Intergranular Failure Mode Research Articles

Dynamic fracture mechanism and behaviour of V-notch white marble specimens with strain gauge and digital image correlation in three-point bending tests

A three-point bending test was conducted on white marble specimens with V-shaped notches. The fracture mechanism of these specimens, including crack initiation toughness and propagation speed under different opening angles, was studied, and their special fracture behaviour was analysed. In this experiment, a falling weight loading device was used to impact the V-notch white marble specimens with different opening angles at the same impact speed. To measure the crack tip initiation toughness, a strain gauge theory is formulated in this study. The calculation results are in good agreement with the digital image correlation (DIC) calculation results. The error is approximately 10%, and the results obtained by the acute angle patching method are stable. The experimental results also show that when the opening angle is large, the specimens crack, and the energy dissipated by the crack initiation is considerable. Owing to the particularity of the white marble structure, the rock fracture behaviour during the dynamic fracture process is clearly observed in the experiment. All the specimens exhibited spreading crack branches at the crack tip under stress. The foregoing occurs because the original defects at the crack tip deform and grow after sustaining stress. Some of the cracks combine to form the main crack and produce branches. In this study, the microcrack areas on the crack tip and fracture path observed in the experiment are collectively referred to as the crack growth area. The expansion of the crack growth area precedes the expansion of the main crack. The expansion of the strain concentration area is calculated by DIC; it reflects the dynamic formation of cracks on the surface of the specimen. Scanning electron microscopy shows that microcracks mainly occur under the intergranular failure mode.

Effect of tool pin geometry and multi-pass intermittent friction stir processing on the surface properties of aerospace grade aluminium 7075 alloy

Multiple pass intermittent friction stir processing was conducted on the surface of an aerospace grade AA7075 alloy using pentagonal and hexagonal profiled pins and its microstructural evolution and mechanical properties were investigated. Results showed a defect-free cross-section with better material mixing for the specimens processed using pentagonal pin. Application of 6 intermittent friction stir processing passes reduced average grain size to 2.32 μm as a result of dynamic recrystallization. Texture index improved after 6 intermittent friction stir processing passes with a higher volume of deformation textures. Microhardness and tensile strength of the alloy increased to 125.5 HV, and 476 ± 10 MPa after 6 intermittent friction stir processing passes due to precipitation strengthening, grain refinement, and dislocation density. Fracture surface investigation revealed an intergranular mode of failure.

Root cause analysis of the failure of the steam generator orifice assembly of a test reactor

Radiographic examination performed based on the increase in flow through one of the steam generators (SG) indicated the physical separation of orifice assembly from the orifice holder. Subsequently, this failed orifice assembly was cut from the feed water inlet header of the SG Module and subjected to further investigation. Both metallurgical and mechanics based post-failure investigations were performed. The failure of orifice assembly is attributed to stress corrosion cracking (SCC). Scanning electron microscopic (SEM) observation of the fractured surface showed an inter-granular mode of SCC failure. SCC occurred due to the residual tensile stresses developed due to the geometrical constraint against the thermal shrinkage during welding.A minimum gap of 160 µm at the top mating portion of the orifice assembly before the welding could have absorbed the thermal shrinkage to prevent the existence of sustained residual stresses and thereby SCC failure. Austenitic stainless steels are highly susceptible to SCC under the combined action of tensile stress and favourable environmental conditions. Under the favourable situation to SCC, cracks propagate slowly without any noticeable deformation until the stresses in the remaining ligament of metal exceed the fracture strength. Based on the insights gained from the root cause analysis, the design of the orifice assembly has been revised to mitigate a similar kind of failure in future.

Impact of annealing treatment on the microstructural-dependent mechanical properties of a cold-rolled Fe–Co–10V alloy

