Understanding how creep deformation evolves during exposure to service conditions is crucial for the design and development of new nickel-based superalloys. The present work aims to improve this understanding through a combination of novel mechanical experiments and damage characterisation methods.
First, small-scale creep tests are carried out on an Electro-Thermal Mechanical Testing (ETMT) system, while accurately measuring the local increase in creep strain over time by direct image correlation (DIC). This setup allows tests to be interrupted after different levels of creep strain; specimens can be easily removed without any additional damage. Three recently developed single-crystal superalloys were tested to assess the influence of gamma prime volume fraction and of solid solution strengthening in the gamma matrix on creep resistance.
Second, damage accumulation is characterised with cross-correlation HR-EBSD and ECCI under controlled diffraction conditions (cECCI) to yield both a qualitative understanding of the emerging dislocation network structures as well as quantitative measurements of changes in GND and total dislocation densities. This approach leads to a more comprehensive description of damage evolution and enables a clear comparison between the three tested materials.
Third, discrete dislocation dynamics (DDD) modelling is employed as a comparison to experimental results, with regards to both dislocation density measurements and to observed dislocation arrangements.
The information derived is used to inform a new physically-based model for creep resistance of single-crystal superalloys in the intermediate temperature and stress regime. This model is applied in the context of Alloys-By-Design methods to identify optimal alloy compositions for certain design constraint parameters and is proven to offer a more accurate description of experimental results when compared to existing merit indices. This combination of small-scale mechanical testing and rapid alloy design has the potential to significantly accelerate design-make-test cycles and to reduce the time necessary to introduce new grades of alloys into industrial applications.