My only credentials are a lifetime of wikipedia, but I think it's because breaks in metals propagate along crystal boundaries. You can harden metal by cooling i

My only credentials are a lifetime of wikipedia, but I think it's because breaks in metals propagate along crystal boundaries. You can harden metal by cooling it quickly which results in small crystals, or compressing or stretching it (i.e. with a hammer), which breaks larger crystals into smaller crystals. Maybe with a completely amorphous structure it's like the smallest crystals ever.