Nimonic 80A vs Inconel 718 Above 700°C: Creep Data Tells the Real Story

Nimonic 80A (UNS N07080, W.Nr 2.4631, AMS 5828, ASTM B637) and Inconel 718 (UNS N07718, W.Nr 2.4668, AMS 5662, ASTM B637) are precipitation-hardened nickel-base alloys used for high-temperature fasteners. Above 700°C, creep-rupture data determine the selection.

Table 1. Nominal composition ranges (wt%)

Element	Nimonic 80A	Inconel 718
Ni	69.0 min	50.0-55.0
Cr	18.0-21.0	17.0-21.0
Fe	3.0 max	balance
Ti	1.8-2.7	0.65-1.15
Al	1.0-1.8	0.20-0.80
Nb+Ta	-	4.75-5.50
Mo	-	2.80-3.30
Co	2.0 max	1.0 max
C	0.10 max	0.08 max

Nimonic 80A is strengthened by gamma prime Ni3(Al,Ti). This coherent phase remains stable in long-term service up to about 815°C. Inconel 718 is strengthened primarily by metastable gamma double prime Ni3Nb. Above 650°C, gamma double prime coarsens and transforms to equilibrium delta Ni3Nb. The loss of coherent gamma double prime reduces creep strength at 700°C and above.

Table 2. Typical 1000-hour creep-rupture strength (MPa) in solution-annealed and aged bar

Temperature (°C)	Nimonic 80A	Inconel 718
650	370	415
700	235	170
750	150	60
815	75	15

These are representative published values, rounded for engineering comparison. At 650°C, Inconel 718 shows slightly higher strength. At 700°C and above, the trend reverses. At 750°C, Nimonic 80A typically carries about 2.5 times the stress of Inconel 718 for the same 1000-hour life. At 815°C, the gap widens further.

Larson-Miller parameter analysis normalizes time-temperature data. Using C = 20, LMP = T(K)(log10 t_r + 20). Plotting stress against LMP allows comparison of rupture life outside the tested temperatures. Nimonic 80A retains higher LMP at a given stress above 700°C, consistent with the 1000-hour data.

Heat treatment also matters. Nimonic 80A is typically solution annealed at 1080°C for 8 h air cool, then aged at 700°C for 16 h air cool. Inconel 718 per AMS 5662 is solution annealed at 980°C for 1 h, then aged at 720°C for 8 h furnace cool to 620°C for 8 h air cool.

Initial cost per kilogram favors Inconel 718 because it contains about 18 to 19 wt% Fe and 50 to 55 wt% Ni. Nimonic 80A has 69 wt% minimum Ni and negligible intentional Fe. Above 700°C, the lower allowable creep stress of Inconel 718 requires larger fastener diameters or more frequent replacement. For creep-limited bolting, labor and downtime dominate lifecycle cost. The higher raw material cost of Nimonic 80A is offset by longer service life at 700 to 815°C.

Selection guidance:

• Below 650°C, Inconel 718 offers higher tensile and yield strength at lower material cost.
• At 700 to 815°C, Nimonic 80A is preferred for creep-rupture-limited bolts, studs, and turbine hardware.
• Above 815°C, use higher gamma-prime alloys such as Waspaloy or Nimonic 105.