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Effet de la présence des phases sur les anomalies dilatométriques dans les alliages métalliques
... Last years, differential dilatometry has been extensively used to study the precipitation reactions in different alloys [9][10][11][12][13][14][15]; it has been shown that this method is very sensitive to this kind of phase transformations. ...
... Differential dilatometry can show effects of precipitation and dissolution by the appearance of various anomalies in the experimental curves, and generally, depending on the lattice parameter and the specific volume variations, these anomalies can be expansions or contractions [9][10][11][12][13][14][15]. However the shape of the dilatometric curve can change if the second phase is a solid solution, which justifies this investigation. ...
... Last years, differential dilatometry has been extensively used to study the precipitation reactions in different alloys [9][10][11][12][13][14][15]; it has been shown that this method is very sensitive to this kind of phase transformations. ...
... Differential dilatometry can show effects of precipitation and dissolution by the appearance of various anomalies in the experimental curves, and generally, depending on the lattice parameter and the specific volume variations, these anomalies can be expansions or contractions [9][10][11][12][13][14][15]. However the shape of the dilatometric curve can change if the second phase is a solid solution, which justifies this investigation. ...
The discontinuous precipitation kinetics and mechanism of the (Ag-rich) phase in Cu-7 wt% Ag alloy has been inves-tigated using dilatometric and calorimetric anisothermal analysis, optical microscopy, scanning and transmission elec-tron microscopy and X-ray diffraction. Dilatometric and calorimetric curves present at ~ 500˚C an important effect related to the (Ag-rich) phase formation and consequently the matrix β (Cu-rich) depletion. The nucleation and growth of the precipitated phase show cells formation at initial grain boundaries; a fine lamellar structure is detected by SEM and TEM and consists of alternate lamellar of the α (Ag-rich) and β (Cu-rich)-solid solutions. Cellular pre-cipitation leads to the simultaneous appearance of two diffraction peaks and occurs apparently according to the Fournelle and Clark's mechanism. Obtained results give an Avrami exponent n = 2.0 ± 0.2 in agreement with an inter-facial controlled process having an activation energy E a equals to 99 ± 7 kJ/mol obtained from anisothermal analysis by using different isoconversion methods. This activation energy expresses the discrepancy between isoconversion methods and the analytical diffusive model. Moreover, the supersaturation rate has an effect on the lamella spacing of the precipitated cells.
... Thus, we had to expect a contraction, not an expansion. However the β′ and β phase formation has another effect: their specific volume (V β′ = 282,51 Å 3 and V β = 19,28 Å 3 ) is superior to that of the matrix (V m = 17,39 Å 3 ) [19] which leads to an expansion. Moreover, this expansion is more dominant than the contraction effect due to the lattice parameter decrease of the matrix. ...
In order to improve the understanding of the sensibility of the internal friction phenomena to different stages of precipitation in Al-Mg alloys, the ageing effect of a supersaturated solid solution on the Temperature Dependent Internal Friction (TDIF) spectra has been studied. The research showed that the dissolution of the β phase and the transfer of Mg atoms to a solid solution and the formation of β′ metastable phase have an effect on internal friction. Thus, it was proved that the precipitation or the dissolution of β phase plays an important role in the anelasticity of Al-Mg alloys. A detailed TDIF analysis revealed the sensibility of the internal friction technique to the formation of semi-coherent and non-coherent phases (β′ and β) but it is not sensitive enough to the formation of coherent phases (GP zones). Several observations on the nature and the shape of the precipitates are also discussed.
... Differential dilatometry can show effects of precipitation and dissolution by the appearance of various anomalies in the experimental curves, and generally, depending on the lattice parameter and the specific volume variations, these anomalies can be expansions or contractions [18][19]. However the shape of the dilatometric curve can change if the second phase is a solid solution, which justifies this investigation. ...
The cellular precipitation in Ag–8wt.% Cu alloy has been studied using dilatometric analysis, differential scanning calorimetriy, optical microscopy and X-ray diffraction (XRD). Dilatometric curve presents at ∼300°C an anomaly identical to that representing an allotropic transformation, which means a formation of a new crystallographic structure. Thus the ageing temperature can affect the initiation mechanism of the reaction. At low temperature thermal migration of grain boundaries and cells formation precede boundary precipitation. At high temperature the structure consists of alternate lamellar of the α (Ag-rich) – solid solution and β (Cu-rich) – solid solution phases. The activation energy Eact equals to 56.5±6.2kJ/mol at low temperature (196°C) and to 109.5±6.7kJ/mol at high temperature (300°C).
Recrystallization, transition phase formation and dissolution of Al-12wt.% Mg alloy were investigated as a function of the defect density introduced by cold rolling and cold compression deformation. Low and high deformation amounts have been used with the objective that the recrystallization takes place combined with the precipitation reaction to attempt to follow an eventual influence of plastic deformation on the dilatometric curves, the sequence of the precipitation and consequently, on the nature of precipitate particles at determined ageing temperatures. Important anomalies resulting from the plastic deformation appeared in the dilatometric curves, which must be influenced by the recrystallization and the precipitation processes. The latter have never been studied through dilatometric analysis. However, no effect on the precipitation sequence has been observed and it has been confirmed that at relatively high deformation amounts a fine crystalline structure is obtained, while at low deformation amounts a coarser structure is developed. Moreover, it has been shown that the recrystallization does not occur but only the recovery stage is observed. Furthermore, the acceleration effect of plastic deformation on particles nucleation and growth is confirmed.
The use of various experimental techniques as differential dilatometry, differential scanning calorimetry, optical and transmission electron microscopy, internal friction, electrical resistance and microhardness measurements, shows their importance and their complementarity in the study of the ageing process in Al–12 wt.% Mg alloy. Indeed, their sensitivity to various precipitation stages is different and can give more information to describe the evolution of the structure, the nature and the morphology of the precipitated phases particles and the transition between them, which is not, yet, well established. The obtained dilatometric curves show numerous effects which have been discussed. During ageing of a supersaturated solid solution Al–12 wt.% Mg it seems that β′ and β phases do not grow at the expense of GP zones (and/or β″) and β′, respectively.
Duplex crystals of Cu and Cu7In3 were produced by directional eutectoid decomposition. The orientation relationship between Cu and Cu7In3 was studied by texture goniometry. Although the Cu7In3 phase has a triclinic structure, the crystals structure can be described as a “pseudo-cubic” one. The cell parameters of the δ-Cu7In3 were found to be a=10.071 Å, b= 9.131 Å, c = 6.726 Å, α = 90.20°, β = 82.84° and γ = 106.82°. The δ-Cu7In3 phase was found to be twinned.
Recrystallization of a precipitation hardening -Fe-alloy was investigated as a function of defect density introduced by cold rolling. At 25% < 90%="" recrystallization="" took="" place="" in="" combination="" with="" precipitation,="" at=""> > 90% only particle growth controlled subgrain growth occurred. The results were explained on the basis of competition between two forces: an increased dislocation density producing an increased driving force for recrystallization and an increased rate of heterogeneous nucleation leading to the individual formation of particles which produce a retarding force.