E. Donoso

Microcalorimetric evaluation of precipitation in Cu-2Be-0.2Mg

Beryllium precipitation from the Cu-rich matrix in a Cu–2 mass% Be–0.2 mass% Mg 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 subeffects: A1 and A2 , and B1 and B2 respectively. Effects A1 and A2 correspond to the precipitation of GP zones and subsequent overlapping and independent precipitation of the 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 the precipitation heats of the phases involved. The heat content of the combined GP zone/γ″ phase precipitation effect was proportional to the number of beryllium atoms precipitated, yielding an average value of 21 kJ mol−1 beryllium for beryllium precipitation. It was shown that the 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′ phases. The apparent activation energies associated with GP zones and γ″, γ′ and γ phases are 1.16±0.08, 1.18±0.07, 1.37±0.08 and 1.74±0.09 eV, respectively. These values are discussed in terms of the mobility of dissolved atoms related to the concentrations of excess vacancies and solute-vacancy complexes, and the direction of plate-like precipitate growth (either normal or perpendicular to the plate). It is inferred that the main roles of magnesium are to decrease the amount and rate of GP formation, to enhance the volume fraction of γ″ and to suppress the discontinuous precipitation of γ′.

Modelling solute segregation to partial dislocations for DSC evaluations

A general model is discussed for assessing the energy release due to the pinning of solute atoms to partial dislocations. The present approach discloses the influence of dislocation character distributions on the magnitude of this energy. In order to test its validity in αCu-Al alloys, differential scanning calorimetry (DSC) evaluations associated with the different peaks involved during linear heating were performed employing both cold worked and quenched materials. Dislocation densities were calculated from recrystallization traces. On the basis of this model it was concluded that the observed energy difference between the deformed and the quenched materials during the exothermic peak designated as Stage 2 corresponds to the pinning process. It was also concluded that nearly equal number of edge and screw dislocations are present in the dislocation configuration of deformed alloys. Nevertheless, it is proposed that dislocation-induced order might also occur as a consequence of enhanced solute concentration around the partials.

Assessment of quenched-in order and vacancies using modelling and differential scanning calorimetry

Using a rate vacancy loss equation and an overall rate constant for order establishment, a model for describing the roles of quench temperature, quench rate and vacancy sink density is proposed. This model allows computation of the retained value of the first short-range order parameter of quenched binary alloys and the prediction that the most highly disordered state obtainable occurs with quenches from intermediate temperatures. Numerical results were tested against experimental data based on energetic analysis of differential scanning calorimetric traces pertaining to a reordering process towards the equilibrium state. Kinetic evaluations of these traces yield experimental estimates of quenched-in vacancy concentrations. Good agreement between modelled and experimental assessments was obtained in a Cu-30 at.% Zn alloy.

Kinetics of short-range-ordering in αCu–10at.% Al conducted through differential isothermal calorimetry as influenced by solute–vacancy complexes

Abstract A modified first-order kinetic law which takes into account defect decay during an ordering process was employed to predict the short-range-order kinetics of a quenched αCu–10at.% Al alloy, in conjunction with experiments performed by differential isothermal calorimetry (DIC). The effective activation energy of point defect migration and its temperature dependence strongly suggests the contribution of bound vacancies to the ordering process. An estimate of 87.6 kJ mol −1 was made for the activation energy of solute–vacancy migration by applying an effective rate constant, a value which is quite reasonable, since it lies between the activation energy of migration of unbound vacancies and the activation energy for complex dissolution. The isothermal curves were utilized to determine the ordering energy: w=−3.66 kJ mol −1 .

DSC evaluations of short-range-order kinetics in Cu–10at.% Al solid solutions as influenced by bound vacancies

Abstract A modified first order kinetic law which describes the roles of bound and unbound vacancies is proposed in order to predict defect decay and short-range-order (SRO) kinetics of quenched binary alloys during linear heating experiments. The model has been applied to differential scanning calorimetry (DSC) thermograms of Cu–10at.% Al quenched from different temperatures. Transient effects at the end of de-freezing when heating after quenched were examined. Activation energy for migration of solute–vacancy complexes was also assessed from the kinetics of SRO using DSC traces. A value of 85.6 kJ mol −1 was obtained. The relative contribution of bound and unbound vacancies to the ordering process as influenced by quenching temperature was determined.

Modelling the kinetics of solute segregation to partial dislocations for isothermal microcalorimetric evaluations

A model is proposed to describe the kinetics of solute segregation to partial dislocations in solid solutions of cold-rolled alloys. The case when half edge and half screw dislocations are present is considered. The model gives account of the kinetic behaviour observed in a deformed Cu-19 at% Al alloy where two unknown processes could be assessed during calorimetric isothermal experiments. The faster process corresponds to segregation to screw dissociated dislocations while the slower one corresponds to segregation to edge dissociated dislocations. Experimental activation energies, larger for edge dislocations, are close to that for pipe diffusion along the partials corrected by pinner binding energy terms. It is also predicted that segregation occurs faster as the dislocation density is increased. A quantitative comparison of experimental results with model predictions is given.

