Predicting failure damage in single-crystals super-alloys
The blades used in gas turbines experience extremely high temperatures and stresses during use. The complexity and high degree of interrelationship between these damaging mechanisms makes their study a difficult task. For instance, start-up, shut-down and change power requirements can give rise to combined creep and fatigue failure with undesirable breaking effects. Increasing protection against failures involves the employment of advanced materials, coatings and cooling schemes and/or combinations of those. Current industrial practices utilise single-crystal nickel based super-alloys for modern gas turbines blades. This is mainly due to their capabilities to increase the turbine's inlet temperature and consequently reduce fuel consumption of power units. Coated with TBCs, these advanced materials can considerably improve the thermodynamic effectiveness of the blades. Despite this, breaks seem to be unavoidable since several undesirable effects are also inferred, including substrate damage, oxidation and TBC degradation. Urged by these needs, this project analysed damage mechanisms for uncoated and coated monocrystalline nickel based super-alloys. Concentrating on the microstructure of these materials, the studies resulted in modelling damage for both fatigue and TBC degradation phenomena. The developed micro, meso and macro mechanical models were used for developing assessment tools that predict lifetimes of actual turbine blades. More specifically, the models are based on the precipitates microstructure, the casting defects and protective TBC properties. On the grounds of structure analysis, validation of several lifetime assessment tools was also performed. The lifetime prediction that is highly related to the developed assessment tools is expected to offer great advantages to gas turbine manufacturers and users. It may contribute to the design of blades with increased durability and reliability and the accurate settings of repair schedules for downtime minimisation and output maximisation.
