Progression of TGO scale in a NiCoCrAlY/EB-PVD TBC system is monitored via analytical SEM and TEM from the first vacuum anneal of the NiCoCrAlY bond coat through the different processing stages of the EB-PVD Y-PSZ coating (preheating; initial and advanced stages of TBC deposition) and upon cyclic fatigue at T > 1100degreesC. During the early stages segregation of OF oxide constituents to the TGO scale is dominated by yttria giving rise to a transient "off-plane" mixed zone (Al2O3, ZrO2) microstructure textured perpendicular to the TGO/TBC interface. Subsequently Y-aluminates and spinel are formed. Local phase incompatibility of coexisting oxides is considered critical for the formation of failure-relevant clustered pores, as verified from the bond coat/TGO interface and the TGO mixed zone adjacent to the TBC top layer.