STRENGTHENING MECHANISMS ASSOCIATED WITH T₁ PARTICLES IN TWO Al-Li-Cu ALLOYS

Abstract
The contribution to the yield strength or critical resolved shear stress (CRSS) of T₁ precipitates in two aged Al-Li-Cu alloys has been measured experimentally and analyzed theoretically. The tensile yield stress, σ_y, of alloys containing 2.30 wt.%Li-2.85 Cu-0.12 Zr (the 2-3 alloy) and 2.90 Li-0.99 Cu-0.12 Zr (the 3-1 alloy) was measured as a function of aging time, t, at 160 and 190 °C after subjecting the samples to reversion heat treatments to dissolve the δ' precipitates while leaving the dispersion of T₁ precipitates relatively undisturbed. The reversion treatments consisted of reheating the aged samples for 60 s at 265 °C for the 2-3 alloy or 315 °C for the 3-1 alloy, followed by quenching into water. The CRSS was calculated by dividing σ_y by the appropriate Taylor factors for the two alloys. The contribution to the CRSS of the T₁ precipitates, Ɗτ_T1, estimated by subtracting the CRSS of the matrix from that of the reverted samples, exhibited normal age hardening behavior. The average values of the diameter, thickness and volume fraction, f_T1, were measured as a function of t using transmission electron microscopy. Ɗτ_T1 was then analyzed quantitatively according to various theories of precipitation hardening. Most of the extant theories are applicable to spherical precipitates and therefore need to be modified for the plate shape of the T₁ particles and for the fact that they lie on {111}, meaning that only 3/4 of them are equivalent obstacles to dislocation motion. The data are in fair agreement with two versions of the theory of chemical strengthening, one based on conventional weak obstacle statistics and the other on strong obstacle statistics. The value of the T₁/matrix interfacial free energy required to fit either version of the theory is rather large (~ 6.58 or ~ 9.27 J/m²). However, the aspect ratio is also very large (40 to 50), implying a significant difference between the energies of the broad and peripheral interfaces of the T₁ precipitates and therefore an acceptable value of the energy of the broad interface (between 0.13 and 0.23 J/m²). The Orowan mechanism is also capable of predicting the experimental results with reasonable quantitative accuracy over the entire range of aging times, provided that allowance is made for the dislocation pairs observed in the microstructures of the reverted samples. The pairs are present because ordered δ' precipitates nucleate during quenching from the reversion temperature. Calculations suggest that the combination of dislocation pair spacings and T₁ particle spacings in this microstructure are such that the paired dislocations can be regarded as single dislocations of Burgers vector 2b. This analysis represents the first indication that the Orowan mechanism can account for a normal age hardening response.