Formation Of Guinier-Preston Zones Research Articles

On the theory of solute clustering in binary alloys

Pseudopotential theory is used to describe the chemical interaction between solute atoms in binary alloys. The binding energies of clusters containing small numbers of solute atoms in aluminium are calculated using this approach. Preferred cluster geometries are predicted in agreement with observed Guinier-Preston zone formation in AlCu and AlZn.

The resistivity maximum during Guinier-Preston zone formation in Al-Zn alloys

Abstract In order to determine experimentally the relaxation time of the conduction electron due to G.P. zone scattering, changes of the Hall coefficient and the electrical resistivity during ageing at 20°C have been measured at 4–2 and 77°K, and in addition, X-ray small-angle scattering measurements were carried out at low temperature. By employing a rigid two-band model, the partial resistivity due to G.P. zone scattering was found to have a maximum during ageing. However, taking account of the log normal size distribution, the scattering power of an individual particle of G.P. zones was shown to increase monotonically with increasing zone size.

The influence of magnesium impurity atoms on the annealing kinetics of low-temperature neutron-irradiated aluminum

Abstract The influencc of magnesium atoms in stages II and III of aluminum are investigated from the recovery of the residual electrical resistivity of aluminum and a series of aluminum-magnesium alloys (nominally 0.02 –0.7 at %) at a total neutron dose of 9.1018 n/cm2 at liquid nitrogen temperature. Isochronal annealing studies were performed for a temperature range 78 to 350°K with different heating rates (0.5 deg/min for 78-235°K and 1 deg/min for 245-345°K). Isothermal annealing for 215°K was carried out for a total period of 136 min. It was found that the annealing of stage III occurs by a second order process with an activation energy E=0.602 ± 0.03 eV for pure aluminum and the aluminum alloys. The consistency of this value with that obtained from quenching experiments is in favor of the vacancy motion model for the interpretation of the mechanism involved in stage III for aluminum and dilute aluminum alloys. Substages in both stage II (127 and 145°K) and stage III (190 and 245°K) are apparent. Substages of stage II are interpreted as being due to the break up of interstitials from impurity atoms. An interesting feature is the substage at the limits of stages III and IV, which appears in the alloys and increases with concentration in magnesium atoms. A mechanism consisting of the trapping of vacancy-type defects during their migation in stage III can explain the experimental results. A reverse electrical resistivity recovery, was observed at the high temperature limits of stage III and was attributcd to the formation of Guinier-Preston zones.

Formation of second phase particles in aluminum-copper alloy films

The formation of second phase particles in Al+3% Cu alloy films was examined in order to find differences of the precipitation in thin films from that occurring in bulk alloys. Compared to bulk alloys in the thin film material the formation of Guinier-Preston zones is retarded. In well quenched thin films 60 h at 100°C or 6 h at 130°C are necessary to produce G.P. zones. θ′ particles are nucleated after approximately 100 h at 120°C without formation of θ″. The stable θ-Al 2Cu precipitate was found at temperatures as low as 150°C on the film surface. The results show that excess vacancies cannot be retained in thin films and that the film surfaces provide preferential nucleation sites for the equilibrium precipitate θ-CuAl 2.

Altering the time cycle of heat treatment by quenched-ln vacancies

The role of quenched-in vacancies in altering the time cycle of heat treatment is best exemplified by the process of formation of Guinier-Preston zones. Phase decomposition can be dramatically accelerated by enhanced solute diffusion which results from the high vacancy supersaturation quenched-in attendant to heat treatment. A review is presented of this field from the earliest work through recent developments. Important contributions, principally of a conceptual nature, have come about from quenching studies on pure metals. Impurity additions to these pure metals, as well as to high-purity binary alloys, have led to an understanding of the vacancy-solute binding energy and to the way in which this binding can affect zone formation. The interrelated diffusion of vacancies and of vacancy-solute pairs introduces coupled processes which are complex and need to be considered in a thorough analysis of the problem. Recent theoretical and experimental approaches are examined.

On the nature of the resistivity maximum during guinier-preston zone formation

An experimental investigation has been made of Labusch's calculations for the resistivity maximum during G.P. zone formation. Simultaneous measurements on the electrical resistivity and Hall coefficient indicate that the one-band model upon which he based his work is inadequate. This implies that changes in the positive carrier mobility resulting from Umklapp processes alone cannot account for the resistivity behavior. The data obtained is in qualitative agreement with the two-band model, but does not rule out changes in resistivity and Hall coefficient from this type of scattering. It indicates however that if critical scattering involving Umklapp processes is indeed occurring, it is not sufficient by itself to account for all of the data.

