Conversion of transgranular to intergranular fracture in NiCr steels

The paper is focused on quantification of causes and characteristics that govern the intergranular fracture initiation and propagation of this fracture micromechanism in competition with cleavage one. A NiCr steel of commercial quality and the same steel with an increased content of impurity elements, Sn and Sb, have been used for this investigation. Step cooling annealing was applied in order to induce intergranular embrittlement and brittle fracture initiation in both steels. Standard bend and the pre-cracked Charpy type specimen geometries were both tested in three-point bending to determine the fracture toughness characteristics. Charpy V notch specimens tested statically in three-point bending supported by FEM calculation have been used for local fracture stress and other local parameters determination. Relation of cleavage fracture stress and critical stress for intergranular failure has been followed showing capability of this parameter for quantification of the transgranular/intergranular fracture conversion. In order to characterise the quantitative roughness differences in fracture surfaces fractal analysis was applied. A boundary level of fractal dimension has been determined to be 1.12 for the investigated steel; the fracture surface roughness with a higher value reflects high level of intergranular embrittlement and thus fracture toughness degradation.     

氢致裂纹慢扩展区断裂表面的分维研究

用垂直剖面法测量了一种含氢高强度钢在静弯矩作用下由裂纹慢扩展形成的断裂表面不同部位的分维 D,发现 D 随断裂表面塑性成分的增加而增加。在平面应变条件下,裂纹慢扩展形成断裂表面的过程是一个增维过程,当分维达到某一临界值时,裂纹失稳扩展。     

Characterization of grain-boundary configuration and fracture surface roughness by fractal geometry and creep-rupture properties of metallic materials

Grain-boundary configuration in heat-treated specimens and fracture surface roughness in creep-ruptured specimens of several kinds of metallic material were quantitatively evaluated on the basis of fractal geometry. Correlations between the fractal dimension of grain boundary, that of fracture surface profile, the creep-rupture properties and the fracture mechanisms of the alloys are discussed. In heat-resistant alloys, the fractal dimension of a nominally serrated grain boundary was always larger than that of a straight grain boundary in the same alloy. The relative importance of the ruggedness of grain boundaries was estimated by the fractal dimension difference between these two grain boundaries. There was a quantitative relationship between the increase of the fractal dimension of the grain boundary and the improvement of rupture ductility and rupture strength owing to grain-boundary serration in the alloy. A similar correlation was also found between the increase in the fractal dimension of the fracture surface profile and the improvement of the creep-rupture properties, since in some cases the fractal dimension of the fracture surface profile was correlated with that of the grain boundary. Both grain boundary and fracture surface profile were assumed to exhibit a fractal nature between one grain boundary length (upper bound) and an interatomic spacing (lower bound). In carbon steels with ferrite-pearlite structure, according to the increase in pearlite volume fraction, the rupture ductility decreased and the fracture mechanism changed from transgranular fracture in pure iron and low-carbon steels to intergranular fracture at ferrite-pearlite grain boundaries in medium-carbon steels, and further to intergranular fracture at pearlite grain boundaries in high-carbon steels. The correspondence between the fractal dimension of the grain boundary and that of the fracture surface was confirmed in ruptured specimens of ferrite-pearlite steels when the grain boundary was the fracture path.     

Characterization of 2-dimensional crack propagation behavior by simulation and analysis

Crack propagation behavior in 2-dimensional polycrystals is simulated and analyzed as a function of the fracture toughness of the grain boundary. The path of a crack impinging on a grain boundary is determined by the competition theory between intergranular and transgranular propagation. With decreasing boundary toughness, the tendency of intergranular propagation increases and the apparent fracture toughness of the polycrystal decreases. The results of the 2-dimensional analysis are compared with the simulation, and the advantages and limitations are discussed. The grain boundary toughness is evaluated by comparing the simulated crack paths with direct observations, resulting in a reasonable value for alumina ceramics. The fracture behavior is characterized in a macro-scale by the percentage of transgranular fracture and also in a micro-scale by the distribution of crack deflection angles.     

Analysis of the fractal dimension of grain boundaries of AA7050 aluminum alloys and its relationship to fracture toughness

Quantitative analysis of the grain boundaries in partially recrystallized microstructures of heat-treated 7050 aluminum alloys has been performed. Fractal dimensions of the extracted grain boundaries were calculated by box-counting method. Five different types of tear-tested materials rolled in different processes each at two orientations of 0° and 90° were studied. Efforts were made to connect the fractal dimensions of grain boundaries in the crack propagation direction to the fracture toughness (unit propagation energy, UPE, in tear test). The results show that there is a linear correlation between UPE and the fractal dimensions of the grain boundaries along the crack propagation direction for both 0° and 90° samples. The dependence corresponds well with the observation of transition from intergranular fracture to transgranular fracture with the increase of UPE. Quantitative analysis has also been performed on the micrographs to estimate the degree of recrystallization and the grain size in crack growth direction. No correlation between the fraction of recrystallized grains and the UPE could be detected.     

