If you are using Explicit Dynamics system, you can use Polynomial EOS and Bilinear Isotropic Hardening directly in Engineering Data to create the material. See attached picture below.
If you want to write your own material model, you need to have Autodyn license and edit Autodyn user subroutines to include your own material model. More information can be found from "Autodyn User-Subroutine Guide" from ANSYS Online Help: Autodyn User's Subroutines Guide (ansys.com)
Here is the reference material from ANSYS Help:
11.9.5. Polynomial EOS (ansys.com)
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