A. Gaber

Investigation of developed precipitates in Al–1·1 wt-%Mg2Si balanced alloy by DSC and SEM techniques

Abstract The effect of temperature on the sequence of the hardening precipitates in Al–1·1 wt-%Mg2Si balanced alloy has been investigated by means of differential scanning calorimetry, scanning electron microscopy and hardness measurements. The non-isothermal DSC thermograms exhibited seven reaction peaks. Out of these seven peaks, five are exothermic (representing precipitation) and two are endothermic (representing dissolution). The activation energy associated with the individual precipitates is calculated. The activation energy of nucleation of GP zones (54·3 kJ mol−1) is close to the migration energy of Si in Al (52·7 kJ mol−1). The activation energy associated with the precipitation of β″ is determined as 77·6 kJ mol−1 and that for the formation of β′ precipitates is 145·3 kJ mol−1. The latter value is close to that for Si diffusion in Al (124 kJ mol−1) and that of Mg diffusion in Al (131 kJ mol−1). It can be concluded that the precipitation of β′ particles might be characterised by both the diffusion of Mg and Si atoms in Al to form β′ precipitates.

Correlative study of the thermoelectric power, electrical resistivity and different precipitates of Al–1.12Mg2Si–0.35Si (mass%) alloy

Characterization of the different precipitates developed in supersaturated Al–1.12Mg2Si–0.35Si (mass%) alloy by thermoelectric power (TEP) and electrical resistivity (ER) measurements was considered. The effect of precipitation of coherent, semi-coherent and non-coherent phases on the TEP was found impressive in describing different precipitates. Upon growing and coherency loss of β′ particles, the TEP increases to reach a maximum value. TEP begins to approach a stable value by the formation of the equilibrium β (Mg2Si) precipitates and then stabilizes with the complete growth of more stable precipitates β phase and Si particles. The first-order coefficient α of the ER–temperature dependence was found dominating in the temperature range 300–635xa0K. Above this temperature range, the second-order coefficient β starts sharing effectively the resistivity–temperature dependence. Furthermore, it has been shown that the range of temperature in which the first-order coefficient α is dominating slightly increases after slow cooling twice to the room temperature in two successive runs. Correlation of α and β with the lattice rigidity of the alloy under investigation was established. Quenching and slow cooling affect strongly the observed correlation. All measurements in the present investigation were taken under non-isothermal conditions.

Investigation of nanoscale precipitates developed in Al–0·73Mg–0·45Si–0·34Cu–0·21Cr–0·20Fe alloy

Abstract In the present work, the developed nanoscale precipitates in Al–0·73Mg–0·45Si–0·34Cu–0·21Cr–0·20Fe (wt-%) alloy have been investigated by means of differential scanning calorimetry, electron microscopy (SEM and TEM) and microhardness measurements (HV). The addition of Cu assists the formation of Q′ phase which positively changes the alloy strength. The precipitation of β′′ nanoparticles is followed by the precipitation of β′ and/or Q′ precipitates. Both coherent and semicoherent precipitates have a positive contribution to the strengthening of the alloy. The average activation energy associated with the precipitation of β(Mg2Si)+Q phases is very close to that for Q″ and/or Q′ phase which suggests that the two phase precipitation might be characterised by the same mechanism. The reaction order of the precipitation processes suggests that β′ and/or Q′ precipitates grow radially; whereas, β-precipitates grow in the three directions.