High temperature bending creep of a Sm-α-β Sialon composite

The four-point bending creep behavior of a Sm-- Sialon composite, in which Sm-melilite solid solution (denoted as M) was designed as intergranular phase, was investigated in the temperature range 1260–1350°C and stresses between 85 and 290 MPa. At temperatures less than 1300°C, the stress exponents were measured to be 1.2–1.5, and the creep activation energy was 708 kJ mol^–1, the dominant creep mechanism was identified as diffusion coupled with grain boundary sliding. At temperatures above 1300°C, the stress exponents were determined to be 2.3–2.4, and creep activation energy was 507 kJ mol ^–1, the dominant creep mechanism was suggested to be diffusion cavity growth at sliding grain boundaries. Creep test at 1350°C for pre-oxidation sample showed a pure diffusion mechanism, because of a stress exponent of 1. N^3– diffusing along grain boundaries was believed to be the rate controlling mechanism for diffusion creep. The oxidation and Sialon phase transformation were analyzed and their effect on creep was evaluated.     

Fabrication and plastic deformation of Y-TZP/alumina nanocomposite ceramics containing oxynitride glass

Dense yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) +28 vol% alumina nanocomposite ceramics with and without 17 vol% oxynitride glass were fabricated at 1380°C using microwave sintering. The specimens were uniaxially compressed in the temperature range 1250 to 1400°C. Strain rates as high as 10^–4 (s^–1) were measured at 1350°C and 90 MPa in the glass-free specimens with the stress exponent of 1.5. Similar strain rates were measured at lower compressive stresses in the counterpart glass-containing specimens. The stress exponent in the glass-containing specimens changed from 1.0 at 1250°C to 2.0 at higher temperatures. Dynamic grain growth of the alumina grains was inhibited in the presence of the oxynitride glass. Plastic deformation at lower temperatures in glass-containing alloy occurred by cooperative grain boundary sliding, aided by viscous flow of the grain boundary glassy phase. The changes in the deformation behavior at higher temperatures were related to crystallization of the glass and simultaneous plastic deformation by grain boundary sliding.     

Enhancement of the diffusional creep of polycrystalline Al2O3 by simultaneous doping with manganese and titanium

The effect of simultaneous doping with manganese and titanium on diffusional creep was studied in dense, polycrystalline alumina over a range of grain sizes (4–80m) and temperatures (1175–1250° C). At a total dopant concentration of 0.32–0.37 cation %, diffusional creep rates were enhanced considerably such that the temperature at which cation mass transport was significant was suppressed by at least 200° C compared to that observed in undoped material. The Mn-Ti (and Cu-Ti) dopant couple was far more effective in enhancing creep rates and suppressing sintering temperatures than the Fe-Ti couple. The enhanced mass transport kinetics are believed to be caused by significant increases in both aluminium lattice and grain-boundary diffusion. When aluminium grain-boundary diffusion is enhanced by increasing the concentration of divalent impurity (Mn²⁺, Fe²⁺) or by creep testing at low temperatures, creep deformation is Newtonian viscous.     

Microstructure and high temperature deformation characteristics of sol–gel derived aluminium titanate–mullite composites

Aluminium titanate (AT)–mullite composites with varying compositions were processed by sol–gel technique. The influence of mullite on the microstructure and creep deformation of AT–mullite composites was investigated. In the composites mullite addition was varied from 0 to 100 vol.%. The AT-80 vol.% mullite composite sintered at 1600 °C resulted in fine-grained microstructure with an average grain size of 2.5 μm. From the steady-state creep analysis of the different AT–mullite composites, the activation energies for the creep deformation and stress exponents were determined. The activation energies in the range 655–874 kJ mol⁻¹ were obtained for various the sol–gel derived AT–mullite composites. Similarly stress exponent values were found in the range 1.5–1.9.     

