Al-Ge Alloys Research Articles

Positron annihilation study of quenched-in defects in dilute AlGe and AlSi alloys

Positron annihilation study of quenched-in defects in dilute AlGe and AlSi alloys

Amorphisation of ge precipitates in an al-ge alloy by electron bombardment

Abstract The results of a TEM study of the effects of electron irradiation on the structure of crystalline Ge precipitates in an Al-Ge alloy are reported. The electron irradiations were performed at−130°C in a HVEM, and dynamic observations on the stability of Ge precipitates were made. A gradual decrease in Ge diffraction spot intensity was observed which recovered on warming at about−30°C provided a dose of 1 × 1023 e/cm2 was not exceeded. The precipitates were crystalline at room temperature. After higher doses, no recovery was seen during warming and the precipitates were amorphous at room temperature. This behaviour is the first reported example of amorphisation of Ge during electron irradiation and shows that amorphisation is possible without the occurrence of collision cascades. It is proposed that amorphisation is the result of a build-up in vacancy concentration within the precipitates which is promoted by the presence of the precipitate-matrix interface acting as a sink for mobile interstitials.

Microstructural Analysis of Rapid Solidification and Undercooling in the Al-Ge System

ABSTRACTSubmicron powders, amorphous films and melt spun ribbons of various Al-Ge alloys have been analyzed to determine the relative roles of undercooling and cooling rate in the production of non-equilibrium structures. All analyses were performed in transmission electron microscopes equipped with energy dispersive x-ray spectrometers. The submicron powders, produced by electro-hydrodynamic atomization, were analyzed in their as-received condition and then annealed and/or melted using the electron beam as a local heating source. Once molten, the liquid droplets were undercooled at different cooling rates by varying the rate of beam obstruction. In this manner, a number of different microstructures were produced. These included metastable crystalline phases and mixed amorphous/crystalline structures. By combining this technique with a microscope heating stage, it was possible to carry out controlled dynamic undercooling experiments and determine phase selection as a function of undercooling and composition. The amorphous films were rapidly heated with the electron beam in the microscope and metastable as well as stable phases were produced. The results of these complementary analyses will be compared and discussed with reference to current models and theories of rapid solidification.

Spatial Resolution of Stem and EDS in an AL-GE Alloy

As part of an effort to better understand beam broadening and its deleterious effect on the spatial resolution of x-ray microchemical analysis employing STEM and EDS, we have been studying apparent concentration profiles in the vicinity of germanium precipitates in an aluminum matrix. Because of the limited solubility of aluminum in germanium, we know that the precipitates are almost completely composed of Ge; accurate knowledge of the composition of such precipitates facilitates the modeling of the apparent concentration profiles via Monte Carlo trajectory simulations.

Initial stages of eutectic solidification

Nucleation phenomena near eutectic temperature were observed in situ by high voltage electron microscopy on AlGe alloys. Non reciprocal nucleation behaviour was observed. This property, commonly observed in metal semi-conductor systems, is explained by considerations of the relative values of the interfacial tensions involved during heterogeneous nucleation. Crystallographic analysis of the epitaxy between the eutectic structures nucleated and the primary crystals allowed us to introduce a nucleation scheme for the coupled growth based upon surface dendrites of the second phase. Moreover a simple evaluation of the coarsening rate of this fine structure obliges to the greatest carefulness when an attempt is made to link the microstructure observed at room temperature with what occurred actually during solidification.

