up or LEFM crack one, the Griffith criterion may not be used for GB microcrack prediction in case of finite thickness. That is why finite crack fracture mechanics is used together with both energy and stress criteria. Taking into account SB finite thickness, t&gt;0, leads to predicted remote stresses to GB microcrack initiation three to six times lower than the ones predicted using the to pile-up theory, in agreement with experimental data.