G. Riontino

Early ageing mechanisms in a high-copper AlCuMg alloy

Abstract Early stages of transformation of a metastable Al4.4Cu1.7Mg alloy have been studied by DSC, microhardness and TEM, during natural ageing. The progressive disappearance of the first exothermal signal of the as-quenched material, the corresponding initial hardness increase, and TEM/SAD observations, suggest the formation of GP+GPB zones.

Grain growth and secondary recrystallization in iron

The effect of impurities and of slight deformations as factors modifying the kinetics of normal grain growth after primary recrystallization in pure iron is investigated. Samples of pure iron and Armco iron, each with or without a content of precipitated oxides, were examined.Inclusions inhibit grain growth up to high temperatures, whilst the influence of strain depends on its amount: very low deformations (∼2%) generally block grain growth; for deformations around 5%, secondary recrystallization takes place; higher deformations (∼10%) accelerate the early stages of growth, leading to not very large final grain dimensions. 2.0% elongation has been found to be, in our conditions, the critical strain limit for secondary recrystallization.

Structural relaxation and irreversible changes of electrical resistivity of Fe‐Ni‐Mo‐B amorphous alloys

The structural relaxation of Fe‐Ni‐Mo‐B alloys with different Mo content is studied through electrical resistivity measurements between the room temperature and the glass transition temperature Tg. The observed behavior of the resistivity during isothermal annealing treatments gives evidence for the presence of competing processes of compositional and topological short‐range ordering, whose nature and role are discussed. The variation of the temperature coefficient of resistivity, α, observed after annealing of ribbons, is related to the variation of the degree of topological short‐range order, and interpreted on the basis of the theories describing the electrical resistivity and the structural relaxation of amorphous metals. A simple relation between the variation of α after annealing, and the reduction of the alloys’s free volume, is derived and discussed.

Hardening nanostructures in an AlZnMg alloy

The formation of nanoscale and sub-nanoscale solute aggregates (clusters, Guinier–Preston zones and precipitates) in an AlZnMg alloy (Al–2.1 at.% Zn–1.5 at.% Mg) has been followed by a combination of experimental techniques with the aim of correlating the properties of the aggregates with their thermal history. The choice of thermal treatments was guided by the results of mechanical and calorimetric characterizations, supported by transmission electron microscopy for the identification of the morphology of the aggregates. Positron annihilation spectroscopy (using two variants of this technique, coincidence Doppler broadening and lifetime spectroscopy) was adopted for determining the local chemistry in the proximity of open volume defects. The geometrical parameters of the distribution (size, volume fraction, numerical density of the solute aggregates) were obtained by small-angle X-ray scattering. The results of the investigation provide new information regarding: two families of vacancy-rich clusters formed during or immediately after quenching; Guinier–Preston zones formed at 95°C after room-temperature pre-ageing; growth of η′ and η phases at 150°C; solute clusters formed at room-temperature in conditions of secondary ageing after preliminary heating at 150°C.

Kinetics of abnormal grain growth in pure iron

Grain growth after primary recrystallization in unstrained and strained specimens of pure iron is examined during isothermal anneals at three different temperatures (664, 680, 690° C). Undeformed specimens undergo continuous grain growth, while the deformed ones show a stage of rapid discontinuous growth. The peculiar characteristics of the abnormal growth kinetics, and in particular the presence of three well-defined stages of growth, are brought out by plotting, versus annealing time, the ratio between the mean grain diameters of deformed and undeformed samples. The parametersn (exponent of the kinetic equation) andQ (apparent activation energy) are determined.

Secondary ageing in Al-Cu-Mg

Secondary ageing, that is microstructural evolution occurring at room temperature after short heating at temperatures above the metastable phase boundary of Guinier-Preston zones, has been studied for an Al-Cu-Mg alloy with a high Cu-to-Mg ratio. Combined data from positron annihilation spectroscopy, Vickers microhardness measurements and differential scanning calorimetry show that, on secondary ageing after 5 or 7min at 190°C, firstly, hardening takes place at a rate nine to 16 times slower than natural age hardening; secondly, vacancies slowly released by Cu-rich aggregates formed during the heat treatment at 190°C promote further formation of solute aggregates, with a time-dependent chemical composition; thirdly, the thermal stability of the structures formed during secondary ageing increases with increasing dwell time at room temperature; and, fourthly, solute aggregates formed at 190°C undergo a structural reorganization and possibly a change in the composition, leading to species with a different thermal stability. The slow release of vacancies from Cu-rich aggregates is proposed as one of the limiting factors of the hardening rate.

Precipitation kinetics of AA4032 and AA6082: A comparison based on DSC and TEM

The objective of this note is to utilize the macroscopic theory in order to obtain an additional criterion for the identification of the nature of the precipitates. To this aim the formulation of the activation energy for the growth of precipitates is used in conjunction with differential calorimetry performed at different scanning velocities. In the following section the authors critically examine the steps leading to the currently adopted kinetic expression da/dt = K(T)f(a), a being the precipitating fraction at time t, K(T) the temperature dependent kinetic activation term and f(a) a function depending exclusively on the reaction mechanism. In section 3 experimental data on the AA4032 and AA6082 alloys are given, including those at several scanning velocities; the data are discussed, showing that the activation energy could be safely used as a parameter for the identification of the nature of the precipitates. For alloys of similar compositions, such as the pseudo-binary Al-Mg-Si alloys considered here, this criterion of identification could prove useful.

Free volume dependence of the electrical resistivity of metallic glasses prepared with different quenching rates

Abstract Measurements of the electrical resistivity ρ and its temperature coefficient α have been performed on amorphous ribbons prepared with different quenching rates. The variation of ρ and α with the degree of free volume frozen in these alloys is interpreted in terms of a theory based on the Ziman approach to the resistivity of metallic glasses.

Kinetics of formation and thermal stability of Fe-X-B metallic glasses

Time-temperature-transformation (TTT) and continuous-cooling-transformation (CCT) curves are derived for Fe-X-B metallic glasses (X = Ti, V, Cr, Mn, Co, Ni, Cu, Pd, C. Si. Ge, Sn). Methods of deriving the relevant parameters for computing the nucleation and growth rate are reviewed and their effect on the shape of TTT and CCT curves is assessed. Critical cooling rates for glass formation arc computed and related to thermal stability against crystallization. Steric and electronic effects on crystallization temperature are discussed.

A computer method to determine the kinetic law of solid-state reactions from DSC curves

Abstract A computer program has been worked out to evaluate the activation energy, the Arrhenius pre-exponential factor and the mechanism of solid-state reactions from non-isothermal measurements. A univocal determination of the reaction mechanism is obtained by the simultaneous application of two selection criteria: (a) activation energy value by the Ozawa method; (b) degree of linearity in the A modification of the Rogers and Smith method was then used to obtain empirically complete agreement between kinetic expression and experimental results when the kinetic law has to be improved to account for a whole experimental curve. The accuracy of the computer method has been checked through a calibration of the program by means of 16 theoretical functions proposed by