Function Of Heat Treatment Temperature Research Articles

Morphological Characteristics of W/Cu Composite Nanoparticles with Complex Phase Structure Synthesized via Reactive Radio Frequency (RF) Thermal Plasma

The W/Cu binary system is characterized by its mutual insolubility and excellent wettability, making W/Cu composite materials ideal for managing thermal and electrical properties in electronic components. To optimize material properties, control over the microstructure is crucial, and nanocomposites with uniform dispersion offer significant advantages. In this study, W/Cu composite nanoparticles were synthesized by feeding a blended feedstock of tungsten trioxide (WO3) micro-powder and cupric oxide (CuO) micro-powder into a reactive radio frequency (RF) argon–hydrogen thermal plasma system. Cu-coated W nanocomposite particles were obtained through the vaporization, reduction, and condensation processes. The resulting nanocomposite particles were composed of body-centered cubic (BCC) α-W, A15 β-W, and face-centered cubic (FCC) Cu phases, with a chemical composition closely matching theoretical calculations. The phase evolution and morphological changes of the synthesized particles were analyzed as a function of heat treatment temperatures up to 1000 °C in a reducing atmosphere. Up to 600 °C, the phase composition and morphology remained stable. At 800 °C, localized diffusion and coalescence of Cu led to the formation of particulate Cu, and a significant phase transformation from metastable β-W to α-W was observed. Additionally, extensive Cu segregation due to long-range diffusion resulted in distinct Cu-rich and Cu-depleted regions. In these regions, notable sintering of W particles and the complete disappearance of β-W occurred. The results showed that the temperature-dependent redistribution of Cu plays a crucial role in the phase transformation of W and the morphology of W/Cu composite particles.

Preparation of Soft Magnetic Composite Using Cold Sintering Technique

Soft magnetic composites are powder metal products under development as an alternative to 2D laminates in electric motor applications. The 3D design of SMCs offers greater energy efficiency but poses a challenge to maintain adequate strength properties required for application. Soft magnetic composites are fabricated through compaction and heat treatment of iron particles coated with insulated coating to form a dense compact. The soft magnetic properties of the compact are affected by the properties of the iron powder, density of compact as well as the efficacy of forming uniform thin insulating coating at the particle boundary. Additionally, the strength of the compact is also affected by the nature of the insulating boundary at the particle interfaces. We report a low temperature process that involves cold sintering phenomenon to densify and strengthen soft magnetic composites. Iron powders have been modified using a co-precipitation method that forms a hydrated copper/iron oxalate coating on individual iron particles. Warm compaction of the modified iron powders at 100°C resulted in a highly dense soft magnetic composite which demonstrated ~ 3x improvement in strength. The compacts can be further heat treated in air at higher temperatures to form high strength oxide coating iron based soft magnetic composites. Results of the AC and DC magnetic measurements of fabricated cold sintered toroid compacts made as a function of heat treatment temperature reveal that DC permeability increased with heat treatment temperature. Analysis of AC core loss data showed that hysteresis power losses dominated the performance at lower heat treatment temperatures (< 300°C) beyond which dynamic core losses significantly increased.

Optical properties of germania and titania at 1064 nm and at 1550 nm

One of the main noise sources in current gravitational-wave detectors is the thermal noise of the high-reflectivity coatings on the main interferometer optics. Coating thermal noise is dominated by the mechanical loss of the high-refractive index material within the coating stacks, Ta2O5 mixed with TiO2 . For upgrades to room-temperature detectors, a mixture of GeO2 and TiO2 is an interesting alternative candidate coating material. While the rather low refractive index of GeO2 increases with increasing TiO2 content, a higher TiO2 content results in a lower threshold temperature before heat treatment leads to crystallisation, and potentially to a degradation of optical properties. For future cryogenic detectors, on the other hand, a higher TiO2 content is beneficial as the TiO2 suppresses the low-temperature mechanical loss peak of GeO2. In this paper, we present the optical properties of coatings—produced by plasma-assisted ion-beam evaporation—with high TiO2 content at 1550 nm, a laser wavelength considered for cryogenic gravitational-wave detectors, as a function of heat-treatment temperature. For comparison, the absorption of pure GeO2 was also measured. Furthermore, results at the currently-used wavelength of 1064 nm are presented.

