Surface Condition of New γ–γ′ Co-Al-Mo-Nb and Co-Al-W Cobalt-Based Superalloys After Oxidation at 800 °C

Abstract

A new type c-c¢ Co-Al-Mo-Nb Co-based superalloys were developed due to limitations of basic Co-Al-W superalloys, related to tungsten alloying. The present study aims to characterization of new c-c¢ Co-10Al-5Mo-2Nb (at.%) cobalt-based superalloy performance in terms of the high-temperature exposure under cyclic conditions, with particular regard to surface condition. Specimens were tested in cycles of hightemperature exposition (25, 50, 75, 100 and 150 h) in air environment at 800°C. Detailed analysis of oxidized surfaces by scanning electron microscopy, energy-dispersive spectroscopy and x-ray diffraction was made at various intervals during testing. The cyclic oxidation behavior of new alloy was compared to the basic Co-9Al-9W (at.%) Co-based superalloy.

Isothermal oxidation behavior of W‐free Co–Ni–Al‐based superalloy at high temperature

“…Obvious cracks parallel to the interface of the matrix and inner oxide layer in 2Mo superalloy formed, which might be caused by mechanical forces, especially the removal of material during cutting. [ 25,29,37 ]…”

“…With increasing temperature to 1173 K, the outer oxide Co 3 O 4 with a thermodynamically unstable state will transform to CoO. [ 25 ] Meanwhile, alloying elements, reacted with oxygen and diffused to subsurface and surface of the superalloys, aggravate the depletion of the related elements during oxidation, which promotes the formation of the γʹ‐free zone closed to the matrix. In addition, the continuous Al 2 O 3 layer approximately disappeared at 1173 K, indicating that the accelerated depletion of the Al element might indirectly provide the paths of rapid diffusions of Co and Ni with the increase in temperature.…”

“…Obvious cracks parallel to the interface of the matrix and inner oxide layer in 2Mo superalloy formed, which might be caused by mechanical forces, especially the removal of material during cutting. [25,29,37] According to EDS point-scanning analysis and XRD results, three obviously different oxides layers can be distinguished for all the samples. Oxides of 2Cr and 2Mo oxidized at 1073 K include a Co 3 O 4 outer layer, an inner layer with complex oxides, and an IOZ with heterogeneous Al-containing and Ti-containing oxides.…”

“…[15,23] Generally, oxides formed on the surface of Co-based superalloys at high temperatures include a Co-containing outer layer, an inner layer with complex spinel oxides containing Co, Al, and an Al 2 O 3 -rich internal oxidation zone (IOZ). [23][24][25][26][27][28] Although Al-rich spinel oxides can play a significant role in inhibiting element diffusion, unfortunately, the production of spinel oxide is accompanied by the invalidation of Al 2 O 3 and the Cr 2 O 3 protective layer. Co 3 W with D0 19 structure is usually formed in the internal Al 2 O 3 precipitation zone due to excessive consumption of Al during oxidation of W-containing Co-based superalloys, which affects the strength of the superalloys.…”

“…[23,27,32,33] However, Al + Cr content decreasing to be less than 10 wt% might significantly accelerate the oxidation of materials. [25,26,34] In addition, as a parameter to evaluate the energy barrier of the oxidation process, the oxidation activation energy can reflect the oxidation resistance of the superalloy. Understanding the situation of oxidation activation would be beneficial to perfect the design parameters of Co-Ni-Al-based superalloys.…”