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Pores and carbides inherited from SX superalloy manufacturing processes usually act as stress concentrators and are preferential sites for fatigue crack initiation. In this study, pore & carbide size, morphology and distribution in a SX superalloy MD2 has been evaluated by X-ray CT. Strain accumulation and fatigue cracking behaviour in MD2, particularly around the pores and carbides, has been investigated by ex-situ SEM-DIC at room temperature along with image-based modelling of the observed MD2 defect populations obtained through X-ray CT imaging. The deformation structures have also been examined by electron channelling contrast imaging under controlled diffraction conditions. The results indicate that the pores & carbides with complicated three-dimensional features are the dominant fatigue crack initiation sites. Deformation is concentrated within intense slip bands and an enhanced strain …