Tension—compression asymmetry of the (001) single crystal nickel base superalloy SC16 under cyclic loading at elevated temperatures

Abstract

Fully reversed low cycle fatigue tests were conducted on 〈001〉 oriented single crystals of the nickel base superalloy SC16 at a constant total strain range of 2.0%. The strain rates were varied from 10 −2 to 10 −5 s −1 at 750°C and the temperatures were changed from 650 to 950°C under the strain rate 10 −3 s −1. During cycling under the high strain rates 10 −2 and 10 −3 s −1 at 750°C or under the strain rate 10 −3 s −1 at 650°C, the SC16 single crystals show that the tensile stress ( T) is higher than the compressive stress ( C). During cycling under the low strain rates 10 −4 and 10 −5 s −1 at 750°C or under the strain rate 10 −3 s −1 at 850°C, the cyclic tension—compression asymmetry of C > T was observed. At 950°C under the strain rate 10 −3 s −1, no tension—compression asymmetry ( T = C) was present during fatigue testing. The deformation mechanism corresponding to T > C determined by transmission electron microscopy (TEM) is that the γ' precipitates are sheared by pairs of a/2〈110〉 matrix dislocations coupled by antiphase boundaries (APB) within γ' phase. The reversed asymmetry behaviour ( C > T) was found to be associated with the (111)〈112〉 viscous slip producing superlattice intrinsic/extrinsic stacking faults (SISF/SESF) in γ' precipitates. At 950°C ( T = C) dislocation climbing over the γ' precipitates becomes the dominant deformation mechanism. Models which can explain the tension—compression asymmetry behaviour were discussed.