Microstructure and oxidation behaviour of Inconel 625 manufactured by laser powder bed fusion

Lewis, Emily Rose ORCID: 0000-0001-9723-007X (2024). Microstructure and oxidation behaviour of Inconel 625 manufactured by laser powder bed fusion. University of Birmingham. Ph.D.

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Abstract

The high temperature oxidation of L-PBF IN625 was investigated following heat-treatment and surface finishing. Sub-parabolic oxidation behaviour was shown, demonstrating similar growth rates to wrought IN625. Regardless of surface morphology and stress-state, chromia (Cr2O3) scale was formed alongside a near-continuous δ-phase layer (Ni3Nb) at the scale/alloy interface. The continuity of the δ-phase layer was influenced by surface finishing via changes to the oxide morphology. Less δ-phase at the scale/alloy interface was correlated to increased spallation. This was related to modelled values for the temperature change required for spallation (ΔTC) alongside physical measurements of stress (XRD sin^2(ψ)) within the scale. It was concluded that surface finish is key in the maintenance of a continuous δ-phase layer to ensure relaxation of Cr2O3 upon cooling.

Internal oxide (Al2O3) was detected at grain boundaries in the sub-scale. Significant fractions of Coincident Site Lattice (CSL) boundaries formed in L-PBF IN625 following heat-treatment and oxidation (up to 70%). Nucleation of internal oxide was strongly influenced by grain boundary character, forming preferentially along random orientation grain boundaries. Higher fractions of CSL boundaries in the substrate corresponded to a shallower depth of internal oxidative attack.

Corrosion of L-PBF IN625 was investigated following heat-treatment and surface finishing in simulated Pressurised Water Reactor (PWR) hydrogenated water. Oxidation was not influenced significantly by L-PBF microstructure, but by changes to surface finish. Rougher and/or stressed surfaces led to scale spallation at early exposures. Mass gain was observed after 1500 hours (h), with similar oxide crystallite size and morphology to wrought IN625. No pitting corrosion was observed. A duplex layered oxide formed, comprised of an Fe-rich outer scale (Ni(Fe,Cr)2O4), on a Cr-rich dense inner scale. Grain orientation directly influenced oxidation. EBSD revealed the following grain orientation-oxidation relationship, in order of increasing oxidation rate: [100] < [101] < [111]. Overall, electropolished surfaces were deemed most suitable for PWR environments.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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