|High purity button, broken.
This button of tungsten started out as tungsten powder, and was then partially molten from the outside before being broken in half. It demonstrates very nicely the difference between the sintered structure in the middle, which is around 96% of theoretical density, and the true crystal structure of tungsten in the fully molten outside, which is at 99+% of theoretical density. I highly recommend the 3D view of this sample (click the camera/turntable icon above), which allows you to see clearly how the crystals growing from the inside form a pattern of facets on the surface.
The furnace that made this button was fully capable of melting it all the way through (which says a lot about the furnace, as that is no mean trick for tungsten, the highest-melting element of all). The electron beam was throttled back to only about 20,000 volts to limit the depth of penetration, intentionally preserving the sintered structure in the middle.
See the sample above for more about the company that made this button.
Source: Kaistar R&D
Contributor: Kaistar R&D
Acquired: 6 August, 2004