Castings and Popsicles
Author: Harold Sharon
Grains growing inward from the surface, and grain boundaries coming right to the surface. The grains, of course, were ice crystals. If this had been a tensile failure of a litle cast iron, cast steel, or cast aluminum part, it would have looked the same.
Castings are articles made by the simple freezing of molten metal ( castings can be other material, too, like popsicles). Under metallurgical examination, we see grains and grain boundaries. Think of crushed rock, held together with glue.
The conspicuous observation to make is the orientation of the grains. The grains begin to freeze out on the surface, and freezing progresses inward. Grain boundaries come right to the surface. Parts that are highly stressed will eventually fail, and the cracks will almost certainly start at the grain boundaries.
Forgings have grain with "flattened" shape and orientation, like corn flakes that have been left to dry in the bowl. Forging makes for stronger parts. But that's another story.
So, quick, while the weather's warm; bite into a popsicle ( or a piece of metal), and have a look.
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