> He ruled out magnesium, which is best per unit weight in compressive buckling but is brittle and difficult to extrude.
There's a fascinating, and very new, class of nano-laminate magnesium alloys called Long Period Stacking-Ordered (LPSO) alloys. These are very lean -- the standard version is 97% Mg + 1% Zn + 2% Y -- and they have outstanding mechanical properties. At an equal weight, they're much stronger and stiffer than 6061 aluminum, and the kicker is that this is generally true only if they're extruded. If they're not extruded, the laminate-like grain structure doesn't form properly.
Could make excellent bike frames.
Magnesium corrosion would still be a problem, though. I got some LPSO-Mg samples from Fuji Light Metals, in Japan, and they were quite badly degraded within weeks.
There was a magnesium bike frame back in the '90s, made by Kirk:
https://www.elmycycles.co.uk/m21b0s365p4804/1992-Kirk-Revolu...
https://www.bikeforums.net/classic-vintage/1279777-kirk-prec...
https://www.independent.co.uk/news/uk/magnesium-in-frame-to-...
https://www.flickr.com/photos/11521783@N05/albums/7215764801...
A friend had one. It cracked.
> It cracked.
> "Kirk Revolution cast magnesium"
Cast magnesium is really weak/brittle compared to forgings and extrusions. Its use was not a great design decision on Kirk's part. I suppose they could have wrapped the casting in carbon fiber or something like that, to give it extra bending strength and spread out loads that might cause fractures, but then it would get expensive.
I had a Merida Magnesium 909 road bike back in the day. They were common in Australia. Was (wrongly) convinced magnesium was going to overtake carbon. Never had any issues in 10 years of ownership and a lot of kms. Welds looked shocking and it was very rigid and unforgiving though.
Can modern material science model this computationally, or does everything have to be observed experimentally? This kind of insane just-so recipe - are researchers just iterating on hundreds of thousands of different alloy compositions and production techniques or are there strong theoretical principles on which some of this can be derived?
Its been some time detached from the mat sci folks deeply involved in the space but its both. There is a bunch of theoretical underpinnings but ultimately a lot of throwing darts on the board as well.
Yeah. It depends a lot on the type of material, too. Conventional metal alloys -- like LPSO-Mg -- are the toughest to model. Too many variables. Ceramics and intermetallics are a lot easier to model in principle, but they can have surprising properties on an atomic level, and there's really no predictive method for that sort of thing. Modeling does get you pretty far with high-entropy alloys -- because to a substantial extent their properties hinge on how a bunch of different atoms might fit together randomly, and that's something that can be computationally predicted. A lot of the recent interest in HEAs is because they're relatively easy to model.
Interesting. I haven’t heard of them. Joining would still be a problem for a bike frame. Any idea on how well they work with other severe plastic deformation processes?
How would you fabricate a frame from extruded tubing? Welding would destroy the grain structure.
That looks like it's an active research problem: https://www.sciencedirect.com/science/article/pii/S266633092...
At a glance, though, the problem doesn't seem insurmountable. FSW appears to work.