The role of diffusion in determining the controlling creep mechanisms in Al-Zn solid-solutions: Part II

The theoretically appropriate diffusion coefficients for creep in binary solid solutions are verified in Part I. Furthermore it is observed that the observed composition dependence of the steady-state creep rate for Al-Zn alloys may be described by these diffusion coefficients. Part II further examines the creep behavior of the Al-Zn system. Stress exponent and activation energy measurements indicate that a transition from climb-controlled to glide-controlled creep occurs as the Zn concentration increases. It is found that the observed creep behavior can be explained by a combination of climb and glide controlled creep processes. Glide creep is dependent on the chemical diffusivity which approaches zero at the miscibility gap composition and temperature. This causes a minimum in the creep rate at the miscibility gap composition. By using a sequential summation of a semiempirical climb equation and the Cottrell-Jaswon glide equation, the absolute magnitude of the observed creep rate can be rationalized for all compositions. The observed behavior of the stress exponent and activation energy is also predicted. Formerly Graduate Research Assistant     

Diffusivity of hydrogen in dilute alloys of copper and niobium in palladium

An electrochemical technique has been used to measure the diffusion coefficient of hydrogen in palladium-copper and palladium-niobium alloys between 0 and 50°C. The range of concentration for the copper and niobium additions was 0–10 at.%. For all experiments the hydrogen content was kept below 0.1 at.% and it was found that the diffusivity of hydrogen is remarkably decreased by additions of niobium. This behavior is discussed within the framework of different trapping models. In agreement with these models no dependence of the diffusivity on hydrogen concentration was observed. Copper additions also decrease the hydrogen diffusivity in palladium, but the effect is less pronounced than niobium. A measurable change of the activation energy of diffusion was observed only for the niobium alloy with the highest niobium content of 6.5 at.%. The activation energy increased by abour 7 kJ/mol compared to pure palladium.     

Hydrogen diffusion in Al-Li alloys

The diffusion coefficients of hydrogen in binary Al-Li alloys containing 1,2, and 3 wt pct Li have been determined from desorption curves of samples saturated with hydrogen at 473 to 873 K. Within this temperature range, the diffusivity of hydrogen in the binary Al-Li alloys investigated has an Arrhenius-type temperature dependence and follows the equation of the general formD = DT) whereD ₀exp(-Q/R is the diffusion coefficient (m²/s),D ₀ is the preexponential or frequency factor (m²/s), R is the gas constant (J/K mol),Q is the activation energy (J/mol), andT is absolute temperature (K). The rate of diffusion of hydrogen in aluminum decreases with increase in lithium additions. This is provisionally attributed to the stronger local binding energy between hydrogen and lithium atoms in the aluminum metal lattice.     

The kinetics of diffusion of hydrogen in niobium

The kinetics of hydrogen diffusion in niobium were studied in the temperature ranges of 200° to 300°C and 600° to 700°C by gas-metal absorption experiments. The hardness profile was correlated with the concentration of hydrogen on specimens of various grain sizes. It was found that the diffusion kinetics of both temperature ranges could be represented by a single Arrhenius-type rate equation. The activation energy and the preexponential factor were found to be 10.000±320 cal per mole, and 1.8±1.0×10⁻² cq cm per sec., respectively. No observable effect of grain size was found on the diffusivity.     

Application of the square root diffusivity to diffusion in Ni-Cr-Al-Mo alloys

The square root diffusivity matrix [r] and diffusivity matrix [D] have been measured for a Ni-4.0 at. pct Cr-6.3 at. pct Al-3.5 at. pct Mo quaternary alloy at 1100 °C. Using a constant diffusivity analysis, the diffusivities were obtained from three diffusion couples having small initial concentration differences of 5 at. pct or less. When elements of [r] were substituted into the error function solution of the diffusion equation, it was found that the predicted concentration profiles were accurate to within ±0.1 at. pct of measured values. Also, it was found that Mo tends to reduce the diffusion kinetics of Cr and Al in Ni-base alloys. These results suggest that these methods of analyzing diffusion data and predicting interdiffusion data can be applied to alloys containing more than four components. Formerly Graduate Student, University of Connecticut.     

