Solute Atom Clusters Research Articles

Pressure effect on elastic after-effect due to oxygen in niobium—II. Clustering

A theory is proposed to derive the binding and formation volumes of solute atom clusters from the pressure effect on multiple relaxation processes, found in an elastic after-effect study. The theory is applied to the Nb-0.70 at.%O system previously studied. A small binding volume of 0.2 ± 0.2cm 3/mol per O atom is obtained for all kinds of the O clusters present. The formation volume of single O atoms is calculated to be 3.85 ± 0.1 cm 3/mol by combining the binding volume and the X-ray data of Taylor and Doyle.

A positron study of precipitation phenomena in AlGe and AlSi alloys

The nucleation of precipitates and the precipitation process are studied in AlGe (0.2 and 1.2 at.%) and AlSi (0.5 at.%) by positron lifetime and angular correlation measurements during post-quench isochronal annealing. In the as-quenched samples small clusters of vacancies and solute atoms are observed. The vacancies disappear during first annealing steps, thereafter positrons are localized at incoherent Ge and Si particle-matrix interfaces. In AlGe (1.2 at.%) and in AlSi (0.5 at.%) above 150°C an increasing number of positrons are trapped by precipitates. Above 250°C the coarsening and dissolution of precipitates is detected. The density of Ge or Si particles estimated from the positron trapping rate agrees well with earlier SAXS and TEM results.

111In-Pd interactions in defect cascades in AgPd and CuPd

AgPd and CuPd alloys, containing solute concentrations of 1 at.% or less, have been implanted with radioactive 111 In atoms below and above the temperature of stage III recovery in order to create defect cascades and to study their influence on solute atom clustering. The experiments show enhanced clustering at the projectile atom as soon as vacancies become mobile, whereas at 100 K the clustering just corresponds to that expected for a random distribution of Pd atoms.

A lattice imaging study of nitrogen precipitation in α-iron and Fe-2.17 at.% Mn

Abstract Lattice imaging provides direct evidence to support the existence of solute-atom clusters on {001} matrix planes in α-iron. Nitrogen and manganese-nitrogen atom clusters (GP zones) are shown to precede the formation of the intermediate precipitates α″-Fe16N2 and η-Mn3N2 in nitrogen-ferrite and an Fe-2·17 at.% Mn-1.3 at.% N alloy, respectively. The manganese-nitrogen zones form as flat platelets analogous to those observed in Al-Cu alloys. However, nitrogen GP zones form with a complex undulating platelet morphology which is retained during the continuous transition to the α″-Fe16N2 precipitate.

Al-4.5 at%Zn合金の時効過程とその計算機シミュレーション実験

Changes in electrical resistivities of Al-4.5 at%Zn alloys in an initial stage of aging were measured and compared with results obtained from simulation experiments. There was a qualitative agreement in both results. Then the rate equation of growth of G.P.zones based on the vacancy pump model derived by Girifalo and Herman was examined with the simulation experiments, and its validity was confirmed. Furthermore, the scaling laws for size distributions of clusters of solute atoms and the structure functions for the spatial configurations of the solute atoms in Al-Zn alloys were also satisfied in the simulation experiments.

On the origin of the first peak of acoustic emission in 7075 aluminium alloy

The origin of acoustic emission (AE) in 7075 aluminium alloy was investigated in tension and compression tests. It is suggested that the first peak associated with the root-mean-square (rms) AE plot against time (or strain) is due to the unpinning of dislocation segments from solute atom clusters and is not due to the fracture of particles. Alteration in the value of the AE first peak and also in electrical resistivity values due to heat treatment of this alloy support the proposed origin of the AE first peak near yield.

Thermoelectric power vs. electrical conductivity plot for strain-ageing of low-carbon aluminium-killed steel.

The thermoelectric power vs. electrical conductivity plot enables us to separate an isothermal ageing process into distinct stages and to evaluate relative magnitude of the lattice strains caused by clustering of solute atoms or the coherency strains induced along precipitate-matrix interfaces. It is suggested that, under the influence of dislocations, the lattice strains caused by clustering of carbon atoms are relaxed, the coherency strains induced around e-carbide particles remain almost unchanged, and those around cementite particles are slightly enhanced. Prestraining before ageing promotes clustering of carbon atoms in place of the precipitation of e-carbide in the early stages of ageing at medium temperatures.

