Precipitation Behavior in High‐Purity Aluminium Alloys with Trace Elements – The Role of Quenched‐in Vacancies

Abstract

The main mechanism for the strengthening of aluminium‐copper alloys of the 2xxx type is hardening by copper‐rich precipitates. However, their size, distribution, and crystal structure determine the final mechanical properties of the material. It has been shown that alloying additionally small amounts of cadmium, indium, or tin influences the precipitation behavior as well as the final strength of Al‐Cu alloys. The binding energy of quenched‐in vacancies to trace elements in the aluminium matrix is recognized as an influence on the diffusion behavior of the copper atoms and thus the preferred type of precipitate changes. A precondition for this influence is the transition of trace elements into solid solution during the solution heat treatment. In the present work, solubility and interaction with quenched‐in vacancies is analyzed for the elements In, Sn, Sb, Bi, and Pb in high‐purity binary alloys using differential scanning calorimetry (DSC), positron annihilation lifetime spectroscopy (PALS) as well as scanning and transmission electron microscopy (SEM, TEM). The results confirm on one hand literature data and deliver on the other hand new structural details. A subsequent anneal at moderate temperature leads to finely distributed precipitations on the nanoscale.

The decomposition process in high-purity Al-1.7 at.% Cu alloys with trace elements: preservation of quenched-in vacancies by In, Sn and Pb influencing the $$\theta \, '$$ formation

“…However, the presence of 100 ppm Pb obviously hinders the diffusion of copper atoms slightly so that the hardness always lags a bit behind the pure base alloy. This is in agreement with our findings on binary alloys [21], where Pb shows either a very weak binding (below 50 meV) to vacancies or the solubility of Pb in aluminium is below 2 ppm. Natural ageing leads-for both Al-Cu and Al-Cu-Pb-to the formation of Cu clusters and GP-zones, which are typically detected by XRD after a few hours ageing at RT.…”

“…However, in this study the purity of the used aluminium was only 4N. Recently, it has been confirmed by the present authors that quenched-in thermal vacancies in high-purity binary alloys (5N5 Al with trace elements) are responsible for the diffusion of solute atoms at room temperature or slightly higher temperatures [21]. The latter study employed PALS, which is sensitive to relevant concentrations of vacancies and vacancies-like defects, but also coincidence Doppler broadening spectroscopy (CDBS), which can deliver information about the atom types attached to vacancies [22][23][24].…”

“…9a and b we show the microstructure after a shorter ageing time (1 h) at 150 C of the In and Sn containing samples, respectively. One can see clearly in comparison to ageing for 48 h that the decomposition of the alloy has just started-visible by platelike Cu precipitates forming on the f100g-planes and In/Sn precipitates as seen in binary alloys [21]. While there are obviously differences in the kinetics of Cu diffusion depending on the type of trace element during the first 24 h of ageing at 150 C, one can clearly notice that the microstructures for the Al-Cu base alloys with indium or tin look pretty similar after 48 h ageing at 150 C (see Fig.…”

“…Approaching the melting point of aluminum (660 C) a solubility between 0.091 and 0.026 at.% for cadmium, indium and tin is predicted. For other elements in that region of the periodic table (Sb, Pb and Bi) the data base is poor (see [21] and Table 1).…”

“…This has been done by employing the well known pseudopotential plane wave code VASP, determining the binding energies of vacancies to many elements in the periodic table including Cd/In/Sn [25]. As discussed in [21] there are still significant discrepancies between experimental studies [20,26] and ab-inito calculations [25], especially concerning the binding energy of vacancies to Mg-atoms.…”