Effect of Boron Addition on Oxide Scale Formation of Ni-Base Superalloys

Abstract

Ni-base superalloys are commonly used as construction materials for high temperature components e.g. in the hottest sections of aero engines and industrial gas turbines due to a combination of excellent creep and high-temperature oxidation resistance at elevated temperature. Apart from major alloying elements such as Cr, Co, Al, Mo, W, Ti, modern Ni-based superalloys contain minor (between 0.01 and 1 at.%) additions of elements such as Hf, Zr or B for obtaining improved mechanical properties, in particular grain boundary strengthening. However, these minor elements also form thermodynamically very stable oxides and can therefore participate in the oxidation process. Whereas several investigations are available on the effect of Hf and Zr additions on the oxidation behaviour of high temperature alloys, hardly any publications deal with studies related to the effect of B additions on oxide formation mechanisms. In the present investigation the air oxidation behaviour of two commercially available B containing Ni base alloys (Rene80, CM247) was studied at temperatures in the range 850°C to 1050°C. After various exposure times up to 100h the oxidized samples were investigated with a number of analysis methods, including glow discharge optical emission spectroscopy (GD-OES), optical and scanning electron microscopy (SEM combined with EDX/WDX and EBSD) as well as X-Ray diffraction. B was found to become incorporated into the oxide scale whereby the participation in the oxidation process strongly depended on alloy composition. The oxidation of B resulted in its depletion from the bulk alloy and was found to be dependent on the exposure conditions, being stronger at higher exposure temperatures and longer exposure times. A modelling approach is presented to correlate B incorporation in the surface oxidation process with its depletion in the bulk alloy.