On the development and investigation of the high-temperature oxidation behavior of Co-Re-base alloys

As an alternative potential materials class of so-called “Beyond Ni-base Superalloys”, Co-Re-base alloys were firstly proposed by Rösler from the Technical University of Braunschweig in 2007 for applications at temperatures beyond 1200°C. Various studies have been consequently carried out in the framework of the German Research Foundation (DFG) research group “FOR 727” aiming to find additional alloying elements to improve both the mechanical properties and oxidation performance of the Co-Re-base alloys. Based on Gorr’s previous work within the framework, the study presented aims to investigate and develop further alloying concepts (Si(-B), Cr, Al, Y, and Ni) to improve the oxidation resistance of the alloy system. The focus is given to the formation of thermally grown protective SiO2, Al2O3, and/or Cr2O3 scales on the Co-Re-base alloys.
Model alloys Co-17Re-9Si-8B, Co-17Re-23Cr-xSi (x = 0, 1, 2, 3, 4), Co-17Re-23Cr-xAl (x = 5, 10), Co-17Re-xCr-2Si (x = 23, 25, 27), Co-17Re-25Cr-2Si-xY (x = 0.05, 0.1), and Co-17Re-23Cr-15Ni were produced through arc melting followed by proper heat treatment (numbers are given in at.%). The oxidation tests of these alloys were performed in air over the temperature range from 800˚C to 1300˚C with the aid of thermogravimetry. The microstructure of the alloys and the oxidation products was characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), electron backscatter diffraction (EBSD) technique, and transmission electron microscopy (TEM). Thermodynamic studies using commercial software FactSage were employed helping to establish the oxidation mechanism.
Results show that the formation of a thermally grown SiO2 layer on the Co-Re-base alloys through macro-alloying with Si is problematic with respect to high-temperature applications of such an alloy system, since the Si addition to the Co-Re alloys strongly reduces their melting temperature. The preferential formation of Co2SiO4 in the model alloy Co-17Re-9Si-8B restricts the formation of a continuous SiO2 layer. The addition of both 5% and 10% Al significantly improves the oxidation resistance of the reference alloy Co-17Re-23Cr. While the main contribution of 5% Al addition is that it acts as an oxygen “getter” promoting the formation of a protective Cr2O3 layer, the 10% Al addition is almost sufficient for the formation of a continuous Al2O3 layer up to 1100˚C. The severe internal nitridation of Al at 1200˚C and above leads to intensive oxidation attacks due to the lack of a protective Al2O3 layer.
The investigation of Co-17Re-23Cr-(1-4)Si and Co-17Re-(23-27)Cr-2Si alloy series indicates that both the Si and Cr significantly promote the formation of brittle and poorly oxidation-resistant primary σ phase in the alloy. Despite the positive effect on isothermal oxidation resistance, the increase of Si content successively intensifies the spallation of the Cr2O3 layer during cooling. The minor addition of Y up to 0.1% tremendously improves the adhesion of the Cr2O3 scale forming on the alloy Co-17Re-25Cr-2Si, especially under the thermocyclic condition. A 15% Ni addition to alloy Co-17Re-23Cr is not only beneficial for the mechanical properties but also improves the oxidation resistance significantly. The refinement of the σ phase through Ni addition allows short-peening treatment to further facilitate the formation of the Cr2O3 scale on the alloy Co-17Re-23Cr-15Ni.