Effect Of Microstructural Changes Research Articles

Comprehensive study on hydrogen induced cracking of electrical resistance welded API X52 pipeline steel

Different heat treatment cycles were designed in order to investigate the effect of microstructural changes on hydrogen induced cracking resistance (HIC) and mechanical properties of the electric resistance welded steel. The heat treating of the as-welded specimen improved the ductility and impact toughness. After heat treatment, the uniform hardness profile was obtained for the welded specimens. The removal of local hard zones reduced the risk of HIC. The chemical composition and clustering of inclusions have a deleterious effect on cracking resistance in the H2S environment. Aluminosilicate compounds and MnS inclusions were favorite sites for HIC. The most promising post weld heat treatment for improving mechanical properties and the resistance to HIC was the application of two-cycle quenching followed by tempering.

Effect of microstructure change on permeability of flax-fiber reinforced silty clay soaked with zinc-ion solution

With the application of fiber-reinforcement technology, the mechanical properties of silty clay are improved with fiber reinforcement. However, the variation of permeability coefficient and other parameters of flax-fiber reinforced silty clay have not been sufficiently studied. In this study, the permeability of flax-fiber reinforced silty clay soaked with zinc-contaminated solution under different osmotic pressure was tested by a flexible-wall permeameter, and the effects of zinc-ion concentration and confining pressure on the permeability of flax-fiber reinforced silty clay were studied. Genius XRF was employed to detect the types and quantity of metal elements in the specimens, thereafter, the reasons for the change of permeability were explained from chemical and microscopic perspective. The results showed that the permeability coefficient of flax-fiber reinforced silty clay decreased significantly with the increase of zinc-ion concentration in a low concentration (about 1–10 mg L−1). While in a high concentration (about 100 mg L−1), the permeability coefficient of flax-fiber reinforced silty clay changed little with the increase of zinc-ion concentration. While the flax-fiber reinforced silty clay was not soaked with zinc-ion solution, the permeability coefficient of the specimen increased with the increase of confining pressure. However, when the flax-fiber reinforced silty clay was soaked with zinc-contaminated solution, the permeability coefficient first decreased and then tended to be constant with the increase of confining pressure. With the increase of confining pressure, the porosity of flax-fiber reinforced silty clay decreased, and with the increase of zinc-ion concentration, the porosity of flax-fiber reinforced silty clay first increased and then decreased.

Modelling and evaluating thermal conductivity of porous thermal barrier coatings at elevated temperatures

Thermal conductivity of various porous thermal barrier coatings (TBCs) used at elevated temperatures for gas turbines has been evaluated using the proposed six-phase model. These TBCs rely on microstructural properties and yield different types of porosities. This paper studies the thermal conductivity of TBCs based on microstructural features to evaluate the effect of different types of porosities on thermal conductivity. The first part of this paper investigates the microstructural characterization of various TBCs using image analysis (IA) technique. The second part of this paper evaluates the thermal conductivity using the image analysis. The volumetric fraction of porosities along with their orientation, shape and morphology, shows a considerable impact on the overall thermal conductivity of TBCs. The proposed six-phase model can predict thermal conductivity of porous TBCs with a good agreement with the measured values. The model results can help to better understand the effect of microstructural changes on thermal conductivity and can provide useful guide to fabricate TBCs with low thermal conductivity.

Influence of Microstructural Changes’ Effects on the Linear and Nonlinear Ultrasonic Parameters of Cast Stainless Steels

In this research, some nondestructive ultrasonic techniques were employed to inquire into the effect of microstructural changes induced by thermal aging and cold work on the ultrasonic response. As thermal embrittlement is a risk to the safety of nuclear power plants, a nondestructive detection method has to be developed for on-site monitoring. The austenitic stainless steel with δ-ferrite specimens were used to study the behavior of microstructural changes caused by age-treating and cold work and then examined by the velocity, attenuation, and nonlinear ultrasonic technique. The variations of the linear and the nonlinear ultrasonic parameters were related to the microstructural changes. Additionally, the experimental results suggest that the ultrasonic nonlinearity parameter of cast stainless steel is determined by the microstructure evolution caused by spinodal decomposition and the phase precipitation process.