U75V重轨钢弯曲疲劳裂纹扩展行为EI北大核心

为明确珠光体钢轨的疲劳裂纹扩展行为,测定U75V重轨钢轧态和热处理态两种条件下的三点弯曲疲劳裂纹扩展速率,采用光学显微镜、扫描电镜、EBSD对钢轨的微观组织、片层、断口形貌及裂纹扩展轨迹进行观察。结果表明:轧态和热处理态钢轨的疲劳辉纹平均间距分别为253,215 nm,轧态钢轨的疲劳断口呈现解理台阶与河流花样形貌,且河流花样趋于合并,而热处理态钢轨的疲劳断口呈现大量的解理台阶及较多的微裂纹和撕裂棱,河流花样以支流为主;热处理态钢轨的疲劳裂纹扩展速率远低于轧态,到达裂纹失稳阶段也较滞后;轧态和热处理态钢轨的疲劳裂纹扩展都是以穿晶断裂为主的穿晶断裂和沿晶断裂混合扩展方式进行,轧态和热处理态钢轨的珠光体片层间距分别为272,148 nm,其中热处理态钢轨的珠光体片层细密且方向多样,存在显著的珠光体团簇,裂纹扩展轨迹中出现较多的分支裂纹和裂纹桥接现象,对扩展起到阻碍作用,是热处理态钢轨抗疲劳裂纹扩展能力优于轧态的重要原因。     

Numerical simulation of intergranular and transgranular crack propagation in ferroelectric polycrystals

We present a phase-field model to simulate intergranular and transgranular crack propagation in ferroelectric polycrystals. The proposed model couples three phase-fields describing (1) the polycrystalline structure, (2) the location of the cracks, and (3) the ferroelectric domain microstructure. Different polycrystalline microstructures are obtained from computer simulations of grain growth. Then, a phase-field model for fracture in ferroelectric single-crystals is extended to polycrystals by incorporating the differential fracture toughness of the bulk and the grain boundaries, and the different crystal orientations of the grains. Our simulation results show intergranular crack propagation in fine-grain microstructures, while transgranular crack propagation is observed in coarse grains. Crack deflection is shown as the main toughening mechanism in the fine-grain structure. Due to the ferroelectric domain switching mechanism, noticeable fracture toughness enhancement is also obtained for transgranular crack propagation. These observations agree with experiment.     

Fractal dimension of metallic fracture surface

In this study, a complete method of determination of the fractal dimension for fracture surfaces of ferrous alloys has been proposed. This dimension is determined for the vertical profile obtained by the profile technique cross-section. The image of the profile, seen through the microscope coupled with a camera, is recorded in a computer, where numerical processing is performed. For calculation of the same fractal dimension, the fd3 program has been used, which is available through the Internet. The essential element of the method is optimisation concerning microscopic magnification (scale of a length), resolution of the recorded image and selection of the grey level threshold at binarization. The tests for the stability of discretization, which enable minimization of the error of the measurement, have also been carried out. These tests consist in checking the difference in fractal dimensions for the same profile obtained in two different methods of contouring as well as the difference between capacitive, informative and correlative dimensions. In both cases, too big difference suggests that the determined dimension is not reliable. This method allows determination of the fractal dimension with an absolute accuracy of 0.05 in non-dimensional units. The method has been employed in many studies. In this paper the following tests have been presented: a “fractal map” of the fracture surface was made, an influence of the mechanical notch radius in a compact specimen on the fractal dimension of the fracture surface, an influence of the distortion rate on the fractal dimension, an effect of fatigue crack propagation rate on the fractal dimension and influence of the stress-intensity factor on the fractal dimension of the fracture surface. The following materials were examined: Armco iron, P355N steel and 41Cr4 steel in different states after the heat treatment. The measurements have been made for the specimens of the compact type. There was considered an influence of location of the place of measurement on the fractal dimension being determined. The dimension was determined on the profiles lying longwise and crosswise the crack propagation direction. It has been found that the fractal dimension of the fracture surface does not depend on a place of measurement. This suggests, among other things, that a distinction between the places, which were created under conditions of the plane stress, and the places, which were created under conditions of the plane strain state, cannot be made with the help of the fractal dimension. When testing an influence of the radius of the mechanical tip notch on the fractal dimension of a fracture surface, this dimension was determined in the places located at different distances from the tip of the mechanical notch. With respect to the radii up to 1.0 mm, no significant differences in fractal dimensions have been found. The fractal dimensions of the fracture surface for all examined materials were practically the same and they ranged from 2.02 to 2.10. However in some ranges of da/dN rate the dimension was changing inversely proportional to da/dN. Obtained results confirm that fractal dimension do not depend on the investigated material.     

