Scale Cracking Research Articles

Oxidation behavior of Fe-20Cr-25Ni-Nb austenitic stainless steel in high-temperature environment with small amount of water vapor

The oxidation behavior of a Fe-20Cr-25Ni-Nb austenitic stainless steel was investigated in an oxidizing environment at 1000 °C. Results show that an accelerated oxidation occurs after a transition point and some oxide nodules with two layers are formed on the steel surface. The oxidation of intergranular Fe2Nb phases can induce micro-cracks and promotes the formation of re-generated Cr2O3 phases. The formation and growth of the re-generated Cr2O3 phases are related to the cracking and spallation of the oxide scale, which result in the formation of these oxide nodules and the associative accelerated oxidation.

Model for SiC fiber strength after oxidation in dry and wet air

AbstractThe strengths of oxidized SiC fibers were modeled from the effects of SiO2 scale residual stress on fracture. Surface tractions from scale residual stress were determined for SiC surface flaws. The residual stress was the sum of the growth stress from oxidation volume expansion, thermal stress from SiO2‐SiC thermal expansion mismatch, and stress from phase transformations in crystallized scale. The partial relaxation of tensile residual stress from scale cracking was also calculated. Scale thicknesses were determined using Deal‐Grove oxidation kinetics for glass and crystalline scales. Kolmogorov‐Johnson‐Mehl‐Avrami (KJMA) kinetics was used to determine scale crystallization rates. Strengths of fibers with glass and with crystalline scales formed by oxidation in dry and wet air between 600° and 1400°C were modeled. The effects of partially crystallized scales were calculated using Weibull statistical methods. Modeled strengths were compared with measurements. Slight strength increases after glass scale formation, large decreases that accompany scale crystallization, and some differences between dry and wet air oxidation were accurately modeled. This suggests that under some conditions the scale residual stress dominates the changes in strength after SiC fiber oxidation. However, modeled strengths were significantly higher than those measured for some fibers oxidized in wet air, which suggests another degradation mechanism is active for these conditions. Modeling assumptions and implications for SiC fiber strength after oxidation for long times are discussed.

Corrosion of Zircaloys: Relating the microstructural observations to the corrosion kinetics

Understanding the corrosion behaviour of zirconium alloys is essential in providing cost effective and safe operation of nuclear power plant. Long-term corrosion testing of Zircaloy-4 and Zircaloy-2 in 350 °C pressurised water is presented with detailed analysis of the oxide's development. The correlation of the corrosion and analytical data provides a detailed understanding regarding the point at which different microstructural features, such as cracking, occur during the cyclic corrosion process. The results from this work show that the large scale cracking, which is often associated with the cyclic transitions, evolve following, rather than prior to, the onset in rapid corrosion.

An atom probe tomography study of Pb-caustic SCC in alloy 800

Stress corrosion cracking (SCC) of Alloy 800 was induced in 330 °C Pb-caustic solution (pH330°C 9.5). These laboratory tests simulated extreme crevice environments in nuclear plants, but do not occur under normal operating conditions. Atom probe tomography (APT) was used to examine crack tips, a powerful but scarcely used technique for this application. APT revealed sub-nanometre scale crack chemistry: successive Ni-rich, de-alloyed regions and segregated Fe- and Cr-rich oxides. Also, APT allowed for quantification of impurities, such as Pb which segregated to oxide-metal interfaces (<4 at.%). Discussion highlights new mechanistic insights from APT, complementing prior transmission electron microscopy (TEM) analysis.

3D-printed PEEK-carbon fiber (CF) composites: Structure and thermal properties

CF-PEEK composites were manufactured by 3D-printing using a novel FDM methodology and customized printer and were compared with their cast counterparts. The characterization of composite thermal properties in the range 25–300 °C revealed that 3D-printed CF-PEEK composites manifest 25–30% lower thermal conductivity than cast composites. Short carbon fibers used for reinforcement showed orientation along the polymer flow both in cast and 3-D printed samples causing the anisotropy of thermal properties. The hierarchical nature of 3DP CF-PEEK porosity was observed by SEM imaging, which allowed the identification of large scale inter-layer gaps and cracks, and fine scale intra-layer defects that are likely to be induced by the thermal and mechanical gradients within the deposit that arise during fabrication. Purposeful lay-up of long continuous carbon yarns during 3D-printing opens a way to fabricate tailored mechanical parts with desired anisotropy of properties.