Microstructures and mechanical properties of oxide dispersion strengthened CoCrFeNi high-entropy alloy produced by mechanical alloying and spark plasma sintering

1.2 Hf and 1.5 Y2O3 (wt.%) were added into CoCrFeNi (CCFN) high-entropy alloy (HEA) to fabricate oxide dispersion strengthened CoCrFeNi (ODS-CCFN) composite by mechanical alloying and spark plasma sintering. Microstructures, including XRD, SEM, EBSD, and TEM of CCFN and ODS-CCFN HEAs were investigated to identify the effects of introducing nano-sized oxides into CCFN HEA. These HEAs compose of FCC matrix and a small amount of Cr7C3 and Cr2O3, and a few of MnCr2O4 is only detected in CCFN HEA. High-density Y2Hf2O7 with the average diameter of 6.9 ± 4.2 nm uniformly distribute in the matrix and Cr7C3 of ODS-CCFN. Lattice misfit calculations indicate that Y2Hf2O7 particles exhibit a coherent interface with the matrix, and a semi-coherent interface with Cr7C3. The average sizes of all phases, including grain, Cr7C3 and Cr2O3, are highly refined in ODS-CCFN. The effects of microstructure changes on properties of CCFN HEA were also detected. Compared with CCFN, the tensile yield strength of ODS-CCFN increases from 978 MPa to 1.25 GPa with a reasonable elongation decreases from 2.6% to 1.9%. The increase in yield strength for ODS-CCFN is mainly ascribed to the precipitation strengthening of Y2Hf2O7 and highly refined grains, Cr7C3 and Cr2O3. The electrical resistivity of ODS-CCFN is lower than that of CCFN, the reason may be that the existence of Y2Hf2O7 particles reduce the lattice distortion of the matrix, and make the electrical resistivity decrease. The corrosion resistance of ODS-CCFN HEA is highly improved in both sulfuric acid solution and sodium chloride solution because its passive current density is largely reduced by the addition of Hf and Y2O3.

Effect of heat-treatment on microstructure, mechanical and tribological properties of Mg-Li-Al based alloy

In the present study, the microstructure evolution and mechanical properties of as-cast Mg-9 wt.%Li-7 wt.%Al-1 wt.%Sn (LAT971) alloy during solid solution treatment are investigated. Further, the age softening of solid solution treated alloys is analyzed at 50 °C and 100 °C from 0.5 h to 5 h. The hardness of the α- and β-phase of LAT971 alloy is observed to increase by ∼61% and ∼136%, respectively after solutionizing, when compared to that of as-cast condition. Further, the hardness of both the phases has decreased after aging treatment due to age-softening effect. Additionally, finite element modeling (FEM) is used to elicit the effects of microstructural changes on stress distribution of thermally treated LAT971. In addition, the solutionized condition has educed higher wear resistance than as-cast condition of LAT971 alloy. The higher scratch hardness (∼1.35 and ∼1.74 times for α- and β-phase, respectively) resulted in the lower wear volume, and, subsequently, lower specific wear rate is obtained for solutionized alloy when compared to that of as-cast condition.

Effect of microstructure of simulated heat‐affected zone on low‐ to high‐cycle fatigue properties of low‐carbon steels

AbstractTo clarify the effect of microstructural changes on the fatigue property of the weld heat‐affected zone (HAZ), low‐ to high‐cycle fatigue tests were conducted on 16 types of simulated HAZ specimens that had been prepared using thermal processes. The results showed the fatigue S‐N curves of the HAZ to be widely scattered as a function of strength level. These fatigue data were divided into two groups: coarse grain (CG) and fine grain (FG) HAZ, when strain amplitude was used to represent S‐N curves. The fatigue data for the CGHAZ group showed a relatively short fatigue life. Based on surface observations, the initiated fatigue crack size of CGHAZ was larger than that of FGHAZ as a function of microstructural unit size. Hence, fatigue crack growth life, which is almost the same as total fatigue life of CGHAZ, decreased.

