Microstructural Damage Evaluation in Ni-based Superalloy Gas Turbine Blades by Fractal Analysis

Ni-based superalloys are used as turbine disc and blade material in which creep, fatigue and creep-fatigue are the important damage mechanisms. Mechanical properties of these alloys depend upon the amounts of γ - γ’ present in the microstructure as well as precipitation of carbides along the grain boundaries. The distribution of γ’ depends on the chemical composition, operating temperature and the length of service exposure. During service exposure, as damage accumulates progressively, the morphological characteristics of microstructure change which needs to be assessed using metallographic technique. Conventionally, the extent of damage resulting in deterioration of mechanical properties is quantified by hardness measurement. The variation in hardness is correlated with the morphological features in the metalographic images by identifying precipitation of carbides, presence of cuboidal γ’ and the structural changes that occur in the matrix. In this paper, we report fractal dimensions of the insitu metallographic images which can correlate the progressive damage accumulation at various locations of the blades.