Dwell Temperature Research Articles

Temperature-Dependent Dwell-Fatigue Behavior of Nanosilver Sintered Lap Shear Joint

A series of dwell-fatigue tests were conducted on nanosilver sintered lap shear joint at elevated temperatures from 125 °C to 325 °C. The effects of temperature and loading conditions on dwell-fatigue behavior of nanosilver sintered lap shear joint were systematically studied. With higher temperature and longer dwell time, creep played a more important part in dwell-fatigue tests. Creep strain accumulated during maximum shear stress hold was found partly recovered by the subsequent cyclic unloading. Both the fracture mode and silver particle growth pattern were characterized by X-ray tomography system and scanning electron microscope (SEM). The mean shear strain rate γ˙m synthesized the effects of various factors, such as temperature, shear stress amplitude, mean shear stress, and dwell time, by which the fatigue and dwell-fatigue life of nanosilver sintered lap shear joint could be well predicted within a factor of two.

Key parameters for spark plasma sintering of wet-precipitated iodate-substituted hydroxyapatite

A possible strategy for the management of radioactive iodine-129 is to incorporate it under the form of iodate in hydroxyapatite matrices (HA-CaI) intended to be stored in deep geological repositories. To meet disposal requirements, powder materials that were here obtained by a wet-precipitation route required shaping. In this work, we demonstrate that sintering of HA-CaI could be achieved by spark plasma sintering (SPS) at low temperature (T <300°C) without iodine volatilization. Such densification was only possible if a non apatitic hydrated calcium phosphate layer was present at the surface of grains, the content of which directly depended on synthesis conditions. The higher the proportion of hydrated layer, the better uniaxial cold compaction and densification during sintering. For an optimised HA-CaI powder, a relative density of 88% could be reached using a pressure of 100MPa, a dwell temperature of 250°C and a heating rate of 30°Cmin−1.

Effect of notches on creep–fatigue behavior of a P/M nickel-based superalloy

A study was performed to determine and model the effect of high temperature dwells on notch low cycle fatigue (NLCF) and notch stress rupture behavior of a fine grain LSHR powder metallurgy (P/M) nickel-based superalloy. It was shown that a 90second (s) dwell applied at the minimum stress (“min dwell”) was considerably more detrimental to the NLCF lives than similar dwell applied at the maximum stress (“max dwell”). The short min dwell NLCF lives were shown to be caused by growth of small oxide blisters which caused preferential cracking when coupled with high concentrated notch root stresses. The cyclic max dwell notch tests failed mostly by creep accumulation, not by fatigue, with the crack origin shifting internally to a substantial distance away from the notch root. The classical von Mises plastic flow model was unable to match the experimental results while the hydrostatic stress profile generated using the Drucker–Prager plasticity flow model was consistent with the experimental findings. The max dwell NLCF and notch stress rupture tests exhibited substantial creep notch strengthening. The triaxial Bridgman effective stress parameter was able to account, with some limitations, for the notch strengthening by collapsing the notch and uniform gage geometry test data into a singular grouping.

Dissolution behaviour of the γ′ precipitates in two kinds of Ni-based superalloys

The effects of high heating temperature and short dwell time on the evolution of γ′ precipitates were investigated in two kinds of Ni-based superalloys. The microstructural evolutions of the samples were characterised by qualitative and quantitative metallography during various heating cycles, and the dissolution kinetics were then evaluated. The results illustrated that the heating temperature plays a more predominant role in the dissolution of γ′ precipitates than the dwell time. Meanwhile, the γ′ morphology in the dissolution process was strongly influenced by the elastic strain energy associated with the lattice mismatch between γ and γ′ phases. For IN 100 alloy, the γ′ evolution was dominated by the elastic interaction energy between adjacent precipitates, but for DS Rene 125 alloy, the γ′ precipitates kept a regular arrangement along the [001] softest directions in the dissolution process. On increasing the dwell time for the two kinds of superalloys, the dissolution rate of γ′ gradually decreased, and the dissolution activation energy gradually increased. However, the dissolution activation energy for IN 100 alloy was different from that of DS Rene 125 alloy under the same heating conditions.

