Grain boundaries and creep deformation mechanisms in a high performance nickel-based superalloy

Abstract

This dissertation is focused on the description of creep deformation in the new Rolls-Royce Alloy 12-1 polycrystalline nickel-based superalloy for disc application in gas turbine engines. In particular, this study aims to progress the understanding of the effect of grain boundaries in relation to the content of minor additions such as C, B and Zr. This was achieved by creep testing at conditions representative of service in gas turbine engines and extensive use of electron microscopy. Alloy 12-1 microstructure was characterised before and after interrupted and failed creep tests at 700°C. Grains exhibited intragranular precipitation of large cubic MC carbides and tetragonal M3B2 borides. Grain boundaries showed a serrated character with large γˈ particles and fine tetragonal M5B3 precipitates before creep testing. Analysis after creep testing revealed a progressive evolution of grain boundary microstructure which is related to the localised accumulation of strain. Orthorhombic M2B, tetragonal M2B and orthorhombic MB borides were detected at the grain boundaries after creep deformation. It is suggested that the combination of strain accumulation and atomic diffusion during creep leads to the formation of these types of secondary precipitates. Analysis of the deformation mechanisms revealed that these varied depending on the testing temperature. Dislocation and stacking fault formation are the main mechanisms at 700°C. or below, whereas perfect dislocations glide and bypass of secondary γˈ particles is predominant at 800°C. However, deformation always initiated at grain boundaries independently from the testing conditions. Grain boundary serrations promoted the formation of dislocation networks which were linked to both the nucleation of stacking faults and the inhibition of their impingement on opposite grain boundaries. This promoted an homogeneous stress distribution preventing grain boundary sliding and cavitation, the latter being limited to triple points. Grain boundary migration and γˈdirectional coarsening were commonly observed after creep at 800°C. However, grain boundary borides showed a pinning effect that limited the migration. Finally four alloys based on Alloy 12-1 composition and varying contents of C, B and Zr were creep tested at 700°C and at 800°C. Despite showing a similar initial microstructure, mechanical testing results showed that grain boundary composition strongly affects creep resistance. A lower B content was found to be beneficial during creep at 700°C and 800 MPa, whereas increasing the amount of C was found to improve creep resistance for any testing condition. This was attributed to the increased carbide and boride intragranular precipitation which disrupted dislocation and stacking fault propagation. Presence of Zr was found to be critical to promote the precipitation of carbides and borides. A reduced content of Zr was associated with higher grain boundary mobility, particularly during creep at higher temperature.