The formation and reversion of guinier-preston zones in an aluminium-6.7 at. % zinc alloy and the effects of small concentrations of magnesium and silver

The X-ray small angle scattering method, measurements of electrical resistance and transmission electron microscopy have revealed the behaviour of G.P. zones on the aging, reversion, and re-aging process in Al-6.7 at.% Zn alloys with or without trace elements of Mg and Ag. The critical zone radius associated with the electrical resistance maximum has been observed to be 9–10 Å on the aging process at room temperature and 10–15 Å on the re-aging process at 80°C after reversion. Addition of Mg atoms retards the rate of the formation of G.P. zones, while in the Ag containing alloys two kinds of zones, either including Ag atoms or not, were observed. G.P. zones in the Al-Zn alloys, which contain trace elements such as Mg or Ag or were pre-aged for longer times, are so stable that the time for completion of reversion takes longer. A change of the shape of the G.P. zones during these processes was observed to occur in the region of a larger zone radius.

On the anomalous electrical resistance of G.P. zones

Theories proposed to explain the increase of electrical resistivity in the early stages of Guinier-Preston zone formation, are discussed, none of the theories are able to explain all the experimental results now available. A new theory is proposed based on an analogy with optical diffraction phenomena. The scattering for zones of critical size is then calculated for aluminium and copper base alloys. The results indicate that the theory can explain the observed anomaly.

A model for the growth of Guinier-Preston zones-the vacancy pump

Vacancies highly in excess of equilibrium are thought to be responsible for the high rates of formation of Guinier-Preston zones (G.P. zones) after the quench from high temperatures. A kinetic model is developed for the growth of spherical G.P. zones based on the diffusion of solute-vacancy complexes to the zones. Under certain favorable physical conditions, when the complex arrives at the zone-matrix interface, the solute and vacancy are no longer bound; and the vacancy can now contribute to enhancing a vacancy concentration gradient directed radially outward from the zone. The vacancies are thus driven back into the matrix to again collide with solute atoms, once more becoming complexes. In this way the zone acts as a vacancy pump, maintaining a high vacancy concentration for long periods of time. Mathematical equations are derived based on this model, and kinetic data for Al-rich Zn is considered. A rather good comparison with experiment is found for early in the zone-forming process.

The effects of irradiation on the formation of guinier-preston zones

Resistivity measurements and small-angle scattering of X-rays were employed to study the effects of (10.5 MeV.) deuteron irradiation on the formation of spherical coherent Guinier-Preston zones (G.P. zones) in Al-5.3 at. % Zn. Specimens were irradiated at 77°K after different thermal treatments: asquenched from 300°C, as-reverted from 200°C, and as-reaged. Irradiation of the specimen quenched from 300°C has the effect of slightly retarding the rate of zone formation. For specimens reverted at 200°C and then irradiated with 0.5 to 1.9 × 10 15 deuterons/ cm 2, the initial part of the reaction is accelerated, the rate of zone formation increasing with dose. The later part of the reaction is not greatly influenced, the final zone sizes, as determined using small-angle scattering of X-rays, are only slightly larger. Irradiation of an alloy containing zones (as formed on reaging) does not result in enhanced growth. It is concluded that vacancies as generated by irradiation with deuterons can aid G.P. zone formation, hut the defect distribution as obtained by quenching is different from that resulting from irradiation. For the case of the present alloy, deuteron bombardment does not give rise to enhanced nucleation.

Electron-metallographic studies of precipitation in an aluminium-zinc-magnesium alloy

The mechanism of sub-microscopic precipitation in an Al-Zn-Mg alloy selected for its maximum response to ageing has been studied by a standardized oxide-replica technique in a 100 kV. Philips Electron Microscope. Contrary to earlier conclusions, examination of the oxide replicas has been shown to reveal details of the precipitation process almost as clearly as the thin-foil transmission technique. The reported formation of spherical Guinier-Preston zones followed by the development of a Widmanstaetten pattern of precipitated platelets has been confirmed. The zones have, however, been shown to grow into the platelets and not to dissolve in the matrix as reported earlier. The precipitation process has been correlated with the Hardness/Ageing Time curve and the structure of the precipitates has also been discussed.

Structural Changes During the Aging in An Al-Mg-Zn Alloy

The structural changes during age hardening were investigated in a high purity aluminum alloy containing 6 pet Zn and 2 pet Mg by means of X-ray diffraction and dilatometry. It was found that when aging at low temperatures (up to 200°C) the precipitate which formed was MgZn2; above 300°C, (AlZn)49Mg32; at intermediate temperatures both phases formed. The precipitation at low temperatures started with the formation of Guinier-Preston zones in the (111) planes, followed by the formation of a transition lattice very similar to MgZn2 but with a = 4.96 and c = 8.68; this was replaced eventually by MgZn2. The dilatometric work confirmed the X-ray results; the length changes determined experimentally coincided with the changes calculated for the foregoing transformations.

Nucleation of a solid precipitate along a dislocation

Models for nucleation along a dislocation are described. It is assumed that diffusion takes place via vacant lattice sites. It is shown that different mobilities and different sizes of the atoms in the alloy give rise to different possibilities of growth of the nuclei. Sometimes growth is very unlikely. Under certain circumstances formation of flat Guinier-Preston zones can be expected.