Modelling of austenite grain growth kinetics in a microalloyed steel (30MSV6) in the presence of carbonitride precipitates

Austenite grain growth kinetics in the presence of secondary precipitates of a microalloyed steel (30MSV6) was studied employing quantitative metallographic techniques. Austenitizing experiments were carried out at 1,000, 1,100 and 1,200?°C. According to the experimental data, abnormal grain growth behaviour is observed at 1,100?°C while it is normal at 1,000 and 1,200?°C. TEM observation represents multicomponent carbonitride precipitate, (Ti,V)(C,N), in the as-received steel with a mean radius of 35?nm. A mathematical model is proposed considering austenite grain growth along with dissolution and coarsening kinetics of the multicomponent precipitates. The austenite grain growth model for short-term non-isothermal and subsequent long-term isothermal heating stages was developed using a statistical approach. The model includes an algorithm to estimate the size distribution of austenite grains. Precipitate mean field dissolution and Lifshitz–Slyozov–Wagner coarsening models were integrated in the proposed model to calculate the pinning pressure retarding the grain boundary movement. The mean austenite grain size and the grain size distribution (normal and abnormal) calculated by the model are in good agreement with experimental data.     

Texture weakening in pure magnesium during grain growth

The microstructure and texture evolution during annealing of rolled pure Mg, at temperatures ranging from 150 to 400°C, was characterised in the present study. A grain growth exponent of n?=?13 was observed and the activation energy for grain growth kinetics was found to be 95.6?kJ?mol^?1. Further, broadening of the normalised grain size distributions, indicating abnormal grain growth, was also observed at all temperatures of annealing. The sample had a dominant basal texture before annealing. However, after annealing up to a temperature of 300°C, the alleviation of basal texture was observed in the samples. On further annealing at a temperature of 400°C, a strong basal texture was developed in the samples. The mobility of high angle grain boundaries, which is proportional to correlated misorientation distribution, was observed to be responsible for texture strengthening of the material. The grain boundary mobility changes during grain growth led to the growth of either small or large grains. It was further observed that the growth of small grains caused the formation of basal fibre and large grains led to the weakening of basal texture.     

Synthesis of nano-polycrystalline diamond from glassy carbon at pressures up to 25 GPa

ABSTRACT

Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15–25?GPa and temperatures 1700–2300°C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from ～1700°C at 15?GPa to ～1900°C at 25?GPa. The NPD having grain sizes less than ～50?nm was synthesized at temperatures below ～2000°C at 15?GPa and ～2300°C at 25?GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10?nm is obtained in a limited temperature range around 1800–2000°C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.     

Densification and microstructure of lead zirconate titanate ceramics fabricated from a triol sol–gel powder

Lead zirconate titanate (PZT) nano-powder was prepared by a triol sol–gel process. X-ray diffraction and transmission electron microscopy results showed that as-synthesized amorphous powder started to crystallize at the calcination temperature above 500 °C. The crystalline powder was formed into pellets and sintered at temperatures between 900 and 1300 °C. Co-existence of tetragonal and rhombohedral phase was observed in all ceramics. Microstructural investigation of PZT ceramics showed that uniform grain size distribution with average grain size of ∼0.8–2.5 μm were received with sintering temperature up to 1200 °C. Further increasing the temperature caused abnormal grain growth with the grain as large as 13.5 μm. An attempt to optimize densification with uniform grain size distribution was also performed by varying heating rate and holding time during sintering. It was found that dense (∼97%) sol–gel derived PZT ceramic with uniform microstructure was achieved at 1100 °C with a heating rate of 5 °C min⁻¹ and 6 h dwell time.     

