Boundary Fracture Research Articles

Influence of Grain Boundary Properties on Microcracking of AlN in Sliding Test

Grain boundary microcracking in sliding test using a silicon pin was evaluated on several kinds of AlN to investigate the behavior of damage generation and the influence of material properties on it. The grain boundary fracture toughness (KIC gb ) was estimated from a percentage of the intergranular fracture as a grain boundary property for each specimen. KIC gb was greatly influensed by an amount and kinds of additives. After the sliding test, silicon debris accumulation and grain boundary microcracking of AlN were observed at the wear traces on the surface of AlN. The density of grain boundary microcracks increased linearly with increasing the contact load. Empirical relationship indicated that its density depended on KIC gb and the AlN grain size, and decreased mainly with increasing KIC gb. The density of grain boundary microcracks greatly degreased at the KIC gbvalue above 1.9 (MPa·m1/2).

Fatigue Crack Propagation under Combined Cyclic Mechanical Loading and Electric Field in Piezoelectric Ceramics

Fatigue crack propagation tests of PZT specimens were performed under cyclic four-point bending with and without superposition of electric fields. The specimens were poled in the longitudinal direction (PL specimens) perpendicular to the crack plane. The crack propagation rate for the case of open circuit was faster than that for the case of short circuit. The application of a negative or positive electric field parallel to the poling direction accelerated the crack propagation rate, and the amount of acceleration was larger for the case of the negative field. The change of the crack propagation rate with crack extension can be divided into three regions. In the region I, the crack propagation rate decreases with increasing crack length, and then turn to increase in the region III. In the region II, the propagation rate is nearly constant. The mechanisms of fatigue crack propagation were correlated to domain switching near the crack tip. The grain boundary fracture was predominant in the low-rate region, while transgranular fracture became abundant on the unstable fracture surface.

Structure-dependent grain boundary deformation and fracture at high temperatures

This paper gives an overview of structure-dependent intergranular deformation and fracture of metallic bicrystals and polycrystals at high temperatures. Structure-dependent grain boundary sliding and migration during high temperature deformation are discussed in connection with grain boundary structural change by the interaction with lattice dislocations and by intrinsic and extrinsic grain boundary structural transformation due to temperature and segregation, respectively. “Grain boundary engineering” for improvement in creep strength, development of superplasticity and control of oxidation-assisted intergranular brittleness are introduced which were successfully achieved by engineering the grain boundary microstructures characterized by the grain boundary character distribution (GBCD), the grain boundary connectivity and grain size.

Grain boundary segregation and fracture

Abstract This paper describes a combined thermodynamic and micro-mechanical model for the propagation of a grain boundary crack through a solid in which stress concentrations ahead of the crack and plastic deformation around the crack tip are considered. It is shown that critical factors are impurity segregation concentration, grain boundary structure, reflected in the Σ value of the boundary, and temperature. Comparisons of the model predictions are made with other models of grain boundary binding by Sutton. Additionally, the model predictions are compared with several experimental observations of grain boundary fracture strength. It is shown that ductile to brittle transition temperature data for steels are a valuable source of experimental grain boundary fracture information for this important range of commercial materials.

Equilibrium crystal shape of Bi-saturated Cu crystals at 1223 K

The equilibrium crystal shape (ECS) of Bi-saturated Cu at 1223 K has been studied, after quenching the material to room temperature, by a scanning electron microscopy investigation of the shapes of pores (or negative crystals) that are present on grain boundary fracture surfaces. Bi-saturation of Cu surfaces at 1223 K results in dramatic changes in the ECS compared to pure Cu. Whereas all possible surface orientations are stable in the case of pure Cu, only the {1 1 1}, {1 0 0}, {1 1 0}, and {3 2 0} orientations are stable in Bi-saturated Cu crystals. In addition, the weak maximum anisotropy of pure Cu of 2% is increased fourfold by Bi saturation of the surface.

Prediction and Control of Grain Boundary Fracture in Brittle Materials on the Basis of the Strongest-Link Theory

This paper discusses micropstructural aspects of brittleness fracture of polycrystalline materials caused by intergranular fracture. Structure-dependent intergranular brittle fracture in bicrystals and polycrystals are discussed and predicted theoretically. Experimental evidence for the structure-dependent intergranular fracture is shown and some general features are discussed to demonstrate the relationship between grain boundary structure/character, grain boundary energy and intergranular fracture strength. Theoretical prediction of the fracture toughness based on the strongest-link theory is introduced for polycrystals with different grain boundary microstructures, primarily defined by the grain boundary character distribution, grain boundary connectivity. Finally recent achievements of successful control of intergranular brittleness by grain boundary engineering based on the strongest-link theory are introduced for different materials.

