The thermal stability of topologically close-packed phases in the single-crystal Ni-base superalloy ERBO/1

In Ni-base superalloys, the addition of refractory elements such as Cr, Mo, Co, W, and Re is necessary to increase the creep resistance. Nevertheless, these elements induce the formation of different kinds of intermetallic phases, namely, the topologically close-packed (TCP) phases. This work focuses on intermetallic phases present in the second-generation single-crystal (SX) Ni-base superalloy ERBO/1. In the as-cast condition, the typical γ/γ′ structure is accompanied by undesirable intermetallic phases located in the interdendritic regions. The nature of these precipitates as well as their thermal stability between 800 and 1200 °C has been investigated by isothermal heat treatments. The investigation techniques include DSC, SEM, EDX, and TEM. The experimental information is complemented by (1) comparison with a structure map to link the local chemical composition with phase stability, as well as (2) thermodynamic calculations based on the CALPHAD method to determine the occurrence and composition of phases during solidification and in equilibrium conditions. The TCP phases Laves, µ and σ were identified in various temperature/time ranges. © 2015, Springer Science+Business Media New York

The role of local chemical composition for TCP phase precipitation in Ni-base and Co-base superalloys

The precipitation of topologically close-packed (TCP) phases in single-crystal superalloys is highly undesirable due to the detrimental effect on the mechanical properties. The TCP phases bind atoms responsible for the solid solution strengthening of the γ′ phase (Re, W, Mo), as well as elements that are important for the formation of the γ phase (Ti, Ta). A thorough understanding of TCP phase precipitation is therefore a prerequisite for the design of future superalloys. The thermodynamic stability of TCP phases as bulk material is meanwhile well understood. However, little is known about the factors that govern the stability of the experimentally observed precipitates of TCP phases within the superalloy matrix. The focus of this paper is the role of the local chemical composition for the stability of TCP phase precipitates. We combine experimental measurements of the local chemical composition of TCP phase precipitates in the Ni-base superalloy ERBO-1 and the Co-base superalloy ERBO-Co0 with a theoryguided interpretation of the structural stability of bulk TCP phases. The experimental characterization of the microstructure and the crystal structure of the precipitates are based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The local chemical composition is determined by energydispersive X-ray spectroscopy (EDX) and electron probe microanalysis (EPMA). The measured local chemical compositions are assessed regarding the likelihood of TCP phase formation by determining their location in a structure map of the stability of bulk TCP phases. This establishes a direct link between the measured local chemical composition of TCP phase precipitates and the phase stability of bulk TCP phases at this composition. By converting the measured local chemical composition into structure-map coordinates, we effectively construct a compound map that indicates the regions in the microstructure that are prone to TCP phase precipitation. Analyzing the intermetallic precipitates in more detail, we find that the chemical compositions of the TCP phase precipitates would be expected to form the same TCP phase as bulk material. This suggests that the observed precipitates of TCP phases can be regarded to be in a local thermodynamic equilibrium. The challenge of predicting TCP phase precipitation in superalloys is hence reduced to the prediction of the local chemical composition during casting, heat treatment and service