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INTRODUCTION Hardening by copper-rich precipitates can be a source of embrittlement in commercial steel alloys. In particular, reactor containment vessels for the nuclear power industry are subject to such embrittlement, due to radiation-enhanced diffusion and precipitate formation [1]. Currently, reactor vessels are qualified for operation by periodic testing of in-situ surveillance specimens. Nondestructive methods to monitor vessel properties directly could improve on this conservative approach and extend the lifetime for vessel operation. Here, we report on ultrasonic experiments to evaluate these microstructural properties.

EXPERIMENT Sample Preparation Our experiments used a steel that could be embrittled through thermal treatment to produce microstructures similar to those created through irradiation. This surrogate material was a high-strength, low-alloy steel with 1.13 mass % copper (ASTM A710). As in the reactor steel, embrittlement in this material occurs by formation of ultrafine (nanometer-sized) copper-rich precipitates. The precipitates restrict the motion of dislocations and thus increase the material's hardness and yield stress [1]. Two sets of specimens were produced. The three specimens in series I were heated to 900 "C for 1 h and air-cooled to room temperature. This solution treatment left the copper in the matrix while maintaining a small (-,10 inm) grain size. Specimen 1-2 was measured without additional aging; specimens I-I and 1-3 were subjected to further aging treatments for 1 h at 700 'C (I-1) and 525 °C (1-3). These conditions had been previously determined [2] to produce material of much lower hardness (overaged) and close to maximum hardness (peak aged), respectively. The seven specimens in series II were furnace-cooled to room temperature after the inital 900 'C solution treatment. Specimen 11-1 was measured without additional aging. The remaining specimens were subjected to aging at 450 'C for the following lengths of time: 11-2, 1.3 h; 11-3, 4.0 h; 11-4, 10.8 h; 129 Mat. Res. Soc. Symp. Proc. Vol. 591 C 2000 Materials Research Society

11-5, 25.0 h; 11-6, 66.7 h; 11-7, 166.7 h. This process was expected to produce a range of material aging conditions and a corresponding range of hardness. Microstructural Characterization Specimen hardness was measured with standard indentation methods. The Rockwell A hardness (HRA) values are given in Table I and represent an average of several measurements taken at different locations in the sample. As expected, the hardness depended on the thermal treatment. For the three samples in series I, hardness values ranged from 53.2 HRA to 58.5 HRA. In series II, specimen hardness started at 50.5 HRA and gradually increased with increasing aging time to a maximum of 59.0 HRA. Aging beyond 25 h caused the hardness to decrease slightly. Small-angle neutron-scattering (SANS) measurements were made on the specimens in series Ii. The difference scattering curves were compared to a theoretical scattering cross-section that assumed a log-normal size distr