Effects of a Mixed Zone on TGO Displacement Instabilities of Thermal Barrier Coatings at High Temperature in Gas-Cooled Fast Reactors

Thermally grown oxide (TGO), commonly pure α-Al2O3, formed on protective coatings acts as an insulation barrier shielding cooled reactors from high temperatures in nuclear energy systems. Mixed zone (MZ) oxide often grows at the interface between the alumina layer and top coat in thermal barrier coatings (TBCs) at high temperature dwell times accompanied by the formation of alumina. The newly formed MZ destroys interface integrity and significantly affects the displacement instabilities of TGO. In this work, a finite element model based on material property changes was constructed to investigate the effects of MZ on the displacement instabilities of TGO. MZ formation was simulated by gradually changing the metal material properties into MZ upon thermal cycling. Quantitative data show that MZ formation induces an enormous stress in TGO, resulting in a sharp change of displacement compared to the alumina layer. The displacement instability increases with an increase in the MZ growth rate, growth strain, and thickness. Thus, the formation of a MZ accelerates the failure of TBCs, which is in agreement with previous experimental observations. These results provide data for the understanding of TBC failure mechanisms associated with MZ formation and of how to prolong TBC working life.

The Role of Microstructure on Microwave Dielectric Properties of (Ba,Sr)TiO3 Ceramics

Microstructural effects on the microwave dielectric properties of (Ba0.4Sr0.6)TiO3 (BST) polycrystalline ceramics were investigated, focusing on the grain size. Sintering temperatures between 1350°C and 1500°C have a strong effect on the permittivity (880 &lt; εr &lt; 990), quality factor (570 GHz &lt; Q×f &lt; 1150 GHz), and temperature coefficient of resonant frequency (3920 ppm/°C &lt; τf &lt; 4560 ppm/°C) at microwave frequency (≈1.4 GHz). The tunable permittivity characteristics (measured at 10 kHz), were also found to be sensitive to the sintering process, demonstrating the possibility of tailoring material property by designed processing. In addition, the effect of the sintering process on grain structure was investigated by XRD calculation and Raman scattering characterization. Less confined phonons were believed to contribute to the enhanced microwave performance as an intrinsic effect (grain size effect), for samples with higher sintering temperature and longer dwell time. On the contrary, the macroscopic properties tend to saturate (or deteriorate), for samples at intensified sintering condition, being thought to be dominated by the extrinsic factors (such as the abundant defects in large grains), as confirmed by the SEM observations.

Reliability Assessment of Packaging Solder Joints Under Different Thermal Cycle Loading Rates

In assessing the solder joint reliability using an accelerated thermal cycling (ATC) test methodology, which is widely used in the microelectronics industry, a common type of solder joint failure is the formation of fatigue cracks at the interface between the solder joints and the components. However, ATC tests are quite time-consuming and can take more than a month to complete. Hence, its efficiency has become an important issue. The parameters of the ATC test, such as temperature ramp rate and dwell time, significantly influence fatigue failure in solder joints. Fast temperature cycles can reduce the testing time, but the effects of ramp rate cause variations in solder joints' material properties and mechanics behavior due to strain rates and stress changes. Commonly used lead-free materials with high homologous temperatures induce creep-related solder joint failures. This paper used a temperature-dependent Young's modulus and Garofalo-Arrhenius creep equation to describe the solder deformation response. An energy-density-based empirical equation was used, along with the optimized mesh size in a finite-element model calculation with different solder joint geometries and packaging types such as power modules, and wafer-level chip-scale packages to determine how to accommodate the strain rate effect given various ramp rates.

Ultrahigh-Temperature Regeneration of Long Period Gratings (LPGs) in Boron-Codoped Germanosilicate Optical Fibre.

The regeneration of UV-written long period gratings (LPG) in boron-codoped germanosilicate “W” fibre is demonstrated and studied. They survive temperatures over 1000 °C. Compared with regenerated FBGs fabricated in the same type of fibre, the evolution curves of LPGs during regeneration and post-annealing reveal even more detail of glass relaxation. Piece-wise temperature dependence is observed, indicating the onset of a phase transition of glass in the core and inner cladding at ~500 °C and ~250 °C, and the melting of inner cladding between 860 °C and 900 °C. An asymmetric spectral response with increasing and decreasing annealing temperature points to the complex process dependent material system response. Resonant wavelength tuning by adjusting the dwell temperature at which regeneration is undertaken is demonstrated, showing a shorter resonant wavelength and shorter time for stabilisation with higher dwell temperatures. All the regenerated LPGs are nearly strain-insensitive and cannot be tuned by applying loads during annealing as done for regenerated FBGs.

