Chemical interdiffusion and kirkendall shifts in silver- cadmium alloys

Chemical interdiffusion in α-phase silver-cadmium alloys was investigated in the tem-perature range 1072 to 1179 K with solid-solid, single-crystal couples. Interdiffusion coefficients and Kirkendall shifts were experimentally determined. Kirkendall shifts were measured directly by traveling microscope measurements and were compared to those shifts calculated from Matano analyses and those predicted by the theories of Manning and Darken. The experimentally-measured Kirkendall shifts were found in all cases to be greater than those predicted by Darken and equivalent to or slightly larger than those predicted by Manning. This difference from the results predicted by Darken is attributed to the vacancy wind effect. The experimentally-determined interdiffusion coefficients are in reasonable agreement with those predicted by theory.     

An investigation of interdiffusion in a two-component mixture of powders of different degrees of dispersion

Conclusions The method in [1] was modified so as to permit calculation of concentration distributions for mixtures of powders of different dispersity.A comparative study of a number of histograms obtained experimentally [3] and calculated for the same conditions theoretically enables one to assume the presence of higher diffusion mobilities of atoms in the investigated objects than the values given in tables in [4], and indicates formation of conglomerates of nickel particles.It is also to be assumed that at annealing temperatures of over 700° ordering takes place, leading to retardation of homogenization in the case of real objects compared with homogenization when the Cu-Ni alloy is an ideal solution.     

Hydrogen diffusion in Nb-Ta alloys

Gorsky effect measurements of the diffusivity of hydrogen were carried out in Nb-Ta alloys over the entire composition range and over the temperature range 165 K to 500 K. Measurements were made in the solid solution range of hydrogen solubilities and the diffusivities were corrected to zero hydrogen concentration using mean field interaction energies determined from the composition dependence of the diffusivities. Significant deviations from an Arrhenius temperature dependence were observed over the entire composition range. Over most of the temperature range the hydrogen diffusivity exhibited a monotonic decrease as the Ta concentration was increased. The mean field H-H interaction energies also varied with Ta concentration. G. MATUSIEWICZ, formerly with the University of Illinois     

Solvent diffusion in silver-antimony alloys

It is well known, that the solvent diffusion is enhanced by alloying faster diffusing elements to a pure matrix. This enhancement usually is assumed to be linear in the impurity concentration. From the linear slope, impurity correlation factors have been calculated in the tight binding approximation in the framework of the five frequencies model by some authors. It is shown here, that earlier interpretations in this alloy system AgSb have largely overestimated the range of validity of this linear dependence. It becomes apparent, that particularly the non-linear increase in the vacancy concentration, as function of the impurity content, governs essentially the behaviour of the solvent diffusion. A model, which has been successfully developed earlier to describe the vacancy formation in such alloys in a fairly wide range of concentrations, can be easily amended to cover also the solvent diffusion in essential parts of the non-linear range of vacancy formation. This model is found to account very well for the marked dependence of solvent diffusion on temperature and impurity concentration observed experimentally.     

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.     

Multiphase diffusion in Ag-Zn alloys

Multiphase diffusion was studied in Ag-Zn alloys at 600°C with vapor-solid diffusion couples. The experiments were performed with a β(bcc) alloy as vapor-source material and with diffusion disks (fcc) of either pure silver or an α alloy. The diffused couples were studied metallo-graphically and by electron microprobe analysis. Interdiffusion coefficients for each of the phases were calculated from data on the expansion of the couple and the motion of the α/β interface on the basis of new relations developed from an analysis of two-phase vapor-solid couples. Also, the compositions at the α/β interface differed little from those determined from an equilibrated two-phase alloy.     

An interdiffusion study of a NiAl alloy using single-phase diffusion couples

The intermetallic compound NiAl is highly ordered at stoichiometry, even at high temperatures. For instance, its point-defect concentration at 900 °C is on the order of 10⁻³ at this composition. Moreover, its enthalpy of formation is highly negative and exhibits a minimum at stoichiometry. All of these data suggest that the interdiffusion coefficient should exhibit a minimum at this composition. Yet, the literature data show that the minimum interdiffusion coefficient occurred between 48 and 49 at. pct Al. Accordingly, we have carried out diffusion-couple measurements employing single-phase NiAl wafers over the temperature interval from 700 °C to 1000 °C. The measured concentration profiles were analyzed in terms of the lattice mole fraction, yielding interdiffusion coefficients as a function of composition and temperature. For all the temperatures studied, the interdiffusion coefficients were found to exhibit a minimum at the stoichiometric composition, with a corresponding maximum in the activation energies obtained from the temperature dependence of the interdiffusion coefficients.     