Performance of a Single-Family Heat Pump at Different Working Conditions Using Small Quantity of Propane as Refrigerant

The performance of a domestic heat pump that uses a low quantity of propane as refrigerant has been experimentally investigated. The heat pump consists of two minichannel aluminium heat exchangers, a scroll compressor, and an electronic expansion valve. It was charged with the minimum amount of refrigerant propane required for the stable operation of the heat pump without permitting refrigerant vapor into the expansion valve at incoming heat source fluid temperature to the evaporator of +10°C. The inlet temperature of the heat source fluid passing through the evaporator was varied from +10°C to ?10°C while holding the condensing temperature constant at 35°C, 40°C, 50°C, and 60°C, respectively. The minimum refrigerant charges required at above-tested condensing temperatures were found to decrease when the condensing temperature increased and were recorded as 230 g, 224 g, 215 g, and 205 g, respectively. The results confirm that a heat pump with 5 kW capacity can be designed with less than 200 g charge of refrigerant propane in the system. Due to the high solubility of propane in compressor lubrication oil, the amount of refrigerant which may escape rapidly in case of accident or leakage is less than 150 g.     

High‐Temperature Stable Zirconia Particles Doped with Yttrium,Lanthanum, and Gadolinium

Zirconia microspheres synthesized by a wet‐chemical sol–gel process are promising building blocks for various photonic applications considered for heat management and energy systems, including highly efficient reflective thermal barrier coatings and absorbers/emitters used in thermophotovoltaic systems. As previously shown, pure zirconia microparticles deteriorate at working temperatures of ≥1000 °C. While the addition of yttrium as a dopant has been shown to improve their phase stability, pronounced grain growth at temperatures of ≥1000 °C compromises the photonic structure of the assembled microspheres. Here, a new approach for the fabrication of highly stable ceramic microparticles by doping with lanthanum, gadolinium, and a combination of those with yttrium is introduced. The morphological changes of the particles are monitored by scanning electron microscopy, ex situ X‐ray diffraction (XRD), and in situ high‐energy XRD as a function of dopant concentration up to 1500 °C. While the addition of lanthanum or gadolinium has a strong grain growth attenuating effect, it alone is insufficient to avoid a destructive tetragonal‐to‐monoclinic phase transformation occurring after heating to >850 °C. However, combining lanthanum or gadolinium with yttrium leads to particles with both efficient phase stabilization and attenuated grain growth. Thus, ceramic microspheres are yielded that remain extremely stable after heating to 1200 °C.     

Damage mechanism of nickel-based creep-resistant alloys strengthened by the Laves phase at the grain boundary

ABSTRACT

This study proposes a design guideline for polycrystal Ni-based model alloys with high ductility and 100-MPa creep rupture strength beyond 800°C and 10^5?h. These alloys are strengthened by both the precipitation of fine γ′ particles inside the grain and the Laves phase at the grain boundary. For investigating the damage mechanism, transformation from the non-equilibrium Laves phase to the σ phase at the grain boundary and formation of the equilibrium needle-like Laves phase inside the grain are promoted by increasing the Fe concentration. The rupture time of Fe-free alloys significantly increases because of the equilibrium Laves phase at the grain boundary owing to a suitable Mo equivalent. In particular, W addition can help achieve high-temperature creep strength. The precipitate-free zone (PFZ) is predominantly formed by prior migration at the grain boundary without precipitation. Creep rupture occurs at the precipitation/matrix interface in the PFZ. Therefore, transformation control from the Laves to the σ phase at the grain boundary suppresses creep degradation. Consequently, a Ni-based alloy with strength >100?MPa and rupture elongation >20% at 800°C and 10^5?h is fabricated using Larson–Miller parameter conversion, and the alloy design guideline’s validity is confirmed.     

Use of pulsed-high hydrostatic pressure treatment to decrease patulin in apple juice

This study was aimed at reducing patulin content of apple juice using a non-thermal method, namely pulsed-high hydrostatic pressure (p-HHP). Commercially available clear apple juice was contaminated artificially with different concentrations of patulin (5, 50 and 100?ppb). Then, the samples were processed 5?min at different pressure treatments (300–500?MPa) in combination with different temperatures (30–50°C) and pulses (6 pulses?×?50?s and 2 pulses?×?150?s). To compare the impact of pulses, single pulse of high hydrostatic pressure (HHP) treatment was also applied with the same pressure/temperature combinations and holding time. Results indicated that pressure treatment in combination with mild heat and pulses reduced the levels of patulin in clear apple juice up to 62.11%. However, reduction rates did not follow a regular pattern. p-HHP was found to be more effective in low patulin concentrations, whereas HHP was more effective for high patulin concentrations. To the best of our knowledge, this is the first study using p-HHP to investigate the reduction of patulin content in apple juice.     

