Test of Superalloy Oxide Scale Growing Stress by Rectangle Wedge Slice Moment Method

Stress is the essential factor to cause surface oxide scale exfoliation to lose oxidation resistance for alloys. So the research on oxide scale mechanical behaviors plays the key role to explore the oxidation resistance of alloys. In this paper, a new kind of experimental technique “rectangle wedge slice moment method” is studied to test oxide scale stress of ferro based superalloy GH2038 during high temperature oxidation. Oxide scale growing stress could be calculated directly by detecting the free end displacement of the rectangle wedge slice using the derived formula. Having been oxidized at 700°C from 10 to 50 h, the oxide scale growing stress versus time is regressed to follow power equation, and the oxidation weight gain and scale thickness with time both abide parabolic law. By SEM, EDS and XRD analyses, oxide scales are highly composed by Cr₂O₃ and spinel Fe(Ni)Cr₂O₄ with even fine grain size and compact structure. By calculating the high temperature standard Gibbs forming free energy of oxides, analyzing the processes of oxidation and reduction reactions, the generating model of oxide scale growing stress is established and the mechanism is studied. New born oxides from reduction and secondary oxidation growing up at the interface of matrix and scales create the oxide scale growing stress. More oxides bring higher oxidation weight gain and bigger growing stress in total, however, oxidation weight gain rate decreases gradually with time, so does the increment of oxide scale growing stress, endowing stronger oxidation resistance for superalloys.