Vacancy-Solute Interactions in Al-Cu-Mg

Abstract

This is an initial report of a multi-technique study on the effect of Mg alloying on solute-vacancy interactions during the early stages of ageing of dilute 2xxx Al-Cu-Mg alloys so as to better understand the early rapid hardening (RH) that occurs in certain compositions of these alloys and the more general phenomena of secondary hardening (SH) at ambient temperatures. Therefore, RH at 150 °C and SH at room temperature from the as-quenched condition and after 60 sec ageing at 150 °C were studied in Al-1.1Cu and Al-1.1Cu-0.5Mg (at. %) variously by positron annihilation lifetime spectroscopy (PALS), coincidence Doppler broadening (CDB) spectroscopy and atom probe tomography (APT) and monitored by Vickers hardness measurements. The present results indicate that Cu-Cu, Mg-Mg and Cu-Mg clusters are formed in the ternary alloy already in the asquenched state and that they persist during ageing at 150 °C. The fraction of the solutes Cu and Mg that were associated with vacancies after ageing was increased 10-fold and double, respectively and the strength of the Cu clustering is enhanced greatly after 60 sec at 150 °C.

Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations

“…Up to four Cu atoms, it is reasonable to assume that the Cu atoms are on the same ͓100͔ plane around the Al vacancy, maximizing the Cu-Cu distances. This is because according to atomic probe field ion microscopy experiments, Cu tends to form platelets perpendicular to the ͗100͘ direction, 1,14,36 and also because according to our calculated total energies, these configurations are favored in contrast to more dense Cu clusters. However, as listed in Table II, in addition to these planar configurations, we have considered three Cu atoms as nearest neighbors to each other and four Cu atoms in a tetragonal configuration.…”

Advances in positron studies of age hardening in light alloys

“…3), with a concomitant increase of the trapping fraction (F=85% in Al-1.1Cu-0.5Mg (in at.%) [11,28]). Figure 4 Local concentrations at the trapping site [11].…”

“…In the example of Ref. [11], the increase of the trapping fraction from 70% to 85% implies the increase of the trapping rate by a factor 2.4; however, surprisingly, the new positron traps contain on the average only 2% Mg (see Fig. 4); put in a different way, this means that no more than 24% of all trapping sites are in contact with Mg atoms.…”

“…The general features brought out by positron annihilation measurements can be summarised as follows. Cu clusters existing in binary Al-Cu alloys, after a solution treatment at high temperature followed by quenching at RT, display a clear role of vacancy stabiliser, demonstrated by evident symptoms of positron trapping: in asquenched Al-Cu samples the positron lifetime is higher than in bulk Al and in bulk Cu (values from 195 to 208 ps, for Cu atomic concentrations from 1.1 to 2 % [5,[28][29][30][31][32][33], instead than 164 ps for bulk Al and 110 ps for bulk Cu), and the CDB spectra reveal substantial trapping (trapping fraction F = 70% [11,28]) with a clear Cu fingerprint [11,28,30], although the vacancy environment is predominantly Al (Cu/Al atomic ratio ≈ 22/78 [11,28], corresponding to less than 3 Cu atoms on the average over the 12 nearest-neighbours of a lattice site). As solute aggregation proceeds toward the formation of ordered GP zones of pure Cu, the positron lifetime gradually tends to the bulk value and the trapping fraction from CDB data decreases, demonstrating that the vacancies are released from the clusters and eventually lost.…”

“…Since then, the study of secondary ageing has been intensely pursued by positron annihilation (PALS and CDB) in combination with other techniques (see, e.g. [11,17,28]). It is now clear that secondary ageing is the indication of the independent formation of a fine distribution of coherent solute aggregates, additional to the precipitates formed during the preliminary treatment at elevated temperature [2].…”