Strengthening mechanisms in two Al-Li-Cu alloys

The room temperature yield strength of two alloys, Al-2.30Li-2.85Cu-0.12Zr (the 2-3 alloy) and Al-2.90Li-1.00Cu-0.12Zr (the 3-1 alloy) was investigated as a function of aging time at 160 and 190°C for times up to the peak strength condition. Reversion experiments were used to separate the contributions of the T₁ and δ^' precipitates to the overall strength of the alloy. The reversion heat treatments produced complete dissolution of the δ^' phase, enabling the individual contribution of the T₁ precipitates to be determined experimentally. The contribution of the δ^' precipitates was then obtained using a generalized addition rule and analyzed according to the theory of strengthening by ordered coherent precipitates. The superposition law used was Δτ_p^q=Δτ_T1^q + Δτ_δ'^q, where Δτ_p is the total contribution of all the precipitates to the CRSS (taken as one-third of the yield stress); Δτ_T1 is the contribution of T₁ particles, Δτ_δ' that of the δ^' particles and q is a constant. Only those values of Δτ_δ' extracted from the data using q = 1.5 were consistent with theoretical calculations employing a single value of the δ^' antiphase boundary energy on {111} (ϒ_apb = 0.19 J/m²)for both the 2-3 and 3-1 alloys.  This value of ϒ_apb predicts values of the dislocation pair spacing and the peak strengthening that are in good agreement with experimental observations.  While q appears to be an arbitrary adjustable parameter, its magnitude is justified by the published results of computer simulation experiments on strengthening by two distinct obstacles, one of which is intrinsically much stronger than the other.