Oxidation Resistance Of Superalloys Research Articles

Evolution of Recrystallization Texture in A286 Iron-Based Superalloy Thin Plates Rolled via Various Routes

The A286 iron-based superalloy has wide-ranging applications in replacing expensive nickel-based and cobalt-based superalloy to manufacture the turbine disk as well as the pipelines and valves of the fourth-generation supercritical water reactor (SCWR) working below 650 °C. The recrystallization texture importantly affects the mechanical properties and oxidation resistance of superalloys. However, very few investigations are available on the recrystallisation texture of A286 alloy. The current work reports the texture of A286 alloy fabrication using various rolling routes, including one-stage cold rolling with the rolling rates of 83% (Route I) and 91% (Route II), and two-stage cold rolling with the rolling rate of 83% + 50% (Route III). Route III is preferentially recommended to manufacture A286 alloy thin plates due to the obviously reduced work hardening as well as the weakened recrystallization texture and anisotropy; moreover, compared with other routes, the recrystallized grain size of Route III did not significantly coarsen. We also revealed the mechanism for the effect of cold rolling textures on the final recrystallization texture and the role of the deformation twin in {110} <115> grain. In this study, recrystallization texture develops through two independent mechanisms related to different deformation microstructures, including the recrystallization texture inherited from deformation texture and the recrystallization texture depended on deformation twins.

Effects of Si variation and Zr addition on the microstructure, oxidation resistance and mechanical property at 1523 K of Nb-Si-Ti/Zr/Hf-Cr/Al high-entropy superalloys

The present work investigated the influences of the variation of Si content and the minor amount of Zr addition on the microstructure, oxidation resistance and mechanical property at 1523 K of Nb-Si-Ti/Zr/Hf-Cr/Al high-entropy superalloys systematically. Alloy compositions with Nb-21.6(Ti/Zr)-3.1Hf-(9.4–15.6)Si-6.3Cr-3.1Al-1.0B (at. %) were designed using a cluster formula approach to realize the substitution of similar elements, in which these alloys were solid-solutionized at 1573 K for 24 h and then aged at 1273 K for 24 h. It was found that all these aged alloys are mainly composed of BCC-Nb solid solution (Nbss), Nb5Si3 and Cr2Nb phases, where the increase of Si content and the addition of Zr could significantly increase the volume fraction of Nb5Si3 in a form of coarse and primary particles. Consequently, high-Si alloys possess outstanding mechanical property at high temperature, as evidenced by the fact that the yield strength (σYS = 373 MPa) at 1523 K of high-Si/Zr alloy containing 15.6 at. % Si and 1.6 at. % Zr is much higher than that (σYS = 266 MPa) of the low-Si alloy with 9.4 at. % Si and 0 Zr. It is mainly ascribed to the existence of primary Nb5Si3 particles, besides the eutectic microstructure composed of Nbss and Nb5Si3, as well as fine Cr2Nb precipitates in Nbss. Moreover, a minor amount of Zr substitution for the Ti is conducive to the phase transition of α-Nb5Si3 to γ-Nb5Si3. Thus, this high-Si/Zr alloy exhibits an improved oxidation resistance after 50 h-exposure at 1523 K, due to the lowest mass gain of about 80.9 mg/cm2. The constitutions of oxide scales in these alloys were characterized and compared, from which it was found that the better oxidation resistance is closely related to the formation of dense oxide scales. This work will provide a new approach to obtain an optimum combination of mechanical property and oxidation resistance of superalloys through multi-component alloying.

Phase Stability, Structure and Thermodynamics of Modified Ni- and Fe-Aluminides

The Ni-aluminides are integral constituents of thermal barrier coatings applied over Ni-based superalloys. These aluminides provide oxidation-resistance by forming a protective α–Al2O3surface layer. The Pt-modified β–NiAl bond coat has been developed with an impetus to increase the service-life of Ni-based superalloys. The Pt-modified β–NiAl bond coat significantly improves the oxidation-resistance of superalloys. An interdiffusion zone containing topologically closed packed phases develops at the bond coat/superalloy interface. This eventually leads to Al-lean γ′–Ni3Al transformation, whose oxidation resistance is inferior to that of β–NiAl. The Pt-group metals Ir and Ru delay this transformation and impart creep-resistance to the bond coat. Recent investigations demonstrate that alloying with transition metals such as Cr, Mo and Fe enhance the mechanical strength. The functional stability of bond coat-superalloy assembly counts on the interfacial reaction and associated local structural variations which is a function of bond coat composition. This chapter elucidates the effect of various alloying elements on phase constitutions, crystallographic structural stability and thermodynamics of Ni-and Fe-aluminides to engineer a prospective bond coat.

Relation between the residual stresses and the high-temperature oxidation resistance of superalloys protected by plasma-sprayed coatings. II

Previous studies on the oxidation behavior of two superalloys, IN100 and CMSX2, protected by LPPS MCrAlYTa coatings, were the basis to relate the high-temperature oxidation resistance to the oxidation stresses. Details are given on the choice of the X-ray diffraction conditions and on the calculations of the crystallographic elastic constants of each phase studied. Dilatometric tests were used to determine the expansion coefficient of the coatings and substrates; a difference in expansion creates stresses during cooling. It has not been possible to determine the Al2O3 oxidation stresses by X-ray diffraction on such systems. Calculations give an Al2O3 stress which agrees with values found by other authors in Al2O3, NiO, or Cr2O3, taking into account the differences in expansion coefficients of the various systems. The as-sprayed coatings are subjected to tensile stresses, due to the Δα between the coating and the substrate. During oxidation, these stresses decrease, owing mainly to the interdiffusion phenomena between the coating and its substrate. This confirms that the more resistant system must consist of CoNiCrAlYTa-coated IN100.

Improvement of the oxidation resistance of superalloys by low-pressure, plasma-sprayed MCrAlYTa coatings. I

The oxidation behavior of two superalloys (CMSX2 and IN100) protected by low-plasma-pressure-sprayed MCrAlYTa coatings (M = Co, Ni) was studied with emphasis on the kinetics and microstructural aspects. The oxidation behavior of each alloy (substrate and coating) was studied separately in order to obtain a better understanding of the oxidation mechanism of duplex systems (coated substrate) under both isothermal and cyclic conditions (mainly at 1100°C and in 1 atm O2). Although all these systems develop an alumina scale, differences were observed as a result of several factors: (1) the coating and substrate composition, thus the presence, amount and distribution of addition elements (Y, Ta, Ti, C,...); (2) the interdiffusion phenomena that induce microstructural modifications; and (3) the porosity amount and roughness of the coating. The system that appears to be the most resistant against cyclic oxidation consists of CoNiCrAlYTa-coated IN100. This cannot be explained by the present study. Further work was conducted, particularly on the oxidation stresses, as described and discussed in paper II.