Precipitation Strengthening of Binary Al-Li Alloys by δ' Precipitates

The published data on the strengthening of binary Al-Li alloys by δ′ precipitates are analyzed according to a recent version of the theory of order strengthening. The results are found to agree with the data provided that the antiphase boundary energy γ_apb on {111} is assigned a value in the range 0.140-0.160 J m^-2. The present results are in essentially perfect agreement with a recent analysis of the contribution of δ′ precipitates to the strengthening of ternary Al-Li-Cu alloys, which contain T₁ precipitates in varying proportions; in that study, γ_apb was found to be 0.150 J m^-2. The success of the current analysis indirectly supports the validity of the generalized superposition rule used to extract the δ′ contribution from the overall strength of the ternary alloys. The results of this study show that the δ′ contribution can be characterized uniquely by the order-strengthening mechanism. It is also shown that with this value of γ_apb the theoretically predicted dislocation pair spacings and average particle sizes at peak strength are in very good agreement with most of the available experimental results.