Modification on near-surface precipitation in AlGE alloy by ion irradiation

A model including precipitate dissolution and solute segregation is used to explain the changes in precipitate distribution during 100 keV Al + ion irradiation of AlGe alloy. It is found by TEM that Ge precipitates in the peak damage region i.e. <100 nm from the surface are destroyed by prolonged irradiation to doses greater than 28 dpa, whereas precipitates near the tail of the damage profile i.e. 100–250 nm from the surface, continue to grow. SIMS has shown that the dissolution of near-surface precipitates is accompanied by segregation of Ge to depths below the damage peak, indicating that Ge segregates down defect gradients. The results show that solute segregation is an important factor in determining precipitate stability. It is thought that the reversal in precipitate behaviour from growth to dissolution occurs because the solute depletion in the matrix near the surface reduces the solute arrival rate at the precipitate sufficiently for recoil dissolution to determine the precipitate behaviour. A preliminary study on low temperature irradiation of AlGe shows that Ge solute re-distribution rate is limited by the concentration and mobility of vacancy-solute complexes, and not by the rate of precipitate dissolution.

Magnetoresistance measurements in Al-Ge alloys after neutron irradiation at 4.6 K

The influence of different structural defects, introduced by irradiation in a series of AlGe alloys is established by measuring the longitudinal magnetoresistance. The validity of Kohler's rule is absolute for both pure and alloyed samples that have been irradiated by neutrons at 4.6 K, while the rule breaks down after annealing at different temperatures. The origin of the breakdown of Kohler's rule appears to be the anisotropic scattering of the electrons on the clusters formed after annealing. This scattering depends on the size of the clusters which varies with the concentration of foreign atoms.

Possible Mechanism of Superconductivity in Metal-Semiconductor Eutectic Alloys

The effect due to the tunneling of conduction electrons into the negative-$U$ centers at the disordered metal-semiconductor interfaces on the superconductivity of metal-semiconductor eutectic alloys is considered. Gorkov's formalism is used to calculate ${T}_{c}$ for (i) ${n}_{I}\ensuremath{\ll}1$ and $\ensuremath{\lambda}\ensuremath{\ne}0$, (ii) $\ensuremath{\lambda}=0$. Where ${n}_{I}$ is the concentration of the pairing centers and $\ensuremath{\lambda}$ is the phonon-mediated electron-electron interaction in BCS theory. Our results can qualitatively explain the increase of ${T}_{c}$ in Al-Si, Al-Ge, and Be-Si eutectic alloys.

Ostwald ripening of Ge precipitates in Al-3.46wt.% Ge alloy

The coarsening behavior of Ge precipitates in the matrix and near the precipitate-free zone (P.F.Z.) along the grain boundary in an Al-3.46wt% Ge alloy was electron microscopically investigated. The coarsening data of the Al-Ge alloy aged at 523 and 573K were analyzed on the basis of the Lifshitz-Wagner theory on diffusion controlled coarsening. The coarsening kinetics of the Ge precipitates in the matrix follows the time-law predictions of the thoery. However, pronounced departure from t1/3 kinetics is found near P.F.Z. The number density of the Ge precipitates near P.F.Z. and the width of P.F.Z. were measured as a function of aging time. The particle-matrix interfacial free energy was determined.

Determination of the effective size of germanium particles in an Al-Ge alloy with small and wide-angle X-ray methods

Determination of the effective size of germanium particles in an Al-Ge alloy with small and wide-angle X-ray methods

An improved method for fabrication of Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;(Al,Ge) wire

Technologies for fabrication of Nb 3 (Al .75 Ge .25 ) wire by reaction of mechanically fabricated composites of Nb, Al and Ge have had very limited success due to the lack of plasticity of elemental Ge, which leads to gross compositional inhomogeneity and low transition temperatures. However, it is possible to develop considerable ductility in Al-Ge alloys, and by using such alloys composited with Nb, composites may be mechanically fabricated which, when reacted in wire form, have transition temperatures ca. 20°K. Due to the nature of the ternary phase diagram, such transition temperatures may be achieved for all core compositions between Al- 14% Ge and Al-30% Ge. However, high reaction temperatures, ca. 1800°C, are necessary, which suggests low-temperature stability problems with the A15 phase which are similar to those in the binary Nb-Al system.

