The Final Chapter
The end is nigh. This is the seventh and final part of our six-part series on metallurgy as applied to bicycles; obviously, we’ve extended the series, but this is the final installment – I promise. In this finale, we’ll finish detailing exotic material, and then give you a mystery material on which to chew (cerebrally, that is).
I can see the ad guys go crazy with this one: “Cure for manic depression! Try the new lithium bike! Feeling psychotic? Lick the top tube!” That’s right, lithium, as in the drug used to treat manic-depressive psychosis, is also used to enhance the mechanical properties of aluminum alloys. Actually, the lithium used as a drug is lithium carbonate, a derivative of metallic lithium.A look at “the numbers” for lithium aluminum alloys reveals some extremely impressive claims – among them high strength and stiffness. So why don’t we see any lithium bikes out there? My attempts to find out more about alloys with lithium were met with lots of secrecy, misinformation and contradiction. It turns out that lithium aluminum alloys have been around for many years, but not much has made it into bike tubing – or many commercial applications at all, for that matter.
For those who have to work with lithium in its metallic form, it’s more likely to cause manic-depression than it is to cure it. You see, lithium is a pain to work with. Lithium and aluminum together have even more pitfalls. Minute amounts of lithium can cause contamination in a processing facility. Lithium is unstable … and it loves oxygen. So you need to extrude it more slowly, and heat treat it longer.
The heat treating is critical … and easy to screw up. If you heat treat for too long, or at too high a heat – even by a small amount – the lithium can oxidize; then you’re left with a soft, almost pure aluminum. Since the alloys have only about one or two percent lithium in them, it doesn’t take much to make all the lithium go away. And the processing requirements and potential problems for lithium all mean one thing: expensive.
Although we can’t use pure lithium to make a bike frame, check out how it compares with other metals. Lithium is number three on the periodic chart; it’s the lightest of all metals; and far less dense than beryllium. Beryllium and magnesium have two thirds the density of aluminum, which is two thirds the density of titanium, which is half the density of steel. Wow, that’s a lot of fractions (they’re approximate, but close). You can see why these materials are enticing.
One thing you need to be aware of with lithium’s numbers is that you may be seeing it in a T-8 condition. That’s fine and dandy if you’re making a hockey stick or baseball bat that doesn’t get welded, but if you’re using it to build a bike, it won’t be in a T-8 (heat-treated, aged and work-hardened) condition anymore. When you look at more realistic conditions, like T-6, the strength numbers then come back down to earth.
There is excellent potential for this material, but based on my research for this article, there seem to be some processing problems to overcome. One question is, “Can you get hold of it?” If the answer is yes, then you need to ask the next question, “Can you manufacture it?” I’m still waiting for two consecutive “yes” answers.
Boron Carbide and Silicon Carbide
Other materials that get thrown into the aluminum vat to make a metal matrix composite are boron carbide (B4C), which is what the Boralyn material has in it, and silicon carbide (Si4C). When you add these materials to aluminum, you get some excellent theoretical enhancements. But the processing these materials require has some pitfalls. Silicon carbide is quite reactive and can break down in the weld zone. When molten, some of the silicon carbide can react and form aluminum carbide. Aluminum carbide is weak in strength and reactive – so reactive, in fact, that it dissolves in water. (Not a good thing for a weld to do, the last time I checked.) This kind of reaction with silicon carbide occurs due to poor welding technique; but that can cause trouble with the aluminum oxide MMCs as well, though to a lesser degree. For obvious reasons, silicon carbide hasn’t seen much use in bicycle applications, though its mechanical properties make it look tempting.
Boron carbide is the material used in Boralyn, and other boron carbide-enhanced aluminum alloys are one their way; several are currently being tested by different manufacturers. You may not see them until 1995, but there’s a good chance that they’ll be out there. Pacific Metal Craft is producing an alloy they call B4C, and if that company’s claims are true, this is a promising alloy. By putting 15 percent boron carbide in a 6013 base alloy, PMC claims yield numbers of 52-56 KSI, ultimate in the 65-72 KSI range, with a modulus of 14-15 MSI (high for an aluminum material) and an elongation of 4.5-6 percent. The 6013 alloy is a high-strength alloy with good fracture toughness (for an aluminum). When you add ceramic (B4C), the fracture toughness diminishes.
One of this alloy’s benefits is that working with it is supposed to be easy. But keep in mind that all boron carbide is not created equal, nor are base alloys, so we haven’t heard the final word on this material.
You may find this hard to believe, but there is a metal out there that is significantly more expensive than titanium. It’s called beryllium. Beryllium has about two-thirds the density of aluminum, so it certainly fits into the category of non- density-challenged metals. Furthermore, beryllium has some amazing mechanical properties – and density is only one of them.The specific strength (strength divided by density) of beryllium is very high. The specific stiffness (modulus divided by density) is the highest of any metal on the face of the earth … or within the earth for that matter. But beryllium is rare: Its concentration in the earth’s crust is approximately 6 ppm. No rich deposits exist, and one of the results of this low concentration is the aforementioned high cost – compared to aluminum, it’s about 200 times as expensive!
Here are some of the specific numbers for a tube of extruded beryllium: 40 KSI ultimate and 44 MSI modulus – which when combined with the low density, gives you the phenomenal specific stiffness numbers … many times higher than any other metal. By comparison, the modulus of steel is only about 30 MSI, and the density of steel is nearly five times that of beryllium.
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 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.