Constitutive relation of FCC crystals

Have you looked at :
Finite Plastic Deformation of Crystalline Solids by K. S. Havner
# Publisher: Cambridge University Press; (March 1992) # ISBN: 0521392454
Havner had a lot of interesting outlooks in his computational studies. Maybe he addressed your problem.
I think that you will find that the polyaxial constitutive equations for FCC crystals are likely to be non-existant or theoretical only.
Have you looked much at the older work on the plasticity and deformation of FCC single crystals in uniaxial tension? In particular the relationship between the flow mechanics or slip systems and relative resolved shear stresses (from the crystal orientation relative to the tensile axis).
It sounds like it could be interesting work. It seems as if it is now a challenging thing to do numerically.
The numerical technology may far exceed the micromechanical understanding science, and result in GIGO situations.
Jim Buch
Jim Buch

AFAIK Paul Van Houtte is working in this field. You should lokk for his papers.
Why do you want to invent the wheel once more?
Michael Dahms