Intergranular oxidation and its influence on the high temperature mechanical properties of new nickel-based superalloys with different Nb/Ti ratios for turbine disk applications

• Grant number: 22/09466-1
• Support Opportunities: Scholarships abroad - Research
• Start date: March 06, 2023
• End date: March 05, 2024
• Field of knowledge: Engineering - Materials and Metallurgical Engineering - Physical Metallurgy
• Principal Investigator: Carlos Angelo Nunes
• Grantee: Carlos Angelo Nunes
• Host Investigator: Roger Charles Reed
• Host Institution: Escola de Engenharia de Lorena (EEL). Universidade de São Paulo (USP). Lorena , SP, Brazil
• Institution abroad: University of Oxford, England

Abstract

Jet engines correspond to complex machines, being used in the propulsion of airplanes and electric power generators. In the turbine region of these machines, very high temperatures are reached, and several components are subjected to high levels of stress, such as the discs. To obtain greater engine efficiency, with a reduction in fuel consumption and pollutant gas emissions, the inlet temperature of the hot gases in the turbine has been increased. With this, the operating temperature of the outermost part of the disk (rim) of the high-pressure turbine should reach values close to 800oC. Using an Alloy-By-Design (ABD) protocol, the Group of Prof. Roger Reed, from University of Oxford, defined new alloy compositions targeting these more critical applications. In terms of chemical composition, some of these alloys only vary in the values of the Nb/Ti ratio. However, despite the high mechanical strength of these polycrystalline alloys, their grain boundaries are a source of weakness at high temperature due to their interaction with oxygen-containing gaseous environments. In another study carried out in the Group of Prof Roger Reed, the sensitivity of these alloys with respect to embrittlement due to oxygen present in the air was investigated, through tensile tests carried out in air at a temperature of 700ºC. A significant increase in ductility and toughness was observed for higher values of the Nb/Ti ratio. However, despite carrying out a systematic work, it was not possible to find a conclusive explanation of the reason for this increase. Thus, we intend to advance in the understanding of the effect of the Nb/Ti ratio on the mechanical behavior of these alloys, by carrying out tensile tests using miniaturized specimens, at a low strain rate, in air, at room temperature, 600ºC; 700ºC and 800ºC. Tests will also be carried out in an argon atmosphere, to serve as a reference. The entire investigation will be accompanied by a systematic microstructural characterization of the materials before and after tests. A preliminary work should be carried out in order to determine heat treatment conditions that lead to a similar grain size for the three alloys, which was not the case in the work previously carried out (700ºC), since the alloys had different grain sizes. (AU)