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

> Creep-rupture comparison of Nimonic 80A vs Inconel 718 at 650-815°C. Larson-Miller parameter analysis, gamma-prime vs gamma-double-prime strengthening, and lifecycle cost for high-temperature creep-resistant bolts.

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.