Comparative Investigation of the Undercooling Capacity and Single-Crystal Castability of Some Ni-Based Superalloys

Abstract

The undercooling capacity of a superalloy is an essential physical property to determine its single-crystal (SC) castability, because stray grains (SGs) will be formed if the geometrical undercooling established at the platform extremities exceeds the undercooling capacity of the applied alloy. In the present work, both the undercooling capacity of eight Ni-based superalloys and their SC castability were experimentally investigated. The liquidus temperature, the critical temperature for grain nucleation, and hence the undercooling capacities of the investigated alloys were evaluated based on the temperature evolution during the heating and cooling processes. The current experimental study revealed a significant difference in undercooling capacity for the superalloys. In the production of SC blade castings, the tendency to form SG defects was found to be highly related to the alloy’s undercooling capacity. The alloys having a low undercooling capacity of around 10 K were very prone to the formation of SGs. In comparison, the alloys with a moderate undercooling capacity from 20 K to 30 K could be easily cast into SC blades without SGs, exhibiting the best SC castability. Other factors influencing the SG formation were also analyzed. As a result, a criterion for the formation of geometry-related SG defects was proposed, in which the influence of the alloy undercooling capacity, casting geometry, and solidification condition are involved.

Eliminating Equiaxed Grain Defects in a Ni‐Based Single‐Crystal Blade Platform by Flattening the Liquidus Isotherm

“…When the solidification front is slightly below the abrupt widening of the cross section, the melt in the platform volume becomes undercooled due to the slope of the liquidus isotherm, caused by the underheating of the mold region facing the central rod (Figure 11a). [ 42 ] For such undercooling of the melt, heterogeneous nucleation of stray grains does not occur. Also, the growth of side branches of the primary dendrite arm is blocked by the lateral surface of the mold, and these branches cannot develop into the platform area yet.…”

The Stray Grains from Fragments in the Rejoined Platforms of Ni-Based Single-Crystal Superalloy

“…In 1997, Bussac et al [10] made a mathematical deduction and derived a criterion to predict SG formation at abrupt cross-sections, which was based on several assumptions rather than an actual casting process. Later, a lot of efforts were paid to investigating the influencing factors of SG formation: withdrawal rates and isothermal inclination angles [7], the undercoolabilities of superalloys [11], alloy composition [12], platform dimension [13], and grain orientation [14]. However, all these studies were based on the presumable fact that SGs are caused by heterogeneous nucleation.…”