How do cooling rates and thermal gradients contribute to sliver formation?

Thermal Gradient Stability

Sliver defects in single-crystal castings are closely linked to the stability of the thermal gradient during solidification. A strong, consistent thermal gradient ensures that dendrites grow directionally along the intended crystallographic orientation. When the gradient weakens—due to uneven heat extraction, mold insulation variation, or furnace temperature fluctuations—the solidification front becomes unstable. This instability allows stray nuclei to form at the metal–mold interface, producing thin, misoriented plate-like regions identified as slivers.

Effects of Low Thermal Gradients

A low thermal gradient widens the mushy zone and reduces directionality in dendrite growth. This slower directional transition at the surface permits localized remelting and re-nucleation events that generate misoriented grains. Surface regions with complex geometry, corners, or rapid cross-sectional changes experience the steepest gradient fluctuations, making them hotspot zones for sliver initiation. Alloys with narrow solidification ranges—such as CMSX-4—are particularly sensitive to these variations.

Cooling Rate Influence

The cooling rate directly influences how quickly the solid–liquid interface progresses. Excessively fast cooling can cause thermal shock or surface chilling, which disrupts stable dendrite growth and encourages the formation of thin misoriented grains. Conversely, cooling that is too slow leads to prolonged mushy-zone conditions and increases the likelihood of interface instability. This imbalance between surface cooling and internal heat flow is a key factor in sliver development.

Interaction Between Cooling and Thermal Gradient

Sliver formation typically results when rapid surface cooling coincides with a weakened thermal gradient. This combination creates non-uniform solidification rates across the casting, especially near mold walls. Local undercooling at the surface allows premature nucleation, while insufficient gradient strength fails to suppress these misaligned grains. Precision control of mold insulation, furnace zoning, and withdrawal speed in directional casting helps maintain optimal cooling and gradient conditions, reducing sliver risk.