Ari Varschavsky

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.

An electron microscopic and diffraction study of Cu-9 wt pct Al alloy

An electron microscopic investigation of Cu-9 wt pct Al alloy low thermal treated at 250°C for 30 min was performed in the deformed and non-deformed condition. For this composition and ordering treatment the alloy exhibits the higher increase in strength. It was found that the images formed in the non-deformed alloy, reveal the presence of ordered domains of an average size of 80Å, being the amount of order dependent of whether or not quenched-in vacancies are present. It is probable that the domains nucleate preferentially at stacking faults in the deformed condition. A periodic antiphase structure was determined from computations and comparison with electron diffraction data. The superlattice cell is based on the LI2 type, tetragonal face centered with a period approximately three times the lattice parameter of the matrix, having three variants of orientation within an ordered region.

Kinetics of short-range ordering in αCu-Al alloys

Abstract The ordering behaviour in quenched αCu-Al was investigated by differential scanning calorimetry (DSC) under rising temperature conditions and by electron diffraction. It was found that the ordering processes can be better explained in terms of a homogeneous short-range order (SRO) model rather than a heterogeneous disperse order (DO) model as previously interpreted for the same type of experiments. The DSC traces indicate that the ordering takes place in two stages: the stage 1 ordering at lower temperatures is associated with the migration of excess vacancies and the stage 2 ordering at higher temperatures is associated with the migration of equilibrium vacancies. At higher temperatures, a marked surge of energy absorption occurs (stage 3) which is attributed to the destruction of order. For furnace-cooled alloys, only stage 3 appears. The relative dominance of stages 1 and 2 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, is proposed. A method for evaluating boundary values for this parameter is developed, making use of the features displayed by the DSC thermograms. The mobility of the vacancies, which controls their life-time and the short-range ordering rate, was evaluated by computing frequency factors and activation energies. Values for activation energies controlling the short-range ordering rate are somewhat smaller than the effective values obtained for stage 1 from the DSC traces, suggesting that the presence of solute-vacancy complexes may be important mainly as the aluminium concentration increases. From estimations of solute-vacancy and divacancy binding energies, it is inferred that divacancy formation is unlikely in the alloys under study. Process frequency factor values are in very good agreement with those calculated for a short-range order state developed during two-stage ordering by a vacancy mechanism. A flow diagram of the factors controlling the fractional increase of short-range order during anisothermal experiments is proposed.

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.

Influence of disperse order on the SN fatigue behavior of a Cu-9 wt.% Al alloy

Abstract When a Cu-9 wt.% Al alloy, previously subjected to an ordering treatment at 250 °C for 30 min, is cyclically stressed, an improvement in its fatigue life with respect to that of annealed alloy is observed at low stress amplitudes. The effect is enhanced if the alloy is deformed prior to the low thermal treatment. This behavior can be interpreted by assuming that ordered domains nucleate preferentially at stacking faults reducing greatly the dislocation activity. At high stress levels the ordering treatment becomes ineffective, since crack initiation observed at grain boundaries appears to be essentially independent of whether or not order is present.

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.