Comparing physicochemical properties related to thermal stability of caprine and bovine milk protein concentrate dispersions

Thermal stability and physicochemical properties as a function of heating temperature, pH and salt addition was compared between caprine and bovine milk protein concentrate dispersions. Caprine milk protein concentrate (MPC) had a lower amount of heat coagulation time and a higher percentage of sediments than bovine MPC. Calcium ion activity of caprine MPC was higher than that of bovine. The amount of κ-casein in the serum of caprine MPC dispersions was higher than that of bovine, and increased with increasing temperatures, pH and the addition of NaCl and citrate. The amount of whey protein in serum was also higher in heated caprine MPC compared with bovine. Particle size of caprine MPC was larger than bovine MPC and was more susceptible to induced aggregation. The lower thermal stability of caprine MPC might be associated with the higher calcium ion activity and the higher amount of κ-casein dissociation promoted by whey protein denaturation and complexation in serum.

Photoelectrochemical, optical and magnetic properties of magnetite nanoparticles

AbstractMagnetite magnetic nanoparticles are prepared using olive leaf extract as a green reducing and stabilizing agents. After reaction the product is heated up to get rid of the organic compounds and get pure magnetite nanoparticles. Differential scanning calorimetry is used to study the phase transformation as a function of heating temperature. Scanning electron microscope and high resolution transmission electron microscope show spherical and crystallized nanoparticles with a size of 5 nm. X‐ray diffraction and Raman and x‐ray photoelectron spectroscopy indicate the formation of Magnetite phase with high cristallinity and purity. The synthesized Magnetite nanoparticles are semiconductors with gap energy around 2 eV. Observed by transmission electron microscope graphite rods with stacked carbon disks are decorated with the prepared nanoparticles and show enhanced photocurrent. The vibrating sample magnetometer measurements indicate that the prepared Magnetite nanoparticles have superparamagnetic behavior. These results are very promising for clinical and water splitting applications.

Composition and phase structure dependence of magnetic properties for Co2FeCr0.5Alx (x=0.9, 1.2) multi principal component alloys

This article investigates the microstructure evolution, phase formation, and magnetic properties of Co2FeCr0.5Alx (x = 0.9, 1.2) complex component alloys, as a function of heat treatment temperatures (at 500, 600, 700, and 1150°C), using XRD, optical microscopy, electron microscopy and vibrating sample magnetometry (VSM). The alloy with 20.4 at% Al (x= 0.9), identified here as C1, consisted of microscale BCC1 phase and BCC nanoscale particles containing mainly Fe and Cr, and B2 matrix with mainly Al and Co. Partial transformation of the BCC1 phase to an FCC phase was observed at 700°C and full transformation at 1150°C, through twinning. For the sample with 25.5 at% Al (x= 1.2), identified as sample C2, there were only nanoscale BCC particles in the B2 matrix with the same element segregation between the phases as C1. This increase in Al (from x= 0.9 to 1.2) content stabilised the B2 matrix phase, reduced the grain size, and increased both saturation magnetisation (Ms) and coercivity (Hc). Moreover, increasing the heat treatment temperature resulted in an increase in grain size of the B2 matrix, volume fraction and average size of the micro BCC 1 and nanoscale BCC phases for both C1 and C2, which also modified the soft magnetic properties, with Ms and Hc increasing up to 600°C followed by a decrease until 1150°C. Using the structural information as inputs for density functional theory calculations of Hc and Ms, it has been found that the Hc is influenced by the grain size of the matrix, and the volume fraction and size of the BCC1 phase at temperature higher than 600°C for C1 and 700°C for C2, but is controlled by nanoscale BCC particles below these temperatures. The Ms is controlled by the elemental diffusion and segregation. Thus, the best combination of Hc and Ms was seen with antiferromagnetic Cr segregated and partitioning in the microscale BCC1 phase, and a B2 matrix with less Cr rich precipitation, formed at 500°C, where the misfit strain between B2 matrix and nanoscale BCC was low.

