Metallurgy for Cyclists VI: Try Something Exotic
by Scot Nicol
You probably thought that with this installment -- part VI of our six-part series on bicycle metallurgy -- we'd be done with the subject. You were wrong, and you should know me better than that by now. I've added a seventh part, because there is too much remaining to discuss. And because I'm having too much fun doing it...
This time we'll start to talk about exotics: those materials that we didn't include in our coverage of aluminum, steel, titanium and carbon-fiber composites. The final episode of our series will have more on these exotics, plus a wrap-up and maybe even a quiz: I'll present you with some materials that have fantastic "numbers," and you can try to determine what they are, and why they would stink as bicycle materials.
The previous installment covered the subject of carbon-fiber composites (it's not a metal, but we explained that last time). In order not to confuse things, I didn't distinguish between thermoset and thermoplastic composites. The carbon-fiber composite bikes that you've seen on the trail for the last few years are of the thermoset variety. Thermoplastic composites are newcomers. The differences in fabrication techniques are analogous to those between making bread and making chocolate.
Making a structure out of thermoset composite is is like making bread. You mix up some ingredients (flour, water, yeast); put them in a mold (bread pan); apply heat (oven); and a chemical reaction occurs (yeast rising). Voilà: Wonderbread. It's a one-way process. You can't re-melt the bread and start over.
Thermoplastic construction is more like molding chocolate. You mix ingredients, then heat them up until they melt. You then mold the material into the shape you want. Make a mistake? No problem. Re-melt and re-mold. Although the number of times you can do this before it degrades varies with the material you're using, it's a simple process and apparently doesn't even smell much. This emission-free reform/recycle capability has led some people to call thermoplastic a green material.
A very important attribute of thermoplastics is that they have much better impact resistance than thermosets. On the other hand, it's hard to bond anything to a thermoplastic. Given the number of bits that you need to attach to the average bicycle frame, this is be a formidable hurdle. Perhaps because of this, the only application of thermoplastics to cycling that I am aware of to date is the Yeti bike, a collaboration between Kaiser Aerospace and Penske.
Imagine a metal with half the density of aluminum, strength better than 6061, and elongation around 10 to 11 percent. I'm describing a magnesium alloy, currently being tested by Easton, who say it looks promising. Although the alloy has a low modulus, in the range of 6MSI, that really shouldn't be an insurmountable problem -- as we've seen, it's easy to build a stiff frame from aluminum, despite its low modulus.
However, that leaves the issue of corrosion. Leave a magnesium part out in the rain and it will disappear faster than just about anything except unpainted steel. This problem may be overcome through proper surface treatment, either painting or anodization. Surface treatment might however cancel one of the side benefits of magnesium -- its usefulness as a firelighter. If you need to start a fire for some reason, you can just scrape some flakes off your dropouts. The material's reactivity means that they'll easily burn. For a mini-Hindenburg effect, just add water! By the way, titanium does the same thing, but it's a little harder to get it started.
I'm sure you've heard of aluminum MMCs, or metal matrix composites. In fact, we've already briefly discussed the Duralcan MMC tubing which Specialized has been using in its M2 bikes for years. The Duralcan material is an alloy of aluminum (for bike-industry purposes, manufacturers use either 6061 or 7005 base material) combined with a ceramic-aluminum oxide (Al2O3). Duralcan has patented a process in which the Al2O3 is added while the aluminum is molten and in a vacuum.
The benefits of the process are apparently numerous, but for we tight-wad bikies, the big advantage is that it's cheap. If aluminum oxide sounds familiar to you, it might be because you've sanded something with Al2O3 sandpaper in the past. If so, you've used essentially the same stuff that goes in these tubes: 600 grit aluminum oxide. Different percentages of Al2O3 yield different results. The M2 bikes have about 10 percent Al2O3 (by weight) in their mix. Which means they're 90% aluminum. Changing the volume fraction of the ceramic allows you to adjust the mechanical properties. Add more Al2O3 and stiffness goes up, but elongation and fracture toughness suffer. With a 10-percent mix, the material has about 8-percent higher yield strength, and 20-percent greater stiffness, than the base alloy. The trade-off is that the elongation will be reduced, but the claimed value of 10% is within acceptable limits.
Aluminum bikes are stiff enough already, you say. True, but as you also know, this stiffness is a function of design. Suppose you are designing the rear end of a bike. Since you'll want plenty of clearance for mud, heels, tires and chainrings, smaller stiffer tubes may be the way to go. For the same reason, you can use increased modulus to reduce the size of your main frame tubes so they don't resemble giant sausages. Being able to change the modulus of the different tubes means that you can put stiffness exactly where you want it. Small, incremental advances like these continue to drive the evolution of the bicycle frame.
Heat treatments for MMCs are virtually to those for 6061 alloys. There's even a 7005 version if you don't want to heat treat, although at the time of writing it hasn't seen much commerical use. The strength numbers don't really change over a standard 7005, but you can get those increases in modulus mentioned previously.
I've refrained from getting into the huge subject of MMCs that can't be welded, but exhibit excellent mechanical properties. These are viable for bicycle use, but the designer is required to use lugs, or some other method of tube joining.
Overall, when you look at the aluminum oxide MMCs, there's no earth-shattering news -- just some minor enhancements to the mechanical properties, and some drawbacks. What matters most, as I keep pointing out, is intelligent design of the entire structure. Had enough yet? For the final installment, I'll take a look at three other aluminum MMCs, do an overview of beryllium and AerMet 100, throw in a mystery metal, and provide that long-awaited wrap-up.