LCF and TMF of Superalloys Used for IGT Blades and Vanes

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. In particular, the blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified or cast as single crystals. Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature and operating environment. The types of tests and data used to evaluate these components are collected from an extensive search of the open literature. The compilation includes smooth specimen, strain-controlled TMF, low cycle fatigue (LCF) and LCF with hold times either at peak compression or tension. The key elements that have greatest influence on the life of this class of alloy are identified. Comparisons between the more complex TMF test and simpler isothermal LCF with hold times as a cyclic test method for qualification of alternative repair, rejuvenation, and heat-treatment procedures is discussed. Generating fatigue crack initiation models that account for these several materials, temperature, and cyclic loading factors is challenging. Hence, a general model that covers a broad range of possible TMF loading scenarios useful for evaluating designs or different operating scenarios is still lacking. Using the data gathered, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, in addition to temperature, and several other cycling and loading parameters.