Low Shear Strength and Shear-Induced Failure in Ti3SiC2

Shear strength and shear-induced Hertzian contact damage in Ti3SiC2 were investigated using double-notched-beam specimen and steel spherical indenter, respectively. The shear strength of 40 MPa that was only about 10% of bending strength was obtained for this novel ceramic. The SEM fractograph of specimens failed in shear test indicated a combination of intergranular and transgranular fracture. Under a contact load, plastic indent without cone crack could be formed on the surface of Ti3SiC2 sample. Optical observation on side view showed half-circle cracks around the damage zone below the indent, and the crack shape was consistent with the contrail of the principal shearing stress. The low shear strength and the shearing-activated intergranular sliding were confirmed being the key factors for failure in Ti3SiC2.     

Stochastic analysis on crack path of polycrystalline ceramics based on the difference between the released energies in crack propagation

The crack path of polycrystalline ceramics has been theoretically analysed with a stochastic model based on the difference between the released energies in intergranular and transgranular crack propagation. Assuming that the path with the lowest released energy should be realized as the actual crack path, the expected values of the fraction of transgranular fracture on fracture surface and the fracture toughness of polycrystalline ceramics were formulated as functions of grain size and the critical energy release rates of grain and grain boundary. By comparison between the theory and the experimental results it was shown that the stochastic model proposed here expressed the change of the crack path and the fracture toughness of polycrystalline Al₂O₃, relative to grain size. This revised version was published online in November 2006 with corrections to the Cover Date.     

高氮奥氏体钢低温脆断中的三种断裂刻面

用扫描电镜对18Cr-18Mn-0.7N高氮奥氏体钢低温脆断中裂纹的形成、扩展及断面进行了分析.结果表明,低温脆断中,首先沿退火孪晶界及晶界形成微裂纹,微裂纹穿晶连接使裂纹扩展,其结果导致断口上形成三种断裂刻面,即光滑平面状退火孪晶界断裂刻面、光滑曲面状沿晶断裂刻面及粗糙不平的穿晶断裂刻面.     

On the stress corrosion cracking mechanisms of austenitic stainless steels

In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.     

Failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting. Part I: Material and fractographic characterization

The failure analysis of a martensitic stainless steel (CA-15M) roll manufactured by centrifugal casting and used in cast glass rolling was carried out by means of traditional characterization techniques (optical metallography, SEM, EDX microanalysis, tensile testing and XRD). The roll was in the as-cast condition and its microstructure featured large proportion of δ ferrite (between 20% and 27%) in a martensitic (α′) matrix, with the δ/α′ interfaces presenting an intergranular network of M₂₃C₆ carbides. The crack propagation began in the internal surface of the roll, with δ/α′ intergranular and transgranular cleavage in the “equiaxed region” of the casting, progressing to δ/α′ intergranular ductile fracture in the “columnar” and “chilled regions”. Tensile thermal stresses in the internal surface of the roll associated with microstructural embrittlement (network of interfacial carbide and microporosities) are thought to be the main causes for the premature failure of the roll. Finally, materials selection was performed to replace the CA-15M stainless steel with another class of stainless steel for centrifugal casting.     

The Influence of Hydrogen on the Low Cycle Fatigue Behavior of Medium Strength 3.5NiCrMoV Steel Studied Using Notched Specimens

The influence of hydrogen on low cycle fatigue (LCF) of 3.5NiCrMoV steel electrochemically hydrogen charged in the acidified pH 2 0.1 M Na₂SO₄ solution is studied. In the presence of hydrogen, the fatigue life decreases significantly by ≈70 to ≈80% by: (i) the crack initiation period is decreased; and (ii) the crack growth rate is accelerated. SEM observation indicates that in the presence of hydrogen, the fracture surface shows flat transgranular fracture with vague striations and some intergranular fracture at lower stresses. The fatigue crack growth rate increases with increasing cyclic stress amplitude and with hydrogen fugacity. Once the fatigue crack reaches a critical length, the specimen becomes mechanical unstable and fracture due to ductile overload occurs. The hydrogen contribution to the final fracture process is not significant.

