Oxidation behavior of wrought, cast and additive manufactured Co-base alloys for high temperature applications

Various high temperature industrial processes require construction materials that possess an optimal balance of mechanical strength, corrosion resistance and fabricability. Among the main candidate materials considered for the long-term service at temperatures exceeding 600°C are metallic Fe-, Co- and Ni-base alloys. The present study followed the oxidation mechanisms of wrought, cast and additive manufactured (AM) Co-base alloys during air exposures at 950-1050°C to address the effects of alloy microstructure, minor alloying elements and specimen thickness on oxidation kinetics and subscale depletion processes. Substantial differences in oxidation behavior between cast and AM (produced by Selective Laser Melting) versions of alloy MAR-M-509 were correlated with the alloy grain morphology and size as well as distribution of the strengthening precipitates. Faster oxidation of the cast alloy was shown to be mainly induced by: (1) oxidation of coarse primary Ta-rich carbides, that oxidized rapidly forming thick oxide nodules and promoting development of cracks/pores in the adjacent Cr2O3. Cracking of the scale and its spallation provided direct access of nitrogen to the Cr-depleted base metal causing internal nitridation. (2) Incorporation of other alloy constituents (Co, Ni, W) into the scale was observed from Cr-depleted alloy thus decreasing the scale grain size and increasing its growth kinetics. In contrast, the microstructure of the AM MAR-M-509 featured fine carbide precipitates that were found to dissolve in the subscale region forming internal oxides and resulting in more homogeneous oxide scale compared to the cast alloy. EDX analyses in the subscale zone of AM specimens further revealed virtually flat Cr concentration profiles, contrary to the cast alloy which exhibited strong Cr depletion. Such behavior was proposed to be the result of enhanced Cr diffusion due to contribution of grain-boundary transport in the fine-grained AM alloy. High temperature exposures of the wrought Co-Cr alloys Haynes 25 and Haynes 188 showed significantly faster oxidation of the former material. Relatively high Mn content in Haynes 25 facilitated formation of Cr/Mn-spinel and increased the scaling rate of Cr2O3, which was found to exhibit a significant part of inward growth. TEM and APT studies of the chromia scale revealed a presence of intragranular Cr/Mn-particles possibly nucleating simultaneously with Cr2O3. Substantially better performance of Haynes 188 was interpreted in terms of differences in composition of Cr, Ni, Mn and especially La. The chromia scale formed on the alloy surface featured a distinct duplex morphology with coarse columnar oxide grains in the outer part and fine equiaxed grains in the inner layer. Observed sub-parabolic oxidation of Haynes 188 was attributed to the slow inward growth of the inner chromia layer as a result of reactive element incorporation into the scale, which could suppress outward cation transport. Dependence of oxidation kinetics on specimen thickness was analysed by oxidizing 0.15-1mm sections of wrought alloy Haynes 188. Exposures at 950 and 1000°C showed higher oxidation kinetics of thinner specimens with more pronounced Cr depletion in agreement with the common observations reported for Fe-Cr and Ni-Cr alloys. Higher oxidation rate of thin Haynes 188 specimens could be clearly observed already after initial hours of isothermal exposures in a thermobalance. The possible explanation of this effect was referred to low intrinsic creep strength of thin sections resulting in more pronounced response to oxide growth stresses. In contrast, during the testing at 1050°C the increase of oxidation-induced Cr loss with decreasing section thickness was not monotonous and showed a maximum in the 0.5 mm thick specimen. This result was correlated with rapid exhaustion of Mn reservoir in thinner specimens that appeared to reduce Cr depletion kinetics due to less pronounced Mn incorporation into the scale