Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y₂O₃, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO₃ and Al₂O₃ will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y₂O₃-77 mol% Al₂O₃ and 1702±7° C. YAlO₃ (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO₃ formed during metastable solidification transforms to YAG in the presence of Al₂O₃ at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.     

Clustering phenomena in Al-4.4 at. % Zn alloy with 0.03 and 0.08 at. % Ag additions

The influence of 0.03 and 0.08 at. % Ag additions on the clustering of Zn atoms in an Al-4.4 at. % Zn alloy has been studied by resistometry. The effect of quenching and ageing temperatures shows that the ageing-ratio method of calculating the vacancy-solute atom binding energy is not applicable to these alloys. Zone-formation in Al-Zn is unaffected by Ag additions, but the zone-reversion process seems to be influenced. Apparent vacancy-formation energies in the binary and ternary alloys have been used to evaluate the v-Ag atom binding energy as 0.21 eV. It is proposed that, Ag and Zn being similar in size, the relative vacancy binding results from valency effects, and that in Al-Zn-Ag alloys clusters of Zn and Ag may form simultaneously, unaffected by the presence of each other.     

Simulation of thermal stress in reinforced concrete at early ages with a simplified model

This paper is primarily concerned with stresses and cracking in reinforced concrete caused by restrained thermal deformations during the hydration process. Simulation of thermal stresses and crack occurrence is carried out with a simplified model. By validation against tests, it is shown that the simple, one-dimensional model gives a reasonable estimate of the thermal stress development. The model is based on a few input parameters like temperature history, stiffness development and tensile strength development. By means of a parametric investigation, the age-dependent stiffness of the concrete and the temperature history are identified as the decisive material characteristics regarding the thermal stress level. It is also found that the reinforcement has no effect prior to cracking but a clear crack redistributing role in the post-cracking state.     

Change of the ultrasonic characteristics with stress in some steels and aluminium alloys

Under application of tensile stress to specimens of several steels and aluminium alloys up to a stress close to their yielding stress, a continuous ultrasonic wave of approximately 5 MHz was propagated through the specimens in directions parallel and perpendicular to the tensile axis. The change of ultrasonic resonance frequency with applied stress was measured. The results of the measurement of the ultrasonic resonance frequency showed that the frequency ratio varied with the magnitude of applied stress, the material of the specimen, and ultrasonic wave propagating direction with respect to the direction of the stress. They also showed that the ultrasonic wave resonance frequency measuring method is useful in finding the ultrasonic characteristics of materials under applied stress.List of symbols density - , Lame's constants - l, m Murnaghan's constants - stress - wavelength - n order number of resonance frequency - f _n resonance frequency ofnth order - elastic strain - v Poisson's ratio - C ultrasonic velocity - L length of specimen     

The effect of microstructure and environment on fatigue crack growth in 7049 aluminium alloy at negative stress ratios

The influence of environment and microstructure on fatigue crack growth has been investigated on a high strength 7049 aluminium alloy. This aluminium alloy was artificially aged to underaged (UA) and overaged (OA) microstructures. The heat treatment procedure was performed in order to obtain an UA and OA microstructure having the same yield strength properties, but differing in the mode of slip deformation: the UA alloy deforms by planar slip and that of the OA alloy by wavy slip. The crack growth measurements were performed in MT specimens at constant load ratios for R=0, −1, −2, −3 near-threshold and Paris regime in ambient air and vacuum conditions. Crack closure loads were measured in order to determine the P_open for each R ratio. Micromechanisms of near-threshold crack growth are briefly discussed for several concurrent processes involving environmentally assisted cracking with intrinsic microstructural effects. The results showed that the presence of humid air leads to a larger reduction in ΔK_th for both the ageing conditions, but the UA specimens were superior probably because of crack branching. The role of environmental effect and microstructures near-threshold regime seems to be more significant than any mechanical contributions to the crack closure, such as plasticity, roughness, oxide, etc.     

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.     

Formation and reversion of G-P zones in Al-1.3 at. % Ag alloy

The kinetics of clustering and reversion of G-P zones in an Al-1.3 at. % Ag alloy have been investigated by resistivity measurements, with special attention to the initial parts of reversion. From an analysis of clustering kinetics, vacancy formation, solute migration and solute-vacancy binding energies have been deduced to be 0.70, 0.53 and 0.10 eV respectively.The activation energy for reversion, which is identified as the activation energy for solute atom diffusion, is found to vary from 0.98 to 1.46 eV with increasing reversion time. This is attributed to variations in the vacancy concentration in equilibrium with small dislocation loops.     