Various contributors have been suggested so far to explain the variation of mechanical properties of Fe–Co–V magnetic alloys, but a comprehensive understanding is still lacking. In the present study, the variation of hardness, tensile yield strength (YS), and elongation at room temperature were determined for an 86% cold-rolled Fe–50Co–10V (wt.%) alloy after annealing at the temperature range of 300–1050 °C for holding times from 10 to 240 min. The potential contributing factors were analyzed and discussed. Detailed microstructural evolution and magnetic properties have been reported earlier for this alloy. The results showed an increase in hardness and YS but a reduction in elongation after annealing at temperatures between 450 and 550 °C for all holding times. The activation energy of the contributing process in increasing the hardness after annealing between 300 and 550 °C was 132–144 kJ/mol, being a typical value for the ordering transition in ferrite in Fe–Co and Fe–Co–2V alloys. The YS increased up to an annealing temperature of 450 °C, followed by a decreasing trend at higher annealing temperatures even in the ordered state, obviously due to i) the recovery and recrystallization of ferrite, ii) decreasing the number of (Co,Fe)3V precipitates and iii) increasing fraction of retained austenite. The elongation decreased to nil after annealing at 450–550 °C, but it was improved even in the ordered state at 600 °C and above due to the same factors which decreased the YS. The intergranular failure mode after annealing below 600 °C converted to ductile mode at 650 °C. Considering the combination of mechanical and magnetic properties, the annealing treatment at 650 °C for 10 min can be recommended for this alloy, providing high YS, a few percent elongation, and high magnetic hardness.

Mechanical properties and fatigue crack growth in tungsten deposited by RF-plasma

Fracture toughness and fatigue crack growth rates of free-standing thick W deposits produced in a protective atmosphere using radio frequency inductively-coupled plasma process were investigated. Stress-strain behavior of the deposits was studied using four-point bending and their fracture toughness was estimated from three-point bending following the ASTM E1820 standard. The fatigue crack growth rates were measured in symmetrical bending using a resonance approach. The results were compared with commercially available bulk tungsten prepared by powder metallurgy that served as a reference. The results showed that the deposits had significantly lower stress-strain properties: the elastic modulus and tensile strength ranged from 30% to 50% of the reference bulk values. The crack growth rate tests together with fractographic analysis suggested that a static failure by inter-granular decohesion predetermines the fatigue properties of the deposits and causes high slopes of the corresponding fatigue crack growth rate curves. Contrary to this, the reference sheet failed trans-granularly. The inter-granular failure mode of the deposits was caused by grain boundary embrittlement, a phenomenon probably related to oxides. The lowest obtained oxygen concentration in deposit was nearly by two orders of magnitude higher, than in reference sheet. The distribution of oxygen was studied by TEM and the amorphous and crystalline oxide phases were identified at splat interfaces. The presence of oxygen in the deposits produced by the inherently non-oxidizing RF technology calls for control of oxygen content in the feedstock powder. The performed characterization provided baseline data of mechanical and fatigue properties of RF plasma sprayed tungsten.

Effects of γʹ size and carbide distribution on fatigue crack growth mechanisms at 650 °C in an advanced Ni-based superalloy

Two distinct and different unimodal γʹ size distributions have been produced in a disc alloy RR1000 to understand the combined effects of γʹ size (linked to slip character), grain boundary (GB) precipitates/carbides and dwell time on fatigue crack growth (FCG) mechanisms at elevated temperature. The FCG behaviour has been investigated on single edge notched bend samples of both γʹ variants at 650 °C under trapezoidal waveform loading with dwell times of 1 s and 90 s at maximum load. The fine γʹ variant has better FCG resistance for both dwell times, and the difference in FCG rate becomes even more marked with longer dwell time. The coarse γʹ variant exhibits faster and more time dependent FCG behaviour, with more evident intergranular fracture modes and rougher fracture surface than the fine γʹ variant. Intergranular failure modes become dominant with the 90 s dwell time in both variants. A more continuous distribution of carbides is seen on the GB in the coarse γʹ variant, which is likely to influence oxidation behaviour on GB and intergranular fracture behaviour. The effects of dwell time and consequent oxidation damage at the crack tip on fatigue behaviour are further investigated by transitioning between cyclic and time dependent regimes to assess the damage zones formed. The results show that an oxidation damage zone contributes to enhanced FCG after switching frequency from cyclic to time dependent regimes while a tortuous crack path is related to crack wake (shielding due to prior crack path tortuosity), resulting in improved FCG resistance.

High-temperature mechanical behavior of proton-conducting yttrium-doped barium zirconate perovskite

The high-temperature mechanical properties of yttrium doped barium zirconate obtained by two different routes are reported in this work. Polycrystals with composition BaZr0.85Y0.15O2.925 have been fabricated from nanopowders synthesized by a modified EDTA-citrate complexing method and conventionally sintered at 1600 °C for 24 h. The material exhibits a single cubic perovskite phase, with a homogeneous, dense and fine-grained microstructure consisting of equiaxed grains with an average size of 0.2 μm. Mechanical tests were carried out in compression between 1100 and 1325 °C in air at constant initial strain rate and at constant load. As the temperature increases and/or the strain rate decreases, a gradual transition from brittle-to-ductile behavior was found. In the brittle regime, the fracture is governed by an intergranular failure mode. In the ductile region, grain boundary sliding is the main deformation mechanism, characterized by a stress exponent of 2 and an activation energy of 570 kJ/mol.