Strength enhancement of magnesium alloy through equal channel angular pressing and laser shock peening

AM80 magnesium alloy was processed by Equal Channel Angular Pressing up to 2 passes under route BC and C, to study the effect of change in microstructure. Microstructures were examined under optical microscope. Samples processed by route BC showed heterogeneous grain structure with good tensile strength compared to that processed by route C. Tensile tests of 2-pass equal channel angular pressed samples showed high tensile strength under route BC. Laser shock peening without coating was carried out on route BC sample for further grain refinement on the surface. Laser pulses with power density of 8 GW cm−2, under different percentages of cover, were used to peen the surface at high strain rate. Microstructures were analysed through scanning and transmission electron microscope, and fine grains of less than 100 nm were observed. Tensile tests indicated that the laser shock peened samples had increased strength and ductility. Fracture details from tensile tested specimens, were examined under SEM. Mixed mode of brittle and ductile fractures was observed in ECAP processed samples. Fracture surface of Laser shock peening without coating treated on equal channel angular pressed samples showed small dimples near the peened surface and intensity of dimples increased with increase in percentage of cover. Wear test was carried out on before and after Laser shock peening without coating processed samples, on pin-on-disc wear test machine. Increase in friction coefficients and wear rate was observed due to roughness induced by peening effect and it decreased after increase in sliding distance due to increase in hardness. Nano indentation experiments were carried out to examine the mechanical characterization at nano level, and it expose the effect of LSPwC in terms of increase in hardness at peened region.

Influence of Isothermal Holding on the Microstructure and Mechanical Properties of Electron Beam Welded Dissimilar Inconel 718/ATI 718Plus\xae Joint

Microstructural changes of the Inconel 718 and ATI 718Plus® base materials and the fusion zone caused by the isothermal hold at 760 °C for up to 500 h were investigated. The evaluation was performed using a scanning electron microscope with backscattered electrons detector and scanning transmission electron microscopy supported by energy-dispersive x-ray spectroscopy. Furthermore, to evaluate the effect of microstructural changes on mechanical properties Vickers microhardness measurements were performed. Exposure of welded joint at high temperature led to coarsening and/or dissolution of main strengthening phases’ particles and nucleation and growth of plate-like precipitates. In the interdendritic region of the fusion zone, the transformation of the Laves phase to a complex cluster of precipitates contained σ phase had taken place. Analysis of microstructure and microhardness measurements showed that ATI 718Plus has better mechanical properties and microstructural stability at elevated temperature than Inconel 718.

Stress Rupture Properties of Welded Joints of Austenitic Heat-resistant Super304H Steel at 650 °C

With the help of metalloscope, scanning electron microscope and its energy dispersive spectroscopy, this paper investigates the stress rupture properties at high temperature and microstructure changes of the welded joints of austenitic heat-resistant Super304H steel at 650 °C as well as the effects of microstructure changes on stress rupture properties by high temperature creep rupture testing. The results show that at 650 °C, the welded joint of Super304H steel has high breaking strength at elevated temperature, and the fracture position is on the base material away from the fusion line, and the fracture type is mainly a transcrystalline rupture. The microscopic process of creep rupture of Super304H steel welded joints is the nucleation and growth of the void and crack, while the large bulk Nb(C,N) is the main nucleation site of void. As the careep time increases, M23C6 on the grain boundary is also the main nucleation site of the void.

