Alloys based on Cr-Cr2Ta containing Si

Dual phase alloys containing Cr-Laves phases have been considered as potential candidates for high temperature service. In this paper, the effect of Si additions on site occupancies in Cr-Cr2Ta alloys has been studied. Calculations using density functional theory predict that Si substitutes for the Cr atoms rather than Ta within the structure and that Si and Cr are relatively free to occupy 2a and 6h sites interchangeably. However, slight non-uniformity in the 6h Si-Cr minimum charge density and bond length leads to Si favouring 2a occupation within C14 (Cr2−xSix)Ta. These predictions were supported by studies using high-resolution synchrotron X-ray powder diffraction. The effect of quaternary additions on the microstructures and elevated temperature oxidation characteristics of Cr-Ta-Si-X alloys, where X = Ag, Ti, Hf, Mo, Al and Re have also been investigated. The microstructure of the alloys comprised primarily of a eutectic mixture of a Cr-rich solid solution and Cr2Ta-based Laves phase along with primary dendrites of C14 Cr2Ta. Upon isothermal exposure of the alloys for 100 h at 1000 °C, the Re-containing alloy was found to possess the best oxidation resistance, followed by the Hf-containing alloy. The alloys exhibited a significantly superior oxidation resistance at 800 °C owing to the formation of an adherent chromia layer. The oxidation behaviour of the alloys could be described using a combination of linear and parabolic growth laws.