Electrical resistivity and thermoelectric power during clustering in aluminium solid solutions

The resistivity behaviour during the first stages of precipitation in solid solutions is commonly used and has been recently conveniently explained; on the other hand, only a few experimental results concern the thermoelectric power evolution during such a phenomenon; no theoretical approach has been developed. In the present work, we calculate the electrical resistivity and the thermoelectric power of an aluminium solid solution, either perfectly disordered or containing small clusters of solute atoms. The calculations are made with free electrons and taking into account the intra-cluster interference effect; only the variations of the diffusion part of the thermopower with the relaxation time are considered. It is shown that the clustering leads to a resistivity increase δρ, and to a thermoelectric power decrease ΔS, for solute atoms with a positive contribution; a linear relation between δρ/ρ and ΔS is predicted as long as the clusters contain less than a few solute atoms. Experiments are carried out for Al-Zn alloys near equilibrium or quasi-equilibrium conditions; a good agreement is obtained with theoretical results.

PECULIARITIES EXHIBITED BY ALUMINIUM-SILVER ALLOY IN THE PROCESS OF FATIGUE LOADING

Torsional fatigue experiments were performed with Al-7.27% Ag alloy in the quenched state. The △E-N curves were determined under various torsion strains and metallographie observations were made on the specimen surface after various numbers of stress cycles.Experimental results showed that, when the torsion strain is small, △E was found to decrease with the increase of stress cycle N. The change of the △E-N curves is similar to those previously reported for Al-Cu and Al-Mg alloys under lower torsion strains. However, when the torsion strain is large, AE decreases slightly at the beginning, increases subsequently to some higher value, and drops again until the specimen is fractured. The shape of the △E-N curves is very different from those of Al-Cu and Al-Mg alloys at same strains. The metallographie change of the specimen surface during cyclic deformation was found to be divided into two distinct stages. In the initial period of fatigue loading, fine straight slip lines are homogeneously distributed within certain grains. However, some of them suddenly become highly concentrated and prominent after a certain number of cycles. With increasing fatigue cycles, newly generated localized slip bands occur successively between the existing ones. This observation is markedly different from that observed on pure aluminiun and the quenched Al-Mg alloys, where only the existing slip regions appeared to become broader with the increase of cyclic straning. In addition, fatigue cracks were frequently observed to initiate and propagate along grain boundaries.The results obtained was discussed in terms of the electrical interaction of silver atoms with dislocation and the cutting effect of the to-and-fro moving dislocations though spherical clusters of silver solute atoms.

Modulated substitutional-interstitial solute–atom clustering in nitrided austenitic Fe–34Ni–V–alloys

The microstructure of Fe–34–Ni–V alloys internally nitrided in NH3/H2at 500–900°C is investigated by electron microscopy and X-ray methods. At low solute supersaturations and high temperatures (above 780°C) homogeneous precipitation of the equilibrium face-centred cubic nitride VN occurs in austenite. However, above a critical supersaturation determined by the nitrogen activity (aN), the vanadium activity (av), and the temperature, a very fine modulated microstructure consisting of meta­ stable substitutional-interstitial solute–atom clusters, periodically dis­tributed along <100>, is developed. On overageing, the modulated state transforms either directly to spheroidal VN precipitates in nitrogen-saturated austenite or via a grain boundary discontinuous precipitate reaction to thin laths of VN in austenite. The metastable modulated state is characterized by (i) periodic electron microscopy images, (ii) sidebands in both electron and X-ray diffraction patterns and, (iii) unit-cell dimen­sions increased with respect to those of the equilibrium Fe–34Ni–N austenite. The effects of the thermodynamic factorsaNandavand of temperature on the development and overageing behaviour of the modulated structures are described and discussed. It is concluded that the mechanism by which the periodic substitutional-interstitial solute–atom clusters develop in nitrided austenite is consistent with spinodal decomposition.

Solute atom clustering during ageing an aluminium–magnesium–silicon solid solution under stress

A permanent increase of stress, Δσ, is found on the stress-strain curve after straining the quenched AlMgSi alloy at 25 °C. Ageing it under stress for a time at 25 °C (or 52) and straining again at 25 °C, Δσ is measured as a function of ageing time. It is concluded that the solute atom clustering to dislocations is responsible for Δσ, and that the „time dependent diffusion coefficient”︁ of the solute via the decay of quenched in vacancies, determines the rate of the process of Δσ change. Ein permanenter Anstieg der Spannung Δσ auf der SpannungsDehnungskurve wird nach Belastung getemperter AlMgSi-Legierungen bei 25 °C gefunden. Nach Alterung unter Spannung für eine gewisse Zeit bei 25 °C (oder 52) und weiterer Deformation bei 25 °C wird Δσ als Funktion der Alterungszeit gemessen. Es wird geschlossen, daß der „zeitabhängige Diffusionskoeffizient”︁ der gelösten Atome die Prozeßgeschwindigkeit der Δσ-Änderung über das Abklingen der eingetemperten Leerstellen bestimmt.