Theoretical characterization of mesoscopic dynamic fracture behaviors of concrete based on an extended self-consistent finite stress model (XSFSM)

This study presents a theoretical approach to characterize the mesoscopic dynamic fracture behaviors of concrete. Firstly, different fracture behaviors are determined from energy criteria. Secondly, by considering the energy equivalence principle, the Saint-Venant’s principle and the Newton’s laws of motion, an extended self-consistent finite stress model (XSFSM) is established for dynamic fracture analysis of concrete-like cementitious materials. The XSFSM is validated and modified by comparing with the existing models in design codes and the curve fitting models in literatures. Thirdly, the XSFSM is utilized to propose the meso-level fracture criterion of concrete based on a morphological material model. Parametric study illustrates that the criterion can predict the transgranular and intergranular failure modes of concrete under quasi-static to dynamic loading. At last, assumptions and limitations of the present theoretical approach and its prospects in computational methods are discussed.

Failure of copper wires in a railway Motor Operation Contact

Many engineering alloys can fail by environmentally assisted cracking (EAC). The risk of failure is dependent on the specific combination of material, the environment and the stress. Identification of the stresses (applied and residual) can be difficult because of the retrospective nature of failure investigation and the commonly complex character of stresses in real engineering applications. A routine inspection of a Motor operation contact installed aside a railway line in the British countryside found that the copper wire contact had failed. Standard failure investigation techniques combined with fractography were applied to identify an intergranular failure mode. Experimental observations combined with a review of the relevant literature allowed the root cause to be assigned to environmental assisted cracking in the presence of gaseous ammonia in the local environment.

An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Superalloys

Cyclic near-threshold fatigue crack growth (FCG) behavior of two disk superalloys was evaluated and was shown to exhibit an unexpected sudden failure mode transition from a mostly transgranular failure mode at higher stress intensity factor ranges to an almost completely intergranular failure mode in the threshold regime. The change in failure modes was associated with a crossover of FCG resistance curves in which the conditions that produced higher FCG rates in the Paris regime resulted in lower FCG rates and increased ∆Kth values in the threshold region. High-resolution scanning and transmission electron microscopy were used to carefully characterize the crack tips at these near-threshold conditions. Formation of stable Al-oxide followed by Cr-oxide and Ti-oxides was found to occur at the crack tip prior to formation of unstable oxides. To contrast with the threshold failure mode regime, a quantitative assessment of the role that the intergranular failure mode has on cyclic FCG behavior in the Paris regime was also performed. It was demonstrated that even a very limited intergranular failure content dominates the FCG response under mixed mode failure conditions.

Strength of pure alumina ceramics above 1 GPa

Up to now, commercially available alumina ceramics were claimed to have strength between 400 and 550MPa. However, our study shows strength ~ 2 times higher for commercially available alumina than commonly believed. The average and characteristic strength, measured on 31 pure alumina ceramic discs by ball on three balls (B3B) test, were 1205 ± 93MPa and 1257MPa, respectively, with a Weibull modulus of m = 11.8. Tested specimens were in form of discs with a diameter of 5mm and thickness 0.5mm. The grain size distribution of the alumina is bimodal with an average grain size of ~ 850nm measured at the surface. The fracture reveals a mixed transgranular / intergranular failure mode. To avoid incorporation of additional flaws, the discs were tested as sintered. The characteristic flexural strength measured in B3B was recalculated according to Weibull theory for standard 4-point bending bars of size 3 × 4 × 45mm as bend 856MPa. The measured strength of nearly 900MPa shows the potential of strength for high purity alumina ceramics.

Metallurgical Analysis of Cracked AA7075 Aluminum Alloy Component Used in Control System of a Satellite Launch Vehicle

High-strength aluminum alloy AA7075 forgings are widely used in various control system components of liquid engines of satellite launch vehicles. In one such application, a forged component was found to crack under storage. Multiple cracks were noticed on the surface of the component. Fractographic features indicated intergranular mode of failure. Optical microscopic examination revealed dendritic coring in the material and crack propagation to be along the grain boundaries. Anodic dissolution of grain boundaries was also noticed. Based on detailed metallurgical investigations, it was concluded that the failure was due to ‘stress corrosion cracking.’

