Snoek relaxation and dislocation damping in aged Fe-Cu-Ni steel
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MANY studies of thermal-aging embrittlement of copper-containing ferritic steels have appeared. They include transmission electron microscopy (TEM),[1–4] field-ion microscopy,[2,5] computer simulation,[6,7] electrical resistivity,[8] and atom-probe characterization.[9] Their motivation came not only from scientific interest in lattice defects but also from the necessity of understanding the irradiation embrittlement in reactor pressure-vessel steel, because such a material is widely used in energy power plants, and the microstructural change caused by irradiation relates to that caused by isothermal aging. Detailed studies with TEM revealed the evolution of copper clustering in the ferriteiron matrix: in the early stages of aging, small bcc copper precipitates (⬃1 nm) nucleate and grow,[1,2] which strain the lattice and increase the hardness. At a critical size of about 5 nm, they transform to a twinned-9R-crystal structure, similar to the bcc-to-9R transformation observed in Cu alloys.[3] Further aging and growing produces a stable fcc structure following a heavily distorted fcc structure.[3] The material shows the maximum hardness at the phase-transformation stage. In the present research, we studied the effect of copper clustering on the behavior of the Snoek relaxation and on ultrasonic attenuation. We used a commercially available alloy steel (A710) containing 1.29 at. pct Cu, subjected to various isothermal heat treatments at 723 K. Using a forcedvibration torsion-pendulum method, we studied two properties relevant to the Snoek relaxation: the internal-friction spectrum and decay rate of the maximum internal friction
after quenching. Also, we studied the evolution of the ultrasonic shear-wave attenuation after quenching, using electromagnetic acoustic resonance (EMAR). The internal-friction spectrum suggested favorable binding between copper substitute atoms and 2nn-sited carbon atoms. The decay measurements provided a view of dislocation-density evolution with aging, which is supported by TEM observation. II. MATERIAL Table I shows the chemical composition. After a heat treatment of 1173 K for 1 hour and furnace cooling, we machined seven plate-shaped specimens. We then exposed them to different aging times at 723 K (specimen numbers S1 through S7). The specimens were 1-mm thick and 76mm long. Figure 1 plots the Rockwell hardness vs aging time. The hardness changed as expected. The S5 specimen showed the maximum hardness, 17 pct higher than that before aging. III. MEASUREMENTS A. Isothermal Mechanical Spectroscopy Low-frequency internal-friction measurement allows us to understand point-defect diffusion and point-defect interactions with dislocations and substitutional solutes in a host metal.[10] Anelastic phenomena arise from the stress-induced reorientation caused by jumps of such a defect, to cause the internal friction (Q⫺1) expressed by the Debye function: Q⫺1() ⫽ ⌬
HIROTSUGU OGI, Associate Professor, is with the Graduate School of Engineering Science, Osaka University, Osaka 560-8531, Japan. H
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