Order hardening: comparison between revised theory and experiment

Abstract

The theory of order-hardening is recast in a form which makes no assumptions about the constancy of the dislocation line tension at any point in its derivation. It thus overcomes an objection raised by Melander and Persson in a recent publication. The data of Melander and Persson on precipitation hardening in underaged Nimonic 80A are analysed in the light of the present theory, as are earlier data on underaged Ni-Al alloys. It is demonstrated that the contribution of the γ′ precipitates to the critical resolved shear stress of underaged Nimonic 80A can be predicted nearly exactly by the present theory using a value of 0.159 J m⁻² for the energy of the antiphase boundary on the {111} slip planes of the γ′ particles. The agreement between the present theory and the data on underaged Ni-Al alloys is also very good. It is therefore possible to explain these data without invoking a contribution of coherency hardening, which should be nearly the same in both systems since the γ/γ′ lattice mismatches are similar. It is concluded that the temperature independence of precipitation hardening in underaged Ni-Al alloys provides the most convincing experimental evidence that coherency hardening makes virtually no contribution under these circumstances. The appropriate experiments on underaged Ninomic 80A have not yet been done.