Heat Extraction and Dendritic Growth during Directional Solidification of Single-Crystal Nickel-base Superalloys.

Abstract

Single crystal (SX) growth requires directional solidification (DS) with a sustained thermal gradient in the withdrawal direction. By increasing the thermal gradient, the dendrite structure in nickel-base superalloys is refined resulting in a mechanically-superior SX with reduced defect occurrence. One method to achieve higher thermal gradients is the utilization of the Liquid Metal Cooling (LMC) process. Optimization of the process has been limited by a lack of the fundamental understanding of the various heat transfer processes associated with the technique.

Solidification experiments of nickel-base superalloys have been conducted in a DS furnace capable of Bridgman or LMC modes. The degree of structure refinement has been investigated in castings with varying cross-sectional areas. The feasibility of a mono-crystalline ring comprised of multiply-seeded, SX segments has been investigated.

Three-dimensional, finite-element (FE) solidification modeling has been used to investigate the thermal characteristics of the Bridgman and LMC DS processes. The dominant heat-transfer step during solidification is radiation from the mold for the Bridgman process and conduction through the mold for the LMC process. In the LMC process, the primary role of the floating baffle is to provide a thermal barrier between the furnace environment and the coolant. Relationships between dendritic structure and the local thermal field are presented. The preferred process conditions occur when the solidification-front position is just above the baffle for both the Bridgman and LMC processes.

The stability of unidirectional dendritic growth is dependent on the curvature of the solid-liquid interface. Some processing conditions cause substantial lateral heat extraction that leads to a change in dendrite morphology, resulting in grain nucleation or lateral growth – the formation of long secondary dendrite arms overgrowing favorably aligned primary dendrites. The conditions under which lateral growth occurs have been studied experimentally and via solidification modeling using FE and diffusional dendrite growth modeling. Lateral growth of dendrites occurs at interface inclination angles as low as 25 deg., and grain nucleation and breakdown of the solidification front occur when the interface inclination reaches 45 deg. The misorientation of the [001] SX orientation from the withdrawal axis significantly contributed to the onset of lateral growth.