Erosion Resistance and Particle Erosion-Induced Tensile Embrittlement of 3D-Selective Laser Melting Inconel 718 Superalloy

We used selective laser melting (SLM) Inconel 718 (coded AS) to carry out three heat treatment processes: (1) double aging (coded A), (2) solid solution + A (coded SA), (3) homogenization + SA (coded HSA) in order to investigate the effects of microstructure changes and tensile strength enhancement on erosion resistance. The as-SLM IN718 and three heat-treated specimens were subjected to clarify the effects of erosion-induced phase transformation on tensile mechanical properties. All heat-treated specimens showed better erosion resistance than as-SLM IN718 did at all impact angles. The as-SLM IN718 and the three heat-treated specimens produced new γ′ phase or metal-oxide via particle erosion, which increased the surface hardness of the material. The thickness of the erosion affected zone is 200 μm, which is the main cause of tensile embrittlement.

Modeling mechanical behaviors of plant stems undergoing microstructural changes

This study presents a constitutive material model for describing nonlinear and hysteretic responses of plant tissues subjected to mechanical loadings. The nonlinear and hysteretic response is associated with the viscoelastic nature of the constituents and microstructural changes of plant tissues during loadings. In order to incorporate the effect of microstructural changes on the macroscopic response of plant tissue, we assume that as the tissue is deformed, an additional micromechanism arises affecting the mechanical response of the tissues. The plant tissue is assumed to consist of two networks, the initial network which is associated with an original reference configuration and the new network that is formed during the deformation. The newly formed network has a new natural configuration. The responses obtained from the proposed model are compared with available experimental data of plant tissues. The model is easily extended to incorporate responses of different tissues, i.e., outer strengthening skin and inner soft core, in determining the overall nonlinear behaviors of the plant stems, which are composite materials. Thus, we can examine the contributions of different constituents in the plant tissues on their macroscopic mechanical responses.

Effect of Carbon Corrosion on Wettability of PEM Fuel Cell Electrodes

Large transport resistances at high current densities hinder the proton-exchange-membrane fuel cells from reaching performance-cost-durability targets. Extensive carbon corrosion in the cathode catalyst layers has been shown to correlate to pore closures and increase in surface oxides of carbon support, which has been associated with the increase in wettability of the catalyst layers. This study investigated effects of changes in microstructure and surface oxides on the change of wetting properties of cathode catalyst layers after different accelerated stress tests. The apparent contact angles were measured over a broad range of cathode carbon mass losses at 0.3 mgPt/cm2. The apparent contact angles fell sharply after small extent of carbon loss followed by a plateau, and decreased again after 123 µg/cm2 (35% carbon support loss in the cathode). The microstructural changes were characterized by surface roughness (Atomic Force Microscopy) and porosity (focus-ion-beam scanning electron microscopy and thermogravimetric analysis). The surface roughness and porosity were used to parameterize Wenzel’s and Cassie-Baxter’s models, respectively, to elucidate the changes in Young’s contact angle. The fittings of the two models revealed that the cathode catalyst layers sustained their wettability after up to 35% of carbon support loss due to the increases in the proportion of hydrophobic fluorocarbon chains on the surface of the catalyst layer from XPS. The results suggest that the mass-transfer resistance cannot be ascribed to greater liquid saturation of the pore structure before the cell significantly fell below the DOE’s technical target.

Investigation of the effect of microstructural changes on thermal transport in semicrystalline polymer semiconductors

Great progress in the development of new semiconducting polymers over the last two decades alongside improved understanding of electron transport mechanisms have resulted in a dramatic increase in the electron mobility of these materials making them promising candidates for electronic and thermoelectric applications. Heat transport phenomena, on the other hand—which govern thermal conductivity—have not received as much attention up to date. In spite of the simplicity of the principle behind the measurement of thermoelectric properties, the combined uncertainty in thermoelectric figure of merit zT could easily reach 50% with the largest uncertainty coming from thermal conductivity measurements. Such a high measurement uncertainty, often comparable to relative variations in zT encountered when optimizing within a given class of materials, prevents the study of structure-thermal property relationships. Here we present a protocol for the measurement of the thermal conductivity of thin films with reduced measurement uncertainty, which allowed us to investigate the effect of microstructural changes on the thermal conductivity of the conjugated polymer P(NDI2OD-T2). We show that the enhancement of the thermal conductivity upon annealing is much less pronounced than the corresponding increase in the electron mobility that has been reported under the same annealing conditions in the literature. This suggests that semicrystalline conjugated polymers in which thermal transport remains limited by the amorphous domain boundaries in between crystalline grains could be a suitable system for realizing the electron-crystal phonon glass concept and enable higher performance thermoelectric materials.

