Vacancy‐solute association in coherent nanostructures formed in a commercial Al‐Zn‐Mg‐Cu alloy

Abstract

The role of vacancy-solute association in the formation of coherent precipitates at room temperature (RT) and at 70 °C was studied by means of coincidence Doppler broadening (CDB) spectroscopy. The formation of clusters and GP zones was monitored by differential scanning calorimetry (DSC). Two conditions were explored at low temperature: a) one-step aging after solution treatment and quenching and b) secondary aging (after solution treatment and quenching + 5 days at RT + 8 min at 150 o C). The assessment of the average fractional concentration of solute in contact with the vacancies was obtained by analyzing the CDB data. The results show considerable aggregation of Cu to vacancies at the very early stage of decomposition of the supersaturated solid solution; the contribution of Zn and Mg increases progressively with the formation of coherent precipitates. The preliminary exposure at 150 °C does not alter this sequence, although a reduction in the dissolution enthalpy observed during DSC scans shows retarded formation of coherent precipitates. 1 Introduction The Al-based alloy 7012 has been the subject of several investigations using positron lifetime spectroscopy (PALS) [1][2][3][4][5] with the purpose to correlate the strong hardening response to aging of this alloy with the sequence of precipitation. The value of PALS in the study of precipitation kinetics comes from the sensitivity of the positron lifetime to changes in the distribution of vacancy-like defects associated to different precipitates (solute clusters, GP zones and η' phase in the present study). The present investigation includes PALS measurements at the very early stage of aging at room temperature (RT) on a time scale never explored before. However, the main attention is shifted on the interaction of vacancies with the different atomic species, with the aim to provide basic elements of knowledge regarding the mechanisms by which the vacancies affect the transport of the solute and the stability of the precipitates. To this end, the chemical environment of the vacancies has been characterized by low-noise measurements of the momentum distribution of the positron annihilation radiation by the coincidence Doppler broadening (CDB) method [6]. Differential scanning calorimetry (DSC) has been adopted to identify the species of precipitates formed during the progression of a multi-step heat treatment including secondary aging at moderate temperature [7].

Advances in positron studies of age hardening in light alloys

“…The surprise given by these alloys was a non-monotonic trend of the positron lifetime during natural ageing, a behaviour that is not known for other systems and had never been reported before Ref. [4], but has then been definitely confirmed as a peculiarity of Al-Zn-Mg [15,17] (see Fig. 6).…”

“…The effect of different sequences of thermal treatments and of Mg, Cu and/or Ag additions as ternary or quaternary elements has been investigated for commercial and laboratory Al-Zn-based alloys by PALS and CDB in combination with DSC, SAXS, TEM [4,15,17]. The surprise given by these alloys was a non-monotonic trend of the positron lifetime during natural ageing, a behaviour that is not known for other systems and had never been reported before Ref.…”