Model Of Intergranular Fracture Research Articles

Microstructure and mechanical properties of Mn-Cu based damping alloy fabricated by laser melting deposition

The M2052 (Mn-20Cu-5Ni-2Fe, at%) damping alloy specimen was prepared by laser melting deposition (LMD). And we studied the mechanical properties and microstructure of M2052 alloy. Columnar dendrite penetrating multiple cladding layers appeared in the as-deposited sample, and the size of dendrite tends to coarsen with the increase of laser power. Microsegregation and martensitic transformation occurs in the as-deposited M2052 alloy. The tensile strength of samples in different directions under the same process, and in the same direction under different processes is all exceeds 500 MPa. The fracture surfaces exhibit intergranular fracture model. This work shows great potential for fabricating Mn-Cu based damping alloy by LMD technology.

ОЦІНКА ЕВОЛЮЦІЇ ПОР У α-ЗАЛІЗІ ПІД НЕЙТРОННИМ ОПРОМІНЕННЯМ

Changes in the mechanical properties of the reactor body of nuclear power plants are due to a number of physical processes at nano-, micro-, meso- and macro-levels both in time and space. The molecular dynamics method, the Monte Carlo method and the quantum mechanical method (ab-initio method) are used to study the cascade stage of interaction of irradiation (electrons, neutrons and ions) with reactor materials, which lasts from 10-15 to 1012 seconds in the size range up to 10 nm. The cluster dynamics method allows to study the longterm kinetics of point defect clusters (vacancies and internodes), as well as clusters and precipitates, which include point defects and alloying elements of reactor alloys. Based on the results of cluster dynamics such as concentration, average size, and numerical density of clusters and precipitates, it is possible to calculate the change in yield stress caused by irradiation. But so far, the urgent task of reactor materials science is to take into account the effects of irradiation on the physical mechanism of their plastic deformation and destruction. This article is devoted to one of the aspects of destruction of irradiated materials, which is based on taking into account the origin and evolution of pores during loading. A physical and mechanical model of intergranular fracture, which is caused by the evolution of pores, is proposed to evaluate the mechanical stability of reactor steels. The time dependence of the increase of the yield strength for the irradiation regime in the BR-2 research reactor (Mol, Belgium) was determined from the results of cluster dynamics for the evolution of the average size and numerical density of clusters for a constant temperature of 300 oC of the reactor vessel and its decrease from 300 oC to 123 oC. The evolution of the pore size and the magnitude of the relative plane of pores are calculated from the kinetic equations.

Construction of Theoretical Curves of the Long-Term Strength for Neutron-Irradiated Austenitic Steels Kh18N9 and 08Kh16N11M3

Abstract—Having analyzed experimental data on the long-term strength of unirradiated austenitic materials and predictive dependences obtained in the context of a physical and mechanical model of intergranular fracture, the authors have determined the assurance factor for plotting the long-term strength curves for unirradiated and irradiated materials. The above model, as well as experimental results, has made it possible to calculate the normative curves for the initial and irradiated steels Kh18N9 and 08Kh16N11M3. Their validity has been verified on the basis of external and intrareactor tests.

Simulation of fatigue damage in ferroelectric polycrystals under mechanical/electrical loading

The reliability of smart-structures made of ferroelectric ceramics is essentially reduced by the formation of cracks under the action of external electrical and/or mechanical loading. In the current research a numerical model for low-cycle fatigue in ferroelectric mesostructures is proposed. In the finite element simulations a combination of two user element routines is utilized. The first one is used to model a micromechanical ferroelectric domain switching behavior inside the grains. The second one is used to simulate fatigue damage of grain boundaries by a cohesive zone model (EMCCZM) based on an electromechanical cyclic traction–separation law (TSL). For numerical simulations a scanning electron microscope image of the ceramic’s grain structure was digitalized and meshed. The response of this mesostructure to cyclic electrical or mechanical loading is systematically analyzed. As a result of the simulations, the distribution of electric potential, field, displacement and polarization as well as mechanical stresses and deformations inside the grains are obtained. At the grain boundaries, the formation and evolution of damage are analyzed until final failure and induced degradation of electric permittivity. It is found that the proposed model correctly mimics polycrystalline behavior during poling processes and progressive damage under cyclic electromechanical loading.To the authors’ knowledge, it is the first model and numerical analysis of ferroelectric polycrystals taking into account both domain reorientation and cohesive modeling of intergranular fracture. It can help to understand failure mechanisms taking place in ferroelectrics during fatigue processes.

