Eduardo Donoso

Calorimetric study of the dissolution of Guinier-Preston zones and η′ phase in Al-4.5at.%Zn-1.75at.%Mg

Differential scanning calorimetry was used to study the energetics and the dissolution kinetics of Guinier-Preston zones and η′ phases in Al-4.5at.%Zn-1.75at.%Mg alloys aged for various times at 298 or 393 K. The results show that for samples aged at 298 K the dissolution enthalpies, which are proportional to the pre-existing particle volume fractions, increase with the aging time, whereas for samples aged at 393 K the ΔHr values exhibit a maximum after aging for 16 days. However, the peak and reversion temperatures always increase with the aging time regardless of the aging temperature. Using an integrated kinetic function, developed on the basis of a difusion-limited dissolution kinetic model under non-isothermal conditions, the calculated curves were fitted to the experimental data, allowing precipitate sizes to be computed. Furthermore, good correlation was observed between the strength of these alloys and the measured particle size.

Short-range ordering by excess and thermal vacancies during linear heating experiments in α-CuAl alloys

Abstract The ordering behaviour in quenched α-CuAl was investigated by differential scanning calorimetry (DSC) under increasing temperature conditions. It was found that the ordering processes can be better explained in terms of an homogeneous short-range order (SRO) model rather than a heterogeneous disperse order model as previously interpreted for the same types of experiment. The DSC traces indicate that the ordering takes place in two stages; stage 1 ordering at low temperatures is associated with the migration of excess vacancies and stage 2 ordering at high temperatures is by the migration of equilibrium vacancies. The relative dominance of each stage is influenced by the quenching temperature, the quench rate, the density of vacancy sinks and the sample shape before quenching. A relationship describing the overall SRO kinetics for both stages together in terms of either the reacted fraction, or the first SRO parameter was proposed. A method for evaluating boundary values for this parameter was developed, making use of the features displayed by the DSC thermograms. The mobility of vacancies, which controls their lifetime and short-range ordering rate, was evaluated by computing frequency factors and activation energies. The resulting values for activation energies controlling the short-range ordering rate were somewhat smaller than the effective values obtained for stage 1 from the DSC traces, suggesting that the presence of vacancy-solute complexes may be important especially as the aluminium concentration increases. From estimations of vacancy-solute and divacancy binding energies, it was inferred that divacancy formation is unlikely in the alloys under study. The process frequency factor values were in very good agreement with those calculated for an SRO state developed during two-stage ordering by a vacancy mechanism.

Fatigue crack propagation in a deformed Cu-9 wt. pct. Al alloy

Abstract Measurements of fatigue crack propagation rates in a 50% deformed Cu-9 Al wt.% alloy were studied as influenced by disperse order under a varied range of mean loading conditions. Although the ordering treatment does not influence the fatigue crack growth rates, the number of cycles required to attain failure is somewhat lower when order is present. Irrespective of the presence of disperse ordering, the alloy exhibits mean stress sensitivity at constant applied cyclic stress; FCP rates increase for higher mean loads. A model which combines the monotonic and cyclic stress-strain response with Elbers concept of effective stress intensity factor is adapted to give account of the observed FCP behavior of this alloy.

Ordered domain characterization in α-Cu-Al alloys from dissolution kinetics studies

Through microcalorimetric experiments, disperse order (DO) dissolution kinetics in preannealedα-Cu-Al alloys containing 19, 13 and 6.5 at % aluminium were adequately described by the integrated kinetic model function arising from the steady-state part of the diffusion field:f(y)=1 − (1−y) 2/3. Domain sizes after the annealing treatment, and also critical radii, were determined from differential scanning calorimetry data analysis at different heating rates. The existence of critical radii indicates that the disperse-order dissolution process is a first-order transition. Nevertheless, it was inferred that for very dilute alloys, only short-range order is present. After pre-annealing the alloys at different temperatures, volume fractions and domain concentrations were computed by employing the above kinetic model under high heating-rate conditions. On the basis of appropriate time constant and diffusion time calculations, the range of such temperatures compatible with equilibrium attainment was established. Prolonged pre-anneals alter the particle distribution, but do not influence either volume fractions or domain sizes. A semi-quantitative particle radius-particle concentration-temperature diagram was proposed forα-Cu-Al alloys.

Modelling the kinetics of solute segregation to partial dislocations in cold-rolled copper alloys

A model describing the kinetics of solute segregation to partial dislocations in cold-rolled copper alloys is proposed. The case when half-edge and half-screw dislocations are present is considered. The model covers the kinetic behaviour detected in the deformed Cu-5 at % Mn alloy where two processes were assessed. The faster process corresponds to segregation to screw while the slower one corresponds to segregation to edge dissociated dislocations. Effective activation energies, larger for edge dislocations, are close to that for pipe diffusion along the partials corrected by pinner binding energy terms.