Vacancy-Driven Stabilization of Sub-Stoichiometric Aluminate Spinel High Entropy Oxides

Despite significant recent developments in the field of high entropy oxides, previously reported HEOs are overwhelmingly stoichiometric structures containing a single cationic site and are stabilized solely by intermixing increasing numbers of cations. For the first time, we demonstrate here that cationic vacancies can significantly increase configurational entropy and stabilize phase-pure HEOs. Aluminate spinel HEOs with AB2O4 stoichiometry are used as a model crystal structure. These spinels tolerate large divalent cation deficiencies without changing phase, allowing for high concentrations of cationic vacancies. Stoichiometric and sub-stoichiometric spinels (with A:B molar ratios <0.5), which contained various mixtures of Co, Cu, Mg, Mn, Ni, and cationic vacancies in nominal equimolar concentration, were systematically compared as a function of heat treatment temperature and number of unique cationic species. We found that the same number of cationic species were needed to stabilize both stoichiometric and sub-stoichiometric nickel-containing spinels at 800 °C calcination, as exemplified by (CoCuMgNi)Al2O4 and (CoMgNi)0.75Al2Ox samples, signifying that vacancies stabilize phase-pure spinels similarly to cations. The chromatic, structural, and chemical properties of these complex spinels were highly tunable via incorporation of cationic vacancies and multiple divalent metals, promoting their potential application as unique pigments, catalysts, and thermal coatings.

Triethylammonium thiocyanate ionic liquid electrolyte-based supercapacitor fabricated using coconut shell-derived electronically conducting activated charcoal electrode material

Organic solvents are commonly used as commercial electrolytes in supercapacitors, but they have several drawbacks, such as high volatility, toxicity, and environmental issues. To overcome these problems, ionic liquids are used though their high costs hamper commercial applications. In this study, a novel and low-cost ionic liquid based on triethylammonium cation and thiocyanate anion were synthesized and used as the electrolyte in a low-cost supercapacitor fabricated using electronically conducting activated carbon derived from cleaned coconut shells as electrode material. The temperature dependence of electrolyte conductivity was investigated, in the frequency range of 1 Hz-500 kHz, using AC impedance analysis, in the temperature range from 30 °C - 90 °C in ten-degree increments and found to be in the range of 4.5 to 14.4 mS cm−1. The ion transport kinetics behaves according to the Vogel-Tammann-Fulcher (VTF) model with an activation energy (Ea) and the pre-exponential factor (AσT) of 0.0158 eV and 114.9 S m−1 K1/2, respectively. The activated carbon material developed has a high porosity and high electronic conductivity. The electrode fabrication methodology was optimized by varying the polyvinylpyrrolidone (PVP) binder amount with respect to the mass of activated carbon on a titanium sheet as a function of heat treatment temperature. The supercapacitor that gives the highest specific capacitance is based on the electrode constructed using 5 % w/w PVP in 0.50 g of activated carbon deposited on a titanium sheet by heat treating at 200 °C for 20 min. At the scan rate of 5 mV s−1, two-electrode CV studies revealed a specific capacitance of 36.8 F g−1 for the optimized supercapacitor assembly. The electrolyte triethylammonium thiocyanate (TAT) demonstrated good electrochemical stability and a large operating voltage window of 1.8 V and high stability, with 94.4 % capacity retention after 1000 cycles. The AC impedance studies also gave comparable results. Prime novelty statementThe manuscript describes a method to prepare a novel and low-cost ionic liquid material triethylammoniumthiocyanate starting from triethylamine and ammonium thiocyanate, and its application as the ionic liquid electrolyte in a low-cost supercapacitor. The electrode material used in the supercapacitor was fabricated using electronically conducting activated carbon derived from cleaned coconut shells, which were deposited firmly adhering to titanium sheets using polyvinylpyrrolidone binder under optimized heat treatment conditions. As such, both the ionic liquid electrolyte and the activated carbon electrode material are novel and attractive materials for the commercial production of low-cost supercapacitors.

