Precipitation in solution-treated aluminum-4wt%copper under cyclic strain.

摘要

With the aim of assessing the effects of cyclic plasticity on the precipitation behavior of Al-4wt%Cu, solutionized specimens were cycled at epsilon p=+/-0.001, 0.0025 and 0.005 at 25°C, 100°C, 175°C and 200°C. The temperatures and strain amplitudes sampled showed a broad range of behaviors, encompassing both the effects of the cyclic strain on the precipitation behavior, and of the precipitation behavior on the cyclic stress-strain response of the material. The excess vacancies produced by cyclic strain accelerated the diffusion kinetics of precipitate growth, while the presence of a high dislocation density provided ample sites for heterogeneous nucleation of precipitates.;The combination of heterogeneous nucleation with accelerated growth led to the formation of theta' precipitates under conditions which would ordinarily be associated with the growth of theta" precipitates. It appears that the dominant factor in the promotion of theta' over theta" is heterogeneous nucleation on dislocations, although precipitate scissioning may play a role by suppressing formation of theta".;Increasing strain amplitudes also led to an acceleration of precipitation kinetics, with decreasing effect at higher strain amplitudes where vacancy supersaturations are consumed by recovery. Above 100°C, recovery initially reduces the strain hardening of the sample material, but the onset of precipitation hardens the material, leading to different mechanical behaviors than observed at 100°C and room temperature.;The effects of texture at 25°C and 100°C appear qualitatively similar to what would be expected on the basis of monotonic tests, but become more complicated at higher temperatures as hardening effects of precipitate formation interact with texture effects. Greater precipitation rates offset the effects of dynamic recovery at 175°C and 200°C, leading to continuous hardening throughout life.;The expected mode of transgranular fatigue fracture was observed at 25°C and 100°C, but at 175°C and 200°C an unexpected mode of intergranular failure occurred at all strain amplitudes sampled. Although intergranular cracking is common in quench-aged Al-4wt%Cu, it is not commonly associated with very fine precipitation. The presence of ductile dimples on the fracture surface suggests that the formation of grain boundary precipitates may also be enhanced proportionally to the increase in diffusion under cyclic strain.