Thermal analysis of precipitation reactions in a Ti–25Nb–3Mo–3Zr–2Sn alloy

A study was undertaken on a Ti–25Nb–3Mo–3Zr–2Sn alloy using differential scanning calorimetry (DSC) in order to improve understanding of the precipitation reactions occurring during aging heat treatments. The investigation showed that isothermal ω phase can be formed in the cast and solution treated alloy at low aging temperatures. An exothermic peak in the temperature range of 300 to 400°C was detected for precipitation of the ω phase, with approximate activation energy of 176 kJ/mol. The ω phase begins to dissolve at temperatures around 400°C and precipitation of the α phase is favoured at higher temperatures between 400°C and 600°C. An exothermic peak with activation energy of 197 kJ/mol was measured for precipitation of the α phase. Deformation resulting in the formation of the stress induced α″ phase altered the DSC heating profile for the solution treated alloy. The exothermic peak associated with precipitation of the ω phase was not detected during heating of the deformed material and increased endothermic heating associated with recovery and recrystallisation was observed.     

Three zone furnace for crystal growth under high pressure

Abstract

A special furnace with programmable temperature gradient was contructed. It can be arranged inside an internally heated gas pressure chamber. In this work, the application of the furnace to obtain mercury telluride crystals is presented. Experiments were carried out under gas pressure of argon or nitrogen up to 1,5 GPa in a gas chamber of 30 nun internal diameter; the temperature range used was 25°C–800°C. Since graphite heating elements are used, higher working temperatures are possible. Quasi linear temperature gradient determined by three independent thermocouples can be programmed by the power control systems (i.e. Eurotherm units for the three regulation zones).     

Observation of transcrystalline growth of PLA crystals on sisal fibre bundles and the effect of crystal structure on interfacial shear strength

Hot-stage microscopy was used to characterise crystal growth at the interface between sisal fibre bundles and a polylactic acid (PLA) matrix in order to better understand the mechanical properties of sisal fibre–PLA composites. Cooling rates and crystallisation temperatures and times were varied to influence crystalline morphology at the interface. Single sisal fibre bundles were evaluated in their as received state or treated with 6 wt.% caustic soda solution for 48?h at room temperature. A microbond shear test was used to characterise the shear strength of the interface as a function of fibre surface treatment. These tests were performed on sisal fibre bundles carefully embedded in flat films of PLA supported on card mounts. Fibre bundles in a PLA matrix were cooled from 180?°C at rates from 2 to 9?°C/min and then crystallised isothermally. For as received fibre bundles uneven growth of PLA spherulites occurred at all cooling rates and crystallisation temperatures. For caustic soda treated fibres, uneven spherulitic growth was observed at crystallisation temperatures at and above 125?°C. In contrast, transcrystalline growth was observed for samples cooled to 120?°C at cooling rates from 2 to 6?°C/min and then allowed to crystallise. The microbond shear strengths of untreated and caustic soda treated fibre bundles were evaluated using Weibull statistics and the caustic soda treated fibres exhibited higher interfacial shear strengths in comparison to untreated fibres, reflecting the development of a transcrystalline layer at the fibre to matrix interface.     

Growth and characterisation of single grain Al-Cu-Ru icosahedral quasicrystals from self-fluxes