Nanoscale precipitation and mechanical properties of Al-0.06 at.% Sc alloys microalloyed with Yb or Gd

Dilute Al-0.06 at.% Sc alloys with microalloying additions of 50 at. ppm of ytterbium (Yb) or gadolinium (Gd) are studied with 3D local-electrode atom-probe (LEAP) tomography for different aging times at 300 °C. Peak-aged alloys exhibit Al₃(Sc_1−x Yb _x ) or Al₃(Sc_1−x Gd _x ) precipitates (L1₂ structure) with a higher number density (and therefore higher peak hardness) than a binary Al-0.06 at.% Sc alloy. The Al–Sc–Gd alloy exhibits a higher number density of precipitates with a smaller average radius than the Al–Sc–Yb alloy, leading to a higher hardness. In the Al–Sc–Gd alloy, only a small amount of the Sc is replaced by Gd in the Al₃(Sc_1−x Gd _x ) precipitates, where x = 0.08. By contrast, the hardness incubation time is significantly shorter in the Al–Sc–Yb alloy, due to the formation of Yb-rich Al₃(Yb_1−x Sc _x ) precipitates to which Sc subsequently diffuses, eventually forming Sc-rich Al₃(Sc_1−x Yb _x ) precipitates. For both alloys, the precipitate radii are found to be almost constant to an aging time of 24 h, although the concentration and distribution of the RE elements in the precipitates continues to evolve temporally. Similar to microhardness at ambient temperature, the creep resistance at 300 °C is significantly improved by RE microalloying of the binary Al-0.06 at.% Sc alloy.     

Influence of stress ratio at baseline loading on delayed retardation phenomena of fatigue crack growth in A553 steel and A5083 aluminium alloy

The delayed retardation phenomena of fatigue crack growth following a single application of tensile overload were investigated under the baseline loading with the stress ratio, R = σ_min/σ_max, ranging from −1 to 0.5 for A553 steel and A5083 aluminium alloy. Two different overload cycles were applied; the one is the case that the ratio of peak stress range to baseline stress range, r = Δσ₂/Δσ₁, is equal to two and the other is the case that the ratio of maximum peak stress to maximum baseline stress, σ_2max/σ_1max, is equal to two. The retardation took place stronger in aluminium than in steel. Under the condition of r = 2 the normalized number of cycles, N_D/N_C, (N_D: the number of cycles during retardation, N_C: the number of cycles required for propagation through the overload-affected-zone size) decreased slightly as the R ratio increased from −1 to 0.5, while under the condition of σ_2max/σ_1max = 2 the N_D/N_C-values increased drastically as the R ratio increased from −1 to 0 (or the overload ratio, r, increased from 1.5 to 2) in both the materials. These retardation behaviors were expressed theoretically according to the model proposed by Matsuoka and Tanaka [1, 3] by using four parameters: the overload ratio, r, the exponent in Paris equation, m, the overload-affected-zone size, ω_D, and the distance at the inflection point, ω_B.     

High temperature stress relaxation in single crystals of Mo,W, and Mo-3 wt.% Nb alloy

Strength of Materials -     

The influence of simultaneous mechanical and thermal loads on the stress distribution in molars with amalgam restorations

A finite element analysis (FEA) of a mandibular molar restored with Class II amalgam restoration was conducted to determine the stress distribution which results from a superposition of simultaneous mechanical and thermal loading. A fully crossed three-level four-factor experimental design was used to evaluate the relative influence of crown temperature, time of thermal loading, occlusal force, and cavo-surface margin adhesion on the stress distribution. It was found that occlusal force and temperature had significant influence on the stress distribution and particularly on the maximum principal stress. Over the range in oral conditions considered, thermal loading contributed for over 35% of the stress within the restored molar subjected to simultaneous mechanical and thermal loads. Furthermore, thermal loading had significant effects on the magnitude of normal stress that develops parallel to the pulpal floor. Although marginal bonding of amalgam reduces the stress resulting from occlusal forces, thermal loading promotes the development of significant interfacial shear stresses along the bonded margin. Stresses related to the thermal component of loading concentrate near the pulpal floor and lingual surface margin, the site most prominent in cusp fracture. Hence, results from this study clearly indicate that an evaluation of new dental materials and/or restorative designs should consider the effects from a superposition of simultaneous mechanical and thermal loads on fracture resistance ©©2000©Kluwer Academic Publishers     

