Rate-controlling Process Research Articles

Superplastic Characteristics of Fine-Grained 7475 Aluminum Alloy

A 7475-aluminum alloy was subjected to a thermomechanical heat treatment that resulted in a final recrystallized grain size on the order of 10 µm. Tensile specimens of dimensions 10 × 4 × 2.3 mm were machined such that the tensile axis was parallel to the rolling direction. Tensile tests were carried out at high temperatures in the range of 773 to 803 K at different cross-head speeds corresponding to initial strain rates in the range of 10−4 to 10−2 s−1. Elongations of several hundred percent were observed at strain rates of <10−3 s−1. The correlation between flow stress and strain rate suggests that the strain rate sensitivity m is close to 0.5 at the lower strain rates. The value of m decreases to ≈0.2 at high strain rates. The decrease in m suggests a transition in the rate-controlling process from superplastic deformation (m ≈ 0.5) to dislocation creep (m ≈ 0.2) with increasing strain rate. The calculated activation energies in the two deformation regions are consistent with the suggested rate-controlling processes.

Oxidation Kinetics of Ni-Dispersed MgO Composites at Elevated Temperatures

Oxidation behavior of MgO-based composites with 5 vol% Ni particle dispersoid was investigated at temperatures ranging from 1100 to 1300°C in air. Oxidized zone consisted of (Mg, Ni)O solid solution and included voids. Growth kinetics of the oxidized zone followed a parabolic law, which meant that mass transport in the oxidized zone was the rate-controlling process. An oxidation model of Ni-dispersed MgO composites was established as a basis of cation diffusion via vacancy diffusion with defect chemistry of formation of cation vacancy in the MgO-NiO system. The parabolic rate constant and its apparent activation energy estimated from the proposed model agreed with experimental results.

Kinetic Features of Reactive Diffusion between Sn&amp;ndash;5Au Alloy and Ni at Solid-State Temperatures

In order to examine microstructure evolution at the interconnection between the Sn-base solder and Au/Ni/Cu multilayer conductor alloys during energization heating, the kinetics of the reactive diffusion between a binary Sn-5 at% Au alloy and pure Ni was experimentally observed at solid-state temperatures. Sandwich (Sn-Au)/Ni/(Sn-Au) diffusion couples were prepared by a diffusion bonding technique, and then isothermally annealed at temperatures of T = 433, 453 and 473 K for various periods up to 1057 h. During annealing, AuNiSn 8 and Ni 3 Sn 4 compound layers are formed along the (Sn-Au)/Ni interface in the diffusion couple. The total thickness of the compound layers is expressed as a power function of the annealing time. The exponent of the power function is close to 0.5 at T = 453-473 K, and equal to 0.7 at T = 433 K. Therefore, volume diffusion is the rate-controlling process for the growth of the compound layers at T = 453-473 K, but interface reaction contributes to the rate-controlling process at T = 433 K. The growth of the compound layers occurs slower for the reactive diffusion between the Sn-5Au alloy and Ni than for that between Sn and Au, but faster for that between the Sn-5Au alloy and Ni than for that between Sn and Ni.

Thermolytic investigations on nickel(II) complexes of 4-N-(4′-antipyrylmethylidene)aminoantipyrine with various counterions

Phenomenological characteristics and kinetic and mechanistic aspects of the thermal decomposition of nitrate, chloride, bromide, and iodide complexes of nickel(II) with 4-N-(4′-antipyrylmethylidene)aminoantipyrine have been studied using thermogravimetric (TG) and differential thermogravimetric (DTG) techniques. Kinetic parameters such as activation energy, preexponential factor, and entropy of activation were quantified. The rate-controlling process in all stages of decomposition was found to be based on random nucleation with one nucleus on each particle according to the Mampel model. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 53–58, 2007

Thermo-erosional notch development at fresh-water-calving Tasman Glacier, New Zealand

AbstractControls on glacier calving rates are receiving increased scientific interest. At fresh-watercalving glaciers, limnological factors might be more important than glaciological ones. Measurements of thermo-erosional notch development at the calving ice cliff of Tasman Glacier, New Zealand, suggest that the calving rates at this glacier are directly controlled by the rate of thermal undercutting. Notch formation rates typically vary between 10 and 30 cm d–1 (maximum rate 65cmd–1) in summer, corresponding to an average calving rate of 34 m a–1. Notch formation is slower than waterline melt and is controlled by water temperatures and circulation, cliff geometry, debris supply and water-level fluctuations. The latter shift the position of undercutting, resetting the level of the notch formation process and thereby slowing it. The geometry of the notch and the debris supply determine the extent of influence of the lake on notch water temperatures and circulation. Hence, water temperatures in the lake are not necessarily indicative of the rate of notch formation. The prediction of rate of notch formation from far-field variables is hampered by the complex interaction of the influencing factors. The significance of thermal undercutting as a calving rate-controlling process decreases with increasing ice velocities, calving rates and surface gradients.

