Center Segregation. Freckles and Development Directions for Niobium-Containing Superalloys

Abstract

Macrosegregation defects, such as freckles and center-segregates, are currently one of the main problems encountered by attempts at scaling-up ingot diameters in order to supply industrial gas turbine manufacturers. Focusing on niobium-containing superalloys (IN7 18, IN706 and IN625), interdendritic liquid segregation and density profiles are presented, and the occurrence of fluid flow leading to macrosegregation is linked to the solidification sequence of these alloys. This paper also mentions the possible use of the Rayleigh number as a criterion to predict detrimental fluid flow patterns. Finally, it is suggested that slight modifications of alloy chemistry may be the only way to eliminate fluid flow related macrosegregation in conventional remelting processes.

Quenching Differential Thermal Analysis and Thermodynamic Calculation to Determine Partition Coefficients of Solute Elements in Simplified Ni-Base Superalloys

“…Furthermore, experimental studies suggest that the existing experimental data of partition coefficients are insufficient for accurately predicting the alloy behavior, with regard to the promptness of forming macrosegregation defects. Using alloy 718, Aubertin et al [11] found an experimentally measured Nb partition coefficient of 0.35, while Knorovsky et al [14] reported experimental results for the Nb partition coefficient of approximately 0.5 for the same alloy. This indicates that a critical need exists to systematically investigate the partition coefficients by experimentation, for verification of the theoretical calculations.…”

“…[11,12] Because the change in the liquid density is induced by the redistribution of solute elements during solidification, the partition or distribution coefficients of the solute elements defined by Eq. [3] are very important metallurgical parameters for freckling prediction affecting the Rayleigh number:…”

Primary Carbides in Alloy 718

“…M6C has also been reported (3) in samples which have undergone longterm heat treatment, forming in small amounts at grain boundaries. Carbon is almost insoluble in the primary gamma phase of 718 and segregates strongly during solidification (4). As a result, although there are no carbides precipitated in the liquid above the liquidus temperature (1330C) (5) When the temperature falls to 1280C, corresponding to approximately 50% solidification (6,7,8), the carbon concentration in the remaining liquid rises to the point where the solubility product of NbC is exceeded and the precipitation of NbC therefore commences.…”

“…As a result, although there are no carbides precipitated in the liquid above the liquidus temperature (1330C) (5) When the temperature falls to 1280C, corresponding to approximately 50% solidification (6,7,8), the carbon concentration in the remaining liquid rises to the point where the solubility product of NbC is exceeded and the precipitation of NbC therefore commences. This process is aided by the strong segregation of Nb, which leads to an Nb concentration of 10 wt% at the point of 50% solidification (4). The solubility product of NbC has been computed (5) at this temperature on the basis of the assumed and measured segregation coefficients of Nb and C. The carbide precipitation continues during the remaining solidification.…”

Powder Characterisation, Microstructure, and Mechanical Property Evolution of IN625 and IN718 During Selective Laser Melting and Heat Treatment

“…Once solidified, there is still the potential for the movement of atoms via diffusion however the relatively large size of Nb makes this process slow, meaning that little to no solid solute redistribution of Nb, to move towards an equilibrium distribution, occurs, following the Scheil equation (Auburtin et al, 1997;DuPont et al, 2009). This process of segregation results in the structures observed in Figure 40 and Figure 41 which show the solidification sub grains (SSG).…”