Recommended values for the thermal conductivity of aluminium of different purities in the cryogenic to room temperature range, and a comparison with copper

The thermal conductivity of pure aluminium at cryogenic temperatures varies by many orders of magnitude depending on purity and treatment, and there is little information in the literature on the likely values to be obtained for samples of a given purity. A compilation of measurements from the literature has been assembled and used to provide recommended ranges of values for aluminium of different purities (4N, 5N and 6N) in the normal (non superconducting) state. The number of direct thermal conductivity measurements is too limited to be used alone. Electrical resistivity measurements have thus also been used by converting to thermal conductivity using the Wiedemann-Franz law, which is shown to be valid. Since low temperature measurements can easily be extrapolated to higher temperatures, the results cover the range from 1.2 K (the superconducting transition temperature) to room temperature. Values for 5N purity copper have also been examined in a similar manner, to allow a comparison between the two materials. The main application of these results is in the design of cryogenic thermal links; a discussion of the advantages and disadvantages of both materials for this use is given. The use of silver is also investigated briefly. Trends in the behaviour of the conductivity of aluminium in the superconducting state (to temperatures as low as 50 mK) are also discussed.     

高温下不同银含量微电子胶连点的力学性能及膨胀系数不匹配热应力

采用热机械分析仪和微压入测试系统对不同银含量微电子互连导电胶进行测试表征,并基于数值模拟分析其用于倒装芯片封装时胶层的不匹配热应力。结果表明,较高银含量固化导电胶的玻璃化转变温度滞后于较低银含量固化导电胶,且其热膨胀系数较低;随着温度的升高,导电胶的模量与硬度由玻璃态时的较高值降低到高弹态时的较低水平,且银含量较高时的刚度与强度较大;各不匹配热应力分量随温度的变化呈现出"蝌蚪状"或"半蝌蚪状",玻璃态时用于倒装芯片封装的导电胶未发生屈服。     

The optical properties of Al-8 at.% Ce alloy in the liquid, amorphous, and crystalline states

The method of spectral ellipsometry is used in the spectral range of 0.52–2.7 eV for measuring the optical constants of liquid, amorphous, and polycrystalline alloys of Al-8 at.% Ce. The results are used to calculate the dispersion dependences of light conduction, of reflectivity, and of functions of characteristic loss of electron energy of these alloys. It is found that the band-to-band transitions have a significant effect on the optical properties of alloy in the amorphous and polycrystalline states. Two absorption bands are observed in the spectra of light conduction of these alloys, namely, in the amorphous alloy at photon energy of 1.72 eV and 0.69 eV, and in the polycrystalline alloy at 1.55 eV and 1.03 eV. The reflectance spectra of liquid and amorphous alloys almost coincide, which is due to the similarity of their atomic structures.     

Effective thermal conductivity of aluminum oxide with metallic fillers in gaseous media and a vacuum at various temperatures

Experimental data on of granular porous aluminum oxide as a function of copper concentration and temperature in various gaseous media at atmospheric pressure and in a vacuum (P=8·10^–3 mm Hg) are presented.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 50, No. 3, pp. 465–471, March, 1986.     

Some aspects of splat-quenching in an inert atmosphere and of the formation of non-crystalline phases in Al-17.3 at. % Cu,germanium and tellurium

An apparatus for splat-quenching by the gun technique in a sealed, inert atmosphere is described. The importance of a low-oxygen quenching atmosphere in promoting efficient spreading of liquid particles and good thermal contact with the quenching surface is shown. A cooling rate of ～10¹⁰ K sec^?1 was estimated from the interlamellar spacing in a quenched Al-17.3 at. % Cu alloy. The process mechanisms of the gun technique are discussed with particular reference to the atomized droplet size and the effective specimen thickness for heat transfer. New non-crystalline phases are reported in electron-transparent areas of splat-quenched foils of Al-17.3 at. % Cu (eutectic composition) pure Ge and pure Te. The glassy Al-Cu phase was also observed in specimens which were chemically thinned from the thicker regions of foils; lattice image studies by high-resolution electron microscopy tentatively suggest that this phase has an amorphous, liquid-like atomic configuration. The peak positions in the electron diffraction patterns of the Ge and Te phases were compared, where possible, with those for the corresponding liquid and vapour-deposited phases. The results for Ge suggest that significant structural rearrangement took place during cooling and freezing from the liquid to give a paracrystalline, tetrahedral short-range order whereas, for Te, the liquid structure was probably largely preserved on freezing.