The effect of semicoherent precipitation on void swelling in Al-Cu alloys

Aluminum-4 wt pct copper alloys previously heat treated (50 h at 473 K) to contain semi-coherent θ′ precipitate were neutron irradiated to a fast fluence of 2.8 × 10²⁶ n/m² (E > 0.1 MeV) at 328 K to determine the effect of the precipitate on void swelling relative to swelling observed in elemental aluminum under the same irradiation conditions. The ob-served void swelling values were ∼1 and ∼12 pct for the alloy and elemental aluminum, respectively. The broad faces of the θ′ precipitate underwent at least partial coherency loss, the matrix dislocation density increased somewhat, and some of the transmutation-formed silicon segregated to θ′ interfaces and some voids during the irradiation. The θ′ particle size distribution did not change significantly during the irradiation. The swell-ing behavior of the alloy under irradiation was analyzed by extending the rate theory of swelling to include reduction of irradiation-caused point defect concentration by indirect recombination, a result of trapping free point defects at θ′ interfaces. Calculated swell-ing in the alloy using the model agreed well with the experimental observation. Accord-ing to the model the suppression of swelling by θ′ precipitates under the irradiation con-ditions used occurs by extension of the knee of the swelling curve, not by changing the dose exponent for swelling.