Creep of polycrystalline alumina,pure and doped with transition metal impurities

Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.     

High-temperature deformation behaviour of YBa2Cu3O7−x

High-temperature compression tests were performed in air for YBa₂Cu₃O_7–x polycrystals with grain sizes of 3 and 7 m at various strain rates between 1.3×10^–5 and 4×10^–4s^–1 and at temperatures between 1136 and 1253 K. Steady state deformation appeared above 1203 K for both samples. A stress exponent of 1.3 and an activation energy of 150 kJ mol^–1 were evaluated. The compression tests and microstructural observations revealed that there was a difference in deformation mechanism above and below 1203 K. The dominant mechanism was diffusional creep associated with grain-boundary sliding above 1203 K, and dislocation glide accompanied with grain-boundary sliding below 1203 K. The growth of anisotropic grains and their preferred arrangement were enhanced by deformation.     

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.     

Creep mapping in a polycrystalline ceramic: application to magnesium oxide and magnesiowustite

The construction of deformation mechanism maps for a polycrystalline ionic solid in which anion and cation transport are coupled has been demonstrated. Because of anioncation ambipolar coupling, two regimes of Coble creep are possible in systems where anion grain boundary transport is rapid: (1) rate-controlled at low temperatures and small grain sizes by cation grain-boundary diffusion, and (2) rate-limited at high temperatures and large grain sizes by anion grain-boundary diffusion. A new type of deformation mechanism map was introduced in which the temperature and grain size were primary variables. This map was shown to be particularly useful for materials which deform primarily by diffusional creep mechanisms. Ambipolar diffusional creep theory was used to construct several deformation mechanism maps for polycrystalline MgO and magnesiowustite over wide ranges of stress, grain size, temperature and composition.     

High-temperature creep response of a commercial grade siliconized silicon carbide

Creep studies conducted in four-point flexure of a commercial siliconized silicon carbide (Si-SiC, designated as Norton NT230) have been carried out at temperatures of 1300, 1370, and 1410°C in air under selected stress levels. The Si-SiC material investigated contained 90% -SiC, 8% discontinuous free Si, and 2% porosity. In general, the Si-SiC material exhibited very low creep rates (2 to 10×10^–10 s^–1) at temperatures 1370°C under applied stress levels of up to 300 MPa. At 1410°C, the melting point of Si, the Si-SiC material still showed relative low creep rates (0.8 to 3 × 10^–9 s^–1) at stresses below a threshold value of 190 MPa. At stresses >190 MPa the Si-SiC material exhibited high creep rates plus a high stress exponent (n=17) as a result of slow crack growth assisted process that initiated within Si-rich regions. The Si-SiC material, tested at temperature 1370°C and below the threshold of 190 MPa at 1410°C, exhibited a stress exponent of one, suggestive of diffusional creep processes. Scanning electron microscopy observations showed very limited creep cavitation at free Si pockets, suggesting the discontinuous Si phase played no or little role in controlling the creep response of the Si-SiC material when it was tested in the creep-controlled regime.     

Some observations of grain-boundary sliding in aluminium bicrystals tested at constant strain rate and constant rate of stress increase

Bicrystals of pure aluminium, aluminium/0.05 wt% and aluminium/0.30 wt% copper have been deformed in shear within the temperature range 350 to 600°C at a constant rate of grip displacement (300 m/h) and constant rates of increase of grain-boundary shear stress (0.30 to 1.04 g/mm^–2 min^–1). The stress/time, sliding/time and sliding/stress curves for these tests are presented together with metallographic observations. The stress/time curves exhibited changes in the strain-hardening rates which were accompanied by the occurrence of extensive crystal deformation. In many cases, following the change in strain-hardening rate and onset of extensive crystal slip, the extent of grain-boundary sliding was proportional to the shear stress on the boundary. The ratio of the extent of sliding to the grain-boundary shear stress increased with increasing test temperature. The temperature-dependence of the sliding behaviour, as reflected by the sliding/stress results, yielded apparent activation energies of 31 Kcal/mole in the temperature range 600 to 475°C and 8 Kcal/mole in the range 475 to 350°C.     