Peridynamic Modeling of Granular Fracture in Polycrystalline Materials

A new peridynamic (PD) formulation is developed for cubic polycrystalline materials. The new approach can be a good alternative to traditional techniques such as finite element method (FEM) and boundary element method (BEM). The formulation is validated by considering a polycrystal subjected to tension-loading condition and comparing the displacement field obtained from both PDs and FEM. Both static and dynamic loading conditions for initially damaged and undamaged structures are considered and the results of plane stress and plane strain configurations are compared. Finally, the effect of grain boundary strength, grain size, fracture toughness, and grain orientation on time-to-failure, crack speed, fracture behavior, and fracture morphology are investigated and the expected transgranular and intergranular failure modes are successfully captured. To the best of the authors' knowledge, this is the first time that a PD material model for cubic crystals is given in detail.

Fatigue resistance of a hot corrosion exposed disk superalloy at varied test temperatures

The fatigue resistance of the hot corrosion pitted ME3 disk superalloy was investigated. Low cycle fatigue specimens were subjected to hot corrosion exposures that produced pits on the gage sections. These specimens were tested at varied temperatures and strain ranges. Corrosion pitting influenced fatigue life and failure mode by varying degrees, depending on temperature and strain range. As observed through interrupted tests, fatigue cracks initiated at a smaller fraction of life for high-temperature tests, in comparison to that at low temperatures. Correspondingly, the crack initiation failure mode changed significantly with test temperature. While cracks initiated from the hot corrosion pits for all test conditions, at 704 °C the intergranular initiation failure mode was dominant, whereas at the lower temperatures cracks initiated within the pits from crystallographic facets. Finite element analyses were performed to quantify the effect of varying pit dimensions and spacing on elastic stress concentration. The highest stress concentration was calculated to occur at the narrow ligaments between overlapping hot corrosion pits. Increasing the number of overlapping pits did not further add to the stress concentration. There was good qualitative agreement between the calculated stress concentrations and the location of crack initiations for tests conducted at 704 °C but not for tests conducted at 204 °C.

Anisotropy in creep properties of DS200 + Hf alloy

The directionally solidified Ni-based superalloy DS200 + Hf is used in aero-engines and industrial gas turbines for the manufacturing of intermediate and low-pressure turbine blades. This material consists of ~<0 0 1> columnar grains oriented along the solidification direction and with random secondary orientations. In this study, the creep behaviour of the DS200 + Hf alloy has been investigated along longitudinal and transverse directions. The main objective was to investigate the durability of this alloy and the damage mechanisms for four different crystalline microstructures: DS200 + Hf composed of fine grains loaded longitudinally, the same alloy composed of coarse grains or fine grains, loaded transversally to the solidification direction and finally, the single crystal version, Mar-M200 + Hf, loaded along a <0 0 1> crystallographic orientation. Creep tests were performed from 750°C up to 1100°C. Under all conditions, a pronounced decrease in the strain to failure is observed for specimens loaded along the transverse direction compared to specimens tested along longitudinal direction or single crystals. Fracture surface observations revealed that the lower creep ductility along such a loading direction results from an intergranular failure mode. Furthermore, the creep life anisotropy significantly decreases when temperature increases. Additional creep experiments under vacuum at 900°C and experiments using single crystalline specimens far away from the <0 0 1> crystallographic orientation show a prominent damaging role of oxidation at very high temperature and a major contribution of the local crystallography at low temperatures. In addition, the grain size effect observed when testing in the transverse direction only results from the grain boundary oxidation.

The role of phosphorus in a newly developed Ni-Fe-Cr-based wrought superalloy

The influence of phosphorus on the microstructures and mechanical properties of a newly developed Ni-Fe-Cr-based wrought superalloy (named as HT700 alloy) for ultra-supercritical power plant applications beyond 700°C was investigated. Results showed that adding phosphorus significantly prolonged the creep rupture life but had little influence on the yield strength of HT700 alloy when phosphorus content ranged from 0.002 to 0.027wt%. Meanwhile, intergranular failure mode dominated in all creep ruptured samples while transgranular failure was the main feature in the tensile tested samples. The effects of adding phosphorus on the nucleation and growth behavior of intergranular M23C6 carbides were found to be the main mechanism responsible for the difference.