An X-ray study by Guinier diffraction camera of Al-Ge alloys rapidly quenched from the melt

Two Al-Ge alloys (6 and 30 at. % Ge) were levitation-melted and quenched in the rotary splat-quencher designed at Sussex University. X-ray diffraction patterns were obtained with crystal-monochromatized CuKα 1 in a precision Guinier camera, and further patterns were obtained after heating to various temperatures in a differential scanning calorimeter; the DSC was used to indicate when one of the metastable phases had completed its transformation. It was clear that there was more than one metastable phase in the concentrated (eutectic) alloy, possibly only one in the dilute alloy. Most of the lines matched those of the patterns reported by Raoet al. and by Koster; the agreement with Koster is very close, both as to lattice spacings and as to groupings of lines belonging to the different phases. Results published by other workers are completely at variance with ours. Koster's attribution of phases is confirmed by this study. (A recently developed iterative computer program was applied in an attempt to interpret the X-ray patternsab initio, but was not found useful.) This study illustrates the difficulty of identifying metastable phases, especially when more than one is present, and points to the importance of using a high-precision X-ray diffraction technique.

Identification of metastable phases in splat-quenched aluminium-germanium alloys

Two AlGe alloys (6 at.% and 30 at.% Ge) were levitation-melted and quenched in the rotary splat-quencher designed at Sussex University. X-ray diffraction patterns were obtained with crystal-monochromatised CuKα in a precision Guinier camera, and further patterns were obtained after heating to various temperatures in a differential scanning calorimeter; the DSC was used to indicate when one of the metastable phases had completed its transformation. It was clear that there was more than one metastable phase in the concentrated (eutectic) alloy and possibly only one in the dilute alloy. Most of the lines matched those of the patterns reported by Rao et al. and by Köster, though these interpreted their patterns quite differently. Results published by other workers are completely at variance with ours. This study illustrates the difficulty of identifying metastable phases, especially when more than one is present.

Influence of gallium and germanium additions on the recovery stages in neutron irradiated aluminum

Abstract Different dilute Al-Ge and Al-Ga alloys, of Ge and Ga from 200 to 20,000 ppm, have been irradiated in the reactor at helium temperature. The total neutron dose was 1.5 × 1018 n/cm2. The recovery curves of electrical resistivity of the Al-Ga alloys are similar to the Al-Ge alloys. When the concentration of Ge and Ga increases, the high temperature part of Stage I (ID and IE) decreases. This decrease should be associated with the trapping of the mobile interstitials by foreign atoms. Stage II is also modified by the Ge and Ga additions. In the substage above 100°K, which does not exist in pure aluminum, several new peaks appear which can be attributed to the dissociation of interstitial impurity complexes. Stage III is represented as a complex structure on the high temperature side, in the most concentrated alloys.

The dependence of the phonon-resistivity on the residual resistivity in the Al-Ge and Al-Ga alloys

The electrical resistivity of aluminum based alloys containing s-p (Ge, Ga) impurities has been measured at 15, 20, 35 and 50 K. It was found that the temperature-dependent part of the resistivity ϱ( T) - ϱ 0 has a logarithmic dependence on ϱ 0 and is independent of the kind of impurities.

Annealing study of gold, zinc and germanium alloyed aluminum neutron-irradiated at liquid nitrogen temperature

Abstract A study of the recovery of the electrical resistivity of the dilute aluminum alloys Al - Au (0.02–0.25 at % Au ), Al - Zn (0.05–1.0 at % Zn ) and Al - Ge (0.02–0.5 at % Ge ) after neutron-irradiation at a total dose of 9 × 10 18 n / cm 2 at 78 K was undertaken. The annealing spectra of Al-Au and Al-Zn alloys showed two distinct substages in stage II, while in Al-Ge alloys a five-peak structure was apparent. At the low-temperature side of the main peak of stage III a shoulder is definitely apparent in the three alloy systems. After stage III, Al-Ge alloys appear to have two doublets and a reverse recovery resistivity, which can be attributed to the formation of Guinier-Preston zones. The break-up of interstitials from impurity atoms or their clusters and the trapping of vacancy-type defects during their migration in stage III, are the mechanisms which can explain stages II and III respectively. The activation energy for the main peak of stage III was in all cases found to be 0.60 eV, and the peak obeys second-order kinetics.