Creep behavior of Mg–2 wt.%Nd binary alloy

A binary magnesium alloy, Mg–2 wt.%Nd, has been prepared. Under the condition of temperature between 150 and 250 °C and applied stress between 30 and 110 MPa, the alloy exhibits good creep resistance due to both solution-hardening and especially precipitation-hardening. Tiny precipitates forming dynamically during creep have been observed, which play an important role in restricting dislocation movements. When the creep tests are carried out at the temperature range between 150 and 250 °C, the stress exponents lie in the range of 4.5–7.1 at low stresses, which is consistent with the “five-power-law”. The values of stress exponent increase up to 9.8–29.5 at high stresses indicate power-law breakdown. When the creep tests are carried out under the applied stress between 30 and 90 MPa, the apparent activation energy values vary from 70.0 to 96.0 kJ/mol at low temperatures, but increase to 199.9–246.1 kJ/mol at high temperature range. Dislocations in basal plane are activated in the primary creep stage, but as creep goes on, they are observed in non-basal plane. The creep is mainly controlled by both dislocation-climb and cross-slip.     

Structure, synthesis and high dielectric property of (NaBi)(NbCr)O6 pyrochlores

Pyrochlores with composition (NaBi)(NbCr)O₆ were sintered in the temperature range of 1100–1250 °C. Dielectric properties of the ceramics were investigated in a wide range of temperatures of − 15 to 200 °C and frequencies of 1–800 kHz. The suitable sintering temperatures for (NaBi)(NbCr)O₆ ceramics are suggested from 1150 to 1200 °C. The dielectric constant behavior reflects, in our opinion, the formation of the space charge and the orientational polarization. The pyrochlores have convex resistivity-temperature characteristics around the transition point, and exhibit the negative temperature coefficient of resistance characteristics. The activation energies of conductivity of samples were obtained at various sintering temperatures.     

Hot deformation behavior and microstructure evolution of twin-roll-cast Mg–2.9Al–0.9Zn alloy: A study with processing map

The hot deformation behavior and microstructure evolution of twin-roll-cast of Mg–2.9Al–0.9Zn–0.4Mn (AZ31) alloy has been studied using the processing map. The tensile tests were conducted in the temperature range of 150–400 °C and the strain rate range of 0.0004–4 s⁻¹ to establish the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (i) the continuous dynamic recrystallization (CDRX) domain in the range of 200–280 °C/≤0.004 s⁻¹ with fine grains which provides a potential for warm deformation such as deep drawing; (ii) the discontinuous dynamic recrystallization (DDRX) domain around 400 °C at high strain rate (0.4 s⁻¹ and above) with excellent elongation which can be utilized for forging, extrusion and rolling; (iii) the grain boundary sliding (GBS) domain at slow strain rate (below 0.004 s⁻¹) above 350 °C appears abundant of cavities, which result in fracture and reduce the ductility of the adopted material; and (iv) the flow instability region which locates at the upper left of the processing map shows the metallographic feature of flow localization.     

Transient Creep Associated with Grain Boundary Sliding in Fine-Grained Single-Phase Al2O3

Transient creep data for high-purity polycrystalline alumina are examined at the testing temperature of 1150–1250 °C. The data are analysed in terms of the effect of stress and temperature on the extent of transient time and strain.In order to explain the observed transient creep, a time function of creep strain is proposed from a two-dimensional model based on grain boundary sliding. The grain boundary sliding is assumed to take place by the glide of grain boundary dislocations accommodated by dislocation climb in the neighboring grain boundaries. The time function for a creep strain obtained from the model is given in a form

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al₂O₃, Al₂O₃-ZrO₂) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10^–8–7.70 × 10^–6s^–1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol^–1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.     

Effect of initial temper on the creep behavior of a Mg–Gd–Nd–Zr alloy

The effect of initial temper on the tensile creep behavior of a cast Mg–Gd–Nd–Zr alloy has been investigated. Specimens in unaged, underaged and peak-aged conditions exhibit a sigmoidal creep stage between the primary and steady-state creep stage, while the overaged specimens have no such creep stage. Transmission electron microscope observations revealed that sigmoidal creep stage was induced by the dynamic precipitation in the microstructure, and the rapid formation of β₁-phase and β-phase plates takes responsibility for the softening of material in this stage. Comparative evaluation of creep properties of the specimens showed that alloy in overaged condition had creep resistance superior to those in other conditions. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of 250–300 °C and stress range of 50–100 MPa, and a dislocation creep mechanism was proposed for the alloy.     