Tetragonality of Fe-Ni-Ti Martensite

Precise X-ray 200, 020 and 002 peak shapes and positions are reported for martensite formed from austenite single crystals containing 2, 4.5 and 6.5 at.% Ti with about 25% Ni. The parent austenite has been rapidly quenched and preaged at 710°C before transformation to martensite and the distribution of intensity in and near the austenite peaks has been measured. In all alloys briefly preaged austenite forms tetragonal martensite with fairly sharp X-ray peaks. Martensite tetragonality increases rapidly with preaging time. In 4.5 and 6.5 at.% Ti alloys even longer preaging treatments produce austenite with satellite X-ray peaks. Martensite subsequently formed from such austenites exhibits a superposition of the fairly sharp peak previously described and a greatly broadened Bragg peak which is positioned to signify reduced tetragonality. With increased preaging time, austenite satellites grow in intensity and the broad martensite peaks grow at the expense of the narrow peaks; these broad peaks sharpen slightly and shift toward still lower tetragonality. In the 2 at.% Ti alloy even extensive preaging failed to produce satellite peaks in the austenite and subsequently formed martensite failed to exhibit broad peaks. These results mean that the very small solute atom clusters first formed are transformed coherently from austenite to martensite and serve as defects which distort the lattice to produce significant martensite tetragonality and probably also significant strain energy in the martensite. In contrast continued aging of the richer Ti austenites produces larger clusters which are sufficiently different in atomic scattering factor of lattice parameter to produce X-ray satellite peaks from austenite. The larger clusters become incoherent during martensite formation, cause defects in the martensite, and lose the cooperative elastic strain which supports tetragonality. The clusters are hypothesized to be Ni 3Ti and the formation of the smaller clusters is modeled to explain the lattice strains and to describe the kinetics of the initial stage of their formation.

Creep of titanium-silicon alloys

Operative creep mechanisms in laboratory melts of Ti-5Zr-0.5Si and Ti-5Al-5Zr-0.5Si have been investigated as a function of microstructure, creep stress, and temperature. From creep rate data and transmission electron microscopy results, it has been shown that an important creep strengthening mechanism at 811 K in Si bearing Ti alloys is clustering of solute atoms on dislocations. All of the alloys investigated showed anomalously high apparent activation energies and areas for creep, and a high exponent (n) in the Dorn equation. In addition, the effect of heat treatment was investigated and it is shown that the highest creep strength was obtained by using a heat treatment which retained the maximum amount of silicon in solution. This is consistent with the proposed creep strengthening mechanism. An investigation of the creep behavior of several other Si containing alloys including two commercial alloys, Ti-11 and IMI-685 indicated similar results.

Further Studies of Dislocation-Loop Annealing in Aluminium-Base Alloys

Abstract The detailed annealing behaviour of large faulted dislocation loops in aluminium and aluminium-base alloys has been studied by quantitative electron microscopy. Changes in loop shrinkage rates, loop character and dislocation pinning have been observed and used to derive information on solute clustering and vacancy/solute interactions. Although generally small, differences between the measured shrinkage rates of loops in pure Al and A1-base alloys were found to be dependent upon the state of solute dispersion. The results are consistent with a model in which extensive clustering occurs during quenching and ageing, and the differences in apparent activation energies are attributed to binding between vacancies and solute atom clusters, or zones. The apparent discrepancy between the large solute atom/vacancy binding energies measured in quenching studies and the much smaller ones measured in equilibrium studies is thus partially explained.

Superparamagnetic hall effect in some very dilute kondo systems

The Hall resistivity of polycrystalline samples of Au Fe, Ag Mn, Cu Fe and Cu Mn shows to varying degrees a contribution which at 4.2°K saturates in a few kG. This is thought to arise from electrons moving in superparamagnetic clusters of solute atoms. The effect is largest in A uFe where the data can be fitted to a Langevin function to give an estimate of 14 atoms per cluster.

The continuous cooling transformation behaviour of zirconium-niobium-oxygen alloys

The continuous cooling transformation behaviour of zirconium alloys containing up to 5 at% niobium and 3 at% (5300 ppm) oxygen has been examined using thermal analysis and metallographic techniques. Two types of reactions were found; nucleation and growth reactions which showed the familiar C-curve kinetics, and the athermal martensite reaction. The nucleation and growth reactions were identified as the formation of α-zirconium at the β-grain boundaries and the transformation βzr → αZr+βNb throughout the grains. Increasing niobium content lowered the temperature and increased the time required to obtain the thermal arrests. Increasing oxygen raised the temperature of both reactions and caused grain boundary nucleation to occur sooner and nucleation within the grains to occur later. The martensite start temperature was not affected by oxygen but decreased linearly with niobium content. Clustering of solute atoms within the β-phase has been suggested by other workers. The results show that this must be an equilibrium condition. Clusters will not disperse with increased solution time as previously proposed.

