Stray grains formation in single-crystal components made of Nickel-based superalloys

Ni-base superalloy single-crystal turbine blades are widely used in gas turbines for aircraft propulsion and power generation as they can be subjected to high service temperature and show high mechanical properties due to the almost total elimination of grain boundaries. Particularly in presence of complex geometry shapes, rare grains nucleating apart from the primary grain, become a serious problem in directional solidification, when characterized by high-angle boundaries with the primary grain, extremely brittle due the elevated amount of highly segregating elements and the absence of grain boundary strengthening elements. It is of fundamental importance analysing the physical mechanisms of formation of stray grains, to understand which thermo-physical and geometrical factors highly influence their formation and to find possible ways to reduce the impact of the problem. In this paper, constrained dendrite growth and heterogeneous grain nucleation theories have been used to model the formation of stray grains in directional solidification of Ni-based superalloys. The study is based on the Hunt's theory, referring to the transition from columnar to equiaxed dendrites in castings. Two main changes have been brought to the original formulation: the heterogeneous nucleation undercooling depending on the cooling rate and a new condition for the incipient transition, based on the dimension of the nucleated spurious grain instead of on the achievement of a threshold value of equiaxed grain fraction. Namely, it has been assumed that a spurious grain develops into a stray grain if its radius is equal to the primary dendrite arm spacing (PDAS) at the time it is reached by the columnar front. Fig. 3 shows the difference produced by these two modifications on the microstructure selection map. The study allows to derive the role played by the most affecting factors: (0 geometrical. angle of primary grain dendrites with the withdrawal direction, responsible for the formation of volumes where the spurious grain undercooling is lower than the undercooling of the columnar dendrite tip; (ii) wettability of foreign substrates, on which the spurious grain undercooling strongly depends, as represented by the trend of the limit velocity above which stray grain forms as function of the wettability factor f(theta) (fig. 5); NO process and alloy: thermal gradient ahead to the solidification front and alloy composition, influencing the columnar dendrite tip undercooling. The convexity of the liquidus isotherm near the shell surface highly influences the columnar undercooling of the side branches, increasing the risk of formation of stray grains (see figg. 7 and 8).