Stress determination by in situ X-ray diffraction – Influence of water vapour on the Zircaloy-4 oxidation at high temperature

A key parameter for the understanding the effect of water vapour on the oxidation mechanism is the determination of the stress level in the zirconia scale formed on Zircaloy-4 (Zy-4) at the operating temperature. In order to provide an accurate description of the structure and microstructure of the oxide layers, X-ray diffraction (XRD) analyses have been performed in situ under dry and wet oxidizing environments at high temperature on Zy-4 flat sheet samples. The aim of the present work is to show the influence of water vapour on the stress developed at high temperature in the oxide scale as well as inside the alloy. In situ growth stress determination in the oxide scale has shown that the compressive stress is larger in dry air compared to the wet air environment at 500 °C. In wet air, the lower growth stress is due to stress relaxation after the pre-transition stage. This is related to the effect of water vapour on the zirconia chemical state and change in the oxide mechanical properties leading to an earlier scale cracking.

Fracture anisotropy in texturized lithium disilicate glass-ceramics

Lithium disilicate glass-ceramics show great potential in the fabrication of anisotropic structures due to their high-strength needle-form crystalline phase subjective to processing-induced alignment. Here we explore this strategy by focusing on the mechanistic aspects of fracture. Through different variations of the fracture toughness test we demonstrate the effect of bulk crystal orientation on fracture energy, toughening mechanisms, crystal size and aspect ratio. Using the anisotropic Poisson's ratio of the Li2Si2O5 crystal phase obtained by Resonant Ultrasound Spectroscopy, we apply the new geometry factor developed by Strobl et al. to provide a more accurate insight into the anisotropic crack propagation behavior in LS2 glass-ceramics. Raman spectra and X-Ray Diffraction patterns are resolved for the Li2Si2O5 phase aligned at plane parallel, anti-plane and random orientations.We show that Li2Si2O5 crystallites oriented perpendicular to the crack growth plane tended to induce large scale crack deflection unless the crack was forced into a straight path, thereby promoting crystallite fracture, increased KIc-values and accentuated R-curve behavior. Crystal aspect ratio and residual stresses in the glass have been identified as important influencing factors on crack growth behavior and toughening mechanisms.

Cavitation erosion behavior of CLAM steel weld joint in liquid lead-bismuth eutectic alloy

The cavitation erosion of weld joint and base metal of China low activation martensitic (CLAM) steel in liquid lead-bismuth eutectic alloy (LBE) at 550 °C was investigated to simulate the cavitation erosion of the first wall and the nuclear main pump impeller in the accelerator driven sub-critical system (ADS). A suit of ultrasonic cavitation facility was self-designed to study the cavitation erosion. By studying the surface micro topography, roughness and mean pit depth of the tested specimens, it was found that some crater clusters and large scale cracks appeared on the tested specimen surface after the formation of numerous single craters, and the base metal exhibited much better cavitation erosion resistance than the weld bead due to the difference in their mechanical properties and microstructures. In addition, by comparing the results of static corrosion and cavitation erosion, it could be concluded that the cavitation erosion and the dissolution and oxidation corrosion in liquid LBE would accelerate mutually.

Elastic crack propagation model for crystalline solids using a self-consistent coupled atomistic–continuum framework

Deformation and failure processes of crystalline materials are governed by complex phenomena at multiple scales. It is necessary to couple these scales for physics-based modeling of these phenomena, while overcoming limitations of modeling at individual scales. To address this issue, this paper develops self-consistent elastic constitutive and crack propagation relations of crystalline materials containing atomic scale cracks, from observations made in a concurrent multi-scale simulation system coupling atomistic and continuum domain models. The concurrent multi-scale model incorporates a finite temperature atomistic region containing the crack, a continuum region represented by a self-consistent crystal elasticity constitutive model, and a handshaking interphase region. Atomistic modeling is done by the molecular dynamics code LAMMPS, while continuum modeling is conducted by the finite element method. For single crystal nickel a nonlinear and nonlocal crystal elasticity constitutive relation is derived, consistent with the atomic potential function. An efficient, staggered solution scheme with parallel implementation is designed for the coupled problem. The atomistic–continuum coupling is achieved by enforcing geometric compatibility and force equilibrium in the interphase region. Quantitative analyses of the crack propagation process focuses on size dependence, strain energy release rate, crack propagation rate and degradation of the local stiffness. The self-consistent constitutive and crack propagation relations, derived from the concurrent model simulation results are validated by comparing results from the concurrent and full FE models. Excellent accuracy and enhanced efficiency are observed in comparison with pure MD and concurrent model results.