Microstructural Modeling of Intergranular Fracture in Tricrystals With Random Low- and High-Angle Grain Boundaries

Intergranular (IG) fracture behavior near triple junctions (TJs) in f.c.c. tricrystals with a variety of grain boundary (GB) misorientations has been investigated. Based on a dislocation-density GB interaction scheme, critical fracture conditions were coupled to evolving dislocation-density pileups and local stresses by using a dislocation-density-based crystalline plasticity formulation within a nonlinear finite-element framework to elucidate the effects of local GB structure, dislocation–GB interactions, and misorientations on IG crack propagation in f.c.c. crystalline materials. Tricrystals with low-angle GBs had higher fracture toughness than tricrystals with high-angle GBs. In TJs with a combination of random low- and high-angle GBs, the formation of dislocation-density pileups in the high-angle GB led to IG crack propagation along the high-angle GB rather than along the low-angle GB. These predictions, which are consistent with experimental observations, indicate that fracture behavior near TJs is controlled by highly local, evolving, and interrelated events, such as dislocation-density pileups and GB misorientations.

Fracture Model of ZrO<sub>2</sub>-Al<sub>2</sub>O<sub>3 </sub>Ceramic Composite

Mixtures of ZrO2-Al2O3 ceramic composite were prepared using zirconia powder, ZrO2-Al2O3 composite powder synthesized by sol-gel process as main starting materials and zirconia gel as a binder. Then the mixtures were pressed into the mold, and then sintered at 1650°C for 3h. Finally, the ZrO2-Al2O3 ceramic composite samples were prepared. Then the mineral phases of ZrO2-Al2O3 ceramic composite were analyzed by X-ray diffraction (XRD), and the fracture model of ZrO2-Al2O3 ceramic composite was analyzed by press, scanning electron microscopy (SEM) and energy dispersive X-ray Spectroscope (EDS). The results show that, zirconia is the main phase and alumina is the secondary phase mainly present in the matrix. The fracture model of ZrO2-Al2O3 material was jointly composed by intergranular fracture and transgranular fracture, of which the coarse particle shows the intergranular fracture model and the fine particle shows the transgranular fracture model.

Atomistically derived cohesive zone model of intergranular fracture in polycrystalline graphene

Pristine single crystal graphene is the strongest known two-dimensional material, and its nonlinear anisotropic mechanical properties are well understood from the atomic length scale up to a continuum description. However, experiments indicate that grain boundaries in the polycrystalline form reduce the mechanical behavior of polycrystalline graphene. Herein, we perform atomistic-scale molecular dynamics simulations of the deformation and fracture of graphene grain boundaries and express the results as continuum cohesive zone models (CZMs) that embed notions of the grain boundary ultimate strength and fracture toughness. To facilitate energy balance, we employ a new methodology that simulates a quasi-static controlled crack propagation which renders the kinetic energy contribution to the total energy negligible. We verify good agreement between Griffith's critical energy release rate and the work of separation of the CZM, and we note that the energy of crack edges and fracture toughness differs by about 35%, which is attributed to the phenomenon of bond trapping. This justifies the implementation of the CZM within the context of the finite element method (FEM). To enhance computational efficiency in the FEM implementation, we discuss the use of scaled traction-separation laws (TSLs) for larger element sizes. As a final result, we have established that the failure characteristics of pristine graphene and high tilt angle bicrystals differ by less than 10%. This result suggests that one could use a unique or a few typical TSLs as a good approximation for the CZMs associated with the mechanical simulations of the polycrystalline graphene.

Phenomenological and microstructural analysis of intermediate temperatures creep in a Ni–Fe-based alloy for advanced ultra-supercritical fossil power plants

A newly developed Ni–Fe-based alloy with high-creep strength and low cost has been developed and evaluated as the promising candidate boiler materials for 700 °C advanced ultra-supercritical coal-fired power plants applications. Three electron microscopy characterization methods–scanning electron microscopy and transmission electron microscopy and high-resolution transmission electron microscopy–were combined to obtain new insights into the microstructural and fracture surface characteristics after creep rupture tests at intermediate temperatures. The alloying elements distribution characteristics have been investigated at nanoscale through EDS mapping, especially Fe element. Fractographic analysis has been also conducted with the finding that the fracture mechanism of the crept specimens at 700 °C/300 MPa and 750 °C/150 MPa are intergranular fracture model. Dislocation configurations resulting from the creep deformation have been also performed on the crept specimens. At 700 °C/300 MPa, Orowan process combining climb of a/2 <110> matrix dislocations was dominant mechanism. At 750 °C/150 MPa, the dominant mechanism is Orowan process combining slip of a/2 <110> matrix dislocations and γ′ precipitates shearing. The formation α-Cr precipitation during the creep process could act as obstacle to impede the dislocation gliding and thus increase the creep strength.

