PRECIPITATION BEHAVIOUR IN THE ALUMINUM-1.5%COPPER-0.75%MAGNESIUM ALLOY

The precipitation behaviour in Al-1.50% Cu-0.75% Mg alloys containing 0, 0.23, 0.49, 0.76 and 1.03% Si have been investigated by transmission electron microscopy (TEM) and differential scanning calorimetry (DSC).; Ageing of the Si free alloy at 130 and 190{dollar}spcirc{dollar}C induced nucleation of the orthorhombic S{dollar}spprime{dollar} phase on dislocation loops and helical dislocations. The S{dollar}spprime{dollar} particles grew as rods parallel to {dollar}langle{dollar}100{dollar}ranglesb{lcub}rm M{rcub}{dollar} matrix directions. The orientation relationships known from the x-ray investigations were used to simulate the S{dollar}spprime{dollar} electron diffraction pattern which was consistent with the morphology and crystal structure. With increase in ageing time the b parameter of the S{dollar}spprime{dollar} phase increased significantly and the c value declined without any change in the cell volume. Grouping of rods on {dollar}{lcub}{dollar}110{dollar}{rcub}sb{lcub}rm M{rcub}{dollar} fcc planes were explained on the basis of expansion of b parameter and contraction of c parameter.; The first exothermic (25{dollar}spcirc{dollar}-170{dollar}spcirc{dollar}C) and endothermic (140{dollar}spcirc{dollar}-240{dollar}spcirc{dollar}C) peaks were due to precipitation of GPB zones and dissolution of GPB zones and complexes respectively. The second exothermic (253{dollar}spcirc{dollar}-367{dollar}spcirc{dollar}C) and enothermic (380{dollar}spcirc{dollar}-434{dollar}spcirc{dollar}C) peaks were associated with the precipitation and dissolution of the S{dollar}spprime{dollar} phase respectively. The processes were analysed using the following relationship. {dollar}{lcub}rm dy{rcub}over{lcub}rm dt{rcub}{dollar} = f(Y) Ko{dollar}sp{lcub}rm e~Q{lcub}sp*{rcub}/RT{rcub}{dollar}.; The expression for f(Y) was (1 {dollar}-{dollar} Y) for all the peaks. The activation energy, Q{dollar}sp{lcub}*{rcub}{dollar}, and the value of k{dollar}sb{lcub}rm o{rcub}{dollar} for the precipitation and dissolution of GPB zones were determined to be 55.6 kJ/mole, 5.7 {dollar}times{dollar} 10{dollar}sp5{dollar} S{dollar}sp{lcub}-1{rcub}{dollar}, 123.9 kJ/mole and 1.1 {dollar}times{dollar} 10{dollar}sp{lcub}10{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}{dollar} respectively. The values of Q{dollar}sp{lcub}*{rcub}{dollar} and k{dollar}sb{lcub}rm o{rcub}{dollar} for the precipitation of the S{dollar}spprime{dollar} phase were 129.9 kJ/mole and 2.6 {dollar}times{dollar} 10{dollar}sp9{dollar} S{dollar}sp{lcub}-1{rcub}{dollar} respectively.; The addition of Si {dollar}geq{dollar} 0.49% caused the formation of insoluble particles. The volume percentage of insolubles in the 1.03% Si alloy was 1.34. The extracted particles analysed by energy dispersive analysis in SEM contained 20.6 {dollar}pm{dollar} 0.4%Cu, 32.6 {dollar}pm{dollar} 0.4%Mg, 30.2 {dollar}pm{dollar} 0.4%Si and 16.6 {dollar}pm{dollar} 0.1%Al by weight. X-ray diffraction analysis yielded an hexagonal structure with a and c parameters, 1.036 {dollar}pm{dollar} 0.004 nm and 0.404 {dollar}pm{dollar} 0.004 nm respectively. The insoluble particles were identified as the quarternary phase, Q. The presence of insoluble particles changed the soluted contents of the matrix from 1.45%Cu, 0.63%Mg, 0.39%Si in the 0.49%Si alloy to 1.16%Cu, 0.05%Mg and 0.44%Si in the 1.03%Si alloy. (Abstract shortened with permission of author.)...