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Nimonic PE16 is an alloy of nickel, iron, chromium, and molybdenum, which is age hardening by precipitation of γ formed with titanium and aluminium. In common with other alloys its high-temperature ductility is reduced after neutron irradiation, presumably by helium from the B10(n,α)Li7 reaction. Boron appears to have dual roles as the source of embrittlement and as augmenter of the alloy's tolerance of helium.
Grain refinement of the alloy results in little change in the tensile properties until grain sizes are reduced to less than 0.005 mm diameter. The alloy in this condition may have a very high ductility, and the strained structure, together with calculations using the Herring equation, indicates that diffusion strain occurs. Irradiation is embrittling but postirradiation ductility can be higher than usual. The strength of the fine grain material is low.
The tensile ductility of the alloy varies widely with strain rate and temperature. When the alloy is unirradiated the ductility trends go through two reversals: as straining time and temperature are increased a peak is ob served, followed by a ductility trough and a second ductility increase. The first peak is not observed in tests on irradiated material. The trough occurs under the same test conditions for the unirradiated and irradiated alloy. The conditions for minimum ductility coincide with those for a constant proportion of diffusion strain but not with those for constant stress nor constant strain rate. The cracking behavior either side of the trough is taken to indicate that diffusion relief of stress on carbides is responsible for recovery from the trough with increased straining time and temperature.
neutron irradiation, nickel-iron alloy, elevated temperature, tensile properties, fracture, strain rate, diffusion strain, grain size boron, irradiation, embrittlement, heat treatment, ductility, microstructure
Broomfield, G. H.
Senior Experimental Officer, United Kingdom Atomic Energy Authority, Atomic Energy Research Establishment, Didcot, Berks