Micro-Melt PD#1 consists of:
From Spyderco's website:
Micro-Melt PD#1 is an air-hardening cold-work die steel. PD#1's unique alloy composition gives it wear resistance significantly superior to conventional steels like D2, while still maintaining excellent toughness. Thanks to Carpenter's advanced Micro-Melt powder metallurgy process, PD#1 possesses an extremely fine microstructure and uniform carbide distribution, as well as the enhanced compressive strength necessary for the demands of tooling applications. Collectively, these qualities give it a unique combination of wear resistance and toughness that make it ideal for use as a blade steel.
-  Spyderco.com product page: Mule Team™ 27 Micro-Melt PD#1