L12 ordering and δ′ precipitation in Al-Cu-Li

Abstract

The precipitation mechanism of the δ′ (Al3Li) phase in Al-Li alloys has been controversially discussed in recent decades, specifically with respect to a conjectured congruent ordering process. However, kinetics in the Al-Li system does not allow to resolve the intermediate stages of precipitation and hence to experimentally clarify this issue. In this paper, we are revisiting the subject in ternary Al-Cu-Li alloys with pronouncedly slower kinetics, employing Transmission Electron Microscopy, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy, Differential Scanning Calorimetry and Atom Probe Tomography. The results show clear evidence for congruent ordering in a selected compositional range, revealing an already strongly L12 ordered microstructure after natural aging with a chemically homogeneous Li distribution and a decomposition of the alloy upon annealing at elevated temperatures. The presented study of the δ′ precipitation evaluates the reaction pathway of this process and compares it to the predictions of the Bragg-Williams-Gorsky model with respect to decomposition and ordering in this alloy system.

Microstructures and the Mechanical Properties of the Al–Li–Cu Alloy Strengthened by the Combined Use of Accumulative Roll Bonding and Aging

“…Finely dispersed nanosized δ 0 -Al 3 Li particles are visible in Figure 5a and θ 0 and T 1 (Al 2 CuLi) phases are in Figure 5b,c for the undeformed sample as indicated by the arrow and the circled area. Such microstructures are typical of decomposition in the Al-Li-Cu alloys, [19][20][21][22][23][24] but it should be noted that increasing lithium content favors the formation of δ 0 precipitates. [25] Similar microstructures are also observed in the ARB-processed samples: for the fine dispersions of nanosized δ 0 -Al 3 Li particles as observed in Figure 5d,g and for the θ 0 phase and T 1 phase as in Figure 5f,i.…”

The Li stance on precipitation in Al–Li-based alloys: an investigation by X-ray Raman spectroscopy

“…The binary alloy was solution-heat-treated at 540 °C and quenched in ice-water and, from a phase diagram perspective, into its (a þ d 0 ) phase region [12,[25][26][27][28]. As-quenched (AQ) samples of this alloy were stored, transported and measured at sufficiently low temperatures below -70 °C, in order to suppress aging kinetics.…”

“…The toolbox commonly used for their analysis ranges from rather classical methods such as hardness measurements [2], differential scanning calorimetry [3,4], diffraction techniques [5], and positron annihilation spectroscopy [4,6] to (transmission) electron microscopy [7][8][9], atom probe tomography [10,11] and small angle X-ray scattering [8,9]. Extensive combinations of these methods are also utilized [4,12]. Despite these experimental efforts, the decomposition sequences of age hardenable Al alloys are still not thoroughly understood due to several reasons.…”

Uncovering the influence of Cu on the thickening and strength of the δ′/θ′/δ′ nano-composite precipitate in Al–Cu–Li alloys

“…This clearly demonstrates the temperature dependence of the h 0 thickness. Generally, the metastable d 0 phase has lower thermal stability and poorer coarsening resistance than the h 0 phase [22,23]. Taking experiences from previous works on the d 0 in high-Li Al-Li alloys, we know that its ordering transformation is through congruent ordering ?…”