Application of Small-Angle X-Ray Scattering to the Study of Problems in Physical Metallurgy

Small-Angle X-Ray Scattering (SAS) has wide application in the field of macromolecules, colloidal solutions etc., while its application to physical metallurgy is restricted. But the smaller particles than 100Å in metals and alloys can be analysed better by the SAS method than by electron microscopy. It was Guinier who first introduced SAS techniques to this field. He had studied the structure of small heterogeneities, named Guinier-Preston Zones. They have the same crystallographic structure as the matrix solid solutions. But it is clear from the theory of SAS that this method can be applied to detect the heterogeneities of atoms in the non-crystalline materials, as glass materials.The advantage in the application of SAS is the fact that lattice strains cause only minor changes of the diffracted intensity.In this summarizing report, it is intended to describe what can be clarified by the SAS method. The determination methods of average particle size, particle number, solute concentrations inside and outside particles, size distribution, shape, the concentration gradient at the particle surface and spacial distribution of particles are described with the theoretical background and some examples.SAS has also been applied to the analysis of lattice defects, as agglomeration of vacancies and dislocations. The scattering intensities with these defects are much weaker than with clusters of solute atoms. As the undesired diffraction effects, double Bragg scattering, incoherent Compton scattering, temperature scattering and fluorescent radiation must be considerd in the quantitative analysis.

Influence of Some Additional Elements on the Aging Kinetics in Al–Cu and Al–Cu–Mg Alloys

Aging kinetics in Al-1.9 at%Cu and Al-1.9 at%Cu-1.7 at%Mg alloys containing small amounts of Zr, Cr, Mn, Ag and Cd have been investigated by the measurements of electrical resistivity and hardness and also by electron microscope observations.The addition of Zr, Cr or Mn retards the clustering of solute atoms in Al–Cu and Al–Cu–Mg alloys and then reduces the age-hardening when these elements are finely distributed as the insoluble compounds. This effect can be reasonably explained by a similar mechanism to that of Al–Zn and Al–Zn–Mg alloys, that is, in terms of the increase in vacancy sinks rather than the decrease in supersaturated solute atoms or the existence of binding energy between a solute atom and a vacancy.The aging kinetics in Al–Cu alloys are little affected by the addition of Ag, despite the well-known fact that Cd reduces remarkably the rate of G. P. zone formation and stimulate the nucleation of intermediate precipitates. This is probably due to the reason that the clustering of Cu atoms takes place on the {100} matrix plane, whereas Ag atoms cluster on the {111} plane and do not participate in the G. P. zones of Cu atoms. The additions of these elements to Al–Cu–Mg alloys decrease the rate of clustering but increase the age-hardening at high temperature. This may be due to the participation of Ag or Cd atoms in G. P. zones of Cu and Mg atoms.

On the Aging Characteristics of Copper-2 wt% Beryllium Alloys with or without Additional Elements

An investigation of the effects of quenching and aging temperatures and of additional elements on the aging characteristics of copper-2 wt% beryllium alloys has been carried out mainly by means of electrical resistance measurements. It has been found that appreciable clustering of solute atoms occurs during quenching and the rate of pre-precipitation during isothermal aging increases greatly with increasing quenching or aging temperature. It has also been observed that the increase in resistivity decreases with increasing aging temperature. The activation energy for the zone formation in the high purity binary alloy after quenching from 800°C is estimated to be 1.1 eV. The effect of additional elements on the zone formation process can be summarized as follows; magnesium and zinc accelerate zone formation, whereas cobalt and iron retard it. The results are interpreted in terms of a rather simple picture of the pre-precipitation phenomenon. The role of vacancy appears to be a fundamental one in determining the rate of zone formation. The variations of the maximum increase in resistivity against the various aging temperatures have been explained in terms of the variations of the number of zones formed. Other related problems such as the reversion and effects of additional elements are also discussed.

Electronic origin of clustering in Al-Mg alloys

The electron charge distribution surrounding a magnesium solute atom in aluminum is computed. The sign of the charge distribution at near-neighbor and next-nearest-neighbor sites to the magnesium solute results in an electrostatic binding energy between magnesium atoms. This result is interpreted as being the origin of solute atom clustering in this alloy system. The electron scattering cross section of a magnesium solute atom is computed by the method of partial waves and is found to be about 1.1 × 10 −16 cm 2.