Amorphous alloys

[Mark.hall]

I'm trying to get my head around a project I'm working on for a materials class and have been reading about non-crystalline alloys as opposed to crystalline alloys.
How would an amorphous alloy, like http://www.nealloys.com/kovar.php for example compare to a crystalline metal with regards to brittleness etc? Are controlled expansion alloys not as brittle? I'm interested in how different cooling techniques can change a metal's properties.

 

[Denbo1991]

it all depends on how dislocations propagate through the material. If the material is perfectly crystalline, a crack or fracture might propagate through the entire material. Since the grains in amorphous materials are smaller, propagating dislocations are more likely to be stopped by grain boundaries.

 

[McWatt]

Denbo's description applies to ordered metals with small crystalline domains. An amorphous metal, on the other hand, by definition has no grain boundaries. It has no grain. I'm no metallurgist, but I imagine this leads to retarded crack propagation. Ref. 9 from the appropriate wikipedia entry indicates that amorphous metals have higher tensile yield strengths and elastic strain limits but lower ductility and fatigue strengths than their crystalline counterparts. That all seems intuitively reasonable given the difference in microstructure, but I recommend you go read ref. 9 or maybe better yet, Johnson, JOM March 2002, Volume 54, Issue 3, pp 40-43.

 

[crashtestdummy]

I'm trying to get my head around a project I'm working on for a materials class and have been reading about non-crystalline alloys as opposed to crystalline alloys.
How would an amorphous alloy, like http://www.nealloys.com/kovar.php for example compare to a crystalline metal with regards to brittleness etc? Are controlled expansion alloys not as brittle? I'm interested in how different cooling techniques can change a metal's properties.

Amorphous alloys are in general quite brittle. If they don't have a crystal structure, dislocations cannot propagate, and the material will not be able to undergo plastic deformation (think glass). They're somewhat less brittle than ceramics, but much more than other metals. Because of the lack of dislocations, though, they tend to have very high yield strength.

This can be great for something like, say, a golf club, where you want the ball to receive as much kinetic energy from the club as possible. Since there is little plastic deformation, less energy is lost in the contact between ball and club than with, say, a titanium driver.

As for the cooling, the faster you quench from the liquid state, the less crystalline your material. There are some alloys that can be cooled in air and still be amorphous, but holding an amorphous metal at an annealing temperature (a little below melting) will generally give you a crystal structure.

 

[Mark.hall]

This is great, thanks. It's all a little clearer now.
Thanks, McWatt, I'll go check that out.

 

[Sho'Nuff]

Mark - check out this Darpa presentation. It provides a pretty good overview (including some material properties comparisons):

http://archive.darpa.mil/DARPATech2000/Presentations/dso_pdf/5ChristodoulouSAMB&W.pdf