FEM‐BEM Code for the Multiscale Modeling and Computer Aided Design of Wire Drawing Technology for Magnesium Alloys

The purpose of the paper is the problem of determination of parameters of hyperfine wire drawing process of biocompatible magnesium alloys. The originality of the considered alloys is the intergranular fracture mechanism associated with small strains (0.07–0.09) in macro‐scale. It was proven that the material state just before appearance of the meso‐cracks is the optimal state of material from the point of view of the restoration of the plasticity by annealing. The forecasting of this material state during drawing process requires the development of the model of intergranular fracture. In this case, new BEM model of the cracks initiation in meso‐scale is proposed. The in situ tests, which allowed observation of the microstructure during deformation, are used for the calibration and validation of the present model. The empirical parameters of the fracture model in micro‐scale for MgCa0.8 and Ax30 magnesium alloys are obtained using tensile and share tests. The developed model was implemented to the Drawing 2d software. The results of meso‐scale simulation were verified by using the experimental drawing process of the hyperfine wires (diameter 70 µm) according to developed technology.

Effects of Si addition on mechanical properties of Ti–5Al–5V–5Mo–3Cr alloy

In this paper, the ageing response and the mechanical properties of Ti-5553 (Ti–5Al–5V–5Mo–3Cr) and Ti-5553s (Ti–5Al–5V–5Mo–3Cr–0.4Si) alloys are comparatively investigated. During ageing at 650°C, the hardness of the alloys first increases in 30min and remains constant from 1h to 8h, and then decreases with further increase the ageing time to 29h. This variation is related to the chemical composition and the volume fraction of α phase. The mechanical properties of the 5h aged Ti-5553 and Ti-5553s alloys are investigated, and the result shows that the ultimate tensile strength of the Ti-5553s alloy is higher than that of Ti-5553 alloy while the ductility of the Ti-5553s alloy slightly decreases. The tensile fracture of the Ti-5553s alloy shows a mixed model of intergranular fracture and cleavage fracture.

A multi-scale model of intergranular fracture and computer simulation of fracture toughness of a carburized steel

Abstract It is well-known that atomic–electronic structures determine materials’ macro-scale physical properties; however, broad varieties in microstructures and heterogeneities from different scales may significantly amplify or dilute the mechanical behavior of an alloy presented at quantum scale. Hence, breakthrough of computational alloy’s design lies in the capabilities to quantitatively and completely integrate key-mechanisms from different scales at each processing step, so as to obtain a unified procedure for establishing quantitative relationships between composition, process, structure, properties, and performance. For this purpose, a multi-scale hierarchical model of intergranular fracture has been developed for polycrystalline systems. As an application example, a carburized steel, which is used for gears and other components in heavy power transmission system, has been analyzed for exploring the ways to improve fracture toughness. As experimental study indicates that the low toughness is often caused by intergranular cracking, this analysis reveals that such a crack initiation can be triggered by the stress concentration at the junctions of grain boundaries when grain boundary adhesion is weak. Therefore, an improved toughness can be achieved by reducing impurities’ grain boundary segregation while increasing grains’ ductility.

In vitro wear behaviour of different ceramic couplings

The overall objective of this study is to introduce a wear map of ceramics which shows the regions of dominant wear modes observed in a wide range of materials and operating conditions. Furthermore, from the wear map, common necessary conditions for the application of various ceramics as wear-resistant tribo-materials in a wide range of operating conditions are discussed. For this purpose, friction and wear tests are carried out using three kinds of ceramics sliding against themselves under various contact pressures, sliding velocities and temperatures. Sliding wear phenomena of ceramics observed in all tests can be classified into two types: “mild wear” and “severe wear”, based on the ration of worn surface roughness Rx to the mean grain size Dg. The specific wear amount is always less than 10−16 mm3 (Nm) −1 when the worn surface roughness becomes relatively small (RyDg < 0.2) in the mild wear region. On the other hand, the relatively rough worn surface (RyDg > 0.5) in the severe wear region gives a wear rate larger than 10−6 mm3 (Nm)−1. The mild wear region is believed to be necessary for the application of ceramics as wear-resistant tribo-materials. The critical condition for mild and severe wear is analyzed with an intergranular fracture model from the view points of both mechanical and thermal aspects. A wear map of ceramics, which can define the regimes of mild and severe wear, is introduced using two dimensionless parameters, namely the mechanical severity of contact (Sc,m) and the thermal severity of contact (Sç,γ), which are used as vertical and horizontal axes, respectively. The availability of the wear map constructed by this method is proven by experimental results observed over a wide range of test materials and operating conditions.

