The I alloys are the Invar family of controlled-expansion iron-nickel alloys. They are not high-temperature superalloys and they are not chosen for strength — all three are soft austenites with tensile strengths around 480–490 MPa. They are chosen because they barely move when the temperature changes. Alloy 36 is the original Invar, holding a coefficient of expansion near 1.4 µm/m·°C from cryogenic temperatures to about 260°C. Alloy 32-5 — Super Invar — adds cobalt to push that down to 0.63 µm/m·°C, but only across a narrow room-temperature band. Alloy 42 is deliberately not the lowest: at 5.3 µm/m·°C it is matched to silicon, alumina and sealing glass, which is what lead frames and glass-to-metal seals need. Exotic Alloy machines and threads these alloys first and performs the stabilising heat treatment at finish size, because the expansion coefficient depends on it.
Try adjusting your search criteria or contact us for custom alloys.
1. Super Invar does not work cold. Alloy 32-5 beats Alloy 36 on expansion at room temperature, so it looks strictly better — but its austenite is only stable to −55°C and martensite starts at −80°C, below which the low-expansion property is lost. For cryogenic and LNG duty you need Alloy 36, not Alloy 32-5.
2. These alloys are not stainless. Chromium is a residual, not an addition — 0.5% max in Alloy 36, 0.25% max in Alloy 42, unspecified in Alloy 32-5. They rust in damp air like plain carbon steel and need plating or dry-film coating for exposed service.
3. Machining changes the expansion coefficient. Machining, forming and thread rolling introduce stresses that alter expansion behaviour, so the stabilising heat treatment must be done at or close to finish size. A component machined after its final treatment no longer carries the coefficient it was bought for.
Controlled-expansion alloys are unusual in that the manufacturing route is part of the specification rather than just a means to a shape. Machining, forming and thread rolling all introduce residual stress, and residual stress shifts the expansion coefficient — so the sequence matters more than the cutting parameters. Exotic Alloy completes all machining and threading first and then performs the stabilising treatment on the finished component. These alloys cut much like the austenitic stainless steels: they work-harden, give gummy stringy chips and need sharp tooling, rigid setups and generous coolant. Alloy 32-5 is the hardest of the three to machine, rated at roughly a quarter of AISI B1112.
Pick on the expansion coefficient you need and the temperature band you need it over — not on the lowest number.
| Grade | Common Name | UNS | W.Nr. | Specification | Nominal Ni / Co | Expansion | Useful Range |
|---|---|---|---|---|---|---|---|
| Alloy 32-5 | Super Invar 32-5 | K93500 | – | ASTM F1684 | 32% / 5.5% | 0.63 µm/m·°C | −55 to 95°C only |
| Alloy 36 | Invar 36 / Nilo 36 | K93600 / K93603 | 1.3912 | ASTM F1684 | 35–37% / – | 1.4 µm/m·°C | Cryogenic to 260°C |
| Alloy 42 | Alloy 42 / Nilo 42 | K94100 | 1.3917 | ASTM F30 / F29 / B753 | 41–42.5% / – | 5.3 µm/m·°C | Room temp. to 300°C |
Expansion figures are mean coefficients over each grade’s stated range. Alloy 42 is specified for its match to silicon, alumina and sealing glass, not for a minimum. Higher-nickel members of the same family — Alloy 48 and Alloy 52, for sealing against softer glasses — are listed under N Alloys. Invar is a registered trademark originating with Imphy Alloys and Nilo is a trademark of the Special Metals Corporation group; Exotic Alloy is an independent manufacturer and is not affiliated with or endorsed by either.
We are committed to delivering zero-defect controlled-expansion components. All products undergo rigorous testing including PMI (Positive Material Identification), stabilising heat treatment at finish size, dimensional inspection, and material traceability with EN 10204 3.1 certification. Dilatometry is available where the expansion coefficient is contractual.
Exotic Alloy exports controlled-expansion fasteners and components to aerospace, composite tooling, cryogenic, optical and semiconductor clients worldwide. Our dedicated logistics team ensures secure export packaging and timely deliveries by sea and air freight with full customs documentation support.