ABSTRACT

In order to obtain high-quality single grains of the Al-Cu-Ru icosahedral quasicrystal (iQC), suitable for a structure analysis, the crystal growth conditions with the self-flux method have been studied. The melts of the master alloys with the compositions of Al_57.0+xCu_39.5-xRu_3.5 (x?=?0, 2.5, 5, 7.5, 10) and Al_62.0Cu_34.0+y Ru_4.0-y (y?=?0, 0.5, 1.5) were held at 1150°C for 2?h, then cooled down to 800, 900, or 1000°C at a rate of ?2?K/h, and subsequently retained for various durations, up to 750?h. Single grain iQCs having several millimetre-sizes, which were evaluated their quality by powder X-ray diffraction (XRD), were grown throughout this study. The peak of (664004) reflection in powder XRD of the iQCs grown at 1000°C has approximately 50% narrower width than that grown at 800°C. The inhomogeneity of the compositions intra- as well as inter-grains grown at 800°C was observed. High-quality single grains with homogeneous composition could be achieved with a long-time annealing at 900°C or regardless of the annealing time at 1000°C. By changing the Al/Cu ratio of the master alloys, the composition could also be controlled for the iQCs grown at 1000°C. Single-crystal XRD experiment with synchrotron radiation on Al_66.6Cu_16.4Ru_17.0 iQC, grown at 1000°C, resulted in the collection of 2680 independent Bragg reflections that confirms the high-quality of the sample. The phase retrieval of the diffraction data resulted successfully in obtaining the structure solution, which reveals some characteristic features of this face-centred iQC structure.     

Crystallization behaviour of ALD-Ta2O5 thin films: the application of in-situ TEM

The microstructure of Ta₂O₅ thin films, deposited onto Si substrates by atomic layer deposition (ALD), was investigated, using in situ transmission electron microscopy (TEM). As-deposited amorphous films crystallize as the orthorhombic phase L-Ta₂O₅ upon heating at 750°C. Two dominant crystallographic orientations are found, one with (0?0?1) and (1?11?0) planes perpendicular to the substrate, while the other has (0?0?1) planes parallel to the substrate. The grains consist of subgrains which are rotated a few degrees with respect to each other. The kinetics of the crystallization were studied by in-situ TEM heating experiments carried out at nominal temperatures of 790°C, 820°C and 850°C. They reveal that the growth and crystallization activation energies are about 4.2?eV and 6.3?eV, respectively. Tilted subgrains keep forming during growth until they come in contact with neighbouring grains. The crystallization behaviour can be approximated by the Kolmogorov–Johnson–Mehl–Avrami (KJMA or Avrami) equation, giving mode parameters of 2.5, 1.9, and 1.7 at 790°C, 820°C and 850°C, respectively. A small value of mode parameters is attributed to decreasing growth and nucleation rates with time.     

High-Rate Microwave Sintering of Ceramics on the Basis of Barium and Strontium Titanates

Radiophysics and Quantum Electronics - Using the method of sintering by microwave heating at a rate of 10–100°C/min to a temperature of 1250 °C without isothermal hold, we obtain...     

Grain boundary wetting phase transition and analysis of the energy characteristics of grain boundaries in the Sns-(Zn/Sn)l system

Regularities of the interaction of tin grain boundaries (special Σ5 and general Σ17 〈001〉) and a Sn-Zn melt of equilibrium composition were studied. The grain boundary wetting phase transition temperature was determined; for Σ5 and Σ17, it is 216°C. More than 90% of the general grain boundaries were completely wetted by the melt over a range of temperatures, from the eutectic melting temperature to the tin melting temperature. It was shown that the anisotropy of interphase energy at the solid tin-Zn-Sn melt interface is 64 ± 10 mJ m^?2 at 216°C. The energies of the Σ5 and Σ17 grain boundaries in the range of 201–216°C were obtained on the basis of the experimental dependence of the dihedral angle on temperature.     

Preparation and dielectric properties of BaZr0.1Ti0.9O3 ceramics with different grain sizes

BaZr_0.1Ti_0.9O₃ ceramics with grain sizes of 0.75 and 2.60?µm have been prepared via solid-state reaction. Optimum parameters for calcination and sintering have been found in order to obtain pure perovskite phase, high density ceramics and homogeneous microstructures. The dielectric data show a diffuse phase transition with a mixed ferroelectric-relaxor character at a maximum at 87–92°C, with a small thermal hysteresis of 2–3°C. A tendency towards a more diffuse character of the ferro–para phase transition towards the full relaxor behaviour is observed as small in the ceramic grain size. Better dielectric properties in the coarse ceramics with higher permittivity up to 14,000 at the transition temperature by comparison with 5000 for the fine one, are observed as a consequence of higher tetragonal distortion and higher density. The differences in the dielectric spectra found for the two grain sizes were interpreted as a consequence of the higher degree of inhomogeneity in the fine ceramics and to different grain boundary properties induced by the different sintering temperatures.     