Creep Characterization of Aluminum-Magnesium Solid-Solution Alloy through Self-Similar Microindentation

Indentation creep tests were performed on an Al-5.3 mol% Mg solid-solution alloy to examine whether mechanical properties can be extracted accurately from a testpiece, as small as a rice grain. A conical diamond indenter was pressed into a test surface with a constant load F at temperatures ranging from 0.60 to 0.65 Tm (Tm: the absolute liquidus temperature). When the representative flow stress � � m and the indentation strain rate _ in the underlying material decreases to each critical value, � � and _ c, as indentation creep proceeds, the stress exponent n for creep varies distinctively from 4.9 to 3.0. The measured � � decreases with increasing temperature, while the corresponding indentation strain rate _ c increases under the same condition. This temperature dependence is in close agreement with the results derived from the dislocation theory. The activation energy Q for creep in range M (n ¼ 5, � � m > � � or _ > _ c) is approximately equivalent to that for the lattice diffusion of pure aluminum, and the Q value in range A (n ¼ 3, � � m � � or _ > _ The findings suggest that the creep rate-controlling process changes from the recovery control (n ¼ 5) to the glide control (n ¼ 3) below � � c. It is thus demonstrated that the indentation testing technique can be effectively used to extract material parameters equivalent to those obtained from conventional uniaxial creep tests in the dislocation creep regime. [doi:10.2320/matertrans.47.2006]

JPEG 2000 Encoding Method for Reducing Tiling Artifacts

This paper proposes an effective JPEG 2000 encoding method for reducing tiling artifacts, which cause one of the biggest problems in JPEG 2000 encoders. Symmetric pixel extension is generally thought to be the main factor in causing artifacts. However this paper shows that differences in quantization accuracy between tiles are a more significant reason for tiling artifacts at middle or low bit rates. This paper also proposes an algorithm that predicts whether tiling artifacts will occur at a tile boundary in the rate control process and that locally improves quantization accuracy by the original post quantization control. This paper further proposes a method for reducing processing time which is yet another serious problem in the JPEG 2000 encoder. The method works by predicting truncation points using the entropy of wavelet transform coefficients prior to the arithmetic coding. These encoding methods require no additional processing in the decoder. The experiments confirmed that tiling artifacts were greatly reduced and that the coding process was considerably accelerated.

Analysis of geometrical effects on graphite oxidation through measurement of internal surface area

To investigate the geometrical effects on nuclear graphite oxidation in the regime where the chemical effect is the rate-controlling process, we introduce the concept of internal surface density into the Arrhenius-type reaction model and suggest how to determine it experimentally. Using the 16 different samples of IG-110 graphite, which have different ratios of external surface to volume, we obtained the value of internal surface density as 17 260 m −1. We found out that the external surface reaction is very small compared to the total reaction for the IG-110 graphite: the ratio of the external surface reaction to the total reaction was within 5%. Finally, we propose the following reaction equation for this material: r ‴ ( kg / m 3 s ) ≈ 2 552 000 · exp - 218 000 R · T · P O 2 0.75 .

Rate-controlling processes in creep of subgrain containing aluminum materials

The creep behavior of aluminum alloys containing Bi, Zn, Ge, Cu, Fe, Mn and Ti has been investigated. Aluminum containing solutes that diffused faster than aluminum has faster creep rates and lower activation energies for creep than observed in pure aluminum. Solutes that diffused slower than aluminum have slower creep rates and higher activation energies for creep than observed in pure aluminum. A dislocation climb model in the subgrain boundary that involves solute atom diffusion as the rate-controlling creep process is proposed to explain the results.

Effect of Ni on reactive diffusion between Au and Sn at solid-state temperatures

Influence of Ni on the kinetics of the reactive diffusion between Au and Sn was experimentally studied at solid-state temperatures. Binary Sn–Ni alloys with Ni concentrations of 1, 3 and 5 mass% were used to prepare sandwich (Sn–Ni)/Au/(Sn–Ni) diffusion couples by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times in an oil bath with silicone oil. After annealing, AuNiSn 8, AuSn 4, AuSn 2 and AuSn compound layers were observed to form at the (Sn–Ni)/Au interface in the diffusion couple. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to the equation l = k( t/ t 0) n , where t 0 is unit time, 1 s. The exponent takes values between n = 0.29 and 0.37 under the present annealing conditions. Such values of n < 0.5 indicate that the grain boundary diffusion contributes to the rate-controlling process and the grain growth occurs at certain rates. The higher the Ni concentration of the Sn–Ni alloy is, the faster the overall growth of the compound layers occurs. This means that Ni is an accelerator for the reactive diffusion between Au and Sn at solid-state temperatures. The acceleration effect of Ni becomes more remarkable at higher annealing temperatures. Such influence of Ni on the kinetics is mainly attributed to the dependencies of the growth rate of the AuNiSn 8 layer on the composition of the Sn–Ni alloy and the annealing temperature.