Time-temperature effects in dual scale crack growth of titanium alloys

Time and temperature effects play substantial role in the microstructural degradation of metal materials. There results in complex creep-fatigue interaction. The explicit representation of material damage is preferably assumed to be a characteristic length such as an equivalent crack length. A dual scale crack growth model that is based on the theory of strain energy density is employed to study the time-temperature effects on the crack growth behaviors of titanium alloys. Three micro/macro parameters defined in the model are used to exhibit the time-dependent variation of material, loading and geometry effects. TA7 and TA19 of titanium alloys are used to particularly address the effects of temperature variation as well as scale parameters. The numerical results show the microscopic Poisson’s ratio is sensitive to the crack growth history of titanium alloys. The inverse approach of creep-fatigue cracking can be possibly applied to the health monitoring of crack growth in fracture control.

Effects of macro-scale corrosion damage feature on fatigue crack initiation and fatigue behavior

The remaining fatigue life for aluminum aerospace alloys is greatly reduced by the presence of corrosion damage; prediction of this deleterious influence via standard fracture mechanics methods is non-trivial. Several corrosion morphologies can develop depending on the local chemistry, pH and electrochemical potential along the geometry of the structure. Previous studies demonstrate that such variations in corrosion morphology can affect the fatigue behavior. This study aims to quantify the effect of corrosion morphologies relevant to a fastener-hole geometry of AA7050-T7451 on the initiation life, propagation life and total fatigue life. Four corrosion morphologies are considered: shallow and deep discrete pits widely distributed on the corroded surface, deep fissure-like features, and general corrosion with surface recession. Corrosion morphologies are carefully characterized using optical microscope, white light interferometer and X-ray computed tomography. The samples are subjected to fatigue using loading protocols that create identifiable marks on the fracture surface at known cycle count. These marker bands are used to determine the small scale crack growth rate (da/dN) of the samples, as well as the cycles to initiate a 10μm crack. Results showed that initiation life plateaus to near constant values for varied corrosion morphologies, and crack growth rates converge to comparable values. The macro-scale corrosion metrics are considered and correlated to the location of the fatigue initiation. Severe macro-scale metrics such as pit or fissure depth, pit mouth surface area, pit volume, pit density, root mean square, peak density, maximum valley depth, do not uniquely correlate with the location of fatigue crack initiation. This suggests a strong role of micro-scale corrosion features and underlying microstructural features on the location of the fatigue crack initiation.

Free Vibration of Edge Cracked Functionally Graded Microscale Beams Based on the Modified Couple Stress Theory

In this study, the free vibration analysis of edge cracked cantilever microscale beams composed of functionally graded material (FGM) is investigated based on the modified couple stress theory (MCST). The material properties of the beam are assumed to change in the height direction according to the exponential distribution. The cracked beam is modeled as a modification of the classical cracked-beam theory consisting of two sub-beams connected by a massless elastic rotational spring. The inclusion of an additional material parameter enables the new beam model to capture the size effect. The new nonclassical beam model reduces to the classical one when the length scale parameter is zero. The problem considered is investigated using the Euler–Bernoulli beam theory by the finite element method. The system of equations of motion is derived by Lagrange’s equations. To verify the accuracy of the present formulation and results, the frequencies obtained are compared with the results available in the literature, for which good agreement is observed. Numerical results are presented to investigate the effect of crack position, beam length, length scale parameter, crack depth, and material distribution on the natural frequencies of the edge cracked FG microbeam. Also, the difference between the classical beam theory (CBT) and MCST is investigated for the vibration characteristics of the beam of concern. It is believed that the results obtained herein serve as a useful reference for research of similar nature.