Chemical diffusion in the β phase of the Zr-Ti alloy system

Interdiffusion studies have been carried out in the bcc phase of the Zr-Ti alloy system using diffusion couples of zirconium and Zr-Ti alloys. Geoscan electron microprobe was employed to establish the concentration-penetration profiles in the diffusion zones, and the interdiffusion coefficients were evaluated by Boltzmann-Matano analysis. The concentration and temperature dependence of the diffusivity have also been examined. Present studies further showed that the diffusivity in the Zr-Ti system in theβ-phase could be expressed by an equation similar to that given by Vignes and Birchenall for vacancy-controlled interdiffusion in fee solid solutions.     

Removal of Toluene Vapor with Dispersed Activated Carbon

This work considers the mechanisms of mass transfer in a process of dispersed sorbent injection. During experiments, an air supply was dosed with toluene vapor, at partial pressures between 4 and 15 Pa. Powdered activated carbon (PAC) was added to remove the toluene from the air, and the resulting mixture was passed through a 3-m-long, tubular, aluminum test section. Toluene concentrations were measured at seven axial locations within the test section. Comparing the measurements with mathematical models indicated the importance of adsorption kinetics. At reduced toluene inlet concentrations the PAC removed a slightly bigger fraction of toluene from the air stream. This fraction increased with PAC concentration. The effect on removal of varying the air temperature between 25 and 85°C was small. Alternative models incorporating either pore diffusion or surface diffusion were fitted to the results. The quality of the fits was fair only, but sufficient to show that the pore diffusivity that gave the best fit was far larger than would be expected from the Knudsen diffusivity in the pores. That is, surface diffusion was an important part of the intraparticle mass transfer.     

Comprehensive Evaluation of Aluminum Diffusivity in Liquid Zinc

Diffusion coefficients of Al in molten Zn in the temperature range relevant to the industrial applications of the Zn-Al alloy were experimentally measured using the long capillary technique. The diffusion coefficients were also predicted using four methods, including a predictive equation derived recently by the present authors, the Enskog expression, the “hole” theory, and the fluid theory. In addition, molecular dynamics simulation was carried out to calculate the Al diffusivity. The experimentally measured Al diffusivity agrees well with the predicted values of the new derived equation and the outcomes of the molecular dynamics computation. An evaluation of the predictions of the four diffusion models favors the density fluctuation model.     

Application of the square root Diffusivity to Diffusion in Ni-Cr-Al-Mo Alloys

The square root diffusivity matrix [r] and diffusivity matrix [D] have been measured for a Ni-4.0 at. pct Cr-6.3 at. pct Al-3.5 at. pct Mo quaternary alloy at 1100 °C. Using a constant diffusivity analysis, the diffusivities were obtained from three diffusion couples having small initial concentration differences of 5 at. pct or less. When elements of [r] were substituted into the error function solution of the diffusion equation, it was found that the predicted concentration profiles were accurate to within ±0.1 at. pct of measured values. Also, it was found that Mo tends to reduce the diffusion kinetics of Cr and Al in Ni-base alloys. These results suggest that these methods of analyzing diffusion data and predicting interdiffusion data can be applied to alloys containing more than four components.     

Modeling of Moisture Diffusion in FRP Strengthened Concrete Specimens

Modeling the movement and distribution of moisture in the fiber-reinforced polymer (FRP) composites strengthened concrete structure is important because the interfacial adhesion between FRP and concrete is susceptible to moisture attack. Using relative humidity as the global variable, the moisture diffusion governing equation was derived for the multilayered system in this study. The moisture diffusivity (diffusion coefficient) and the isotherm curve, which correlates the moisture content to environmental relative humidity, of each constitutive material (concrete, epoxy, and FRP) were experimentally determined. A multilinear diffusivity model was developed for concrete based on desorption test, and a linear diffusivity model was proposed for epoxy adhesive based on absorption test. A simple method was developed to directly measure the FRP/concrete interface region relative humidity (IRRH). Finite-element analysis was performed to study the moisture diffusion in the FRP-adhesive-concrete system. The IRRH values were obtained for different environmental relative humidity in the numerical study. The error between the experimental and numerical results of IRRH at test locations was less than 5% RH. The good agreement between experimental and numerical results indicates that the approach developed in this study worked well.