Wear map of ceramics

The overall objective of this study is to introduce a wear map of ceramics which shows the regions of dominant wear modes observed in a wide range of materials and operating conditions. Furthermore, from the wear map, common necessary conditions for the application of various ceramics as wear-resistant tribo-materials in a wide range of operating conditions are discussed. For this purpose, friction and wear tests are carried out using three kinds of ceramics sliding against themselves under various contact pressures, sliding velocities and temperatures. Sliding wear phenomena of ceramics observed in all tests can be classified into two types: “mild wear” and “severe wear”, based on the ration of worn surface roughness R x to the mean grain size D g . The specific wear amount is always less than 10 −16 mm 3 (Nm) −1 when the worn surface roughness becomes relatively small ( R y D g < 0.2 ) in the mild wear region. On the other hand, the relatively rough worn surface ( R y D g > 0.5 ) in the severe wear region gives a wear rate larger than 10 −6 mm 3 (Nm) −1. The mild wear region is believed to be necessary for the application of ceramics as wear-resistant tribo-materials. The critical condition for mild and severe wear is analyzed with an intergranular fracture model from the view points of both mechanical and thermal aspects. A wear map of ceramics, which can define the regimes of mild and severe wear, is introduced using two dimensionless parameters, namely the mechanical severity of contact ( S c, m ) and the thermal severity of contact ( S c ̧ ,γ ), which are used as vertical and horizontal axes, respectively. The availability of the wear map constructed by this method is proven by experimental results observed over a wide range of test materials and operating conditions.

Wear Map of Ceramics. 2nd Report. Construction of Mild-Severe Wear Mode Map.

The overall objective of this study is to introduce a wear map of ceramics which shows the regions of dominant wear modes obtained in a wide range of materials and operating conditions. In the 1st report, it was clarified that the sliding wear mode of ceramics observed under various contact pressures, sliding velocities and temperatures can be classified into two types called "mild wear" and "severe wear" based on the ratio of worn surface roughness, Ry, to mean grain size, Dg, and specific wear amount, Ws. In this paper, the critical condition of the mild and severe wear is analyzed using an intergranular fracture model from both the view points of mechanical and thermal aspects, and a wear map of ceramics, which can define the regimes of mild and severe wear, is introduced; it uses two dimensionless parameters named as the mechanical severity of contact Sc, m and the thermal severity of contact Sc, t, as the vertical and horizontal axes, respectively. The availability of the wear map constructed in this way can be proved by the experimental results observed over a wide range of materials and operating conditions.

Microstructural model of intergranular fracture during tensile tests

A microstructural model of intergranular fracture in textured materials is presented. In this model, the material is represented by a two-dimensional microstructure with non-regular polygonal grains which represents material's texture and grain shape measured in experiments or calculated from Monte Carlo simulations. The grain boundary character, grain boundary energy, and fracture stress are assigned to each grain boundary according the grain boundary character distribution. Intergranular fracture susceptibility is analyzed by defining the probability of finding a continuous path along the grain boundaries which are intrinsically susceptible to fracture. In this analysis the orientations of the grain boundary with respect to the applied or residual tensile stress axis is considered. The probability of intergranular fracture for each grain boundary depends on the intergranular fracture resistance, the interface orientation relative to the stress axis, and a value of the tensile stress acting on the grain boundary. The crack arrest distance and the fracture toughness are calculated in terms of the frequency of low-energy grain boundaries, fracture stress of low-energy grain boundary, angle distribution of grain boundary interfaces, and anisotropy of grain shape. The results indicate that the fracture toughness increases and the crack arrest distance decreases dramatically with increasing the frequency of the low-energy grain boundaries. Lowering the grain boundary energy can improve the fracture toughness and decrease the crack arrest distance. The angle distribution of grain boundary interfaces and the grain shape factor are also very effective in controlling the fracture toughness. High fracture toughness of polycrystalline materials is related to the presence of a high frequency of low-energy boundaries which are resistant to fracture. The best fracture toughness for brittle materials can be achieved by controlling the frequencies of the low-energy grain boundaries, the grain boundary character, and the boundary inclination.