Metallurgy for Cyclists - Part 7
source www.ibiscycles.com
by Scot Nicol
posted 2006-04-16
Part 7 - The Final Chapter
Had enough good news? The bad news is the horrendous elongation number of about 2 percent in the longitudinal direction, and 0.2 percent in the transverse direction. On planets where the surface temperature might be about 390 degrees F, the elongation number for beryllium goes way up, to 23 percent. Unfortunately, that doesn't do us much good here on earth. (An interesting note that appeared next to the elongation number in one mechanical engineering handbook was "Ductility values in practice will be found in general to be much lower, and essentially zero in the transverse direction." Ouch!)
Fortunately, there is an alternative to extruded beryllium: It's called cross-rolled sheet. The beryllium bike that Brush-Wellman (a vertically integrated beryllium company) made for American Bicycle Manufacturing a couple of years ago was fabricated from sheet, to take advantage of higher elongation (more than 10 percent) and higher ultimate and tensile strength. To make the bike, the sheet was rolled into tubes and welded together.
In further bad news, beryllium wins out over all metals on the toxicity issue - beryllium dust can kill you. Inhalation of dust particles or vapors containing beryllium may cause berylliosis, an inflammation of the lungs.
Due to cost, ductility and toxicity constraints, pure beryllium isn't commercially feasible for bicycle frames. Sure, you can make one, like that $25,000 showpiece that ABM did, but you can't call that project commercially feasible.
Brush-Wellman also has created an aluminum-based alloy with beryllium added to the mix. The patented metal is trademarked as AlBeMet, and it shows some promise for bicycle parts. The material is already being sold commercially in other markets - for computer disk drives, for instance - and Beyond Fabrications of San José, California, has seatposts and handlebars made out of it. Frames are on the way, according to a spokesman for Brush-Wellman. Altogether, the company has four alloys of AlBeMet, and they vary from 30 to 62 percent beryllium in the mix, with the following claimed mechanical properties:
| AlBeMet Alloy | 130 | 140 | 150 | 162 |
| % Beryllium | 30 | 40 | 50 | 62 |
| Density | .086 | .082 | .080 | .076 |
| Yield (KSI) | 23 | 30 | 33 | 40 |
| Ultimate (KS) | 34 | 40 | 50 | 55 |
| Elongation (%) | 17 | 15 | 13 | 7 |
| Modulus (MSI) | 19 | 20 | 25 | 28 |
Brush claims that these alloys are weldable. It's interesting to note that strength is quite low for the materials with less than 50 percent beryllium.A final practical use for beryllium is as a neutron source to charge the initiator in atomic bombs. When bombarded with alpha radiation, beryllium emits neutrons. Thanks to beryllium, that's how the neutron was discovered back in 1932.
AerMet 100
AerMet 100 is a promising material that's been raising eyebrows in the bike industry lately. This new ferrous alloy (steel) was patented in 1992 by Ray Hemphill and Dave Wert of the Carpenter Technology Corporation, and there are now several framebuilders fiddling around with this alloy.
Check out AerMet 100's amazing numbers. The density, at 0.285 pounds per cubic inch, is virtually identical to cro-moly steel. Indeed, if you look at the make-up of AerMet, there's a large percentage of nickel and cobalt, which have slightly higher density than iron. Where this material really blows away the other weldable steel alloys (and all other bicycle fabricating materials) is in strength, though. Our December 13, 1993 VeloNews tubing test included samples provided by Carpenter and showed that AerMet 100 had a yield strength more than 261 KSI, and ultimate strength over 300 KSI. Humm baby!
Combine that with good elongation - the same test revealed 10 percent elongation - and a modulus typical of steels at 28 MSI. Carpenter claims that "the alloy is designed for components requiring high strength, high fracture-toughness, and exceptional resistance to stress corrosion cracking and fatigue."
So what's wrong with this picture? Nothing is wrong, but there are a few hurdles. As you can imagine, AerMet is expensive, but still only about a half to two-thirds the price of titanium. It's not available tapered or butted (not yet anyway - both of those processes are currently being worked on). So far, the manufacturer has been successful in swaging stays, and these should be commercially available soon. In the meantime, you can weld or braze AerMet to 4130 cro-moly - so tapered stays, braze-ons, dropouts, and all of the usual accouterments can be added easily.
On the negative side, I'll make the same claim that I made for steel: AerMet 100 is density challenged. Even so, extremely light frames can be made with this material and have sufficient strength. The biggest obstacle I can see is the old buckling problem, the beer-can effect. To get those frame weights down to the two-pound area, which I consider the next weight milestone in frame fabrication, it's going to take some work. On the other hand, it looks like it will be easy to build an extremely durable three-pound frame. Several framebuilders - including Kellogg, Davidson, Curve and Arrow - are already producing AerMet 100 frames.
And Now for a Mystery Metal...
Gary Helfrich recently told me about a wonderful new material that has some unreal mechanical properties. His claims:
Density: 0.084 KSI, 15 percent less than aluminum (wow!)
Yield strength: 510 KSI, 12 times that of aluminum, double that of AerMet 100 (triple wow!)
Modulus: 18 MSI, 80 percent higher than aluminum (wow again!)
Strength-to-weight ratio 14 times that of aluminum (wow to the fourth!).
If you use 1.25 x 0.030-inch top and seat tubes, and a 1.375 x 0.033-inch down tube to mimic the sweet ride of titanium, when you compare modulus figures, what do you get? Frame weight of 1.3 pounds!
What's wrong with this picture? Here comes that pesky elongation, again, rearing its ugly head. Although it's as good as a carbon-fiber filament, it's barely above zippo percent.
The material? Monocrystalline silicon, the material used to make the chips in the computer that this document was typed on.
Don't be Fooled
I have two simple thoughts to leave with you at the conclusion of this series. When assessing the materials used in bicycle design: 1) look at the whole picture; and 2) remember that bicycle design is probably the subject for a 14-part series - put the material where you need it.