Hydrogen and deuterium diffusion in vanadium-niobium alloys

Hydrogen and deuterium diffusion was studied in vanadium-niobium alloy by a Boltzmann-Matano analysis of concentration profiles. The diffusion coefficients decreased smoothly from both pure metals to a very deep minimum at 75 at. pct niobium. The temperature dependence could be expressed by an activation energy that increased with alloy concentration to a maximum also at 75 at. pct niobium. The terminal solid solubilities increased with both vanadium and niobium concentration, and hydride phases were not observed in the more concentrated alloys. The variation of the diffusion coefficients with hydrogen concentration and with alloy concentration does not follow the predictions of simple local deep trap models. Formerly a Graduate Student, Department of Materials Science and Engineering, Iowa State University     

Hydrogen and deuterium diffusion in vanadium-titanium alloys

Hydrogen and deuterium diffusion coefficients were measured in vanadium-titanium alloys, containing up to 30 at. pct Ti, by Boltzmann-Matano techniques. Hydrogen and deuterium diffusivity decreased rapidly with titanium concentration. The diffusion coefficients showed an Arrhenius temperature dependence in each alloy between 230 and 473 K with activation energies that increased with titanium concentration. The diffusion coefficient decreased linearly with hydrogen concentration in all alloys. The terminal solid solubilities increased markedly with titanium concentration with deuterium showing a larger terminal solid solubility than hydrogen below ten percent titanium. The diffusion results do not fit a localized deep trapping of hydrogen by the titanium atoms. H.M. HERRO, formerly a Graduate Student, Department of Materials Science and Engineering, Iowa State University, is Senior Metallurgist at Nalco Chemical Company, Metallurgy Laboratory, 6216 West 66th Place, Chicago, IL 60638.     

Hydrogen and deuterium diffusion in vanadium-titanium alloys

Hydrogen and deuterium diffusion coefficients were measured in vanadium-titanium alloys, containing up to 30 at. pct Ti, by Boltzmann-Matano techniques. Hydrogen and deuterium diffusivity decreased rapidly with titanium concentration. The diffusion coefficients showed an Arrhenius temperature dependence in each alloy between 230 and 473 K with activation energies that increased with titanium concentration. The diffusion coefficient decreased linearly with hydrogen concentration in all alloys. The terminal solid solubilities increased markedly with titanium concentration with deuterium showing a larger terminal solid solubility than hydrogen below ten percent titanium. The diffusion results do not fit a localized deep trapping of hydrogen by the titanium atoms. Formerly a Graduate Student, Department of Materials Science and Engineering, Iowa State University     

Solubility and diffusion of hydrogen in vanadium-oxygen alloys

The solubilities and diffusion coefficients of hydrogen have been measured in vanadium-oxygen alloys. A small increase in the isopiestic solubility of hydrogen at 297 and 373 K was caused by the addition of oxygen to vanadium, but Sieverts’ law was obeyed in each alloy in the hydrogen concentration ranges studied. Hydrogen diffusion coefficients as a function of hydrogen concentration were measured at 227, 297, and 373 K by a Boltzmann-Matano method that also allowed a determination of the terminal hydrogen solid solubility. The hydrogen diffusion coefficients decreased with increasing-oxygen concentrations. The solubility and diffusion results do not fit a simple trapping model. The terminal hydrogen solid solubility increased slightly with oxygen concentration     

Multiphase diffusion in ternary Cu-Zn-Ni alloys

Isothermal, multiphase diffusion was studied in the Cu-Zn-Ni system at 775°C in order to investigate the development of time-dependent diffusion structures. Solid-solid couples set up with disks of a 46.5 Zn-31.5 Cu-22 Ni wt pct ternary alloy (bcc) sandwiched between disks of either pure nickel or copper (fcc) were diffusion-annealed for times of 2 hr to 2 days and studied metallographically and by electron microprobe analysis. The ternary alloy/nickel couples developed structures that exhibited complex, morphological evolution with time; single phase bands and highly orientation-dependent nonplanar morphologies and nonisolated precipitates observed initially in the diffusion zone gradually transformed to two-phase regions with isolated precipitates. Only minor structural changes in the diffusion zone were displayed by the ternary alloy/copper couples.     

An analysis of interdiffusion in finite-geometry,two-phase diffusion couples in the Ni−W and Ag−Cu systems

The progress of homogenization in finite, multilayer diffusion couples was studied experimentally using both quantitative metallography to measure phase thicknesses and electron microprobe analysis to determine concentration-distance profiles. Ni?W couples having mean compositions in the nickel-rich terminal solid solution range (12.1 and 15.2 at. pct W) were studied after 4.43 to 240.7 hr interdiffusion treatments at 1156° and 1207°C. Ag?Cu couples having mean compositions in both of the terminal solid solution ranges (8.5 at. pct Cu and 2.1 at. pct Ag) were studied after 3.8 to 65.4 hr interdiffusion treatments at 760°C. Experimental data were in good agreement with calculations of interdiffusion based on equilibrium interface concentrations and concentration-independent interdiffusion coefficients. The good agreement between experimental data and calculated values for the Ni?W couples extended to times necessary for the achievement of essentially complete homogenization,i.e., through both the two-phase dissolution process and the subsequent stage of gradient elimination in the terminal solid solution phase.