What defects can material testing reveal in superalloy parts?

Internal and Volumetric Flaws

Material testing is critical for revealing a range of defects that can compromise the integrity of superalloy parts. Non-destructive evaluation (NDE) methods like ultrasonic testing are highly effective at identifying internal, volumetric flaws. These include gas porosity and shrinkage cavities formed during solidification in processes like vacuum investment casting. It can also detect lack-of-fusion defects in 3D-printed superalloy components and non-metallic inclusions, which are brittle ceramic particles that can act as stress concentrators and initiate cracks under cyclic loading.

Surface and Subsurface Defects

Techniques such as liquid penetrant testing and magnetic particle inspection are indispensable for detecting surface-breaking anomalies. These tests can reveal fine cracks, also known as hot tears or stress cracks, that may develop during cooling or from thermal fatigue. They also identify surface-connected porosity and pitting, which can severely reduce fatigue life and serve as initiation sites for corrosion. For forged components, these methods can detect seams and laps—surface imperfections folded over during the forging process.

Microstructural Anomalies and Degradation

Metallographic examination, a key part of material testing and analysis, reveals defects at the microscopic level. This includes chemical segregation, where alloying elements are not uniformly distributed, leading to localized weak spots. It can also identify undesirable phase transformations, such as the formation of brittle topologically close-packed (TCP) phases like sigma and mu, which deplete the matrix of strengthening elements and embrittle the alloy. Furthermore, it detects abnormal grain growth or recrystallization, which can result from improper heat treatment and lead to anisotropic mechanical properties and reduced creep life.

Deviations in Mechanical Properties

Mechanical testing, including tensile and creep tests, does not detect a specific "defect" but reveals the consequential underperformance of the material. Sub-standard strength, ductility, or creep resistance indicates that an underlying issue—such as those mentioned above—is present. This testing is essential for validating that the post-processing, including Hot Isostatic Pressing (HIP), has successfully healed internal defects and that the final component will perform reliably in demanding applications like aerospace and power generation.