Far from potmetal, it's a typical appearance of brittle fracture of a carbon steel. The failure was initiated by fatigue cracking mechanism (the narrow, shinier, smoother band on the top outer, roughly from 9:00 to 3:00-4:00 clock position). Like most fatigue failures, there is an origin. However, without a microscopic-level investigation I wouldn't put down my money yet, but look at around 11:00 position. There is a dark sliver on the fracture surface which might indicate the origin. It might be a near-microscopic crack.
So, the crack grew with each iteration of torque application through fatigue. The crack became longer and deeper and at one point, it let go at once. If I'm not mistaken, there is a sign of ductile fracture around the "peak" and probably extended all the way to 9:00 judging from the chevrons pointing to the peak. The inner portion is all brittle judging from the rough and jagged appearance.
Half of you are probably snoring by now ...