With the increasing power and thermal efficiency of aero-engines, the surface service temperature of turbine blades and other components is getting higher and higher. The harsh service environment makes high-temperature oxidation one of the main factors of component failure, which can lead to catastrophic consequences. The study of high-temperature oxidation behavior of directionally solidified nickel-based high-temperature alloys, which are used as materials for critical parts of aero-engines, has become crucial.
On the one hand, the relationship between the surface weight gain kinetics and the oxide film thickening kinetics of nickel-based alloys needs to be elucidated; on the other hand, the oxide film structure is affected by a variety of factors, such as the surface state, the temperature and the alloying elements, etc., and the relationship between the oxide film structure, the morphology evolution and the oxidized weight gain kinetics under different conditions is still unclear, and it needs to be further revealed.
In this paper, through the study of the oxidation kinetics and oxide film structure of the domestically produced directional solidification nickel-based high temperature alloys DZ125 and DD6 under different conditions, the relationship between the surface weighting kinetics and oxide film thickening kinetics is clarified; the oxide film growth mechanism of the alloys under different conditions is revealed; and the relationship between the oxidation weighting kinetics and the oxide film structure and oxide film morphology evolution is established. The details are as follows.
(1) The oxidation kinetics of DD6 and DZ125 alloys were investigated, and it was found that the kinetics of surface weight gain and the kinetics of oxide film thickening showed similar behavioral characteristics, and there was a good agreement between the two.When Ra=0.6-0.7 μm, the oxidation kinetics of both alloys did not have the phenomenon of phasing, and the oxidation reaction index, n≈0.2, followed the asymptotic horizontal line growth law. At Ra=0-0.1 μm, Ra=0.1-0.2 μm and Ra=0.3-0.4 μm, the oxidation kinetics of the two alloys were phased, and the weight-gaining reaction indices n of the two alloys in the first and the second phases showed large differences. Among them, the first stage of the oxidation kinetics of DD6 and DZ125 alloys followed the cubic growth law (n≈0.3) and parabolic growth law (n≈0.5), respectively; and the second stage of the oxidation kinetics of the two alloys followed the asymptotic horizontal growth law (n≈0.1-0.2).
(2) Through the analysis of the two alloys’ oxide film structure and oxidation kinetics, two oxidation modes are proposed: the single-layer oxide film structure and oxidation weight gain kinetics are not divided into phases is called Ⅰ type growth mode (Type ⅠGrowth Mode); multi-layer oxide film structure and oxidation weight gain kinetics are divided into phases is called Ⅱ type growth mode (Type ⅡGrowth Mode). Growth Mode). It was revealed that the phasing of the surface weight gain kinetics in the Type II growth mode was due to the difference in the growth rates of the different oxide layers, and that the surface weight gain in the first phase was determined by the growth of the outer oxide film and the inner oxide film, while the surface weight gain in the second phase was controlled by the growth of the middle oxide film.
(3) The studies on the morphology, elemental distribution and phase structure of the oxide films of the two alloys in the short and long time in the Ⅰ and Ⅱ growth modes show that the oxide films of the two alloys in the Ⅰ growth mode, DD6 and DZ125, are mainly composed of continuous Al2O3, while the oxide films of the two alloys in the Ⅱ growth mode are composed of multiple phases. Among them, the short-time oxide film of DD6 alloy is composed of NiO, Ni(Co)Cr2O4, and Ta2O5 discontinuous α-Al2O3 from the outer surface layer to the substrate, respectively; the oxide film of DZ125 alloy is composed of Cr2O3, unoxidized region, Ta2O5 and discontinuous α-Al2O3 from the outer surface layer to the substrate, respectively. The long-time oxide film DD6 alloy is composed of NiO, CoAl2O4, CoCr2O4, Ta2O5 and continuous α-Al2O3 from the outer surface layer to the substrate; DZ125 alloy is composed of NiO, NiCr2O4, CoCr2O4, Ta2O5 and continuous α-Al2O3 from the outer surface layer to the substrate, respectively.
(4) Through the analysis of the formation conditions of the alloy oxide film products: the formation conditions of the alloy early oxide film products are elucidated, and the high roughness surface preferentially generates Al2O3, which is a necessary condition for the oxidation behavior to follow the Ⅰ-type growth mode; whereas the difference in the content of Cr, the solute element of the two alloys, is a sufficient condition for the existence of a large difference between the initial oxide products of the Ⅱ-type growth mode. The influence of surface roughness on the critical concentration of Al2O3 in the multiple nickel-based alloys for the occurrence of external oxidation is revealed, and the promotion of short-circuit diffusion on the surface with high roughness leads to the gradual shift of the critical concentration line to the direction of low Al concentration, while the effect of temperature on the critical concentration line is relatively small.
(5) Through the analysis of the evolution of the distribution characteristics of the alloy oxide film, it is shown that: Ⅰ type growth mode DD6 alloy oxide film morphology is characterized by discontinuous spherical Ta2O5 phase randomly doped in the middle of the Al2O3 film, DZ125 alloy HfO2 randomly doped in the lower part of the Al2O3 film; Ⅱ growth mode DZ125 alloy outer surface layer of the short-term Cr2O3 with the increase of oxidation time, gradually replaced by Cr2O3 with the increase of oxidizing time. With the increase of oxidation time, the outer layer of DZ125 alloy is gradually covered by NiO, and the middle layer is transformed into a series of complex spinel phases from the unoxidized region and Ta2O5. It is found that the evolution of the oxide film morphology of the alloy is related to the elemental enrichment and depletion behavior in the region of the non-precipitated phase: the enrichment of Co and Cr solutes in the region of the non-precipitated phase of the type-II model increases gradually with the extension of the oxidation time, which promotes the growth of the oxide film in the intermediate layer.
(6) The analysis of the growth mechanism of the oxide film of the two alloys shows that: the oxide film evolution and the distribution characteristics of the oxidation rate constant k show a good correspondence, Ⅰ type growth mode oxide film is a continuous dense Al2O3, k distribution in the vicinity of the Al2O3, the same way Ⅱ type growth mode follows a similar pattern. It is found that the surface weight gain kinetic characteristics of the alloy are closely related to the growth mechanism of the oxide film and have a direct relationship with the morphology of Al2O3: when Al2O3 is continuous, the growth mechanism of the oxide film of the alloy is dominated by the diffusion of metal cations through the Al2O3, and the index of oxidation reaction is n=0.1-0.2; when Al2O3 is discontinuous and the outer oxide film is not dense, the oxidation mechanism is dominated by metal cations and anions, and the oxidation reaction index is n=0.1-0.2; when Al2O3 is non-continuous and the outer oxide film is not dense, the oxidation mechanism is dominated by metal cations and anions. When Al2O3 is discontinuous and the outer oxide film is not dense, the oxidation mechanism of the alloy is dominated by the bidirectional diffusion of metal cations and anions to control the growth of oxides, and the oxidation index of the alloy is n=0.3; when Al2O3 is discontinuous but the outer oxide film is dense, the oxidation mechanism of the alloy is dominated by the diffusion of metal cations through the outer layer, and the oxidation index of the alloy is n≈0.5.
Post time: Aug-05-2023
