DSC study of precipitation in an Al–Mg–Mn alloy microalloyed with Cu

Abstract

Power compensation DSC has been employed to detect and analyse precipitation reactions in an Al-1.3Mg-0.4Mn and an Al-1.3Mg-0.4Mn-0.07Cu alloy in which very small amounts of precipitate, less than 0.3 at%, are expected to form. Due to the very small heat effects, baseline instability and drift significantly interfere with the measurements. After repeated experiments and careful baseline correction it is demonstrated that in the Cu containing alloy, ageing at 170°C causes the appearance of two endothermic effects: for 2 days ageing a small dissolution effect appears at about 230°C, whilst for 7 and 21 days ageing a dissolution effect peaking appears at about 300°C. The temperature range of the latter is consistent with S phase dissolution. IntroductionDifferential scanning calorimetry (DSC) and isothermal calorimetry are extensively used for the study of precipitation in heat treatable Al based alloys [1]. Studies are conducted on alloys in which the amount of alloying elements that precipitate is typically in the range of 1 to 10 atomic percent. For instance, in commercial Al-Cu-Mg alloys (e.g. AA2024: Al-1.6at%Cu-1at%Mg) the amount of Cu and Mg atoms that precipitate to form S phase is about 2at% [2,3], in an Al-1at%Si alloy the amount of Si that precipitates is about 1at% [4], in commercial Al-Zn-Mg-Cu based alloys the amount of precipitates is in the order of 4 to 7at% [5,6] (but these precipitates will contain some Al), in Al-Li-Cu-Mg alloys the amount of Li that precipitates to form δ' phase (Al 3 Li) is about 2 to 3 at% [7,8] and in Al-16at%Mg the amount of Mg atoms that precipitates to form β'' (Al 3 Mg) phase is about 6at% [9]. The Al alloys with the smallest amount of precipitation which have been studied by calorimetry are probably the Al-4.7at%Mg-0.25at%Cu-0.14at%Si alloy, in which about 0.5at% of Cu and Mg are expected to combine to form S phase (Al 2 CuMg) [10,11] and the Al-0.5at%Mn-0.3at%Fe based AA3003 alloy in which most of the Mn and Fe is thought to form intermetallic Al, Mn and Fe containing precipitates [12]. As far as we are aware no attempts to detect or analyse precipitation with calorimetry have been reported for metallic alloys in which the total amount of precipitating alloying elements is less than 0.5at%.The present paper reports work on a very dilute precipitation system: an Al-1.3at%Mg-0.4at%Mn alloy microalloyed with up to 0.07at%Cu. If S phase (Al 2 CuMg) forms in this alloy, the maximum

The thermodynamics of and strengthening due to co-clusters: General theory and application to the case of Al–Cu–Mg alloys

“…Bookmark not defined.] and references therein, and references [10,11,13,[15][16][17][18][19][20][21][22]44,51,52,53,65]), and concluded that the present model is consistent with all the available data. In particular it should be noted that no microstructure investigation ever showed evidence for precipitates (other than co-clusters suggested by APFIM, 3DAP) for ageing times within about 10 times the time to reach the plateau hardness (or time to stabilise the enthalpy).…”

“…Further corrections for imperfections of the DSC as well as correction for the heat capacity contribution not related to reactions of the samples is needed [43]. The optimum choice for this correction is alloy dependent and was considered in [44]. For the Cu-lean alloys 1 and 2, further correction was achieved by subtracting a linear function of T fitted to the initial part of the curve, where no reactions are thought to occur.…”

Age hardening and softening in cold-rolled Al–Mg–Mn alloys with up to 0.4wt% Cu

“…The model is limited to considering two precipitates. These are the precipitate responsible for peak strength in Al-Si-Mg based alloys, β″ phase [17,18,19], and the precipitate responsible for peak strength in Al-Cu-Mg based alloys, S phase [15,20,21,22]. In support it is noted that quaternary Al-Mg-Si-Cu precipitates (see e.g.…”

Reply to the comments on “Room-temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield-strength development”

“…To provide an alternative estimate of the magnitude of the heat effect we propose the following semi-quantitative comparisons. Estimates based on a regular solution model outlined in [ 32 ], indicate that about 90% of the Mg present in the Al-1.9Cu-1.6Mg alloy can precipitate as Cu-Mg co-clusters during a DSC run 1 . Then the enthalpy of formation in the Al-1.9Cu-1.6Mg alloy is 14 J/g × 27.5 g/mol/(0.016 × 0.9) = 0.27 eV per Mg atom 2 .…”