Developing an optical backscatter method for determining casein micelle particle size in heated milk

A plethora of different factors, such as heat treatment, pH, soluble calcium and phosphate concentrations, colloidal calcium phosphate, ionic strength, redox potential, etc., affect functionally of critical milk components such as casein micelles, fat globules and whey proteins. These physicochemical changes induce fat- or protein-protein interactions that would be associated to changes in particle size that might be revealed using light backscatter measurements. We hypothesized that inline, simple, low-cost light backscatter measurements might have the potential to provide functionally related information, representing an interesting opportunity for process control. Casein micelle particle size and near infrared light backscatter spectra were measured in milks heat treated at 80 and 90 °C and pH 6.3, 6.7 and 7.1 in order to obtain prediction models for estimating changes in casein micelle particle size during milk heat treatment. Light intensity was measured over a spectral range of 200–1100 nm using a simple optical backscatter sensor and was implemented into models for particle size predictions as a function of heat treatment temperature and pH. Models which included an exponential factor containing a ratio of two specific wavebands were found to improve R2 when compared to single wavelength models. The best model exhibited an R2 of 0.993 and SEP of 2.36 nm. The developed prediction models show promise for in-line monitoring of whey protein denaturation and casein micelle particle size.

High-throughput investigation of structural evolution upon solid-state in Cu–Cr–Co combinatorial multilayer thin-film

Cu–Cr–Co combinatorial multilayer thin-films were prepared by a high-throughput ion beam sputtering system. Based on the thickness ratio among the individual nanoscale monolayers (Cu, Cr, Co), the resulting stoichiometry covered the entire phase diagram. The chemical composition and structure of Cu–Cr–Co combinatorial chip upon solid-state reaction were studied by lab-based micro-X-ray fluorescence (μ-XRF) and high-throughput synchrotron X-ray diffraction (XRD), respectively. A composition-structure map for Cu–Cr–Co combinatorial chip was developed through automated data analysis employing hierarchical clustering techniques. The structural evolution of Cu–Cr–Co combinatorial chip as a function of heat-treatment temperature, time, and modulation period were studied systematically. Furthermore, the effect of the elemental distribution in-depth direction was investigated to gain more insights regarding phase transformation. This work provides an efficient method and new perspectives for the design and optimization of the composition and structure of high-performance thin-films.

Bayesian statistical modeling to describe uncertainty of thermal inactivation behaviour of bacterial spores

To produce high-quality ready-to-eat food, excessive heat treatment must be avoided to maintain product quality. However, there is a risk of survival of heat-tolerant bacterial spores, such as those of Bacillus simplex. Therefore, the thermal inactivation process needs optimization, considering the product's properties, such as pH, and water activity (aw). This study aimed to develop a thermal inactivation prediction model for B. simplex as a function of heating temperature, pH, and aw, which expresses the uncertainty of the prediction error as a probability distribution using Bayesian inference. The survival kinetics of B. simplex spores were successfully fitted using the Weibull model, and the number of surviving B. simplex spores was estimated as a probability distribution, where the parameters were described as a function of heating temperature, pH, and aw using Bayesian regression. The B. simplex survival kinetics was successfully described as a 95% prediction band by using the estimated parameter distribution, and 84% of the validation data from bacterial culture agreed with the 95% prediction band. Although validation with real food, such as meat sauce, demonstrated that the predictive model is valid, it was not valid for other fat- or sugar-rich sauces, such as carbonara and Japanese cooked beef stew. These facts suggest that environmental factors other than temperature, pH, and aw should be incorporated into the secondary model. Nevertheless, the prediction of B. simplex survival with uncertainty using a combination of multiple environmental factors would enable a more accurate and realistic evaluation of the optimal thermal inactivation conditions.