Assessment of Neutron Irradiation-Induced Grain Boundary Embrittlement by Phosphorous Segregation in a Reactor Pressure Vessel Steel

Abstract The materials used were two sorts of reactor pressure vessel steels (RPVSs), which contain different amounts of impurity phosphorous (P) and copper (Cu). The specimens for Charpy V-notch impact tests and Auger electron spectroscopy (AES) were irradiated in the Japan Materials Test Reactor (JMTR) at 290°C up to fluences of 6 × 1021 and 1 × 1024 n/m2 using a multi-division temperature control capsule which enables removal of a part of the sub-capsules during operation of the reactor. Neutron irradiation resulted in a significant shift in the ductile-brittle transition temperature (DBTT) accompanied by a large irradiation hardening in the high P and high Cu steel. The AES measurements following the irradiations revealed that almost no phosphorous segregation occurred at grain boundaries. The DBTT shifts by neutron irradiation were reasonably interpreted in terms of a so-called hardening mechanism. The SEM observations of the fractured surface indicated that a very small amount of grain boundary fracture was induced at the irradiation conditions, and resultantly no grain boundary embrittlement was observed. Based on the results of irradiation experiments as well as long-term thermal aging experiments beyond 16 000 h, neutron irradiation-induced grain boundary embrittlement is considered to rarely happen to the RPVS that contains 110 wppm of phosphorous.

Grain Boundary Phosphorous Segregation and Its Influence on the Ductile Brittle Transition Temperature in Reactor Pressure Vessel Steels

Abstract The materials used for this work were two sorts of reactor pressure vessel (RPV) steels, which contain different amounts of phosphorous (P), namely 0.011 and 0.002 wt%. The specimens for Charpy V-notch (CVN) impact test, Auger electron spectroscopy (AES), and tensile test were thermally aged at 400, 450, and 500°C for 1000, 3000, and 5000 h. After the thermal aging, the AES specimens were broken in the AES chamber to measure the P concentration at grain boundaries. The AES measurements for as-received specimens were carried out following hydrogen charging so that grain boundary facets were available even without P segregation. The AES measurements revealed that the peak height ratio (PHR) of P/Fe at the grain boundaries of the high-P steel were 0.066, 0.141, and 0.120 in the specimens aged at 400°C for 3000 h, 450°C for 3000 h, and 500°C for 1000 h, respectively. The ductile-brittle transition temperature (DBTT) was measured for the aged specimens, and the ΔDBTT of 15K was observed only for the specimen aged at 450°C for 3000 h, although no changes in the hardness and tensile properties were observed. The grain boundary fracture ratio (GBFR) increased with increasing the PHR of P/Fe. Grain boundary fracture mode was located at the area close to the V-notch root. There was a good relationship among PHR, GBFR, and DBTT, indicating directly that the shift in the DBTT was due to grain boundary embrittlement caused by P segregation.

3-D Modelling of Campi Flegrei Ground Deformations: Role of Caldera Boundary Discontinuities

Campi Flegrei is a caldera complex located west of Naples, Italy. The last eruption occurred in 1538, although the volcano has produced unrest episodes since then, involving rapid and large ground movements (up to 2 m vertical in two years), accompanied by intense seismic activity. Surface ground displacements detected by various techniques (mainly InSAR and levelling) for the 1970 to 1996 period can be modelled by a shallow point source in an elastic half-space, however the source depth is not compatible with seismic and drill hole observations, which suggest a magma chamber just below 4 km depth. This apparent paradox has been explained by the presence of boundary fractures marking the caldera collapse. We present here the first full 3-D modelling for the unrest of 1982–1985 including the effect of caldera bordering fractures and the topography. To model the presence of topography and of the complex caldera rim discontinuities, we used a mixed boundary elements method. The a priori caldera geometry is determined initially from gravimetric modelling results and refined by inversion. The presence of the caldera discontinuities allows a fit to the 1982–1985 levelling data as good as, or better than, in the continuous half-space case, with quite a different source depth which fits the actual magma chamber position as seen from seismic waves. These results show the importance of volcanic structures, and mainly of caldera collapses, in ground deformation episodes.

