The metallurgical display case

Dave Cunix looked at my hand at the Blogger 11th Birthday Fiesta and commented that my rings seemed to be multiplying. They weren't - I'd just moved one from my right hand to my left - but as of today I have a new one.

On the left is my platinum wedding band, with a few dings because platinum, a noble metal like silver and gold, is soft. Platinum shares its face-centered cubic arrangement of atoms with other soft metals such as aluminum and copper.

In the middle is my tungsten carbide ring. It's not pure tungsten carbide, because that material is really a ceramic and can't be shaped by bending or cutting like metals can. The material we call tungsten carbide is really tungsten carbide powder held together with the metal cobalt, like concrete is pebbles held together with cement. Cemented tungsten carbide is used to cut steel, and is certainly harder than almost anything I'm likely to grab - that's why it's unscratched.

The ring on the right is my new titanium one. They're all the same size, but the titanium one is much lighter than the others because it's only about a quarter as dense. This makes the Metal Guessing Game a lot more interesting, even though I don't wear the silver ring any more. This lightness, together with its resistance to corrosion, is earning titanium some inroads into the world of jewelry. Its strength also makes it a great structural material, but its Achilles' heel is that machinists hate it: its terrible friction and wear properties cause it to weld itself to cutting tools.