焼結過程の数値シミュレーション −マグネタイト鉱石粒子の配合割合が大規模亀裂の発生に及ぼす影響−

焼結過程の数値シミュレーション −マグネタイト鉱石粒子の配合割合が大規模亀裂の発生に及ぼす影響−

焼結過程の数値シミュレーション −マグネタイト鉱石粒子の体積溶融率および粒子間摩擦力が大規模亀裂の発生に及ぼす影響−

焼結過程の数値シミュレーション −マグネタイト鉱石粒子の体積溶融率および粒子間摩擦力が大規模亀裂の発生に及ぼす影響−

Extracting oxide scale cracking data from acoustic emission data using the example of oxidized cobalt

4‐point bending with in situ acoustic emission measurement has proven to be a successful route for determining the critical strain values for various oxide scales. In combination with metallographic investigations, such critical strain data can be used to establish mechanical stability diagrams for an assessment of the maximum tolerable strain before scale failure becomes imminent. The use of the acoustic emission technique enables to detect micro‐cracking in the oxide scale without metallographic inspection, thereby, providing access to the critical strain required to initiate oxide cracking. However, in some cases, acoustic emission is generated not only by oxide cracking but also from plastic deformation processes within the metal substrate. In this work, a tailored analysis procedure is developed for discriminating the signals generated by oxide scale cracking from those generated by plastic deformation in the cobalt substrate. From the resulting critical strain data, the above‐mentioned failure diagrams were deduced for cobalt oxide scales grown on high‐purity cobalt at 650 °C in dry synthetic air, and synthetic air with 10 vol% H2O, respectively.

Failure analysis of a fluidisation nozzle in biomass boiler and the long-term high temperatures oxidation behaviour of 304 stainless steel

The long-term (> 10 years) oxidation behaviour of stainless steels (SS) at high temperatures was previously unknown. The behaviour was studied through a case study of failure analysis. A fluidisation nozzle made of 304 austenitic SS. After over 100,000 h of service at temperatures of 790–820 °C in a biomass boiler, the nozzle fractured. Failure analysis pinpoints that the nozzle wall temperature fluctuation caused the oxide scale cracking, which intensified the oxidation. The brittle fracture was due to fully oxidization. The long-term oxidation behaviours are distinct from the short-term oxidation behaviours. XRD analysis indicates that the scale was mainly Fe+ 2Cr2O4 and (Fe0.6 Cr0.4)2O3. ESEM/EDS analysis indicates internal oxidation and sulfidation along the grain boundaries. The different diffusion rates of the Fe, Cr and Ni atoms formed a Ni-rich (48%) layer underneath the scale, and a Cr-rich (35%) core in the remained SS. A schematic is proposed to describe the diffusion mechanism of the internal oxidization and sulfidation behaviour.

Impact fracture of ZnSe ceramics

Structurally different ZnSe ceramics prepared by various techniques were subjected to fallingweight impact fracture. Mechanoluminescence (ML) pulses generated during the motion and multiplication of dislocations were detected, as well as acoustic emission (AE) pulses produced predominantly during the growth of macroscopic (on the specimen scale) cracks. The luminescence began immediately at the moment of contact of a striker with the surface of the specimen, whereas the emission of sound occurred within 50–100 μs after the impact. The emission maxima in the ML and AE time series coincided with each other. The signal series were used to construct energy distributions upon the emission of light and the generation of sound. It was established that the ML amplitude (the number of emitted photons) is proportional to the energy released due to dislocation rearrangements, and the intensity (the square of the amplitude) of AE pulses is proportional to the energy released due to discontinuities of the material. It was found that the ML energy distribution follows a power law, which indicates the self-organization of an ensemble of dislocations during rapid plastic deformation. The AE energy distribution, on the contrary, was found to be random, i.e., typical of the growth of non-interacting cracks. It was shown that the efficiency of the interaction of dislocations depends, to a certain extent, on the technological prehistory of ZnSe ceramics.

