Multidisciplinary Analysis of Radiographic Indications Observed in Nickel‑Based Superalloy Wrought Parts

Abstract

Non Destructive Evaluation (NDE) of cast, wrought, and welded components of any Ni-based superalloy
is always challenging. Post-machining radiography (RT) is often applied to wrought parts that do not
meet the ultrasonic inspectability requirements. One of such superalloys with 67 wt.% Nickel is selected
to realize critical sub-assemblies for liquid rocket engines, owing to its high ignition and oxidation
resistance in extreme thermal environments. In post-machining RT of certain parts made of this alloy,
typical indications were observed, that resembled low-dense inclusions. Severities of these indications
varied from mere globular to highly linear nature, oriented transverse to hoop direction of the part.
This paper elaborates on the details of the extensive investigation of samples extracted from such a
part, using advanced non-destructive, destructive, and metallography characterization tools. Computed
tomography (CT) was used to precisely locate the indications. Metallography analysis using OM and
SEM-EDS was performed on open-to-surface as well as embedded indications for its characterization.
The structural implication of these indications was studied using micro-tensile specimens. Metallographic
results revealed the network of segregated Ti-rich carbide particles. The low mass attenuation coefficient
(μ/ρ) of Ti against the Ni-rich matrix resulted in relatively higher optical density radiographic indications.
Though no significant drop in tensile strength was observed, the presence of these brittle phases hampered
local ductility. Fractography showed a mixed mode of failure, comprising of brittle cleavage fracture of
carbide particles and ductile dimple rupture of the matrix. It is noteworthy that the elucidations from this
"4D NDE" approach revealed the characteristics of the radiography indication beyond its spatial attributes.