High temperature surface graphitization of CVD diamond films and analysis of the kinetics mechanism

A plasma arc generator was used to heat treat the free-standing CVD diamond films at 1450–1900 °C. An Arrhenius curve was fit according to trends in the measured Raman peak intensity ratio of diamond to graphite as a function of heat treatment temperature, and the curve was extrapolated linearly. It was found that the extrapolation lines intersected at the Debye temperature, ΘD = 2021 K. By comparing the activation energies of diamond graphitization in literature and from different measurement modes in our study, it was found that the graphitization mechanisms were different on both sides of Debye temperature due to differences in the excited lattice vibration modes. When the heat treatment temperature was just below the Debye temperature, the graphitization process was promoted by generating vacancies or the breaking of single CC bonds, and the apparent activation energy was 366 ± 60 kJ·mol−1. When the heat treatment temperature was above the Debye temperature, the graphitization process was promoted by the simultaneous breaking of multiple CC bonds, and the apparent activation energy was 887 ± 141 kJ·mol−1. In combination with the morphology changes on the surface of the CVD diamond film after high temperature heat treatment, the mechanism of the micro-cyclic phase transition on the surface of the diamond film at temperatures lower than Debye temperature was summarized as follows. First, CC bonds on the diamond surface were activated and broken at high temperatures, and some diamond carbon atoms dissociated from solid phase sites to the gas phase by forming hydrocarbon free radicals. Then, the carbon atoms from these hydrocarbons entered the graphite lattice step by step to complete the spontaneous deposition of sp2 clusters.

Temperature driven structural transition in the nickel-based catalytic graphitization of coconut coir

Biomass waste is considered as a renewable, low-cost, and environmental friendly carbon source. However, converting a largely disordered carbon materials from biomass wastes into a highly crystalline graphitic carbon nanostructure with low-energy consumption process remains a challenge. Here, we report a simple and low energy process to transform the amorphous carbon structure of coconut coir from Indonesian biomass waste into graphitic nanostructure. A high crystalline graphitic nanostructure has been successfully obtained through two-step process consisting of (1) carbonization process at 500 °C to produce amorphous charcoal, followed by (2) nickel-based catalytic graphitization process at various temperature of 1000 °C to 1300 °C. It is important to note that the temperature process is a main factor in the structural transformation from amorphous to graphitic nanostructures. The structural transformation during catalytic graphitization process was investigated as a function of heat treatment temperature using X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The results show that the carbon structure of coconut coir start to transform from amorphous to ordered graphitic nanostructure at 1200 °C, a temperature lower than that of conventional graphite production, with the highest degree of graphitization (82.16%) and IG/ID ratio of 1.16. Observation using high resolution transmission electron microscopy (HRTEM) reveal the well-defined lattice fringes of graphitic nanostructures with an interplanar distance of 0.3369 nm corresponding to the (0 0 2) crystal plane of pure graphite (0.3354 nm) and less than the disorder carbon (0.3440 nm). In conclusion, this simple process and low energy successfully convert the coconut coir waste into a higher value-added product of graphitic materials as well as reduce the environmental impact of coconut waste.

