Abstract
The age hardening response of a high-purity Al–4Cu–1Li–0.25Mn alloy (wt. %) during isothermal aging without and with an applied external load was investigated. Plate shaped nanometer size T1 (Al2CuLi) and θ′ (Al2Cu) hardening phases were formed. The precipitates were analyzed with respect to the development of their structure, size, number density, volume fraction and associated transformation strains by conducting transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) studies in combination with geometrical phase analysis (GPA). Special attention was paid to the thickening of T1 phase. Two elementary types of single-layer T1 precipitate, one with a Li-rich (Type 1) and another with an Al-rich (Defect Type 1) central layer, were identified. The results show that the Defect Type 1 structure can act as a precursor for the Type 1 structure. The thickening of T1 precipitates occurs by alternative stacking of these two elementary structures. The thickening mechanism was analyzed based on the magnitude of strain associated with the precipitation transformation normal to its habit plane. Long-term aging and aging under load resulted in thicker and structurally defected T1 precipitates. Several types of defected precipitates were characterized and discussed. For θ′ precipitates, a ledge mechanism of thickening was observed. Compared to the normal aging, an external load applied to the peak aged state leads to small variations in the average sizes and volume fractions of the precipitates.
Highlights
Al–Li base alloys represent an attractive material class for weight critical applications in aviation and space flight industry
In the Al–Cu–Li system, δ0 (Al3 Li), θ0 (Al2 Cu) and T1 (Al2 CuLi) are the main phases that contribute to the precipitation hardening [2,3,4,5,6,7,8]
We extended the previous work by investigating long term aging and aging under external load
Summary
Al–Li base alloys represent an attractive material class for weight critical applications in aviation and space flight industry. These includes, e.g., fuselage and upper wing structures or cryogenic tanks [1]. These alloys are mainly strengthened by precipitation hardening. In the Al–Cu–Li system, δ0 (Al3 Li), θ0 (Al2 Cu) and T1 (Al2 CuLi) are the main phases that contribute to the precipitation hardening [2,3,4,5,6,7,8]. Nobel and Thompson [2] and Cassada et al [10,11] proposed
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