Characterization of precipitation in Al-Li alloy AA2195 by means of atom probe tomography and transmission electron microscopy

TL;DR

This study investigates the nano-scale microstructure of AA2195 alloy after T8 tempering, revealing detailed compositions of precipitates using atom probe tomography and electron microscopy, and suggesting deviations from expected equilibrium phases.

Contribution

The paper provides detailed nano-scale chemical characterization of precipitates in AA2195 alloy, highlighting deviations from equilibrium compositions and the presence of Mg enrichment.

Findings

$ heta'$ platelets are less than 2 nm thick with expected ($Al_{2}Cu$) composition.

$T_{1}$ platelets show lower Li and Cu content than equilibrium, with Mg enrichment.

Results support that thin $T_{1}$ precipitates differ from bulk phase compositions.

Abstract

The microstructure of the commercial alloy AA2195 was investigated on the nano-scale after conducting a T8 tempering. This particular thermo-mechanical treatment of the specimen resulted in the formation of platelet-shaped $T_{1}$ ($Al_{2}CuLi$)/ $\theta^{'}$($Al_{2}Cu$) within the Al-matrix. The electrochemically prepared samples were analyzed by scanning transmission electron microscopy and atom probe tomography for chemical mapping. The $\theta^{'}$ platelets, which are less than 2 nm thick, have the stoichiometric composition consistent with the expected ($Al_{2}Cu$) equilibrium composition. Additionally, the Li distribution inside the $\theta^{'}$ platelets was found to equal the same value as in the matrix. The equally thin $T_{1}$ platelet deviates from the formula ($Al_{2}Li Cu$) in its stoichiometry and shows Mg enrichment inside the platelet without any indication of a higher segregation level at the precipitate/matrix interface. The deviation from the ($Al_{2}Li Cu$) stoichiometry cannot be simply interpreted as a consequence of artifacts when measuring the Cu and Li concentrations inside the $T_{1}$ platelet. The results show rather a strong hint for a true lower Li and Cu contents and hence, supporting reasonably the hypothesis that the real chemical composition for the thin $T_{1}$ platelet in the T8 tempering condition differs from the equilibrium composition of the thermodynamic stable bulk phase.