Grain-boundary sliding measurements in Al2O3 by machine vision photogrammetry

The nucleation, growth and coalescence of grain-boundary cavities is the primary damage mechanism observed during creep of structural ceramics. Furthermore, grain-boundary sliding (GBS) has been identified as the driving force process. Although the creep characteristics of structural ceramics have been extensively studied, very little is known about the details of GBS during creep and how GBS relates to cavitation kinetics. This paper presents the results of a study using a machine vision system to measure Mode II GBS displacements in a Lucalox Al₂O₃. Specifically, sliding displacements as large as 0.4 m were measured. The measured displacements indicate that some grain boundaries experienced shear strains and strain rates of 4200% and 2.3×10^–2 s^–1, respectively. The techniques utilized for these measurements are described in detail, and data gathered during a 2 1/2 h compressive creep test under a stress of 138 MPa at 1600 °C are presented and discussed.     

Grain-boundary de-segregation and intergranular cohesion in Si-Al-O-N ceramics

The influence of heat-treatment on high-temperature creep and sub-critical crack growth in hot-pressed Si-Al-O-N ceramics has been analyzed from microstructural evidence and determination of stress exponents and activation energies. The most significant change is the suppression of cavitation during creep and of the cavity-interlinkage mechanism for slow crack propagation. A creep mechanism of grain-boundary diffusion is characterized by stress exponent n=1 and unusually high activation energy >820 kJ mol^–1. The microstructural origin of the transformation in grain-boundary dominated properties is mainly the removal of triple-junction glassy residues within which cavities are nucleated. This is caused by grain-boundary diffusion of metallic impurities (Mg, Mn, Ca) into a surface silica oxidation layer, and consequent crystallization of the remaining glass components as . There is a continued improvement in grain-boundary cohesion and increased difficulty of grain-boundary diffusion following the stage at which triple-junction glass is removed. The resultant ceramics, in addition to superior mechanical behaviour, have an increased temperature for application due to a marked reduction in susceptibility to dissociation above 1400° C.     

High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy

High temperature compressive properties in AZ31 magnesium alloy were examined over a wide strain rate range from 10^–3 to 10³ s^–1. It was suggested that the dominant deformation mechanism in the low strain rate range below 10^–1 s^–1 was dislocation creep controlled by pipe diffusion at low temperatures, and by lattice diffusion at high temperatures. On the other hand, analysis of the flow behavior and microstructural observations indicated that the deformation at high strain rates of 10³ s^–1 proceeds by conventional plastic flow of dislocation glide and twinning even at elevated temperatures.     

Cavitation and fracture of mechanically alloyed aluminium at high homologous temperatures

The tensile behaviour of mechanically alloyed (dispersion strengthened) IN90211 was characterized at strain rates between 0.0001 and 340 sec^–1 at temperatures between 425 and 475 ° C, At strain rates above 0.1 sec^–1, superplastic elongations were obtained (maximum elongation 525% at 475 ° C, 2.5sec^–1. Large elongations were possible due to the lack of cavitation, even though the strain-rate sensitivity was lower (m 0.25) than usually found in superplasticity. Cavitation was precluded by the morphology of the platelet-shaped grains in which low-angle subgrain boundaries were predominantly perpendicular to the tensile axis. Grain-boundary sliding was observed along high-angle grain boundaries which were generally parallel to the tensile axis. At the high homologous testing temperatures (0.76 to 0.81), concurrent grain-boundary sliding and lattice slip was made possible by the rapid lattice diffusivity and easy climb of lattice dislocations over dispersions in the matrix and grain boundaries.     