Determination of Optimum Process for Thermal Debinding and Sintering Using Taguchi Method

Debinding involves long and delicate processing periods of removing binder components from a green body after injection moulding; failure to completely remove the binder components results in distortion, cracking, blisters and contamination at elevated temperatures. This study focuses on optimising thermal debinding process parameters on the basis of obtaining a defect-free part after sintering and also determining a sintering time that gives high sintering density. Thermal debinding was conducted after solvent debinding. The feedstock used to produce green compacts composed of Ti6Al4V powder and a wax-based binder. The binder’s backbone component is a low density polyethylene (LDPE). Careful selection of thermal debinding parameters was guided by thermo-gravimetric analysis (TGA) results. The Taguchi method was used to determine an optimum debinding process. Thermally debound compacts were analysed for residual binder using a TGA. Archimedes’ principle and optical microscopy were done to analyse the sintering density and microstructure of the sintered product, respectively. Optimum debinding and sintering conditions were identified. The study demonstrated that heating rate during debinding was the most influential factor that contributes to minimum residual binder followed by debinding dwell time and temperature. Longer sintering time of 4 h favoured higher density of 91.6 ±1.55%. A typical radial shrinkage level of 11.1 ±0.0816% was determined.

Effect of cooling path on the phase transformation of boron steel 22MnB5 in hot stamping process

Boron steels have phase transformation in hot stamping process, and the mechanical properties of hot stamped parts are determined by the microstructure generated by phase transformation. Cooling path has a significant effect on phase transformation and microstructure of the deformed parts; in order to obtain the tailored-customized mechanical properties, the relationship among cooling path, the phase transformation, and the corresponding microstructure must be investigated. In this paper, the dilatometric experiment with the cooling rate of 2, 10, and 50 °C/s and various dwell temperatures was conducted to explore the influence of cooling path on the start and finish temperatures of phase transformations from austenite to ferrite, bainite, and martensite; lasting times of transformations; and final microstructures in hot stamping process. The controllable factors which influence the mechanical properties of hot stamped boron steel component were also determined. In addition, the metallographic and Vickers hardness experiments were conducted as well to confirm the analysis result of dilatometric experiment. The knowledge about the relationship of cooling path, phase transformation, microstructure, and mechanical property of the hot stamped parts helps the design of hot stamping process of the stamped parts with tailored-customized mechanical properties.

Taguchi design of experiments for optimization of ionic conductivity in nanocrystalline Gadolinium doped Ceria

Taguchi experiments are designed and carried out for glycine nitrate precursor (GNP) combustion method to study four preparation parameters, viz. Fuel to oxidizer molar ratio, oven temperature, calcination temperature and calcination dwell time for their influence on physical properties and ionic conductivity of sintered pellets of Gadolinium doped Ceria (GDC) powders. These four parameters with their three levels form L9 orthogonal array. Ionic conductivity measurements were done using Nyquist and Bode plots obtained using frequency perturbed impedance analysis in the frequency range of 1Hz to 1MHz at temperatures 500–700°C. The optimum conditions was found out on the basis of ionic conductivity by using ‘larger the better’ analysis. Both, the Analysis of Mean (ANOM) and Analysis of Variance (ANOVA) indicate that Fuel to oxidizer ratio is the only influential process parameter. The ionic conductivity of GDC powder prepared using optimized conditions by Taguchi method was found to be 0.023Scm−1 at 600°C, which is 1.5 times higher than earlier measurements. Significantly, this is highest reported conductivity for GDC at 600°C. The significant improvement in results following Taguchi optimization underlines importance of this method for materials synthesis.

Effects of microstructure on high temperature dwell fatigue crack growth in a coarse grain PM nickel based superalloy

The influence of microstructure on the dwell fatigue crack growth behaviour of an advanced nickel-based superalloy was investigated at a temperature of 700°C. Microstructural variations were induced by heat treatment variables: different cooling rates of quenching from super-solvus solution heat treatment, 0.7 and 1.8°Cs−1, and an addition of a high temperature stabilisation heat treatment (2h at 857°C) between the solution treatment and the final ageing treatment. With a one hour dwell introduced at the peak load of the fatigue cycle, such different microstructural conditions can lead to a difference of up to two orders of magnitude in crack growth rates in air, when compared to those obtained under baseline fatigue loading. By performing such dwell fatigue and baseline fatigue tests in vacuum, it is confirmed that such increases in crack growth rates under dwell fatigue loading in air are mainly environmentally related. Transmission electron microscopy (TEM) was utilised to analyse both crack tip oxides and associated deformation mechanisms in the matrix. A novel mechanism taking into account competing interactions of crack tip oxidation (leading to increases in crack growth rates) and stress relaxation (leading to decreases in crack growth rates) is outlined.

