What is it about?
ABSTRACT Titanium Carbonitride (Ti(C,N)) decomposition in Inconel 617 alloy creep exposed at 650C for 574 hours is reported using analytical electron microscopy techniques. Crenriched M23C6-type carbides enveloped in fine gamma prime particles thought to be precipitated from the decomposition reaction are observed in the alloy. The morphology of the M23C6 carbides is irregular and blocky and the particle size up to 5lm, whereas the morphology of gamma prime particles is mostly spherical and up to 30 nm in size. Intergranular carbides are mostly secondary precipitates of the M23Cc type (M predominantly Cr) and these respond to solution heat treatment and precipitate on the grain boundaries as a result of ageing. The ability of intragranular MX to decompose is sensitive to the N content, high N resists decomposition. Decomposed intragranular MX provides an excess source of C which can react locally with Cr to form heat treatable intragranular fine Cr23C6 precipitates. M6C can segregate in interdendritic locations during melting which may be the reason for high content of Mo in M23C6. These precipitates are generally very small and contribute to an additional hardening effect and are the reason for the onset of voiding and cracking along the grain boundaries that ultimately lead to a reduced creep rupture life.
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Why is it important?
The Ti (C,N) carbide decomposition reaction in creep-exposed Inconel 617 alloy has been illustrated by TEM analysis. There is evidence of the degeneration reaction, TiC + gamma = M23C6 + gamma prime for the sample which has been creep exposed for 574 hours at 650C with intermediate creep stress. The hardness of the failed sample significantly increased for creep-exposed as compared to as-received sample. The rise in hardness observed in the creep-exposed sample is associated with precipitation of M23C6 carbides and fine gamma prime particles and also with work hardening. Moreover, formation of M23C6 carbides at and near the grain boundaries contributed to creep failure due to grain boundary de-cohesion and triple point cracking.
Perspectives
Future generation steam turbines and their components are expected to confront ultra supercritical conditions i.e., working steam conditions up to 760C or more and operating pressure up to 35 MPa, in order to improve the thermal efficiency and reduce the environmental emissions. To accommodate the imposed extended working conditions, the currently used ferritic and austenitic alloys will need to be substituted by alloys with better creep strength and environmental resistance. Nickel-based alloys are likely to be considered as a potential alternatives. Inconel 617 alloy (IN617) belongs to the family of nickel-based alloys and is being actively evaluated as a potential candidate alloy for elevated temperature applications owing to its excellent high-temperature strength, adequate metallurgical stability, good resistance to creep, and better resistance to corrosion and oxidation.
Dr RAM KRISHNA
University of Manchester
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This page is a summary of: Effect of titanium carbonitride (Ti(C,N)) decomposition on failure mechanisms in inconel 617 alloy, Microscopy Research and Technique, April 2015, Wiley,
DOI: 10.1002/jemt.22359.
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