Precipitation Kinetics and Mechanism in Cu-7 wt% Ag Alloy

Abstract

The discontinuous precipitation kinetics and mechanism of the α (Ag-rich) phase in Cu-7 wt% Ag alloy has been investigated using dilatometric and calorimetric anisothermal analysis, optical microscopy, scanning and transmission electron 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 precipitation 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 interfacial controlled process having an activation energy E<sub>a</sub> 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

Precipitation and dissolution of Ag in ageing hypoeutectic alloys

“…The heat flow of proeutectic Cu is expected to dominate the total heat flow. At elevated temperature (>250°C), both exothermic Ag precipitation and endothermic Ag dissolution in proeutectic Cu will occur [14]. Those two reactions are shown in the DSC curves of as-solution treated Cu28%Ag alloys in Fig.…”

“…The fitting gave 55.3 ± 22.4 kJ/mol as [20] the activation energy of Ag-rich precipitation (the first reaction of the DSC curves in Fig. 1), which is lower than that in Cu-7 wt.%Ag alloy (95 ± 4 kJ/ mol) [14], Cu-3.8 at.%Ag bicrystals (144.1 kJ/mol) [10] and Cu-5.7 wt%Ag single crystal (185 kJ/mol). The activation energy of Ag-rich dissolution (the high temperature reaction in the DSC curves in Fig.…”

“…As the Ag composition was less than 8 wt.%, the Cu-rich solid solutions were transformed into structures with rod-like Ag-rich precipitations embedded in Cu-rich matrix by discontinuous precipitation. Coarse precipitates from the high-angle grain boundaries were generated [9,10,12,13]. The estimated activation energy of Ag precipitation [10,12,14], growth direction, morphology and habit planes [15] were reported.…”

“…Coarse precipitates from the high-angle grain boundaries were generated [9,10,12,13]. The estimated activation energy of Ag precipitation [10,12,14], growth direction, morphology and habit planes [15] were reported. Whereas the silver content was more than 8 wt%, a sufficient volume fraction of eutectics suppressed discontinuous precipitation by the absence of grain boundaries and fine precipitates were observed by continuous precipitation mode.…”

Study of the non-isothermal microstructural evolution of deformed Cu–15 wt.%In and Cu–9 wt.%Sb alloys by means of X-ray diffraction and dilatometry

“…1b) can be at least related to d formation and dissolution respectively. This expresses the predominance of lattice parameter effect as it is observed in the Cu7wt.%Ag [20].…”