Investigation of Stress Assisted Grain Boundary Oxidation Cracking in MAR-M002 High Pressure Turbine Blades

Abstract

Modern superalloys have enabled high pressure turbine (HPT) blades in gas turbine engines (GTE) to operate at higher temperatures. Unfortunately, the complexity of these materials can make it difficult to understand the failure mechanisms of these blades. HPT blades made of the nickel-based superalloy Mar-M002 have been found to suffer from stress assisted grain boundary' oxidation (SAGBO) cracking. HPT blades removed from an RB21I-24C aeroderivative industrial GTE were sectioned, atul the cracks and microstructure were studied using scanning electron micro.icopy (SEM) and energy dispersive X-ray spectroscopy (EDS). No cracks were found on the external surface of the blade, which had been coated with an oxidation resistant material. Surface irregularities were found along the walls of the inner cooling channels throughout the entire blade. Larger SAGBO cracks were observed to be near the lower 25% span of the blade and had initiated from the surfaces of the cooling channels. SEM/EDS analyses showed that these cracks had large amounts of alumina and hafnium-rich particles within them. It is evident that these cracks occurred in locations where the combination of high stress and high temperature led to higher rates of oxygen diffusion and subsequent oxidation of grain boundary' carbides. Hafnium carbide precipitates along the grain boundaries e.xpanded as they converted into hafnium oxide, thus further increasing the stress. It is envisaged that this increase in stress along the grain boundary has caused the cracks to initiate and coalesce. Based on this observation, it is believed that the inner cooling channels of these HPT blades could benefit from the application of cm oxidation resistant coating in order to prevent or delay the formation of these cracks.

Performance of High-temperature Coatings : Oxidation and Interdiffusion

“…För att lösa denna kompromiss användes två strategier i denna forskning. Turbine engines play a critical role as power generation and aircraft propulsion supporting our modern civilization [1][2][3][4]. It is an equipment to transfer the heat energy to a mechanical power by burning fuels.…”

The influence of stress on the oxidation of a Ni-based superalloy

“…These recent observations provide persuasive evidence that the damaging effect of an oxidising environment arises from the fracture of these brittle oxides which will lead to enhanced crack growth. SAGBO, however, would also require the intrusion growth rate along the grain boundary to be increased by tensile stress, a detail seldom reported since the term was coined [51][52][53][54].…”

Experimental Study on Coaxial Waterjet-Assisted Laser Scanning Machining of Nickel-Based Special Alloy

“…In modern aero and industrial gas turbine engines, the combustion temperatures are increasing to optimize power and efficiency [ 1 , 2 ]. Nickel-based alloys have been widely applied in the hot end parts of aero and gas turbine engines because of their excellent high-temperature strength, fatigue properties, corrosion, and oxidation resistance [ 3 ].…”