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.
Highlights
The formation mechanism of the δ′ (Al3Li) phase in Al-Li alloys has been the subject of controversial discussion[1,2,3,4], since this process is assumed to take a complex pathway in the parameter space of chemical composition and order parameter as a function of temperature and time
Yoshimura et al.[17] employed in their investigation High Resolution Transmission Electron Microscopy (HRTEM) and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) and found first evidence for congruent ordering in an alloy with 8.9 at.% Li and 1.3 at.% Cu but no such occurrence in an alloy with 6.1 at.%Li and 1.3 at.% Cu
Atom Probe Tomography (APT), giving access to 3D compositional information, has successfully been employed in resolving the early stages of precipitation in Al alloys[18,19,20,21], and its combination with TEM seems the optimal set of experimental techniques to contribute to the open congruent order debate
Summary
The formation mechanism of the δ′ (Al3Li) phase in Al-Li alloys has been the subject of controversial discussion[1,2,3,4], since this process is assumed to take a complex pathway in the parameter space of chemical composition and order parameter as a function of temperature and time. The amount of contradicting studies on the precipitation mechanisms in binary Al-Li compounds casts doubt on the question whether all parameters responsible for the precipitation processes are always in the experimenters’ control This might especially be stressed in the light of new insight into the role of minor additions of supplemental elements such as Sn on the vacancy diffusion kinetics in Al alloys[14]. The Al-Cu-Li family shows here a desirable combination of mechanical properties such as a high Young’s modulus, a low density and a high yield strength[15, 16] Besides these technical benefits, Al-Cu-Li alloys apparently show altered kinetics for Li diffusion, as Yoshimura et al.[17] demonstrated a retardation of precipitation compared to binary Al-Li alloys. Atom Probe Tomography (APT), giving access to 3D compositional information, has successfully been employed in resolving the early stages of precipitation in Al alloys[18,19,20,21], and its combination with TEM seems the optimal set of experimental techniques to contribute to the open congruent order debate
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