Compaction of granular calcite by pressure solution at room temperature and effects of pore fluid chemistry

Pressure solution is an efficient compaction mechanism leading to porosity/permeability reduction in both clastic and carbonate rocks. We performed compaction experiments on crushed limestone and granular calcite at room temperature and 1–4 MPa effective stress using various pore fluids. By applying a dry pre-compaction at 8 MPa, we could largely eliminate deformation mechanisms such as grain rearrangement, sliding and fracturing from runs employing a pore fluid. Compaction creep occurred only in samples filled with a saturated carbonate solution, in contrast to those filled with chemically inert fluids, at ε ˙ = 1 0 - 6 – 10 - 9 s - 1 . The measured strain rates increased with decreasing grain size and with increasing effective stress according to power laws with exponents of about 3 and 1–2, respectively. Samples made from naturally oil-contaminated limestone showed far less compaction than oil-free samples. Addition of dissolution and precipitation inhibitors (magnesium and phosphate ions) to the pore fluid significantly decreased compaction rates of wet samples, indicating that creep must have involved calcite dissolution/precipitation. Pore fluid with a salinity (NaCl) of 0.1–0.5 mole/l increased the compaction strain rate compared to non-saline solutions. The experimental results suggest the main deformation mechanism in calcite under our experimental conditions was pressure solution and that diffusion is likely to be the rate-controlling process in pure solution system.

A mechanistic study of initial atmospheric corrosion kinetics using electrical resistance sensors

This paper describes a novel experimental approach to the study of atmospheric corrosion of iron and zinc, utilising electrical resistance sensors that are sensitive to corrosion losses of the order of one atomic monolayer. Using such devices, a mechanistic study of the initial stages in the atmospheric corrosion of iron and zinc was performed in a rectangular flow cell using controlled relative humidity (RH), temperature and gas flow rate. Additionally, the effects of SO 2 contamination in the gas phase and prior NaCl contamination of the metal surface were studied. It was found that the initial corrosion kinetics of iron and zinc are, not unexpectedly, dominated by the development of surface corrosion product films, but that the growth kinetics vary with metal, humidity, etc. Specifically, in the presence of gas-phase SO 2, activation energies and kinetic and chemical rate orders were consistent with control of the atmospheric corrosion process by solution-phase oxidation of sulphite–sulphate ion. For iron, this implies that the well-known sulphate-nest theory is inoperative at least during the early stages of atmospheric corrosion. In contrast, for chloride–contaminated zinc, the data were consistent with a rate-controlled diffusion of a species, probably water vapour or oxygen, through a thickening corrosion product film. Finally, the kinetic and chemical rate orders for corrosion of chloride–contaminated iron precluded a diffusion-controlled mechanism, but were consistent with a rate-controlling process involving some regeneration of chloride: e.g. as in metal–ion hydrolysis in a pit or similar localised corrosion events.

Reactive diffusion between Pd and Sn at solid-state temperatures

The kinetics of the reactive diffusion between Pd and Sn at solid-state temperatures was experimentally examined using Sn/Pd/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures of T = 433, 453 and 473 K for various times up to 300 h in an oil bath with silicone oil. After annealing, PdSn 4, PdSn 3 and PdSn 2 compound layers were observed at T = 433 K, but only PdSn 4 and PdSn 3 layers were recognized at T = 453 and 473 K. The total thickness l of the Pd–Sn compound layers is mathematically described as a power function of the annealing time t as follows: l = k( t/ t 0) n , where t 0 is unit time, 1 s. The diameter d of the columnar grain in the compound layers is also expressed as a power function of the annealing time t: d = k d( t/ t 0) p . At T = 433–473 K, n is close to 0.5, and p takes values of 0.14–0.17. Such values of p indicate that the grain growth occurs at certain rates in the compound layers. Nevertheless, the total thickness l is almost proportional to the square root of the annealing time t. Thus, it is concluded that the volume diffusion is the rate-controlling process of the reactive diffusion between Pd and Sn.