The temperature dependent part of the impurity resistivity in aluminium alloys with germanium

Values of the electrical resistivity of polycrystalline Al-Ge alloys with different solute concentrations have been measured from 4.2-320K. It is found experimentally that the phonon resistivity of the alloys in the temperature region 10-60K varies as T2<n<3.6. At intermediate temperatures the deviation from Matthiessen's rule passes through a maximum which shifts to higher temperatures with concentrations as c15/. At high enough temperatures the sign of the derivative delta Delta (T)/ delta T is negative. The results obtained experimentally are compared with theory.

The Metallography and Superconductivity of Niobium-Rich Nb–Al–Ge Alloys

Abstract Four alloys of nominal composition Nb x (Al0.75Ge0.25), where x = 3·8, 3·3, 2·9, and 2·4 were prepared by arc-melting. The microstructures of the alloys consisted of dendritic solid solution, A15 phase and two minor phases—one specifically rich in aluminium, the other in germanium. Superconducting transition-temperature measurements, micro-hardness tests, and electronprobe microanalyses were carried out on each alloy before and after heat-treatment at 1023 K. Transition temperatures for the onset of superconductivity increased from ∼ 19 to 20·00–20·34 K as a result of heat-treatment, while the aluminium and germanium content of the A15 phase increased slightly owing to the partial decomposition of the solute-rich phases. Despite this increase in solute content, the A15 phase of each alloy was found to be niobium-rich relative to the stoichiometric composition. For heat-treated alloys, relationships appear to exist between T c and A15-phase micro-hardness and, possibly, between T c and A15-phase e...

On the correlation between resistivity changes and formation of germanium nuclei in AlGe alloys

The formation of germanium nuclei in a supersaturated AlGe alloy at temperatures below 140 °C has been observed by small angle X-ray scattering and resistivity changes. The latter have been analyzed quantitatively using the X-ray results. The resistivity decreases by (1) trapping of free, quenched-in vacancies by germanium atoms and by (2) reduction of the solute content and the vacancy concentration when nuclei form. Both solute atoms and vacancies are needed to form Ge nuclei since Ge has a larger atomic volume than the Al matrix. Simultaneously, the resistivity is increased proportional to the total interface area of these nuclei. This interpretation leads to reasonable parameters and confirms the internal consistency of the atomic model. Die Keimbildung von Germanium in übersättigten AlGe-Mischkristallen bei Temperaturen unterhalb 140 °C wurde mittels Röntgen-Kleinwinkelstreuung und Widerstandsmessungen untersucht. Die Änderung des Widerstandes wurde mit Hilfe der Ergebnisse aus den Röntgenuntersuchungen analysiert. Danach nimmt der Widerstand ab durch (1) Einfang von freien Leerstellen durch Germanium-Atome und (2) durch Abnahme der Konzentration von Germanium und Leerstellen in der Matrix bei der Keimbildung. Beide Defekttypen werden zur Keimbildung von Germanium benötigt, da das Atomvolumen der Ausscheidung wesentlich größer ist als das der Matrix. Gleichzeitig mit der Widerstandsabnahme (2) tritt eine Erhöhung derselben proportional zur gesamten Oberfläche der Keime auf. Diese Interpretation führt zu vernünftigen Parametern und bestätigt das angenommene Modell.

Plasticity of AlGe single crystals containing small fractions of precipitates

The strengthening mechanism of an AlGe alloy containing small volume fractions of very small (∼ 13 Å) precipitates has been studied. The increase in strength due to the presence of the precipitates can be explained more reasonably by a dislocation cutting mechanism than by the Orowan mechanism.