Metallurgy for Cyclists VII: The Final Chapter
by Scot Nicol
The end is nigh. This is the seventh and final part of our six-part series on metallurgy as applied to bicycles. This really is the final installment, I promise. I will use it to finish off my discussion of exotic materials, and then give you a sample of 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, the drug used to treat manic-depressive psychosis, can also be used to enhance the mechanical properties of aluminum alloys, producing new materials with amazing figures for strength and stiffness.
Although no-one is proposing to make a bike frame out of pure lithium, it's worth spending a moment to check out its properties. Lithium is the lightest of all metals, number three on the periodic chart, and far less dense even than beryllium. However, for those who have to work with it in its metallic form, it seems more likely to cause manic depression than to cure it. Lithium is a pain to work with. It's unstable, and it loves oxygen. Minute amounts of the stuff can contaminate an entire processing facility.
This cussedness is carried over into the lithium-aluminum alloys. Heat treatment is critical... and easy to screw up. If you heat for too long, or too hot -- even by a small amount -- the lithium will oxidize, leaving you with a soft, almost pure aluminum. Since the alloys contain only about one or two percent lithium, it doesn't take much to make it all go away.
OK, let's say that you've bought up a batch of heat-treated, aged and work-hardened T-8 lithium-aluminum alloy. It was difficult to produce and so it's ruinously expensive, but the numbers look great. What now?
If you want to cast a hockey stick or baseball bat, fine and dandy. If you want to get it extruded into tubes so that you can weld it into a bike, it won't be in a T-8 condition anymore. When you look at more realistic conditions, like T-6, the strength numbers then come back down to earth.
It's not surprising that, although lithium-aluminum has been around for many years, not much of the stuff has made it into bike tubing -- or many commercial applications at all, for that matter. The first question for erstwhile lithium framebuilders is, "Can you get hold of it?" Only if the answer is "Yes" can you go on to the second, which is, "Can you manufacture it?" I'm still waiting for two consecutive "Yes" answers.
Other materials that get thrown into the MMC vat include boron carbide (B4C) and silicon carbide (Si4C). When you add these materials to aluminum, you get some excellent theoretical enhancements. But processing them is not without problems.
Silicon carbide is quite reactive and can break down in the weld zone, reacting with the base metal to form aluminum carbide. Aluminum carbide is weak in strength and reactive -- so much so, in fact, that it dissolves in water. Not a good thing for a weld to do. For obvious reasons, silicon carbide hasn't seen much use in bicycle applications, despite its tempting mechanical properties.
Boron carbide is the material used in Boralyn. Other boron carbide-enhanced aluminum alloys are undergoing testing. 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%. The base 6013 is a high-strength alloy with good fracture toughness for an aluminum -- though the boron carbide B4C tends to diminish that -- and it is supposed to be easy to work. But keep in mind that not all boron carbides are created equal, and neither are all base alloys. We have yet to hear 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.
In fact, low density is only one of beryllium's amazing mechanical properties. It also boasts an amazing specific strength (strength over density) and specific stiffness (modulus over density). In fact, its specific stiffness is the highest of any metal on the face of the earth... or within it, for that matter.
An extruded beryllium tube has 40 KSI ultimate strength and a 44 MSI modulus. This gives the stuff the most phenomenal specific stiffness numbers, many times higher than any other metal. For instance, the modulus of steel is only about 30 MSI, and its density is nearly five times higher.
OK, now the bad news. First, the metal has a horrendous elongation number, about 2% in the longitudinal direction, and 0.2% in the transverse. (An interesting note that appeared next to the elongation number in one mechanical engineering handbook I found read: "Ductility values in practice will be found in general to be much lower, and essentially zero in the transverse direction." Ouch!)
But the real clincher is beryllium's rarity. Its concentration in the earth's crust is approximately 6 ppm. Since no rich deposits exist, it costs mega -- about 200 times more than aluminum.
Fortunately, there is an alternative to extruded beryllium. It's called cross-rolled sheet. The $25,000 showpiece beryllium bike that Brush-Wellman (a vertically integrated beryllium company) made for American Bicycle Manufacturing a couple of years ago was fabricated from sheet-rolled tubes, taking advantage of the material's higher elongation number (>10%) and higher ultimate and tensile strength. To make the bike, the sheet was rolled into tubes and welded together.
One last piece of bad news. Beryllium wins out over all metals on the toxicity issue. In fact, it can kill you. Inhalation of dust particles or vapors containing beryllium may cause berylliosis, an inflammation of the lungs.
A pure beryllium bike is not commercially feasible, but Brush-Wellman has created an aluminum-beryllium alloy which shows some promise. Trademarked as AlBeMet, 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, varying from 30 to 62% beryllium in the mix, with the following claimed mechanical properties:
Brush claims that these alloys are weldable. It's interesting to note that strength is quite low for the versions with less than 50 percent beryllium.
AerMet 100 is a promising material that's been raising eyebrows all around the industry. This new ferrous alloy (steel) was patented in 1992 by Ray Hemphill and Dave Wert of the Carpenter Technology Corporation, and several framebuilders, including Kellogg, Davidson, Curve and Arrow, are presently fiddling around with it.
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, you'll find a large percentage of nickel and cobalt, which have slightly higher density than iron. But where this material really blows away the other weldable steel alloys (and all other bicycle fabricating materials) is in strength. 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 ductility -- the same test revealed 10% elongation -- and a 28 MSI modulus, comparable with that of other steels. 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, but there are a few clouds on the horizon. As you might imagine, AerMet is expensive, but it's still only about a half to two-thirds the price of titanium. Also, it's not yet available tapered or butted, though work is ongoing -- and, meanwhile, you can always braze it to 4130.
The real problem for a modern framebuilder is that AerMet 100 is density challenged in the same way that steel is. It's going to take some work to get those frame weights down to the two-pound level, which I consider the next weight milestone in frame fabrication, without suffering beer-can effects. On the other hand, it does look like AerMet will make it easy to build an extremely durable three-pound frame.
A Mystery Metal...
Gary Helfrich recently told me about a wonderful new material with 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 -- you get a frame weight of 1.3 pounds!
What's wrong with this picture? Here comes that pesky elongation again, rearing its ugly head. Although Helfrich's material has elongation as good as a carbon-fiber filament, it's still barely above zippo percent.
The material? Monocrystalline silicon, the material used to make the chips in the computer on which this document was typed.
Don't be Fooled
Did you get all excited about the Helfrich material? Or did you learn enough from these lectures to look twice at makers' claims? Two simple thoughts to conclude the series. When assessing materials for use in bicycle design: i. look at the whole picture, and ii. put the material where you need it.