Effects of a pressurized water treatment on internal gelation sol–gel microspheres

Based on previous observations that a pressurized water treatment (PWT) prevented cracking of sol–gel microspheres, we investigated the effects of a PWT on microsphere crystallinity, density, and specific surface area (SSA). Results were used to determine how a PWT alters the properties of microspheres upon drying and heating. Microspheres with diameters near 100–200 µm were prepared with and without a PWT and measured using x-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen adsorption (BET), and pycnometry. Properties of air-dried microspheres processed with and without a PWT are compared. In addition, the properties of microspheres processed using a PWT are reported after heating to 150, 450, 750, 1050, and 1350 °C. XRD measurements indicate that the PWT step improves the crystallinity of air-dried microspheres. XRD data were also used to calculate crystallite size, which increases with higher heat-treatment temperatures. TEM images support crystallite size calculations from XRD data and provide an indication of the range of crystallite sizes, particularly for samples processed at higher temperatures where crystallite sizes are too large for estimation using the Scherrer formula. Density and SSA measurements performed as a function of heat-treatment temperature indicate that a PWT increases the density of air-dried microspheres, creates a pore network, and that significant densification occurs between 450 and 750 °C. These results may be used to inform decisions on internal gelation flowsheet parameters to optimize the microsphere formation and gelation step, prevent microsphere cracking, and produce microspheres suitable for subsequent coating operations or pressing into pellets.

Induced thermoluminescence as a method for dating recent volcanism: The Blue Dragon flow, Idaho, USA and the factors affecting induced thermoluminescence

In order to continue our investigation into the possible utility of induced thermoluminescence as a chronometer for volcanic activity we have measured the induced thermoluminescence properties of twenty-eight samples collected from the almost 3 km long Blue Dragon flow at Craters of the Moon National Monument and Preserve, Idaho. The properties of the samples are consistent with previous conclusions that the major mineral causing the luminescence is feldspar and that variation in the glow curves (light emitted as a function of heating temperature) is caused by variations in the thermal history of the samples. However, in nine of the 28 samples a there is a significant contribution to the TL signal from apatite. The induced thermoluminescence level along the flow is essentially constant, showing at most a factor of four decrease from beginning to the end (from ∼16 to ∼4, on a scale of the Dhajala meteorite = 1000), and this is not statistically significant. Thus sampling distance from the vent should not be a problem for the proposed TL dating method. However, there are details that might pose a problem for dating. (1) There is considerable (factor of 10 or so) scatter in the TL data due to the high heterogeneity of the samples. (2) There are abundant highly luminescent spherules which could explain some of the scatter; however, they are restricted to samples taken very close to the vent and could be screened from TL samples using cathodoluminescence, a microscopic means of identifying components producing the TL. (3) There is a factor of three increase in crystallization along the core (from ∼20 vol% to ∼60 vol%) but no factor of three increase in induced TL value. This is additionally problematic because crystallization is accompanied by a small decrease in anorthite content of the feldspar which should further increase induced TL values. While empirical data suggest that there is a weak correlation between induced TL and age for volcanics from Idaho and Hawaii, our detailed study of the Blue Dragon flow removes one complication for the dating method but reveals many details that need to be kept in mind as this possible dating method is explored. Most importantly, we need a method for reducing the scatter in induced TL displayed by samples from a single volcanic eruption.

Optimizing the structure of layered cathode material for higher electrochemical performance by elucidating structural evolution during heat processing

Improving electrochemical performance of cathode materials for lithium-ion batteries requires comprehensive understanding of their structural properties which could facilitate or impede the diffusion of lithium during charge-discharge. In order to optimize the structure and improve the electrochemical performance of layered cathode material, the detailed structural evolution as a function of heat treatment temperature in LiNi0.8Co0.1Mn0.1O2 was investigated by in-situ and ex-situ neutron powder diffraction methods. We show that both cycling stability and rate performance of LiNi0.8Co0.1Mn0.1O2 can be improved by performing heat treatment at 400 °C, which is attributed to the optimization of surface structure and the enlargement of c/a ratio. Heat treatment of LiNi0.8Co0.1Mn0.1O2 at higher temperature induces a layered-to-rock-salt structure phase transition accompanied with the precipitation of lithium oxide. A 3D phase diagram, which correlates the high temperature phases and room temperature phases, is constructed. The presentation of comprehensive phase diagrams up to 1000 °C could provide the basis for further research on not only synthesis strategy but also thermal stability in Ni-rich layered cathode materials.