     

Three-dimensional analysis of fracture,corrosion and wear surfaces

A brief look at the history of fractography has shown a recent trend in the quantification of topographic parameters through the use of three-dimensional reconstruction techniques, which associate SEM stereoscopy and stereophotogrammetry software, allowing the calculation of the elevation measurement at numerous points of the topography due to the parallax that takes place during the tilting of the sample along the microscope eucentric plane. Several investigators have used reconstruction techniques to correlate some fractographic parameters, such as fractal dimension and fractured to projected area ratio, to the mechanical properties of materials, such as fracture toughness and tensile strength. So far, the search for a clear relationship between the fracture topography and mechanical properties has provided ambiguous results. The present work applied a surface metrology software to reconstruct three-dimensionally fracture surfaces (transgranular cleavage, intergranular and dimple fracture), corrosion pits and tribo-surfaces in order to explore the potential of this stereophotogrammetry technique. The existence of a variation in the calculated topographic parameters with the conditions of SEM image acquisition reinforces the importance of both good image acquisition and accurate calibration methods in order to validate this 3D reconstruction technique in metrological terms. Preliminary results did not indicate the existence of a clear relationship between either the true to project area ratio and CVN absorbed energy or the fractal dimension and CVN absorbed energy. It is likely that each fracture mechanism presents a proper relationship between the fractographic parameters and mechanical properties.     

Self-similarity of the Crack Front in Stress Corrosion Fracture

In this letter, a new method was proposed to visualize the crack front in stress corrosion fracture of high strength steel. The fractal characteristic of the crack front was analyzed by measuring its fractal dimension and roughness exponent at the same time. The experimental results show that the irregularity of the crack front was partly self-affine.     

Conditions of invariance of corrosion crack resistance characteristics

Conclusions It has been established that for a number of materials the condition of invariance of fracture toughness depends not only upon the type of material but also upon its structure.For a high-strength steel with a martensitic structure the kinetics of subcritical crack growth and also the parameter K_Iscc are sensitive to the original austenitic grain size. Heat treatment for coarse grains has a favorable influence on the corrosion crack resistance of such a steel. The creation in coarse-grained steel of serrated austenitic grain boundaries leads to an additional increase in resistance to corrosion crack growth.The generally accepted criterion of invariance of fracture toughness Eq. (1) is unsuitable as a condition guaranteeing no change in corrosion crack resistance parameters. The value of the coefficient A_c in Eq. (2), which characterizes the condition of obtaining the parameter K_Iscc independent of sample thickness, is determined by the structure of the material and for the systems considered is more than 500.Corrosion cracks in steel with a martensitic structure may have a complex morphology dependent upon the subcritical crack growth mechanism, the size of the austenitic grains, and the form of their boundaries. In contrast to fine-grained and overheated steel, for which intergranular subcritical crack growth is characteristic, in steel with serrated grain boundaries the subcritical crack growth mechanism is more complex. It was also observed that in the center layers of thick samples there is primarily transgranular failure replaced by intergranular at the transition to the surface layers.To determine the effective stress intensity factor at the tip of a corrosion crack with a complex trajectory, a method based on determining the pliability of a sample with a crack propagating in a curved trajectory was found to be effective.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 17, No. 3, pp. 24–33, May–June, 1981.     

16MnD5钢板焊接件热影响区开裂原因分析

采用化学成分分析、低倍检验、金相检验、显微硬度测试等方法对16MnD5低合金高强度钢板焊接件热影响区开裂的原因进行了分析。结果表明:裂纹起始于热影响区粗晶区,裂纹以沿晶和穿晶混合方式扩展,为以残余应力和母材淬硬倾向为主因的冷裂纹。     

Modes of failure under creep/fatigue loading of a nickel-based superalloy

Crack growth experiments have been carried out under combined creep and fatigue loading at 700° C on a hot isostatically pressed powder nickel alloy. A fractographic investigation has been undertaken of the modes of failure over a frequency range of 0.001 to 10 Hz. The observations indicate that under static loading and at low frequencies failure is intergranular and controlled by creep processes, whereas at high frequencies a transgranular fatigue fracture is obtained. The transition from creep to fatigue behaviour is found to be progressive, and to begin at a lower frequency the higher the ratio of cyclic to mean load. In the transition region a mixed intergranular and transgranular fracture surface is observed, which correlates well with the recorded proportion of creep to fatigue crack growth.     

Geometrical analysis and pattern recognition using mapping technologies of three-dimensional fracture surfaces in materials

Geometrical analysis of fracture surfaces in materials was made using newly developed computer programs on the three-dimensional images reconstructed by the stereo matching method. The global value of the fractal dimension of the fracture surface was estimated by the box-counting method on a fatigue fracture surface of a Cu-Be alloy and impact fracture surfaces of a SiC and an alumina. The results of the present analysis were well correlated with those of the two-dimensional fractal analysis. The fractal dimension map (FDM) by the box-counting method and the surface roughness map (SRM) proposed in this study can give important information about the local fracture mechanisms, the crack growth direction or the fracture origin in materials. FDM and SRM have interesting characteristics by which one can discriminate the flat regions, the regions of complex geometry or the steeply inclined areas on a given fracture surface. Pattern recognition using mapping technologies of FDM and SRM is also applicable to the extraction of “hidden patterns” on fracture surfaces, which cannot be observed only by microscopes.