Failure mechanism of turbine guide vane and oxide composition analysis on the surface of failure vane cracks

This study deals with the investigation of failure mechanism of turbine guide vane and the oxide composition on the surface of failure vane cracks. The material of vane is Co-based superalloy DZ40M, which has directional solidified grain structure. The investigate methods include optical microscope (OM) observation, scanning electron microscopy (SEM) observation and energy dispersive X-ray spectrometry (EDS) analysis. The results of crack morphology observation show that: the cracks are distributed at almost all the rows of cooling holes at the middle of the vane and most macroscopic morphology of cracks are transverse propagation (perpendicular to directional solidification direction). Due to the high temperature environment and action of cycle thermal stress during service, the recrystallization tissue near the cooling holes which produced by high-energy laser production process can easily become the starting point of cracks. And the cracks propagate along the grain boundaries and sub-boundaries as the large difference of thermal expansion coefficient between carbides and matrix in high temperature. The results of oxide composition analysis show that, oxide layer on the surface of cracks can be divided into two layers. The main composition of outer-oxide layer is mainly the oxides of CoO, CoCr₂O₄ and a small amount of Cr₂O₃, evenly distributed on the crack surface with about 10 μm thickness. The inner-oxide layer discontinuous distribution with spherical and needlelike morphologies, mainly composed by oxides of Cr₂O₃ and Al₂O₃, the thickness is about 30 μm.