Material changes, how to

Hi all,

I'm using Creo 3.0 M010.

On my assembly, depending of certains conditions some parts must change propreties. (using relations and/or program). I've my own material data base.

How can I order a certain part that for a certain condition "1" the material will be "A" and if the condition will be "2" the material will be "B".

I don't know how to do this (nor within parameters nor with program). I just used to go to FILE > PREPARE > MODEL PROPERTIES > MATERIAL and there change my material.

Can anybody help me with this?