Non-isothermal short-range-ordering by excess vacancies in αCu-Zn alloys

The kinetics short-range-order (SRO) in quenched Cu-30 at.% Zn, Cu-25 at. %Zn and Cu-20 at. %Zn was investigated by differential scanning calorimetry (DSC). It was evidenced a growing atomic mobility with increasing Zn content. From the DSC traces it is inferred that ordering is established in one stage, assisted by excess vacancies. As the quenching temperature increases considerable reordering occurs during cooling from the quenching temperature. The variation in the SRO non-isothermal behaviour with quenching temperature and composition is interpreted in terms of the atomic mobility and the degree of disorder together with the concentration of vacancies retained by quenching. Activation energies which control the mean life of vacancies and those which control the ordering rate were very similar, indicating that the mobility of vacancies is highly effective in generating SRO. Such activation energies are somewhat lower than the effective energies which control the kinetics of the process obtained from the DSC traces, suggesting that the presence of solute-vacancy complexes may be important as the Zn concentration increases. This feature was confirmed by an estimation of the solute-vacancy binding energy. It was also inferred that divacancy formation is unlikely in the alloys under study.ZusammenfassungDie Kinetik-Nahordnung (short-range-order, SRO) in abgeschrecktem Cu-30 At%Zn, Cu-25 At%Zn, Cu-20 At%Zn zeugt mit steigendem Zn-Gehalt von anwachsender Atombeweglichkeit. Anhand der DSC-Kurven wird darauf geschlossen, daß die Ordnung in einem Stadium gefestigt wird, unterstützt durch einen Gitterleerstellenüberschuß. Bei Erhöhen der Abschrecktemperatur erfolgt während des Abkühlens von der Abschrecktemperatur eine erhebliche Rückordnung. Die Änderung des SRO nichtisothermen Verhaltens mit der Abschrecktemperatur und der Zusammensetzung wird interpretiert mit der Atombeweglichkeit und dem Grade der Unordnung zusammen mit der Konzentration der durch das Abschrecken zurückgehaltenen Gitterleerstellen. Die Aktivierungsenergien, welche die mittlere Lebensdauer der Gitterleerstellen und die Ordnungsgeschwindigkeit bestimmen waren sehr ähnlich, was darauf hinweist, daß die beweglichkeit der Gitterleerstellen beim Aufbau der Nahordnung sehr effektiv ist. Diese Aktivierungsenergien sind etwas niedriger als die effektiven Energien, welche die Kinetik des Prozesses anhand der DSC-Kurven bestimmen, was darauf hinweist, daß die Gegenwart von gelöster Stoff-Gitterleerstellen Komplexen bei steigender Zn-Konzentration wichtig werden kann. Dieses Merkmal wird durch eine Schätzung der gelöster Stoff-Gitterleerstelle Bindungsenergie bestätigt. Weiterhin wird darauf geschlossen, daß die Bildung von Doppelgitterleerstellen in den untersuchten Legierungen unwahrscheinlich ist.

Short-range-ordering kinetics of cu-5 at.% Zn influenced by solute-vacancy complexes and cold rolling

A modified first-order kinetic law which takes into account defect decay during an ordering process was employed to predict the short-range-order kinetics of a quenched and a quenched-deformed Cu—5 at.% Zn alloy, in conjunction with experiments performed by isothermal calorimetry. The effective activation energy of point defect migration and its temperature dependence strongly suggest the contribution of bound vacancies to the ordering process. An estimate of 91.2 kJ mol−1 was made for the activation energy of solute—vacancy migration by applying an effective rate constant, a value in very good agreement with that obtained from previous non-isothermal experiments. The isothermal curves were utilized to determine the ordering energy: w=−2.90 kJ mol−1. In conjunction, a parametric study of the defect sink density was performed in order to assess its influence on the calculated isothermal curve profiles.

DSC evaluations in F.C.C. solid solutions of short-range-order kinetics as influenced by bound vacancies

A modified first order kinetic law, which describes the roles of bound and unbound vacancies, is proposed in order to predict defect decay and short-range-order kinetics of quenched binary alloys during linear heating experiments. The model has been applied to differential scanning calorimetry (DSC) curves of Cu–5 at%Zn quenched from different temperatures. Activation energy for migration of solute-vacancy complexes was also assessed from the kinetics of short-range-order using DSC traces. A value of 89.5±0.32 kJ mol–1 was obtained. The relative contribution of bound and unbound vacancies to the ordering process as influenced by quenching temperature was determined. In conjunction, a parametric study of the initial total defect concentration and effective energy for defect migration was performed in order to envisage their influence on the calculated DSC profiles.

Kinetics of solute flow to partial dislocation in Cu-3.4 at.% Sb

The kinetics of solute segregation to partial dislocations in a Cu–3.4 At.% Sb alloy was studied by using a phenomenological approach with differential scanning calorimetry and isothermal calorimetry. The material, severely deformed by repeated bending, presented an excess of dissociated edge dislocations with a dislocation density amounting to about 8.5·1014 m−2, calculated using a prior model of the authors, together with calorimetric recrystallization trace analysis. The kinetics was found to be ruled by two overlapping mechanisms: diffusion of solute atoms mostly through dislocation pipes in the initial and middle stages of the reaction process, acting together with bulk solute diffusion in these stages and later. Bulk solute diffusion increases as the reaction proceeds, as shown by the increasing values of apparent activation energy in the reaction. The exponent of the Mehl-Johnson-Avrami equation used in the phenomenological description was successfully fitted to a time—temperature-dependent function, increasing in agreement with the apparent activation energy behaviour, as may be expected.