Finescale Structure of the T–S Relation in the Eastern North Atlantic

Abstract Distributions of temperature (T) and salinity (S) and their relationship in the oceans are the result of a balance between T–S variability generated at the surface by air–sea fluxes and its removal by molecular dissipation. In this paper the role of different motions in setting the cascade of T–S variance to dissipation scales is quantified using data from the North Atlantic Tracer Release Experiment (NATRE). The NATRE observational programs include fine- and microscale measurements and provide a snapshot of T–S variability across a wide range of scales from basin to molecular. It is found that microscale turbulence controls the rate of thermal dissipation in the thermocline. At this level the T–S relation is established through a balance between large-scale advection by the gyre circulation and small-scale turbulence. Further down, at the level of intermediate and Mediterranean waters, mesoscale eddies are the rate-controlling process. The transition between the two regimes is related to the presence of a strong salinity gradient along density surfaces associated with the outflow of Mediterranean waters. Mesoscale eddies stir this gradient and produce a rich filamentation and salinity-compensated temperature inversions: isopycnal stirring and diapycnal mixing are both required to explain the T–S relation at depth.

Growth behavior of Nb 3Sn layer during reactive diffusion between Cu–8.3Sn alloy and Nb

The growth behavior of the Nb 3Sn compound during the reactive diffusion between Nb and a binary Cu–8.3 at.% Sn alloy was experimentally examined. In the experiment, (Cu–8.3Sn)/Nb/(Cu–8.3Sn) diffusion couples were isothermally annealed at temperatures of T = 923–1053 K for various times up to 1038 h. After annealing, a Nb 3Sn compound layer was observed to form at each (Cu–Sn)/Nb interface in the diffusion couple. The Nb 3Sn layer grows predominantly towards Nb but hardly towards the Cu–Sn alloy. Thus, the growth of the Nb 3Sn layer is governed by the migration of the Nb 3Sn/Nb interface. The thickness l of the Nb 3Sn layer is mathematically described as a power function of the annealing time t as follows: l = k( t/ t 0) n , where t 0 is unit time, 1 s. The exponent n is close to unity at T = 923 and 973 K and monotonically decreases from 0.96 to 0.77 with increasing annealing temperature from T = 973 to 1053 K. This means that the interface reaction in the Nb 3Sn/Nb interface is the rate-controlling process for the growth of the Nb 3Sn layer at T = 923–973 K and the interdiffusion contributes to the rate-controlling process at T = 973–1053 K. Furthermore, the volume diffusion may govern the interdiffusion at T = 1053 K, but the grain boundary diffusion will contribute to the interdiffusion at T = 923–1023 K.

Experimental determination of crystal growth rates in highly supercooled aluminosilicate liquids: Implications for rate-controlling processes

The kinetics of crystal growth have been investigated between 100 and 200 K above the glass-transition temperature for four liquids in the ternary system CaO-Al 2 O 3 -SiO 2 . The observed phases are yoshiokaite, gehlenite, larnite, anorthite, and wollastonite. They crystallize either congruently or incongruently through heterogeneous nucleation on gas-liquid interfaces. In all cases, the growth rate at high degrees of supercooling is independent of time, while its temperature dependence is Arrhenian. When the crystal has the same Al/Si ratio as the parent liquid, the growth rate of congruent and incongruent crystallization shows a 1:1 correlation with the viscosity of the melt. We do not interpret this observation to imply that growth rate is controlled by viscosity but, rather, that both phenomena are controlled by the rate of rearrangement of bonds between oxygen and network-forming cations. When Al/Si varies across the crystal-melt interface, the effects of interdiffusion must also be considered and the temperature dependence of growth rate is no longer correlated directly with that of viscosity, the former being smaller than the latter. Our observations show that, at constant viscosity, the fastest growth rates occur when the Gibbs free energy difference between liquid and crystal is greatest, rather than when crystalline and liquid phases are structurally most similar. A simplified equation that describes crystal growth rates from highly supercooled liquids is proposed.

Growth behavior of Ni 3Sn 4 layer during reactive diffusion between Ni and Sn at solid-state temperatures

