Tungsten Tungsten/zirconium alloy screw plug
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3DTungsten/zirconium alloy screw plug.
Neil Lipson, the source of most of my tungsten and molybdenum, sent me this little plug in exchange for me analyzing what it's made of. He was hoping there might be iridium in it, but instead analysis by x-ray fluorescence spectroscopy at the Center for Microanalysis of Materials, University of Illinois (partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439) indicates that it's a strange mixture of primarily tungsten (62%) and zirconium (30%), with the remainder being iron and nickel.
I looked around on the web and could find no references to tungsten-zirconium alloy, but that's no proof that such a thing isn't used in specialized applications. I would guess high temperature, corrosive environments, perhaps a chemical reaction chamber that operates at high pressures and temperatures.

I would, however, be wrong, as was pointed out in an email from John Wechselberger:
By the way, the tungsten alloy three-hole spanner threaded plug on your site is a weight plug for the Callaway Hawk Eye golf club. A chamber in the club head is filled with a measured weight of the aforementioned pellets, then closed with the plug, which is machine-driven with a three-pin spanner drive head. The Big Bertha plug is similar, but has larger diameter, smaller spanner holes spaced further apart, and the head contour comes up as a shallow cone, which then goes to a flat, rather than the blended contour of the Hawk Eye plug.
Shows what I know, eh?
Source: Neil Lipson
Contributor: Neil Lipson
Acquired: 18 December, 2002
Price: Donated
Size: 0.75"
Purity: 62%
Sample Group: Golf Equipment