Calorimetric Study of Precipitation Process in Al–Mg Alloys

Abstract

The precipitation process was examined by measuring specific heat vs temperature (S-T) curves of aged Al-alloys containing 7.6∼12.5 at% Mg. Age-hardening was also studied with Al-12.5 at% Mg alloy. (1) Four heat-absorptions (‘A’, ‘B’, ‘C’ and ‘D’) and one heat-evolution (‘P’) were observed on the S-T curves: ‘A’ was attributed to the decomposition of δ″ (short-range ordered Al₃Mg or AlMg) and ‘B’ to that of δ′ (long-range ordered Al₃Mg or AlMg); ‘C’ and ‘D’ were due to the redissolution of β′ and β phases into the matrix respectively; and ‘P’ was ascribed to the precipitation of β phase. (2) The precipitation in Al–Mg alloys was a four-stage process below 323 K: Solid solution→δ″→δ′→β′→β. (3) Temperature limits for the formation of δ″ and δ′ were determined as a function of the Mg content of alloys. Below the solvus temperature of the solute Mg atom, there existed no temperature limit for β′ formation. (4) Age-hardening of Al–Mg alloys was associated with the precipitations of δ′, β′ and β phases. The formation of δ″ did not cause hardening of the alloys. (5) The δ″ particles were already present in the as-quenched state from 723 K in the alloys containing more than 10.5 at% Mg. (6) The T-T-T diagram determined with Al-12.5 at% Mg alloy shows a double C-curve separated into two parts at 363 K.