Microstructure Evolution of a Precipitation Hardened Cu-Ni-Si Alloy during Low Cycle Fatigue

Abstract

The low cycle fatigue (LCF) resistance of a precipitation hardened Cu-Ni-Si alloy has been investigated. Fully reversed strain controlled LCF tests were performed in air at room temperature at a total strain variation Δεt included between 0.6 and 1.5% and at a strain rate equal to 4.10-3 s-1. The cyclic accommodation of the alloy is composed of a hardening step (for Δεt > 0.8 %) followed by a continuous softening until the specimen fracture, except at Δεt = 0.6 % where only a slight softening is observed. For all tested strain variations, the microstructure study of the fatigued alloy by transmission electron microscope (TEM) revealed some grains with a high density of isolated short dislocations pinned between two nanoprecipitates. For Δεt ≥ 0.8 %, some grains present also deformation bands observed by TEM. The strain localisation into these bands implies an important shearing of the precipitates. It leads to their dissolution into the copper matrix and forms precipitate-free bands. The resulting macroscale cyclic accommodation depends on the ratio between the two mechanisms of plasticity accommodation at the microscale.