Modeling Crack Propagation in Polycrystalline Microstructure Using Variational Multiscale Method

Crack propagation in a polycrystalline microstructure is analyzed using a novel multiscale model. The model includes an explicit microstructural representation at critical regions (stress concentrators such as notches and cracks) and a reduced order model that statistically captures the microstructure at regions far away from stress concentrations. Crack propagation is modeled in these critical regions using the variational multiscale method. In this approach, a discontinuous displacement field is added to elements that exceed the critical values of normal or tangential tractions during loading. Compared to traditional cohesive zone modeling approaches, the method does not require the use of any special interface elements in the microstructure and thus can model arbitrary crack paths. The capability of the method in predicting both intergranular and transgranular failure modes in an elastoplastic polycrystal is demonstrated under tensile and three-point bending loads.

Comparison of fatigue crack propagation behaviour in two gas turbine disc alloys under creep–fatigue conditions: evaluating microstructure, environment and temperature effects

Gas turbine disc materials should possess excellent fatigue and creep performance due to the severe in service conditions experienced. In this study, a comparison of fatigue crack propagation behaviour in two turbine disc alloys, i.e. N18 and low solvus high refractory (LSHR) superalloy, has been made in terms of the propagation rate and fractography observed under equivalent testing conditions. Temperatures of 650 and 725°C are compared for a trapezoidal dwell fatigue cycle (1–20–1–1) in both air and vacuum at an R ratio of 0·1. It is found that coarse grained LSHR superalloy has better fatigue crack propagation resistance than fine grained N18 in vacuum, which is ascribed to its better creep performance. Oxidation causes significant degradation of fatigue performance of these two alloys, especially in the LSHR superalloy at higher temperature (725°C), resulting in its inferior fatigue performance compared with N18. In the LSHR superalloy, it seems that oxidation is the principal contributor to the deterioration of fatigue resistance. This is supported by observations of transgranular fracture in vacuum and intergranular fracture in air. In contrast, creep is a greater contributor to the deterioration in fatigue performance of N18 (as indicated by the intergranular failure modes observed in vacuum). An apparent activation energy analysis is able to provide further insight into the underlying mechanisms of fatigue crack propagation under creep–oxidation–fatigue conditions in these two alloys.

Microstructural examination of high temperature creep failure of Zircaloy-2 cladding in irradiated PHWR fuel pins

Cladding samples taken from the ballooned region of the irradiated Zircaloy-2 cladded PHWR fuel pins which failed during isothermal heating tests carried out at 800–900°C were examined using optical and scanning electron microscopy. The examination of samples from the fuel pin tested at 900°C showed an intergranular mode of failure in the cladding due to formation of cracks, cavities and zirconium hydride precipitates on the grain boundaries in the cladding material. A thin hard α-Zr(O) layer was observed on outer surface due to dissolution of the oxide layer formed during reactor operation. Grain boundary sliding was identified to be the main mode of creep deformation of Zircaloy-2 at 900°C. Examination of the cladding tested at 800°C showed absence of cracks or cavities in the deformed material and no localisation of hydrides was observed at the grain boundaries. The failure of the cladding occurred after necking followed by extensive wall thinning of the cladding tube.

Experimental tests of stereological estimates of grain boundary populations

We present experimental validation of a method for estimating three-dimensional (3-D) relative numerical populations of grain boundaries from measurements of individual two-dimensional (2-D) cross-sections. Such numerical populations are relevant to network topology and the modeling of intergranular failure modes in grain boundary engineered materials, and are distinct from geometrical population measures such as area per volume. We examine 3-D reconstructions of stainless steel and copper, with varying populations of twin-related boundaries, generated by serial-section electron backscatter diffraction and high-energy X-ray diffraction microscopy. We show that 2-D length fractions, 2-D number fractions and 3-D number fractions are all distinct quantities when grain boundary type is correlated with grain boundary size. We also demonstrate that the last quantity may be reliably inferred from the first two, provided the experimental spatial resolution is much finer than the grain size, eliminating the need to use 3-D experimental methods to access at least some information about 3-D network properties. Many of the Σ3 boundaries are extremely complex, with highly re-entrant shapes that can intersect a sample plane many times, giving a false impression of multiple separate boundaries.