A castability model based on elemental solid-liquid partitioning in advanced nickel-base single-crystal superalloys

Abstract

A castability model that accounts for the characteristic segregation behavior of constituent elements in Ni-base superalloys has been developed and experimentally verified in production scale casting trials. The model ranks alloy compositions with respect to their susceptibility to freckle formation during directional solidification. Thirty-nine distinct Ni-base single-crystal superalloys encompassing a broad range of compositions were investigated to assess the influence of the constituent elements on their solidification characteristics. Linear regression was applied to the fitted solid-liquid partition coefficients of the major constituent elements to develop formulas capable of describing elemental interactions. The high-density refractory elements Ta, W, and Re were found to segregate most severely during solidification. Increasing the amount of Cr and Mo in high-refractory single-crystal alloys reduced the extent of W and Re microsegregation during solidification. This effect was found to minimize the occurrence of freckle defects due to the corresponding decrease in the density inversion term, which is effectively the driving force for thermosolutal convective instabilities known to cause macroscopic grain defects during single-crystal solidification. Model predictions were validated using production scale casting trials where additions of 1.5 wt pct (1.9 at. pct) Cr and 3.0 wt pct (2.0 at. pct) Mo to a high-refractory superalloy more than halved the number of solidification-related grain defects formed. These findings suggest that elemental interactions between Cr, Mo, W, and Re need to be considered when optimizing alloys for high-temperature creep properties.

Solute Redistribution during Planar and Dendritic Growth of Directionally Solidified Ni-Base Superalloy CMSX-10

“…Of all solute elements that were presented in Table II, Ti and heavy refractory elements such as Re, Ta, and W exhibited particularly strong segregation tendency. This segregation tendency was found to be comparable with those reported for dendritically solidified commercial single crystal superalloys [3,4,7,11] and model single crystal superalloys [6,23]. (Table II).…”

“…As shown in Table II, the solute elements with i < 1 are Cr, Mo, and γ′ forming elements such as Al, Ta, Ti, and Nb, Therefore, as the solidification proceeds, these elements are continually accumulated in liquid, and if the composition of solid at the S/L interface exceeds the solubility limit of primary γ, the eutectic reaction of L → γ + γ′ would occur. Generally, it has been known that the solidification of the advanced Ni-base superalloys completes with the formation of γ/γ′ eutectic [4,7,17,29,30].…”

“…For this reason, the total amount of refractory alloying addition has been gradually increased in the advanced Ni-base superalloys [1,2]. However, the refractory elements segregate severely during solidification [3,4], which often lead to a number of problems such as (i) interdendritic porosity [5], (ii) misoriented grains [6], (iii) freckle formation [6,7], (iv) localized phase instability [8,9], and (v) formation of topologically close-packed (TCP) phases [9,10]. While some of these problems were mitigated by the complex step-wise homogenization heat treatment, most result in increased scrap rates.…”

“…Of the techniques available, random sampling approach, which is commonly known as point matrix scan technique, is widely used for characterizing microsegregation in Ni-base superalloys [3,4,7,[11][12][13][14]. This technique has become prevalent because it allows characterizing microsegregation with no dependence on the plane of examination and simply provides a continuous profile with respect to the fraction of solid ( s ).…”

Studies on the Solidification Path of Single Crystal Superalloys

“…[25][26][27] The as-cast microstructure consists of c dendrites with interdendritic c¢ present due to the microsegregation of alloying elements. Figure 2 shows a typical microstructure found in directionally solidified CMSX-4.…”