The influence of aging and stabilizing annealing at elevated temperatures on the precipitation structure and hardness of an Al-Cu-Sn alloy

Abstract

Elevation of the heat resistance of the precipitates for wrought heat-treatable alloys of 2xxx series at temperatures higher than 250 °C is one of the most complicated unsolved problems in physical metallurgy of aluminum alloys. The data obtained in recent years demonstrate that, for cast heat-treatable Al-Cu-Mn-Zr-based alloys, the procedure of long-term high-temperature annealing may lead, under certain conditions, to the stabilization of the precipitation structure as a result of significant segregation of the atoms of some alloying elements on the θ′/(Al) interfaces. In the present work, to study this possibility for the case of wrought alloys, we successively perform the operations of homogenization annealing and hot rolling at 450 °C with a total degree of deformation equal to 80%, solid-solution heat treatment at 535 °C/1h with quenching into water followed by aging at 250 °C and stabilizing annealing at 300 °C for 100 h for a new tin-containing alloy with the following composition (wt.%): Al‑4.9Cu‑0.5Mn‑0.14Si‑0.19Zr‑0.1Sn. The level of hardness attained in the alloy as a result of the performed operations was ~87 HV. The analysis of the precipitates carried out by the transmission electron microscopy method demonstrated that, in the process of high-temperature aging at 250 °C/6 h, one can observe the formation of lamellar precipitates with high density of distribution. The subsequent procedure of high-temperature stabilization annealing at 300 °C/100 h leads to a noticeable but nonuniform coarsening of the structure of precipitates. The quantitative analysis performed by the atom-probe tomography (APT) method showed that, as a result of annealing at 300 °C for 100 h, the θ′-phase crystals become saturated with Mn (0.34 ± 0.08 at.%), Si (0.64 ± 0.03 at.%), and Zr (0.04 ± 0.04 at.%). The analysis carried out by the proxigram method revealed high susceptibility of Mn and Zr to segregation on the coherent θ′/(Al) interface; moreover, the most significant role in this case was played by the highest attained concentration of Zr (~0.2 at.%).