Rare-earth doped zirconia fibres by sol-gel processing

Polycrystalline zirconia fibres, doped with 2–8 mol% of oxides of trivalent lanthanum, praseodymium, neodymium, samarium, gadolinium, and dysprosium (in decreasing cation size), were prepared by spinning of acetate-derived sols and baking the gel fibres thus obtained at 900–1300 °C for 1 h. The larger sized dopants lanthanum, praseodymium and neodymium (Group A) gave rise to tetragonal zirconia, with or without cubic zirconia, at 900 °C which converted partly or fully to monoclinic zirconia, in certain cases accompanied by a cubic zirconate phase at higher temperatures. The smaller sized dopants samarium, gadolinium and dysprosium (Group B) generated only tetragonal or cubic, or both polymorphs of zirconia, depending on the cation type, concentration and temperature. This stabilization of higher symmetry polymorphs with Group B dopants was associated with relatively large crystallite size (especially when calcined at 1300 °C). The maximum tensile strength values of usable fibres calcined at 1300 °C were found to decrease with increasing size in dopant dysprosium > gadolinium > samarium > neodymium, praseodymium, lanthanum=0). Although all the dopant cations were larger in size than Zr⁴⁺ (in the same oxygen coordination), the relative closeness in size of Group B cations with Zr⁴⁺ was considered to be the reason behind the obtained differences in properties.     

Creep in pure and two phase nickel-doped alumina

Creep in pure and two phase nickel-doped alumina has been investigated in the stress range 0.70 to 4.57 kgf mm^–2 (1000 to 6500 psi), and temperature range 1450 to 1800° C, for grain sizes from 15 to 45 m (pure alumina) and 15 to 30 um, (nickel-doped alumina). The effect of stress, grain size and temperature on the creep rate suggests that diffusion controlled grain-boundary sliding is the predominant creep mechanism at low stresses and small grain sizes. However, the stress exponents show that some non-viscous boundary sliding occurs even at the lowest stresses investigated. This mechanism is confirmed by metallographic evidence, which shows considerable boundary corrugation in the deformed aluminas. At higher stresses and larger grain sizes the localized propagation of microcracks leads to high stress exponents in the creep rate equation. The nickel dopant, which introduces an evenly distributed spinel second phase into the alumina matrix, increases the creep rate and enhances boundary sliding and localized crack propagation. The weakening effect of the second phase increases with grain size, and tertiary creep occurs at strains of 0.5% and below in large grained material.     

Creep of austenitic 20% Cr/35% Ni stainless steels at low stresses

The present work comprises measurements of the secondary creep-rate at different stress levels with rates between about 2×10^–5 %/h and 10%/h and the grain-boundary sliding at 700° C in two austenitic 20 wt % Cr/35 wt % Ni stainless steels. One alloy was a pure 20 wt % Cr/35 wt % Ni steel, whereas the other contained about 0.5 wt % Ti and 0.5 wt % Al so that it precipitated during creep at 700° C. Special care was taken to assure equivalent microstructure in the specimens and precise creep conditions so as to obtain accurate and reproducible creep-rates. Both materials exhibited decreasing stress-dependence of the creep-rate at low stresses. Neither the stress-dependence of the creep-rate, nor the absolute creep-rate was consistent with diffusion-creep. The amount of grain-boundary sliding was measured separately by means of scribed grid lines on the creep specimens for the pure material at stresses above the creep yield. The values for the component of the creep-rate due to grain-boundary sliding coincide very well with the extrapolated line of the low-stress branch of the creep-rate/stress curve. All these results taken together suggest that the most likely explanation of the creep yield in 20 wt % Cr/35 wt % Ni steels is the one based upon grain-boundary sliding.     

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti₃Al and Ti₃Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m^–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m^–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m^–2, a transition regime with a lower stress-exponent value was obtained.     

