Insight into the failure and healing of oxide scales during cyclic oxidation of powder metallurgy superalloy with various Nb content

Failure and healing behavior of oxide scales for powder metallurgy superalloy with varying Nb contents (0, 0.5, 1, 2 wt%) during cyclic oxidation were studied. The cyclic oxidation test of four alloys was conducted using the discontinuous thermogravimetric method which consists of 100 cycles of 1 h exposure in air from 20 °C to 900 °C. The oxide scales after cyclic oxidation were systematically characterized by scanning electron microscope (SEM), laser scanning microscopy (LSM) etc. It has been found that the spallation of oxide scale in all alloys took place via expanding of the initiated cracks at grain boundaries into grain matrix. After spallation, the exposed alloy matrix would be re-oxidized into a Cr2O3 layer to recover the failed oxide scales. The spallation tendency of oxide scale was significantly inhibited by Nb addition. This can be ascribed to the faster regeneration of Cr2O3 layer in alloy with higher Nb content. •Nb addition significantly decreased the spallation of oxide scales.•The spallation in oxide scales starts in terms of cracks near grain boundaries.•The repetitive regeneration of Cr2O3 of high-Nb alloy resulted in a smaller spallation area of the oxide scales.