Characterization of Precipitation in Al-Li Alloy AA2195 by means of Atom Probe Tomography and Transmission Electron Microscopy

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)/θ ′ (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 θ ′ 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 θ ′ platelets was found to equal the same value as in the matrix. The equally thin T 1 platelet deviates from the formula (Al 2 CuLi) 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 CuLi) 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.3

Precipitation of T1 and θ′ Phase in Al-4Cu-1Li-0.25Mn During Age Hardening: Microstructural Investigation and Phase-Field Simulation

“…This indicates that the slightly brighter dots between the T 1 reflections pairs are overlays of θ ′ reflection rods. The absence of δ ′ is in a good agreement with other publications on AA2195 [ 55 , 56 , 57 ] which showed that coexistence of all three phases ( θ ′, T 1 and δ ′) depends on the Cu/Li ratio and temperature [ 58 ]. Finally, δ ′ precipitates are reported to form only in alloy composition higher than 5 at.…”

“…Finally, δ ′ precipitates are reported to form only in alloy composition higher than 5 at. % Li [ 55 ].…”

Characterising the Microstructure of an Additively Built Al-Cu-Li Alloy

“…Phases such as S and ϴ’ were equally observed at various ageing temperatures. Using APT and Transmission Electron Microscopy (TEM) to characterize a T8 processed AA 2195 Al-Li alloy, the authors [ 15 ] concluded that the composition of T 1 was non-stoichiometric and that it deviated from the stoichiometry (Al 2 CuLi) of the bulk equilibrium T 1 phase. The authors further concluded that the T 1 /matrix interface is one of Al-Cu layer rather than an Al-Li layer.…”

Macromechanics study of stable fatigue crack growth in Al–Cu–Li–Mg–Ag alloy

“…The alloy under study is the most advanced third‐generation Al–Li base alloy having low Li contents as compared with the earlier version of Al–Li base alloys. It is reported that the major strengthening precipitates in these alloys are T 1 (AI 2 CuLi), δ′ (AL 3 Li) and θ′ (AI 2 Cu) or Ω phase . The chemical composition of this alloy (Table ) depicts Cu to Li ratio, such that the alloy is equally stabilized by the Cu‐rich phases (Ω, θ′ and T 1 ) along with the main strengthening phase δ′ (AL 3 Li).…”

“…It is reported that the major strengthening precipitates in these alloys are T 1 (AI 2 CuLi), δ′ (AL 3 Li) and θ′ (AI 2 Cu) or Ω phase. [12][13][14][15] The chemical composition of this alloy (Table 1) depicts Cu to Li ratio, such that the alloy is equally stabilized by the Cu-rich phases (Ω, θ′ and T 1 ) along with the main strengthening phase δ′ (AL 3 Li). Therefore, it is quite interesting and fruitful to investigate the role of ageing precipitate (Cu-rich phases) on the FCG resistance of Al-Cu-Li-Mg-Ag-type alloy.…”