Multiscale Analysis of Defect Structures in Single-Crystalline CMSX-4 Superalloys

Area of Science:

• Materials Science
• Metallurgy
• Solid-State Physics

Background:

• Single-crystalline superalloys are crucial for high-temperature applications like turbine blades.
• Understanding defect formation is key to improving material performance and lifespan.
• The selector-root connection plane is a critical area for potential defect accumulation.

Purpose of the Study:

• To analyze defect creation at the selector-root connection plane in CMSX-4 superalloy turbine blades.
• To characterize the morphology and origin of microscopic defects.
• To investigate positron lifetimes as indicators of large void-type defects.

Main Methods:

• Coupled scanning electron microscopy (SEM) and X-ray diffraction (XRD) topography for macroscopic defect visualization.
• Positron annihilation lifetime spectroscopy (PALS) for microscopic defect characterization.
• Transmission electron microscopy (TEM) for detailed defect analysis.

Main Results:

• Visualization of dendritic arrays and surface defects at the blade root.
• Observation of contrast inversions and internal stress areas.
• Identification of large void-type defects (2-5 nm) attributed to fluid metal contraction during solidification.
• Positron lifetimes exceeding 0.5 ns, indicative of significant void defects.

Conclusions:

• Large void defects, 2-5 nm in diameter, are formed during the liquid-to-solid transition in CMSX-4 superalloys.
• Positron lifetimes above 0.5 ns are characteristic of these large void defects, suggesting the presence of positronium.
• This study opens a new avenue for understanding defect morphology in single-crystalline superalloys using positron spectroscopy.