J. Phys. Colloques
Volume 48, Numéro C3, Septembre 1987
4th International Aluminium Lithium Conference
Page(s) C3-373 - C3-383
4th International Aluminium Lithium Conference

J. Phys. Colloques 48 (1987) C3-373-C3-383

DOI: 10.1051/jphyscol:1987343



1  Materials Science and Technology Division, Mail Stop K-765, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
2  Department of Materials Science and Engineering, University of California , Los Angeles, CA 90024, U.S.A.

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, σy, 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 σy by the appropriate Taylor factors for the two alloys. The contribution to the CRSS of the T1 precipitates, ƊτT1, 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, fT1, were measured as a function of t using transmission electron microscopy. ƊτT1 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.