Swelling and post-irradiated deformation structures in 18Cr–10Ni–Ti irradiated with heavy ions

The current work presents irradiation microstructure evolution in 18Cr–10Ni–Ti SS after Cr ion irradiation and tensile deformation. TEM and tensile samples were irradiated to a total dose of 1, 5, 10 and 100 dpa with 2 MeV Cr 3+ ions at 305 °C (8 × 10 −4 dpa/s) and 600 °C (10 −2 dpa), respectively. Damage structure at 305 °C consists of high density of black dots, loops, which are less than 10 nm in diameter (1–5 dpa), at saturation number density 2 × 10 22 m −3. Deformation behavior of irradiated steel is manifested as a transition from twinning to dislocation channeling with test temperature and dose increasing. Damage at 635 °C is a different volumes of swelling: ∼0.2–0.4% at 5 dpa and ∼20% at 100 dpa. Irradiated samples showed a strong role of voids in the matrix and grain boundary fracture processes.

Chemistry of grain boundaries in mantle rocks

The compositions of olivine grain boundaries have been analyzed with scanning transmission electron microscopy (STEM) via energy dispersive X-ray (EDX) spectrum profiling in three specimens: a peridotite ultramylonite, olivine phenocrysts in a basaltic rock, and synthesized compacts of olivine + diopside. Composition profiles across grain boundaries in both natural and synthetic samples exhibit a characteristic width of 5 nm and a depletion of Mg and concomitant enrichments of Ca, Al, Ti, and Cr. Chemical segregation is known to affect grain boundary processes such as grain boundary diffusion, sliding, fracture, and migration, all of which influence the rheological properties of polycrystalline aggregates. Also, because grain boundaries are enriched in trace elements, the boundaries can be important storage sites for such elements in mantle rocks. Mantlederived melts with unusual compositions, such as those rich in Ca and/or Ti, might be explained by preferential melting of olivine grain boundaries enriched in these elements. The common chemical signatures at grain boundaries in all samples indicate that chemical segregation is an energetically favorable phenomenon and thus should occur elsewhere in Earth’s mantle. Segregation of trace elements to grain boundaries may play an important role in dynamical and geochemical processes in Earth’s mantle.

A factor to predict microcrack nucleation at γ–γ grain boundaries in TiAl

Grain boundary microcracking between γ-TiAl grains has been studied in a duplex TiAl alloy. Microcracking initiated at impinging deformation twins. A predictive factor has been developed which incorporates contributions from deformation twinning and deformation transfer, which correlates with grain boundary fracture.

Interphase boundary fracture and grain boundary precipitation of Ni 3Al(γ′) phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni 3Al(γ′) precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ′-film satisfied the Kurdjumov–Sachs (K–S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ′) interphase boundary deviating from the K–S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ′-film to β crystal across coherent (β/γ′) interface was observed, when the variant of γ′-film satisfied the K–S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K–S relation, the boundary structure of incoherent (β/γ′) interfaces was discussed using O-lattice theory and transmission electron microscopic observations.

Electrolytic Damage in Zirconia Electrolytes

Yttria stabilized zirconia (YSZ) electrolytes, subjected to high current densities, develop significant damage. This paper characterizes the damage that develops from the anode side, in YSZ used in an oxygen pump configuration. The damage is in the form of vacancy loops, precipitates formed on dislocations, pore development, and grain boundary fracture. The damage initiates at elevated current densities, well below the decomposition potential of YSZ. A mechanism, involving mobile oxygen interstitials, is proposed.

Fracture toughness and surface energies of minerals: theoretical estimates for oxides, sulphides, silicates and halides

Theoretical estimates of the ideal fracture toughness and surface energies of 48 minerals have been modelled by treating them as ionic solids, using the Born model of bonding. Development of the toughness model required calculation of the crystal binding enthalpy from thermodynamic data and the use of published elastic constants for single crystals. The principal minerals studied were oxides, sulphides and silicates, plus a few halides and sulphates. The study showed grain boundary fracture is most likely in single-phase polycrystalline minerals. However, the fracture toughness for grain boundary cracking in pure minerals is not significantly lower than that for intragranular cracking. The computed critical stress intensity values for intragranular cracking, K IC, ranged from 0.131 to 2.774 MPa m 1/2. The critical energy release rates for intragranular cracking, G IC , ranged from 0.676 to 20.75 J m −2. The results are discussed with relevance to mineral comminution, including energy considerations, particle impact efficiency, and lower limiting particle size.