Tensile creep behaviour of a fibre-reinforced SiC-Si3N4 composite

The tensile creep behaviour of a SiC-fibre-Si₃N₄-matrix composite was investigated in air at 1350 C. The unidirectional composite, containing 30 vol % SCS-6 SiC fibres, was prepared by hot pressing at 1700 C. Creep testing was conducted at stress levels of 70, 110, 150 and 190 MPa. An apparent steady-state creep rate was observed at stress levels between 70 and 150 MPa; at 190 MPa, only tertiary creep was observed. For an applied stress of 70 MPa, the steady-state creep rate was approximately 2.5×10^–10 s^–1 with failure times in excess of 790 h. At 150 MPa, the steady-state creep rate increased to an average of 5.6×10^–8 s^–1 with failure times under 40 h. The creep rate of the composite is compared with published data for the steady-state creep rate of monolithic Si₃N₄.     

Research on the hot deformation behavior and graphite morphology of spheroidal graphite cast iron at high strain rate

The hot deformation behavior of spheroidal graphite cast iron (SGCI) was investigated quantitatively from 600 °C to 950 °C at high strain rate of 10 s⁻¹ by compression tests on a Gleeble-1500 simulator. The results show that the peak strain increases gradually with increasing deformation temperatures in the range of 600–800 °C and decreases when the temperature is raised to 800 °C and above. The optimum deformation temperature range is determined at 700–900 °C. The graphite particles become spindles or flakes after deformation, even some graphite collapse in the compressed specimens with about 0.7 peak strains. The graphite area fraction decreases as the temperature increases, at the same time, the high peak strain promotes the dissolving of carbon.     

High temperature oxidation of hot-pressed aluminium nitride by water vapour

Hot pressed AIN without additives was oxidized et 1100 to 1400°C in dry air, wet air and wet nitrogen gas atmospheres with 1.5 to 20 kPa of water vapour pressure. AIN was oxidized by both air and water vapour, and formed -Al₂O₃ film on the surface above 1150°C. The oxidation kinetics in air were parabolic end were promoted by water vapour. On the other hand, the oxidation kinetics in wet nitrogen were linear below 1250°C and parabolic above 1350°C. The oxidation rate in wet nitrogen was much greater than that in wet air. The rate of oxidation increased with increasing temperature until 1350°C, and then decreased. The parabolic rate constant decreased with increasing temperature and increased linearly with increasing water vapour pressure. The linear rate constant at 1150 to 1250° C increased with increasing the temperature with the apparent activation energy of 250 kJ mol^–1. The relation between the linear rate constant and water vapour pressure was of the Langmuir type.     

Creep of chemically vapour deposited SiC fibres

The creep, thermal expansion, and elastic modulus properties for chemically vapour deposited SiC fibres were measured between 1000 and 1500°C. Creep strain was observed to increase logarithmically with time, monotonically with temperature, and linearly with tensile stress up to 800 MPa. The controlling activation energy was 480 ± 20 kJ mol^–1. Thermal pretreatments near 1200 and 145O° C were found to significantly reduce fibre creep. These results coupled with creep recovery observations indicate that below 1400°C fibre creep is anelastic with negligible plastic component. This allowed a simple predictive method to be developed for describing fibre total deformation as a function of time, temperature, and stress. Mechanistic analysis of the property data suggests that fibre creep is the result of -SiC grain boundary sliding, controlled by a small percentage of free silicon in the grain boundaries.     

Grain boundary diffusion and wetting in the analysis of intergranular penetration

Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500 °C and Ni at 700 °C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu–Bi, while micrometer-thick films were observed in Ni–Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion-type profile over a distance of the order of 100 μm for Cu–Bi and a nanometer-thick film followed by similar diffusion-type profile for Ni–Bi. In both cases the kinetics of intergranular penetration and embrittlement has been shown to be parabolic. It is concluded that no wetting occurs in Cu–Bi system at 500 °C while Bi wets Ni at 700 °C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena: grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