The solid-state reactive diffusion between Ni and Sn was experimentally studied using Sn/Ni/Sn diffusion couples. The diffusion couples were prepared by a diffusion bonding technique and then annealed at temperatures between T = 433 and 473 K for various times in an oil bath with silicone oil. At the Ni/Sn interface in the annealed diffusion couple, a rather wavy layer of Ni 3Sn 4 was observed at T = 453 and 473 K, but granular particles of Ni 3Sn 4 were recognized at T = 433 K. The average thickness l of the Ni 3Sn 4 layer monotonically increases with increasing annealing time t according to the equation l = k( t/ t 0) n , where t 0 is unit time, 1 s. The experimental results provide n = 0.46 and 0.41 at T = 453 and 473 K, respectively. The values of n smaller than 0.5 indicate that the grain boundary diffusion contributes to the rate-controlling process and the grain growth occurs at certain rates. Under the present annealing conditions, the thickness of the Ni 3Sn 4 layer is more than one order of magnitude smaller than the total thickness of the AuSn 4, AuSn 2 and AuSn layers formed by the reactive diffusion between Au and Sn. This indicates that the Ni layer is an effective inhibitor against the reactive diffusion between the Sn-base solder and Au/Ni/Cu multilayer conductor alloys during annealing at solid-state temperatures after complete dissolution of the Au layer into the molten solder.

Role of Austenite in Weld Toughness of Super Duplex Stainless Steel

Microstructure control for welding super duplex stainless steel SAF2507 was carried out on a welded joint by GTA welding. The toughness of the bond region in the heat-affected zone (HAZ) of the advanced SAF2507 stainless steel was much lower than that of the base metal. The microstructure of the bond region for the as-welded sample was examined and the extreme grain growth of ferrite and the lowering of the amount of austenite phase were observed. In order to improve the toughness of the bond region, microstructure control was carried out using a cooling rate control process during welding. Various cooling times from 1 673 K to 1 073 K in the bond region were selected, which corresponded to the heat input from 1kJ/mm to 6kJ/mm. For the ferrite grain growth, the cooling time from 1 673 K to 1 473 K, that is, t16-14 was controlled using a Gleeble simulator. The ferrite grain size increased with increasing cooling time t16-14. For austenite phase reformation, the cooling time from 1 473 K to 1 073 K, t14-10 was selected, since austenite phase reformation occurs within that temperature range. The amount of austenite increased with increasing t14–10. Increasing the cooling rate caused both ferrite grain growth and an increase of the austenite phase. Improvement of the toughness was accomplished up to 60 s in the cooling time from 1 473 K to 1 073 K, however hardly any change in toughness was accomplished at the cooling time of 120 s, because the slow cooling rate caused both ferrite grain growth and an increase of the austenite phase.

Growth behavior of compound layers in Sn/Cu/Sn diffusion couples during annealing at 433–473 K

The kinetics of the reactive diffusion between Cu and Sn at solid-state temperatures was experimentally examined using Sn/Cu/Sn diffusion couples prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed at temperatures between T = 433 and 473 K for various times in an oil bath with silicone oil. Due to annealing, compound layers consisting of Cu 3Sn and Cu 6Sn 5 are formed at the Cu/Sn interface. In most of the annealed diffusion couples, grains of CuSnO exist in the Cu 6Sn 5 layer. However, the CuSnO grain scarcely influences the growth behavior of the compound layers. The thickness of the Cu 6Sn 5 layer is around twice greater than that of the Cu 3Sn layer. The difference between their thicknesses slightly increases with increasing annealing temperature. At a constant temperature, the ratio of the thicknesses remains constant independent of the annealing time. The total thickness l of the compound layers monotonically increases with increasing annealing time t according to equation l = k( t/ t 0) n , where t 0 is unit time, 1 s. The observations provide n = 0.37, 0.43 and 0.50 at T = 433, 453 and 473 K, respectively. The value n = 0.5 indicates that the volume diffusion is the rate-controlling process for the reactive diffusion at T = 473 K. As the annealing temperature decreases, the grain boundary diffusion will contribute to the rate-controlling process. Thus, the values of n smaller than 0.5 yield that there is the contribution of the grain boundary diffusion and the grain growth occurs at certain rates at T = 433 and 453 K. Since the ratio between the thicknesses of the compound layers is kept constant during isothermal annealing, the same rate-controlling process is expected to work in both compound layers.

Effects of Fuel Volatility on Flame Spread along Fuel Droplet Arrays in Microgravity

Microgravity combustion experiments of fuel droplet arrays were conducted with different fuel volatilities. N-octane, n-decane and n-hexadecane were used to examine effects of volatility. Experimental results showed that the flame spread rate took a maximum value at a specific inter-droplet distance for all the fuels and increased with fuel volatility. Effects of inter-droplet distance on flame spread behavior at room temperature were discussed based on positional relation between the leading flame and an unburned droplet. Effects of fuel volatility were discussed based on the droplet heating time. Simple formulas to estimate flame spread rates were proposed considering rate-controlling processes. Flame spread behavior were explained based on the droplet heating process for small inter-droplet distance and on the droplet heating process and the development of the flammable mixture layer around the next droplet for large inter-droplet distance.