Oxide dispersion strengthened nickel based alloys via spark plasma sintering

Oxide dispersion strengthened (ODS) nickel based alloys were developed via mechanical milling and spark plasma sintering (SPS) of Ni–20Cr powder with additional dispersion of 1.2wt% Y2O3 powder. Furthermore, 5wt% Al2O3 was added to Ni–20Cr–1.2Y2O3 to provide composite strengthening in the ODS alloy. The effects of milling times, sintering temperature, and sintering dwell time were investigated on both mechanical properties and microstructural evolution. A high number of annealing twins was observed in the sintered microstructure for all the milling times. However, longer milling time contributed to improved hardness and narrower twin width in the consolidated alloys. Higher sintering temperature led to higher fraction of recrystallized grains, improved density and hardness. Adding 1.2wt% Y2O3 to Ni–20Cr matrix significantly reduced the grain size due to dispersion strengthening effect of Y2O3 particles in controlling the grain boundary mobility and recrystallization phenomena. The strengthening mechanisms at room temperature were quantified based on both experimental and analytical calculations with a good agreement. A high compression yield stress obtained at 800°C for Ni–20Cr–1.2Y2O3–5Al2O3 alloy was attributed to a combined effect of dispersion and composite strengthening.

Microwave Assisted Synthesis and Sintering of Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) perovskite-type oxide is currently one of the most promising materials for applications in solid-oxide fuel cells (SOFCs), protonic ceramic fuel cells (PCFCs), oxygen separation membranes, and catalytic membranes for methane conversion. BSCF powder synthesis has received considerable attention and new synthesis methods have been proposed to obtain nanoscale powders with high chemical homogeneity. In this study, BSCF perovskite powder has been successfully prepared by microwave assisted combustion in aqueous solution. The synthesized powder was characterized by DSC, BET, XRD, and SEM. BSCF powder presented phase homogeneity, high specific surface area (9.93 g/m2) and nanometric crystallite size (23nm). The microwave sintering has been conducted in different conditions of dwell time and temperature. The influence of the temperature and the dwell time on microstructure was evaluated by optical microscopy. The results showed that microwave sintering could achieve the same densification compared to conventional sintering with only 10% of processing time. This shorter processing time has resulted in a reduction of grain size of up to 46.5%, compared to conventional sintering.

Spark plasma sintering of mullite: Relation between microstructure, properties and spark plasma sintering (SPS) parameters

The spark plasma sintering (SPS) of mullite powder (D50=3.29μm) was investigated. The SPS parameters such as the dwell temperature, applied external pressure, and dwell time were varied. The sinterability of mullite increases with increase in the dwell temperature, dwell time and applied external pressure. The influence of porosity on mechanical properties (hardness, and fracture toughness) of mullite prepared by SPS is measured. Pulsed DC current is found to play a dominant role in the densification of mullite and achieving near theoretical density. Hardness, and fracture toughness of near fully dense mullite are measured as 1256HV, and 2.8MPam1/2 respectively.

Neighbourhood and dwelling characteristics associated with the self-reported adverse health effects of heat in most deprived urban areas: A cross-sectional study in 9 cities

Dwelling and neighbourhood characteristics associated with the prevalence of self-reported heat-induced adverse health effects are not well known. We interviewed 3485 people in the most disadvantaged neighbourhoods of the nine largest cities in Québec, Canada. The prevalence of heat-induced adverse health effects was 46%, out of which one fourth led to medical consultation. Multivariate analyses showed that dissatisfaction with the summer dwelling temperature, which refers to home heat exposure, and perception that the neighbourhood is polluted due to traffic, were determinant, even after adjusting for current health status. These risk indicators can be used to identify subgroups at high risk and as priority-setting criteria for urban renewal programs for the hotter climate to come.

Patterned Deposition of Nanoparticles Using Dip Pen Nanolithography For Synthesis of Carbon Nanotubes

AbstractOrdered carbon nanotube (CNT) growth by deposition of nanoparticle catalysts using dip pen nanolithography (DPN) is presented. DPN is a direct write, tip based lithography technique capable of multi-component deposition of a wide range of materials with nanometer precision. A NanoInk NLP 2000 is used to pattern different catalytic nanoparticle solutions on various substrates. To generate a uniform pattern of nanoparticle clusters, various conditions need to be considered. These parameters include: the humidity in the vessel, temperature, and tip-surface dwell time. By patterning different nanoparticle solutions next to each other, identical growth conditions can be compared for different catalysts in a streamlined analysis process. Fe, Ni, and Co nanoparticle solutions patterned on silicon, mica, and graphite substrates serve as nucleation sites for CNT growth. The CNTs were synthesized by a chemical vapor deposition (CVD) reaction. Each nanoparticle patterned substrate is placed in a tube furnace held at 725°C during CNT growth. The carbon source used in the growth chamber is toluene. The toluene is injected at a rate of 5 mL/hr. Growth is observed for Fe and Ni nanoparticle patterns, but is lacking for the Co patterns. The results of these reactions provide important information regarding efficient and highly reproducible mechanisms for CNT growth.

