Wait...she REPLACE the steel bar with the aluminum bar? I think the dwarves are getting smarter. Well, time to set my PCs on fire.
I am afraid the situation is actually much worse. Since it was an armorsmithing mood I was pretty interested in her work so I watched her gather the materials and I checked the content of her workshop while she was working. I saw her take the aluminum bar after she had already gotten the steel bar first. I thought it was simply to add some decoration to her steel artifact and I was pretty happy with it since aluminum is as valuable as you'll get, short of you-know-what.
The point is; I'm confident she grabbed only one bar of aluminum and my bookkeeper agrees with me. I had just bought two bars from the dwarven caravan a few month prior, I saw two bars in my stock when I forbidden them, they were the only two bars of aluminum I had and the second is still in my stockpile.
But I wasn't wrong about her intentions,
there is an aluminum decoration on that aluminum breastplate...
If I'm not mistaken she should have needed two bars to make both the breastplate and the decoration, yet she only used one. And she didn't use the steel bar either, it's still in the forge she used to make the artifact.
She did replace a steel bar with an aluminum bar, but not with one of mine. She willed an aluminum bar into existence, she materialised 25k dwarfbucks worth of metalsmithing out of thin air.
I have to admit the last fortress I played before this one was back when DF2014 was just called "the current one" and ravens were murder, so I'm a bit out of the loop when it comes to dwarfy behaviours. Is that something that is known to happen ?