Crack growth in a nearly fully-lamellar gamma TiAl alloy at 650 °C and 800 °C under constant load conditions

The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities (K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800 °C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation.

Grain Boundary Segregation in NI-Base Alloys: an Integrated Approach using FIB, TEM and SIMS

Segregation of elements to the grain boundaries in Ni-base alloys can have a large effect on the mechanical and corrosion properties of these materials. The extent of segregation, whether it is equilibrium or non-equilibrium segregation, is dependent upon the thermo-mechanical treatments applied to the alloy. in order to determine if a particular thermo-mechanical process delivers the desired microstructure, a complete microstructural analysis including an examination of grain boundary segregation must be performed.In the past, TEM has been used to identify the various phases and precipitates in these materials through the use of bright- and dark-field imaging, electron diffraction, and EDS. However, one of the elements that can play a large role in determining the properties of these alloys is boron. in this group of alloys, boron is generally present in bulk analyses in only a few tens of ppm, and as a result it is difficult to detect using the aforementioned techniques. More commonly, boron segregation to the grain boundaries is usually studied by means of Auger analysis, whereby the sample is charged with hydrogen in order to promote brittle intergranular fracture in-situ in the Auger microprobe. Boron can then be detected at the surface of the clean grain boundary fracture surfaces.

Grain boundary segregation and fracture resistance of engineering steels

AbstractThe developments in surface analytical techniques and experimental measurements have made it possible to understand grain boundary segregation processes and related fracture mechanisms. This article shows how experimental contributions have led to the concept of grain boundary segregation isotherms for a complete understanding of grain boundary segregation behavior, and its effect on material properties in terms of the kinetics of segregation, the interaction processes amongst trace and solute elements and the influence of solute elements, heat treatment practice and applied tensile stress on grain boundary segregation processes. The determining role of grain boundary chemistry in obtaining a marked improvement in toughness of engineering steels at the specified levels of strength is elucidated here. Copyright © 2001 John Wiley & Sons, Ltd.

Monte-Carlo simulations of life expectancy using the dual boundary element method

Ageing aircraft structures are susceptible to a very typical and dangerous form of fatigue damage named “multiple site damage”. An overall assessment method has been developed in order to improve the understanding of this phenomenon and to provide inspection programs. A full deterministic approach has been completed first. An automatic numerical process, using the dual boundary element method and fracture mechanics laws, computes the crack evolution until failure of the structure. Then, in order to take materials scatter into account, a probabilistic approach has been developed based on Monte-Carlo simulations. Initial cracks scenarios are randomly defined and cracks evolution is computed for each of them. Thus statistical results are provided. The results obtained are in good agreement with experimental tests carried out at the Aerospatiale Matra research center.

Tensile behaviour of type 304 austenitic stainless steels in hydrogen atmosphere at low temperatures

The tensile behaviour of solution annealed type 304L, solution annealed type 304, and solution annealed and sensitised type 304 stainless steels was investigated in hydrogen and helium under a pressure of 1·1 MPa over the temperature range 300–80 K at strain rates ranging from 4·2×10-5 to 4·2×10-2 s-1. For 304L steel, hydrogen environment embrittlement (HEE) increased with decreasing strain rate. For 304L and 304 steels, HEE increased with decreasing temperature, reached a maximum, and then decreased with further decrease in temperature: the decrease was particularly rapid near the minimum temperature for HEE. Sensitisation enhanced the HEE of 304 steel. Above the maximum HEE temperature, the HEE behaviour was similar to the hydrogen embrittlement behaviour of materials in previous studies, but near the minimum temperature for HEE it was different. Three types of hydrogen induced brittle fracture were observed as a result of HEE: transgranular fracture along strain induced martensite laths and twin boundary fracture on the fracture surfaces of solution annealed 304L and 304 steels, and grain boundary fracture on the sensitised 304 steel. It was found that from room temperature to the maximum HEE temperature, the HEE of the materials depended on the transformation of strain induced martensite and below the maximum HEE temperature it depended on the diffusion of hydrogen.