Simulation-Based Optimization of Cure Cycle of Large Area Compression Molding for LED Silicone Lens

Three-dimensional heat transfer-curing simulation was performed for the curing process by introducing a large area compression molding for simultaneous forming and mass production for the lens and encapsulants in the LED molding process. A dynamic cure kinetics model for the silicone resin was adopted and cure model and analysis result were validated and compared through a temperature measurement experiment for cylinder geometry with cure model. The temperature deviation between each lens cavity could be reduced by implementing a simulation model on the large area compression mold and by optimizing the location of heat source. A two-step cure cycle was constructed to reduce excessive reaction peak at the initial stage and cycle time. An optimum cure cycle that could reduce cycle time by more than 29% compared to a one-step cure cycle by adjusting dwell temperature, heating rate, and dwell time was proposed. It was thus confirmed that an optimization of large area LED lens molding process was possible by using the present experiment and the finite element method.

Enhancing ductility of the Al-Si coating on hot stamping steel by controlling the Fe-Al phase transformation during austenitization

In this study, austenitizing heat treatment before hot stamping of Al-10% Si coated boron steel is first investigated through environment scanning electron microscopy (ESEM) equipped with energy dispersive x-ray analysis (EDAX). The cracking behavior of the coating was evaluated using Gleeble 3500, a thermo-mechanical simulator under uniaxial plastic deformation at elevated temperatures. The extent and number of cracks developed in the coating were carefully assessed through an optical microscope. The coating layer under hot-dipped condition consists of an Al-Si eutectic matrix, Fe2Al7Si, Fe3Al2Si3 and Fe2Al5, from the coating surface to the steel substrate. The coating layer remains dense, continuous and smooth. During austenitization, the Al-rich Fe-Al intermetallics in the coating transform to more Fe-rich intermetallics, promoted by the Fe diffusion process. The coating finally shows the coexistence of two types of Fe-Al intermetallics, namely, FeAl2 and FeAl. Microcracks and Kirkendall voids occur in the coating layer and diffusion zone, respectively. The coating is heavily cracked and broken into segments during the hot tensile tests. Bare steel exposed between the separate segments of the coating is oxidized and covered with a thin FeO x layer. The appearance of the oxide decreases the adhesion of the Al-Si coating. It is found that the ductile FeAl is preferred as a coating microstructure instead of the brittle FeAl2. Therefore, the ductility of the Al-Si coating on hot stamping boron steel could be enhanced by controlling the ductile Fe-rich intermetallic phase transformations within it during austenitization. Experiments indicate that a higher austenitizing temperature or longer dwell time facilitate the Fe-rich intermetallics transformation, increasing the volume fraction of FeAl. This phase transformation also contributes to reducing the crack density and depth.

Ni-YSZ cermets for solid oxide fuel cell anodes via two-step firing

The electrochemical performance and dimensional stability of Ni-YSZ cermets, conventionally used as solid oxide fuel cell anodes, depend strongly on their microstructure and therefore fabrication conditions. This work was focused on the assessment of a less common two-step firing procedure for fabrication of Ni-YSZ cermets with comparatively low nickel fraction of 30 vol.%. The impact of different firing parameters including peak temperature (1623–1723 K), heating/cooling rate (4–10 K/min), and isothermal treatment temperature (1473–1573 K) and time (2–8 h), on the porosity and electrical conductivity of cermets was assessed employing Taguchi experimental planning. The applied procedure yielded Ni-YSZ composites with porosity 26–35% and electrical conductivity ranging from 170 to 420 S/cm at 873–1173 K in 10%H2–N2 atmosphere. Microstructural studies indicated that the conductivity is determined mainly by Ni particle size distribution. Analysis of results suggests that, for the studied range of sintering parameters, a higher peak temperature and ramp rate are favorable for the improvement of conductivity, whereas isothermal dwell temperature and time have a rather minor effect on the conductivity level.