Metallurgy for Cyclists - Part 7
source www.ibiscycles.com
by Scot Nicol
posted 2006-04-16
Part 7 - The Final Chapter
Cycling Interactive would like to thank Scot Nicol of www.ibiscycles.com for allowing us to republish this article series.
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).
Aluminum Lithium
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.
Beryllium
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.
