Abstract
We studied the ultrasonic velocity change caused by the accumulative creep damage in polycrystalline pure copper after high-temperature tensile loading. The propagation velocities of bulk waves, longitudinal and shear waves polarized parallel and perpendicular to the stress direction, showed a strong sensitivity to intergranular creep controlled by grain-boundary cavitation and subsequent microcracking. The velocities decreased slowly with creep time up to approximately 60 pct of the lifetime, when the steady creep shifted to the tertiary creep. Beyond this point, they decreased at ever increasing rates until eventual failure. The total velocity changes amounted to several percent of the original velocities. The creep damage also caused velocity anisotropy in the shear waves. Evolution in the anisotropy revealed that formation of cavity arrays, cavity coalescence, and microcracking, which occurred preferentially on boundaries lying normal to the stress axis, were restricted to the last 20 pct of the lifetime. Metallography and measurements of porosity support the ultrasonic observations.
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