Sudden mean stress variations during fatigue crack propagation in Cu-9 wt.% Al

Abstract Sudden mean stress variations were performed during fatigue crack propagation (FCP) in deformed Cu-9 wt.% Al. A rise in the mean stress level produces an instantaneous decrease in FCP rates followed by a continuous increase. A reduction in mean stress causes a large reduction in FCP rates. The observed behaviour is almost insensitive to the value of the stress intensity range at which the mean stress variation is made. Also an increase (decrease) in mean stress is associated with an increase (decrease) in static modes of fracture.

Microcalorimetry of plastic zones in a deformed disperse-ordered Cu−9 wt.% Al alloy during fatigue crack propagation

Abstract Measurements of fatigue crack propagation (FCP) rates in a 50% deformed Cu−9 wt.% Al alloy were studied in order to investigate the influence of disperse order. The ordering treatments were found not to affect crack growth rates. Microcalorimetric analysis of plastic zones during different stages of the process revealed that ordering destruction, defined by a parameter βy, decreases monotonically from a certain threshold value of the stress intensity amplitude. For low and medium crack growth velocities the insensitivity of FCP rates to disperse ordering is ascribed to a high degree of disordering. The onset of the decrease in order destruction is associated with the increasing tendency of the microstructure to become heterogeneous. At high growth rates it is suggested that the overall reversion process is minimized owing to the fact that plastic deformation concentrates mainly in the dimple walls. Microvoid coalescence appears to be largely unaffected by the presence of the ordered domains.

The influence of particle shape on the non-isothermal kinetics of precipitate dissolution

Abstract Based on the theory of precipitate dissolution kinetics proposed by Whelan, two expressions suitable for use under non-isothermal conditions were derived. The first one arises from the transient part of the diffusion field and it is found to be identical to that known as the three-dimensional diffusion kinetic law, being instead the reflection of a one-dimensional diffusion process. The second relationship which arises essentially from the steady-state part of the diffusion field, better describes a three-dimensional diffusion situation, being the corresponding kinetic model function in the integrated form: f(y)=1 -(1-y) 2 3 . The first solution was successfully applied for describing the dissolution behaviour of disc-shaped G.P. zones in 2219 aluminium under non-isothermal conditions, while the second was valid for description, under the same conditions, of the dissolution kinetics of spherical ordered domains in two αCuAl alloys.

Heating rate dependence of disordering in αCu-AI alloys

Microcalorimetric experiments in pre-annealed and furnace cooled αCu-Al alloys containing 19,13, and 6.5 at. pct aluminum were performed at different heating rates. The features displayed by the differential scanning calorimetric traces were studied with reference to the rate of equilibration of the processes associated to a disperse order model (volume fraction and particle number adjustment, and domain dissolution). Such a comparison indicates that the observed features harmonize quite well with this model. The disordering reactions were found to be first order transitions. Volume fractions and radii of the ordered particles as well as latent and molar heats of dissolution were calculated. The decrease of the critical temperatureTc with increasing Al content is explained through a combination of the lattice parameter and the quasichemical effects. The molar entropy of dissolution increases as the alloy becomes more concentrated, indicating that internal order within domains also increases correspondingly.

A differential scanning calorimetric study of precipitation in Cu2Be

Abstract Beryllium precipitation from the Cu-rich matrix in a Cu-2 wt% Be alloy homogenized and quenched from 1073 K was studied by differential scanning calorimetry (DSC). The DSC traces showed two main exothermic effects, A and B, each comprising two sub-effects A1, A2, and B1, B2 respectively. Effects A1 and A2 correspond to the precipitation of GP zones and subsequent overlapping and independent precipitation of γ″-phase. Only at very low heating rates can γ″ be inherited from GP zones. Effects B1 and B2 correspond to heat evolved during transitions to the states with γ′-and γ-phases respectively. Heat effect A can be quantitatively described in terms of solid solubilities before and after precipitation, and of precipitation heats of the phases involved. The heat content of the combined GP zone γ″- phase precipitation effect was proportional to the beryllium atoms precipitated, yielding an average value of 21 kJ mol−1 beryllium for beryllium precipitation. It was shown that γ′-phase arises from the combined transition from states with GP zones and γ″-phases, whereas γ arises from the transition of states with γ″ and γ′. The apparent activation energies associated with GP zones, and γ″,γ′-and γ-phases are 0.94 ± 0.07, 1.26 ± 0.07, 1.31 ± 0.08 and 1.69 ± 0.1 eV respectively. These values are discussed in terms of dissolved atom mobility related to vacancy concentration, and the direction of plate-like precipitate growth (either normal or perpendicular to the plate). Small discontinuous precipitations of γ-phase occurred along with the transformation to the state with γ′-phase.