Zr-Hf interdiffusion in polycrystalline Y2O3-(Zr+Hf)O2

Lattice and grain-boundary interdiffusion coefficients were calculated from the concentration distributions determined for Zr-Hf interdiffusion in polycrystalline 16Y₂O₃·84(Zr_1–x Hf _x )O₂ withx=0.020 and 0.100. The lattice interdiffusion coefficients were described byD=0.031 exp [–391 (kJ mol^–1)/RT] cm² sec^–1 and the grain-boundary diffusion parameters byD=1.5×10^–6exp [–309(kJ mol^–1)/RT] cm³ sec^–1 in the temperature range 1584–2116° C. Comparison of the results with those for the systems CaO-(Zr+Hf)O₂ and MgO-(Zr+Hf)O₂ indicated that the Zr self-diffusion coefficient was insensitive to the dopants in the fluorite-cubic ZrO₂ solid solutions.     

Carbon self-diffusion in tungsten carbide

Dense, hot pressed tungsten carbide specimens were used to study the self-diffusion of ¹⁴C into WC in the temperature range 2238 to 2643 ° K. The necessity for extended diffusion anneal times was eliminated by using a submicron sectioning technique, and the diffusion penetration depth was determined by spectrophotometric W^V thiocyanate analysis of the sample sections. The existence of two clearly delineated diffusion mechanisms was demonstrated from the shape of the activity versus penetration curves. The first obeyed a bulk diffusion law, originated at the specimen surface, demonstrated anomalously low diffusion coefficients, dominated to a depth of about 0.5 m, and could be represented by the expression: D _vol=1.90×10^–6 exp–(88,000/RT).Autoradiography demonstrated that the second mechanism was grain-boundary diffusion which dominated at depths greater than 1 m. The Fisher grain-boundary diffusion analysis and the Suzuoka analysis gave apparent grain-boundary diffusion activation energies of 74 Kcal/mole and 71 Kcal/mole respectively. By using an estimated value of the bulk diffusion coefficient, the Suzuoka analysis permitted direct calculation of the grainboundary diffusion coefficient, and can be represented by: D _g.b.=4.57×10² exp–(71,000/RT).     

The creep of sapphire filament with orientations close to the c-axis

Sapphire filament oriented within 2 1/2° of the crystallographic c-axis underwent creep by a mechanism other than slip on the basal planes at temperatures above 1600° C. There was a stress below which creep could not be detected; this decreased from 180 MNm^–2 at 1600° C to 65 MNm^–2 at 1800° C. The total tensile strain obtained never exceeded 5%. Fracture occurred during a linear stage of creep in which the stress exponent of the strain-rate was approximately 6. The creep mechanism appeared to be slip on {20¯2¯1} 01 T2 (morphological unit cell). A filament in which the c-axis lay at 6° to the filament axis deformed by localized basal slip. The accompanying local latice rotations produced fracture at a small overall strain, usually less than 0.5%. The results demonstrate extreme anisotropy of creep in sapphire crystals.     

Mechanical behaviour,percolation and damage of materials with viscous solid grain boundaries

The mechanical behaviour, percolation and damage mechanism of a aluminium alloy with viscous solid grain boundaries (GBs) at 465 °C have been characterized in experiments performed in tension or compression in the strain rate range of 10^–5-10^–2s^–1. It was found that grain-boundary sliding (GBS) occurs as strain rates below 10^–4s^–1. It was shown that the viscous solid interphase migrates during the process of deformation. In the case of tension, it was squeezed out of GBs parallel with the tension axis into GBs perpendicular to the axis and vice versa in the case of compression. This local percolation is discussed in terms of the viscosity of the interphase, gradient of local stresses and percolation time. The viscosity of the solid interphase is estimated. It was also found that cavitation depends on the type of stress (tension or compression) and the strain rate. Cavity nucleation occurs at multiple points when GBS happens or along G B facets in the absence of GBS. Cavity growth takes place along GBs at high normal stresses and the cavity coalsescence leads to saw-tooth fracture.     

II–VI semiconductor alloy films: Cd1−xZnxTe

Electrical conductivity and optical properties of undoped and copper-doped Cd_1–xZn_xTe (0.1相似文献   

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.