A thermodynamically consistent model for viscoelastic polymers undergoing microstructural changes

This study presents a thermodynamic framework for describing the mechanical response of viscoelastic polymers undergoing microstructural changes within the framework of a body possessing multiple natural configurations. In order to incorporate the effect of microstructural changes on the macroscopic response of polymers, we assume that the polymer under consideration consists of two networks. In its initial configuration, the polymer is comprised of an original microstructure (initial network) and as the polymer is deformed, a new network is formed. The newly formed network has a new reference configuration. When subjected to mechanical loading, the natural configuration of the polymer evolves between these initial and final configurations. We use the proposed constitutive model in order to describe the macroscopic response of a polyoxymethylene (POM) polymer under various loading histories. The predictions of the model developed are in good agreement with experimental data that is available.

Hybrid Carbon Nano-Fibers with Improved Oxidation Resistance

Hybrid Carbon-Silicon Carbide (C-SiC) nano-fibers were fabricated while using a mixture of polyacrylonitrile (PAN) and silicon (Si) nanoparticles as precursors. The microstructure of the material was examined using X-ray diffraction and Raman spectroscopy as a function of processing temperature and holding time. A complete transformation of Si to SiC occurred at 1250 °C. However, for heat treatments below 1000 °C, three distinct phases, including Si, C, and SiC were present. The effect of microstructural changes, due to the heat treatment, on oxidation resistance was determined using thermogravimetric analysis (TGA). Furthermore, the char yield showed exponential growth with increasing the carbonization temperature from 850 °C to 1250 °C. The holding times at higher temperatures showed a significant increase in thermal properties because of SiC grain growth. At longer holding times, the SiC phase has the function of bothcoating and reinforcing phase. Such structural changes were related to fibers mechanical properties. The tensile strength was the highest for fiber carbonized fibers at 850 °C, while the modulus increased monotonically with increasing carbonization temperature.

Enhanced Galvanic Corrosion Phenomenon in the Welded Joint of NiCrMoV Steel by Low-Cycle Fatigue Behavior

The effect of microstructural changes on the corrosion of a NiCrMoV steel welded joint induced by low-cycle fatigue behavior in chloride solution has been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), potentiodynamic polarization curve tests and scanning vibrating electrode technique (SVET) tests. During the low-cycle fatigue tests, cyclic softening takes place in the three zones (base metal, BM/weld metal, WM/heat affected zone, HAZ) of the welded joint due to the decrease of dislocation density or the formation of low-energy structures. Compared to the as-received specimens, the corrosion potential of each zone in the low-cycle fatigue damage specimens moves to the positive direction and the corrosion current density decreases. The corrosion resistance of the individual zone increases, which is related to the reduction of dislocation density or the formation of low-energy structures. However, the difference of corrosion potential between base metal and weld metal in the low-cycle fatigue welded joints becomes lager, which means that the galvanic corrosion susceptibility of the low-cycle fatigue welded joint increases. This may be caused by the difference of microstructural changes in the various regions of welded joints during cyclic plastic deformation.

