X-750 vs 718 1

[RedVette]

Hey guys,

Can someone please tell me if there is any notable difference in the all-around performance between nickel alloys X-750 and 718? From the manuals I've read on the Special Metals website as well as numerous other articles, it seems to me like these two alloys are almost identical in their properties, except that 718 seems to be a more advanced version of X-750's engineering--having better mechanical properties and precipitation hardenability. However, both seem to be ascribed the same level of oxidation, corrosion, creep, fatigue, and stress corrosion-cracking resistance as well as the cost; giving the overall impression that they are fully interchangeable. If this is the case, then one would expect 718 to fully displace X-750, but since X-750 is obviously still in demand, it must have something preferable over 718. So what circumstance would make one a better choice over the other?

Any input will be appreciated.

 

[tbuelna]

718 is widely used in aerospace as a high-strength corrosion resistant fastener material. So one practical advantage 718 has over X-750 is greater availability in raw material forms like bar.

 

[btrueblood]

X-750 has slightly higher stiffness (Young's Modulus) at all use temperatures, than 718. Also, X750 has slightly better (i.e. usable) stress rupture strength at higher temperatures (1400 to 1600 F). Both properties, I think, are due to higher Ni content in the X750 alloy.

 

[btrueblood]

I always do this, hit post to soon...

An old rocket engine designer I worked with spun me a yarn once about working with Inco alloys on new materials, and being one of the first people to try their "new" superalloy, X750, back in the day. Alloy 718 came along some years later. So, you might find some components that use X750 because of their "heritage" as much as any other reason.