Decades ago, Sears sold magnesium stepladders. I've used one, and it's freakishly light, a 6-foot step ladder that you can walk around with balanced on one finger.
I've always wondered what a titanium one would be like.
I've always wondered what a titanium one would be like.
In an application like a stepladder, you have to work with certain minimum dimensions for the stepladder to be practical (eg rungs and sides have to fit in the hands nicely). You also have to have certain minimum thicknesses on the parts to have sufficient resistance to local deformation (eg dropping a hammer on the rungs). That forces the parts to be significantly larger and stronger than they otherwise would be. Which makes very lightweight metals like magnesium and aluminum the better choice, as you can make thick parts at the required dimensions at very little weight.
Climbing gear is a great example of this. Even though there's a segment of that market for which money is no object, the only use for titanium in climbing gear is certain specialized applications where corrosion resistance is important. Eg fixed gear mounted on sea-side cliffs. Because climbing gear has to have certain minimum dimensions to avoid damaging ropes, the very low density of aluminum wins over titanium's higher density/higher strength.
If you made a carabiner out of titanium it'd be stronger than necessary, and a lot heavier.
In terms of (rigid, diamond-frame) bicycles, this is why I’m still firmly in the steel camp. No aluminium, no carbon; just steel. It really does have an excellent combination of nice ride quality, low weight, high strength, good failure mode (I’ve broken a few frames, and they tend to just bend/sag, vs the rapid unscheduled disassembling of carbon/Al).
But complex microstructures can be designed to have non-sudden failures. Eg. you could ensure that a visible crack appears at 0.75x the ultimate strength, yet doesn't fail till 1.0x the strength.
You can also design structures so that a 'crack' is either 1mm wide or not there at all (ie. no hairline cracks).
such features of microstructures are not free though - you will lose strength/weight to get them.
Bicycles don't have the minimum size problem GGP is talking about. Titanium is pretty much the perfect frame material (if you can afford it) - all the nice things you list (a bit stiffer than steel, but ride quality is still decent), but substantially lighter.
Good point.
> Titanium is pretty much the perfect frame material[…]
Anecdotally - I understand it’s tougher to work with at about every single step. I’ve seen too many cracked Ti bikes/parts to sign up, I think. I understand the lust though.
They do, in a slightly different way. Bicycle frames are (broadly) stiffness-critical structures. Wider-diameter tubes have a higher specific stiffness because of the increased moment of inertia - that's why we use structures like tubes and I-beams instead of solid bars. Steel frames have skinny tubes, because they're limited by the minimum wall thickness of the tubing; increase the diameter too much and you have a tube that is very vulnerable to dents and very prone to buckling. Steel racing frames of the 1970s are remarkably flimsy, because framebuilders were pushing wall thickness to the absolute limit.
Aluminium bicycle frames are only lighter because the lower density allows you to retain an acceptable wall thickness on larger-diameter tubes. An aluminium frame with the same tube diameters as a steel frame would be considerably heavier than the steel frame, because an aluminium frame needs to be overbuilt to compensate for the lack of a defined fatigue limit.
All common steel alloys have essentially the same stiffness (~207GPa), but higher-strength steels allow us to use wider-diameter tubes with thinner wall sections; incidentally, this is why it's quite pointless to use an expensive tubeset in a lugged frame. CFRP obviously has immense specific stiffness, but it also allows frame designers to really optimise the geometry and use the material more efficiently.
Titanium is a really nice frame material, but it does have some significant issues in practical use. Titanium is very prone to embrittlement if there is any amount of contamination in the weld. Most framebuilders aren't capable of maintaining the level of cleanliness and the comprehensive gas purging required to produce really good welds in titanium, so it's very common to see titanium frames eventually crack around the welds.
To my mind the perfect material for a non-sporting frame was the superb Reynolds 953 maraging steel, but unfortunately it is no longer available. Reynolds 931 and KVA MS2 are still very good materials, particularly when fillet brazed rather than welded. CFRP obviously wins out in terms of pure performance, but I'm not sure that I'd ever trust an old and battle-scarred carbon frame on a hard descent.
Not directly extruding it, but the end result is metal.
The result was a stunningly fast fighter aircraft, capable of Mach 3.2, though in practice engine overheating restricted operation maximum to Mach 2.83 (3,000 km/h), and even that for only 5 minutes at a time as the airframe and fuel would overheat. The MiG-25's mass necessitated huge wings (and overall dimensions), and limited maneuverability. Steel however provided better thermal-tolerance capabilities than aluminium, and lower cost and easier fabrication than titanium.
First flight 1964, introduced to active service in 1970.
That said, the aircraft is notable as an exception to your generally-applicable rule.
<https://en.wikipedia.org/wiki/Mikoyan-Gurevich_MiG-25>
I suspect carbon fibre would also have thermal limitations for high-speed aircraft.
As a slight aside, magnesium is also a very interesting material. It might be we're on the cusp of a major expansion in magnesium usage due to recent advancements
- Mining from seawater (about 1 kg Mg in 1000L of seawater), or existing brine tailings from other extraction activities. With cheap solar electricity this might drive the cost down considerably (below the extremely dirty production methods being used today in China), providing carbon-emission free production of essentially unlimited amounts.
- thixomolding, a die-casting / injection molding-like process where the material isn't completely melted (thixotropic state), producing parts with much less porosity than traditional die casting.
- New alloys that are less prone to fires and corrosion.
For slightly more details, see https://www.youtube.com/watch?v=OIv_Rfl0L_A
For those curious, titanium is present in sea water, at 1 ppb! (magnesium is 1300 ppm)
The idea is you can use less titanium in the application you would use aluminum, but this has limits. If your ladder was .200” wall thickness, you might in theory get away with a .070” titanium for the same weight, but you start running into mechanical stresses or assembly issues or manufacturing.
Titanium is useful when you need internal volume - most recently as an example by Apple. Aluminum was fine, but had thick walls. Steel allowed thinner was but was heavier. Titanium allowed for thin walls and more internal volume, but at a higher cost.
Basically, if you don’t have a size limit, aluminum is great! But most things have size limits, and titanium allows you to trade size for cost.
https://www.apple.com/newsroom/2023/09/apple-unveils-iphone-... https://www.youtube.com/watch?v=S_W73ouKtjU&t=605s
Thick walls on the iPhone are what are going to prevent X Y area which I suspect they need more than thickness.