Study of the formation of Co3O4 thin films using sol–gel method

Sol–gel derived coatings containing cobalt have been analyzed using impedance and reflection measurements. It is found that during the thermal treatments in air at temperatures in the range of 35–400 °C, cobalt migrates to the front surface of the coating where it is oxidized by the atmospheric oxygen and forms a top layer rich in nanocrystalline Co₃O₄. In samples heated above 260 °C, the current flows entirely through this top layer because it has higher conductivity than the rest of the coating. For these samples, the impedance spectra show two semicircles, related with the electrical properties of grain and grain boundaries of the cobalt oxide layer. Using the brick model, the grain boundary volume fraction was calculated as a function of the heating temperature.     

The system Na2CO3–MgCO3 at 3 GPa

It was suggested that Na–Mg carbonates might play a substantial role in mantle metasomatic processes through lowering melting temperatures of mantle peridotites. Taking into account that natrite, Na₂CO₃, eitelite, Na₂Mg(CO₃)₂, and magnesite, MgCO₃, have been recently reported from xenoliths of shallow mantle (110–115?km) origin, we performed experiments on phase relations in the system Na₂CO₃–MgCO₃ at 3?GPa and 800–1250°C. We found that the subsolidus assemblages comprise the stability fields of Na-carbonate?+?eitelite and eitelite?+?magnesite with the transition boundary at 50?mol% Na₂CO₃. The Na-carbonate–eitelite eutectic was established at 900°C and 69?mol% Na₂CO₃. Eitelite melts incongruently to magnesite and a liquid containing about 55?mol% Na₂CO₃ at 925?±?25 °C. At 1050 °C, the liquid, coexisting with Na-carbonate, contains 86–88?mol% Na₂CO₃. Melting point of Na₂CO₃ was established at 1175?±?25 °C. The Na₂CO₃ content in the liquid coexisting with magnesite decreases to 31?mol% as temperature increases to 1250°C. According to our data, the Na- and Mg-rich carbonate melt, which is more alkaline than eitelite, can be stable at the P–T conditions of the shallow lithospheric mantle with thermal gradient of 45?mW/m² corresponding to temperature of 900 °C at 3?GPa.     

Studying the effect of hydrogen on diamond growth by adding C10H10Fe under high pressures and high temperatures

In this paper, hydrogen-doped industrial diamonds and gem diamonds were synthesized in the Fe–Ni–C system with C₁₀H₁₀Fe additive, high pressures and high temperatures range of 5.2–6.2?GPa and 1250–1460°C. Experimental results indicate similar effect of hydrogen on these two types of diamonds: with the increasing content of C₁₀H₁₀Fe added in diamond growth environment, temperature is a crucial factor that sensitively affects the hydrogen-doped diamond crystallization. The temperature region for high-quality diamond growth becomes higher and the morphology of diamond crystal changes from cube-octahedral to octahedral. The defects on the {100} surfaces of diamond are more than those on the {111} surfaces. Fourier transform infrared spectroscopy (FTIR) results indicate that the hydrogen atoms enter into the diamond crystal lattice from {100} faces more easily. Most interestingly, under low temperature, nitrogen atoms can also easily enter into the diamond crystal lattice from {100} faces cooperated with hydrogen atoms.     

Mössbauer study of turbine blade steels

Mössbauer investigations were carried out on low carbon steels containing 12–13.5% Cr and 3–5% Ni in order to get information about the reason of cracking and fracture which take place during the use of turbine blade wheels. The quantity of retained austenite determined from the Mössbauer spectra of steels was low (<1%) in the cracked and fractured basic materials. Comparing this value with those being considerable in quenched (≈11%) as well as in annealed state (≈5%) of the same sample, we can conclude that the transformation of the austenite taking place during the working of turbine blade wheel can be associated with the cracking and the fracture. We found an anomalous increase of the quantity of the austenite in steel samples (quenched from different temperature between 700 and 1000 °C and aged at 450–600 °C) aged again at 450–550 °C. On the basis of the evaluation of Mössbauer spectra of the steels, information can be obtained about the changes in the concentration of alloying elements being in martensite at the various heat treatments.