Heat-capacity measurements of sandstone at high temperatures

To evaluate geothermal energy generation systems, we must predict the amount of heat present and the rate at which it can be extracted. These two factors—amount of heat and recovery rate—in turn depend on the basic physical properties of the reservoir rocks. The amount of heat present in a reservoir depends on the heat capacity CP of the rocks. The heat capacity of the rocks is needed to evaluate energy storage, Q = (1 − ϕ)ρCP(T)T, where ϕ is the porosity, ρ is the density, CP is the heat capacity, and T is the temperature. For global heat balances, the mean specific heat between two extreme temperatures T1 and T2, can be defined as \(Q = \frac{1}{{T_{2} - T_{1} }}\int\nolimits_{{T_{1} }}^{{T_{2} }} {C_{P} \left( T \right)dT}\), where CP(T) is the temperature-dependence of the heat capacity. For dynamic processes, the value of heat-capacity at each given temperature is needed. The area under a CP(T) curve between any two temperature limits yields an enthalpy change as \(\Delta H = \int\nolimits_{{T_{1} }}^{{T_{2} }} {C_{P} \left( T \right)dT}\). In the present work we have detailed study of the temperature dependence of the heat capacity (CP) of sandstone over the temperature range from 308 to 763 K using a differential scanning calorimeter model of 204 F1. The combined expanded uncertainty of the heat capacity measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 1%. A rapid increase (from 1.06 to 1.85 kJ kg−1 K−1) of the measured heat capacities above the classical value (3R) in the high-temperature range above 575 K was observed. This increase at high temperatures can be explained as being due to microstructural changes during heating of the sample (the microstructural effect), which was confirmed by thermal expansion coefficient measurements. The effect of microstructural changes (“thermal cracking”) on the temperature behavior of heat-capacity of sandstone was studied. Theoretically-based correlations for the heat capacity were adopted to accurately represent the measured data. The measured CP(T) data for sandstone were used to check the accuracy, predictive capability, and applicability of various correlation models proposed in the literature. The proposed correlation equations for the heat capacity are based on the present experimental data that have been critically assessed for consistency with other measured properties and for agreement with theory whenever possible.

Effect of microstructural changes in the biological behavior of magnetron sputtered ZnO thin films

This work is focused on the evaluation of the correlation between the microstructural changes of zinc oxide (ZnO) thin films and its biological behavior. For this study, ZnO thin films were prepared by reactive magnetron sputtering, using different deposition times and reactive gas (O2) flows, and tested against a fungal pathogenic species, Candida albicans. Results showed that the increase of thickness of the films did not affect significantly the surface roughness but changed the crystalline structure of the films (ZnO in the hcp structure), which was followed by a small increase of antifungal properties, leading to a decrease of viable cells. The decrease of O2 flow in the deposition chamber affected the roughness only slightly (roughly 1 nm difference between the different films) but resulted in a clear decrease in the crystallinity of thin films, which improved even further their antifungal activity. These results seem to indicate a correlation between structural features and the antifungal behavior of the ZnO thin films, which will be shown by different analyses. When tested against the fungal species, the films showed to be capable of reducing the growth rate and inhibit their growth, leading to a low number of microbial cells at the end of the experiment. A deeper analysis by flow cytometry on how the thin films affected the microbial cells showed a reduced global enzymatic activity in yeast cells after contact with the materials’ surface.

Temperature dependent fracture toughness of the particulate-reinforced ultra-high-temperature-ceramics considering effects of change in critical flaw size and plastic power

The fracture toughness of particulate-reinforced ultra-high-temperature-ceramics changes but does not decrease by a constant gradient with the increase of temperature. The change in microstructure and the occurrence of brittle-ductile transition affect the fracture toughness of materials at high temperature. In this paper, a novel temperature dependent fracture toughness model for the particulate-reinforced ultra-high-temperature-ceramics was developed based on the Griffith energy theories and the concept of the maximum storage of energy associated with fracture. The effect of change in microstructure was considered by introducing a non-dimensional value, the ratio of critical flaw sizes of materials at room temperature and high temperature. The effect of plastic power was included in the model. It should be noted that the model has no any fitting parameter and which only needs some basic material parameters such as Young's modulus and specific heat capacity. The predictions of the fracture toughness of the composites in argon or air agreed well with the experimental measurements.