Characterization of precipitation in Al-Li alloy AA2195 by means of atom   probe tomography and transmission electron microscopy
  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. 
Muna Khushaim, Torben Boll, Judith Seibert, Fredinand Haider and   Talaat Al-Kassab
cond-mat.mtrl-sci