The evolution of the microstructure of polycrystals can be modeled by coupling a Cosserat model and a phase field one [1]. In such a model, rotational degrees of freedom define grain orientation and evolution, while grain boundaries are governed by the phase field. However, this promising approach is highly complex to implement and induces a high computational cost: test cases are often academic and/or the theory is simplified, assuming for example small deformation or bi-dimensional problem. In this work, the finite strain Cosserat-phase-field theory [1] is assessed on 3D test cases. To reduce the time to solution and exploit modern supercomputers, the domain decomposition framework for the phase field fracture staggered solver [2] is extended to this theory. The Cosserat subproblem and the phase field one are solved with parallel domain decomposition solvers. To be robust multipreconditioned iterative solvers are exploited [3,4]. Preconditioners performances are assessed and scalability studies up to hundreds of cores are presented. REFERENCES [1] A. Ask, S. Forest, B. Appolaire and K. Ammar, Microstructure evolution in deformed polycrystals predicted by a diffuse interface Cosserat approach, Adv. Model. and Simul. in Eng. Sci. 7, 9 (2020) [2] J. Rannou and C. Bovet Domain decomposition methods and acceleration techniques for the phase field fracture staggered solver, Preprint submitted to Int J Numer Methods Eng., https://hal.science/hal-03938084 [3] C. Bovet, A. Parret-Fr´eaud, N. Spillane, and P. Gosselet. Adaptive multiprecondi- tioned FETI: Scalability results and robustness assessment. Computers & Structures, 193:1–20, 2017. [4] C. Bovet, P. Gosselet and N. Spillane, Multipreconditioning for nonsymmetric problems: The case of orthomin and biCG, Comptes Rendus Mathematique, 355(3):354–358, 2017.