I guess it's steel from the photo.
Will at www.63xc.com has done a good article about cog choice, and explains these sudden failures quite well here.

excerpt
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Secondly, consider carefully the implications of heat treatment.

Materials don't cost much. The steel in your cog was probably cheaper than the packaging in which you bought it. The reason why an ounce or two of metal ended up costing as much as a pair of good jeans was that the manufacturer subjected it to various post-machining treatments to make it wear better.

Durability in a component is a complex phenomenon. Consider a samurai sword.

At its edge, the sword needs to be very hard, to take and hold a keen edge. If the metal at the edge of the sword was soft, the sword would blunt too quickly to be of use.

At its centre, the sword needs to be pliable. This allows it to bend and absorb impact. If the metal in the centre of the sword was hard, the sword would likely shatter against other metal weapons.

The skill of the swordsmith lies in judging hardness and softness and how to combine them.

In just the same way, a sprocket needs hard teeth to withstand wear, but must retain enough pliability to cope with road shocks, sudden changes of speed, and so on.

Case hardening, the deliberate creation of a hard crystalline 'skin' on the outside of a metal component, requires very precise control. Since it isn't economic to process sprockets one by one, they must be batched, and there is then the danger that individual sprockets within a batch may be over- or undercooked. An undertreated sprocket will be too soft and will wear quickly. An overtreated sprocket will be too hard and may shatter in use.

This gives rise to an odd paradox: expensive cogs subjected to serious heat treatment wear better than cheapies, but at an increased risk of sudden failure. Brittle failures are rare, but they do happen, and here is a photo to prove it.