I'm only an amateur here, but even I'd say it looks like very poor quality metal that has resulted in catastrophic fracturing. This would be VERY dangerous to use. I have seen this happen before with bosses and even cranks, but usually after hitting something very hard. I cannot remember the brands.
The only bizarre issue I have had with blade discs was on a brand new Supaswift mower. For some inexplicable reason, one day when I was mowing the entire disc suddenly dropped off. It kept rotating and dug a neat hole into the lawn, making a bare round patch which took months to grow over again. I cannot explain it, because I always do the bolts up tightly to the required torque. The grass was perhaps a bit higher than usual and was a bit damp, but not overly wet. Anyway it was the first time something like that ever happened and it left me scratching my head. It has never happened again.
