MP35N and Elgiloy: Cobalt-Alloy Springs for Medical and Aerospace Extremes
Our high-temperature spring materials guide and titanium springs guide cover the upper end of most spring material ladders. MP35N and Elgiloy sit above both on a narrower axis: fatigue strength combined with biocompatibility and corrosion resistance in demanding implantable and aerospace applications, at a cost that only makes sense when the application actually needs it.
What makes cobalt alloys different
MP35N (35% cobalt, 35% nickel, 20% chromium, 10% molybdenum) and Elgiloy (a similar cobalt-chromium-nickel-molybdenum family, originally developed for watch mainsprings) both age-harden after cold working — cold-drawing the wire, then heat-treating, produces exceptional tensile strength and fatigue resistance without the brittleness that similar-strength steels would show. This combination is difficult to replicate with any steel or nickel alloy at comparable strength levels.
Where they earn the premium
- Implantable medical devices — heart valve frames, guidewires, and stent components, where biocompatibility is not optional and MP35N's track record in implant applications carries real regulatory weight alongside its mechanical properties
- High-cycle, small-wire mechanisms — watch and precision instrument springs (Elgiloy's original use case), where extreme fatigue life in a very small wire diameter matters more than raw force
- Non-magnetic, high-strength requirements — both alloys are essentially non-magnetic, relevant for MRI-compatible medical devices or sensitive instrumentation (see our non-magnetic spring materials guide for the broader category)
What they cost you beyond price
- Coiling difficulty. Both alloys work-harden significantly during cold coiling, requiring coiler setup and tooling wear management that a standard steel-wire program doesn't — not every factory runs these materials regularly, and experience matters more here than with commodity steels
- Longer lead time for raw wire. These alloys come from a smaller pool of specialty wire mills than music wire or stainless — confirm wire availability before committing to a delivery date, particularly for unusual diameters
- Documentation expectations. Medical-grade applications using these materials typically require full material traceability and certification to the specific medical-grade specification (ASTM F562 for MP35N is common) — generic "cobalt alloy" wire without that paper trail isn't a substitute
When a cheaper alloy actually solves the problem
If your requirement is high fatigue life without implant-grade biocompatibility or extreme corrosion resistance, chrome silicon or 17-7PH stainless (see our chrome silicon guide and 17-7PH guide) often gets close to the performance at a fraction of the cost — worth confirming which property is actually driving your spec before defaulting to cobalt alloy.
Send your application and any regulatory requirement through our RFQ form — we source these alloys through specialty mills when the application genuinely calls for them.
Frequently Asked Questions
Is MP35N always required for implantable devices?
No — 316L stainless and certain titanium alloys are also used in implants depending on the specific application and required properties. MP35N is chosen specifically when its fatigue-strength-to-size ratio is needed, not as a default for anything implantable.
Can MP35N or Elgiloy be welded?
Both are weldable but require careful process control to avoid degrading the cold-worked properties near the weld — for most spring applications, mechanical joining or crimping is preferred over welding specifically to preserve the material's strength.
What's the typical lead time difference versus stainless spring wire?
Expect meaningfully longer lead times for raw wire sourcing on cobalt alloys, particularly for less common diameters — confirm current availability with your RFQ rather than assuming stainless-wire lead times apply.