In situ Synchrotron X-Ray diffraction study of high-temperature stress relaxation in chromia scales containing the reactive element yttrium

Viscoplasticity and growth strain parameters identification by full modelling optimization during the high temperature oxidation of Ni28Cr modified by the reactive element yttria or zirconium

“…24.b indicate that there is no increase of stress, due to the protection of oxide layer formed at 1273K, which proves that the oxide layer formed at high temperature prevents the formation of oxide layer at low temperature. This phenomena has been discussed in previous works [12]. In the present work, a full model is applied that leads to the same conclution, which comfirms that the oxide layer did not increase when temperature jumps were applied towards lower values.…”

“…To determine the elementary mechanisms that govern the viscoplastic behavior of the oxide layer as a function of the amount of yttria deposited, the growth mechanism of the thin oxide layer should be considered at the same time [12]. It is known that the growth mechanism of the thin chromia oxide layers becomes anionic when a sufficient quantity of a reactive element is introduced; the formation of the oxide layer is thus controlled by the internal diffusion of oxygen along the diffusion short-circuits, i.e.…”

“…As previously discussed for the reactive element Y2O3, to determine the elementary mechanisms that should govern the viscoplastic behavior of the oxide layer as a function of the amount of yttria deposited, the growth mechanism of thin oxide layer should be considered at the same time [12]. It leads to the same conclusions for the reactive element zirconium as for yttria.…”

“…The same approach as described in Refs. [12][13][14] has been applied, with a stress exponent value equal to unity in the Norton flow model. The Norton coefficient J for the oxide is given by:…”

“…This has been verified through the XRD intensity evolution in the oxide phase (not shown). Therefore, D 2 can be chosen equal to zero at 973K, 1073K and 1173K with a good confidence [12]. Additionally, the parabolic kinetics parameter (Ap) is equal to zero, because of no growth of oxide at lower temperature plateau.…”