Acoustic emission characteristics and stress release rate of coal samples in different dynamic destruction time

Considering the importance of the prediction of rock burst disasters, and in order to grasp the law of acoustic emission (AE) of coal samples in different dynamic destruction time, the SH-II AE monitoring system was adopted to monitor the failure process of coal samples. The study of the change rule of the AE numbers, energy, ‘b’ value and spectrum in the micro crack propagation process of the coal samples shows that as dynamic damage time went by, AE presented high-energy counts and the accumulated counts increased during the compression phase. The AE energy and cumulative counts increased during the elastic stage. The AE blank area increased gradually and the blank lines were more and more obvious in the molding stage. The AE counts and energy showed a trend of decrease in the residual damage phase. AE ‘b’ values gradually became sparse, and the large scale cracks percentage compared with micro cracks decreased and the degree of damage decreased. The AE frequency spectrum peak went from the residual damage phase to the molding phase, and finally it was nearly stable, besides the bandwidth of the main frequency is gradually narrowed. Also, the frequency peak changed from single peak frequency to bi-peak frequency and to the single peak frequency. Uniaxial compressive strength is more sensitive than the elastic modulus to dynamic damage time.

Corrosion Performance of Fe-Based Alloys in Simulated Oxy-Fuel Environment

The long-term corrosion of Fe-based alloys in simulated oxy-fuel environment at 1023 K (750 °C) was studied. Detailed results are presented on weight change, scale thickness, internal penetration, microstructural characteristics of the corrosion products, and the cracking of scales for the alloys after exposure at 1023 K (750 °C) for up to 3600 hours. An incubation period during which the corrosion rate was low was observed for the alloys. After the incubation period, the corrosion accelerated, and the corrosion process followed linear kinetics. Effects of alloy, CaO-containing ash, and gas composition on the corrosion rate were also studied. In addition, synchrotron nanobeam X-ray analysis was employed to determine the phase and chemical composition of the oxide layers on the alloy surface. Results from these studies are being used to address the long-term corrosion performance of Fe-based alloys in various coal-ash combustion environments and to develop methods to mitigate high-temperature ash corrosion.

(Henry B. Linford Award for Distinguished Teaching) Determination of Local Hydrogen Concentrations in High Performance Alloys Using Local Probe Methods

Introduction: Techniques for the measurement of hydrogen in high strength metallic alloys and effective hydrogen diffusivity (DH,eff) lack micrometer scale spatial resolution. This is of great importance since the hydrogen-metal interactions and processes occurring at the micrometer to nanometer length scales control hydrogen embrittlement (HE). This presentation discusses three methods which enable spatial resolution and mapping of absorbed hydrogen at sub-millimeter length scales in ultra-high strength steels (UHSS) and stainless steels. The scanning Kelvin probe (SKP), scanning Kelvin probe force microscope (SKPFM), and SKP combined with secondary ion mass spectrometry have recently been developed as useful techniques for spatial H detection with depth (using cross-sectional analysis), lateral detection, and mapping.[1-7] In terms of H resolution, the SKP can detect concentrations as low as 0.01 atomic ppm.[8] This presentation explores three novel techniques for the spatial detection of hydrogen concentrations. These include the SKP, the scanning electrochemical microscope (SECM) and re-scaling methods to produce mm scale cracks or crevices that are electrochemically equivalent to small scale occluded sites [5, 9-11]. Experimental and Results: Two secondary age hardened martensitic steels were examined, UNS K92580 and UNS S46500. The SKP voltage (Φ) dependence on dissolved H concentration was optimized by mapping under conditions producing a low corrosion rate; an essential aspect of successful hydrogen mapping. A calibration curve was developed relating absorbed hydrogen concentration to Φ. High strength steels were pre-charged and exposed to MgCl2 and NaCl droplets, which produced natural acidic pit sites that enabled local hydrogen uptake[12]. The factors controlling calibration of SKP potential and hydrogen concentration as well as those limiting spatial resolution are discussed. Moreover, DH,eff coefficients were determined.[5] Results were corroborated with previous studies of DH,eff.[13] The SECM also was optimized to enable detection of corrosion processes with spatial resolution and utilized to measure spatially a pre-dissolved concentration of hydrogen across pre-charged and uncharged zones [10]. The talk will close with comparison of the length scales of various physical processes occurring during hydrogen embrittlement compared to those accessed by these techniques. Acknowledgement: Research was sponsored by the US Air Force Academy under agreement number FA7000-13-2-0020 and ONR under PROJ0007990. The authors would like to acknowledge the Army Research Laboratories, as well as Monash University, in particular Dr. Sebastian Thomas and Prof. Nick Birbilis. The research was partially supported by a 2014 Australia Endeavour Award Research Fellowship with support from Dr. Kishore Venkatesan and Dr. Ivan Cole at CSIRO.