Dimensionless correlations of frost properties on a cold plate

This study proposes dimensionless correlations for predicting the properties of frost formed on a cold plate. Frosting experiments are carried out to obtain the correlations with various environmental parameters including the air temperature, air velocity, absolute humidity, and cooling plate temperature. The thickness, density, surface temperature, effective thermal conductivity, average heat and mass transfer coefficients of the frost layer are correlated as functions of the Reynolds number, Fourier number, absolute humidity, and dimensionless temperature by using a dimensional analysis. The correlations proposed in this study agree well with the experimental data within a maximum error of 10%, and can be used to predict the average frost properties in the following ranges: the air temperature of 5–15 °C, air velocity of 1.0–2.5 m s⁻¹, absolute humidity of 0.00322–0.00847 kg kg_a⁻¹, and cooling plate temperature of −35–−15 °C.     

Hot deformation behavior of SiCp/AZ91 magnesium matrix composite fabricated by stir casting

The uniaxial compressive deformation behavior of a 10 vol.% SiC particulate reinforced AZ91 magnesium matrix composite (SiCp/AZ91) fabricated by stir casting is investigated at elevated temperature (250–400 °C). Peak stresses and flow stresses decrease as temperatures increase and strain rates decrease. The extent of dynamic recrystallization (DRX) becomes less as temperatures decrease at 250–350 °C or strain rates increase, and recrystallization occurs mainly within the intergranular regions rich of particles. Dynamic recrystallization accomplishes at 400 °C even at the strain rate of 1 s⁻¹. An analysis of the effective stress dependence on strain rate and temperature gives a stress exponent of n = 5 and a true activation energy of Q = 99 kJ/kJ. The value of Q is close to the value for grain boundary diffusion in Mg. It is concluded that the deformation mechanism of SiCp/AZ91 composite during hot compression is controlled by the dislocation climb.     

Surface morphologies and properties of pure and antimony-doped tin oxide films derived by sol–gel dip-coating processing

A simple laboratory technique for the routine preparation of antimony-doped tin oxide (ATO) on float glass substrates (25 mm × 76 mm × 1 mm) was described. As-prepared thin films were dried at temperature of 100 ± 5 °C and annealed at temperatures of 400–550 °C. Microstructural and morphological analyses of as-prepared films were performed at different conditions. The evolution of grain size and the morphologies of ATO films were analyzed by means of atom force microscopy (AFM) and digital microscope. The studies suggested that higher Sb-doped level and higher annealing temperature led to a decrease in the surface roughness of the deposited films. The XRD patterns revealed that as-prepared ATO films were in the crystallization of a tetragonal rutile structure of SnO₂ with highly (1 1 0) preferred orientation. Their optical properties were analyzed by U-3310 spectrophotometer. The transmission of the ATO thin films was obtained as high as 80–90% in visible region, but decreased substantially in IR region. The sheet resistance of the investigated thin films was determined by four-probe method, showing that it was about 85–100 Ω □⁻¹which decreased with the increase of antimony-doped concentration.     

Effect of Mg17Al12 precipitates on the microstructural changes and mechanical properties of hot compressed AZ91 magnesium alloy

During hot compression, Mg₁₇Al₁₂ (β) precipitates show strong influence on the microstructural changes of 415 °C-24 h homogenized AZ91 alloy. When compressed at 300 °C and 350 °C, dynamic recrystallization (DRX) only occurs near grain boundaries with discontinuous β precipitate pinning at the newly DRXed grain boundaries. With increasing compression temperature and decreasing strain rate, the β-precipitating region expands; however, the amount of pinning precipitates decreases, resulting in increases in the DRX ratio and average DRXed grain size. With a compression ratio of only 50%, the specimen compressed at 350 °C and a strain rate of 0.2 s⁻¹ (designated 350 °C-0.2 s⁻¹ compressed specimen) shows an ultimate tensile strength (UTS) of 334 MPa, a 0.2% proof stress (PS) of 195 MPa and an enough elongation of 17.9%. After a subsequent aging treatment at 180 °C, due to the large number of β precipitates, the strength of the compressed specimens are further improved, and the specimen peak aged after compression at 400 °C and 0.2 s⁻¹ shows UTS of 364 MPa and PS of 248 MPa with a moderate elongation of 7.7%.