This is an aluminium oxide though, so it's properties are probably quite different from metallic aluminium. Think about how iron oxide (rust) is a completely different thing from a block of iron. I doubt you could swap it in 1:1 with a cast engine block.
Since you mentioned it though... https://www.youtube.com/watch?v=xflY5uS-nnw
Rubies, sapphires and emeralds are all aluminum oxides which is why the whole transparent aluminum wasn't a real stretch of the imagination. I know they are very hard but I don't know how much so in comparison with metal aluminum.
The space shuttle had a sapphire windshield so it's not even new tech and already used on spacecraft. Also, many high-end watches have a sapphire crystal (not the oscillator, the face glass).
They're also used for sensor platforms on military aircraft where a material has to be see through, but also very abrasion and heat resistant.
Mostly it'd be the bittleness I'd be concerned about.
Yes, hardness isn't everything. So she said, anyway.
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