Reactivity of metallic palladium in a K2CO3-K2O-B2O3-Al2O3 melt

The reactivity of metallic palladium in a K2CO3-K2O-B2O3 melt is investigated as a function of heating temperature and time. The aqueous solubility of heat-treated palladium is enhanced by controlling the heating conditions leading to an optimum palladium extraction of 53% from the heat-treated product. The alumina crucible used as a reactor partially dissolves during the heating of metallic Pd, which requires the analysis of palladium solubilization to be discussed in terms of K2CO3-K2O-B2O3-Al2O3 system. Oxidized palladium combines with boron and aluminum oxyanions as ligands in the molten medium to form water-soluble palladium compounds. Information regarding water-soluble palladium compounds was obtained by analyzing the precipitate collected from the palladium-containing solution produced by water-leaching of the heat-treated product. The precipitate contained Pd (0.19 wt%), Al (0.74 wt%), B (2.6 wt%), and K (45 wt%). Raman spectroscopy confirmed the presence of oxidized palladium in the precipitate. Aluminum oxyanions with AlO4 and AlO6 structures and boron oxyanions as diborate groups also were detected in the precipitate.

Influence of heating and moisture content on sludge drying process

Drying is one of the most efficient processes to reduce sludge volume by lowering moisture content. Vacuum filters are equipment suitable for sludge drying in small wastewater treatment plants. It is known that by reducing pressure in the filter, the vaporization process begins at lower temperature. The aim of this study is to estimate the values of liquid (free water) and gaseous (air) phase ratios in sludge cake during vacuum drying. Numerical results in function of heating temperature and cake initial moisture content are estimated by finite element analysis in a central point situated on cake surface.

Microstructural evolution in (α + βZr) region of Zr-2.5 wt% Nb annealed at different temperatures: Effect on mechanical properties

The present study investigated the microstructural evolution in α + βZr regime of Zr-2.5 wt% Nb (a dual phase alloy with α (hcp) and β (bcc)) as a function of heat treatment temperature and holding time. The microstructures were studied in terms of composition and phase fraction evolution and the morphological changes of the constituent phases. The composition and quantification of phases revealed that, they significantly deviated from the corresponding equilibrium values for all the temperatures and holding times employed in the present study. In addition, an anomalous behavior was noticed in the time evolution of the phase fraction. The observed anomaly was rationalized by suggesting a possible sequence in attaining the equilibrium as: the crystal structure transformation of α to β phase followed by Nb enrichment in the β phase. Widmanstätten and ω phase transformations were exhibited by β phase at certain heat treatment conditions. The critical temperature for the widmanstätten transformation was found to be 750 °C, below which the high temperature β phase retained to room temperature. Despite wide variations in the microstructures, flow curves were characteristically similar for all the heat treatment conditions. However, yield strength was found to be more sensitive to the β phase morphology and the transformations associated with it. The presence of fine transformed products inside the β phase was observed to promote higher extent of deformation twinning.

Effect of Crack Length on Strength Recovery of Silicon-Carbide-Whisker-Reinforced Silicon Nitride Upon Healing Treatment

A silicon carbide whisker (SiCw) reinforced silicon nitride (Si3N4) composite ceramic was selected to study the crack-healing performance. Pre-cracks of various length were introduced on the tensile side of specimens by Vickers hardness tester. The crack-healing performance as a function of heating temperature and crack dimension, as well as the crack-healing mechanism were investigated. The optimal heat treatment temperature for crack-healing of Si3N4/SiCw is 1200–1300 °C. The treatment allows complete healing of a crack of 200 μm after heat treated at 1300 °C, and achievement of a substantial strength recovery for pre-cracked specimens even with much longer cracks (1200 μm). Crack closure and strength recovery of the pre-cracked specimen were considered to be triggered by the creation of SiO2, liquid aluminosilicate glass and silicon yttrium oxide by the oxidation reaction as filling mechanism during heat treatment.