  We develop a fully-coupled, fully-implicit approach for phase-field modeling
of solidification in metals and alloys. Predictive simulation of solidification
in pure metals and metal alloys remains a significant challenge in the field of
materials science, as microstructure formation during the solidification
process plays a critical role in the properties and performance of the solid
material. Our simulation approach consists of a finite element spatial
discretization of the fully-coupled nonlinear system of partial differential
equations at the microscale, which is treated implicitly in time with a
preconditioned Jacobian-free Newton-Krylov method. The approach allows time
steps larger than those restricted by the traditional explicit CFL limit and is
algorithmically scalable as well as efficient due to an effective
preconditioning strategy based on algebraic multigrid and block factorization.
We implement this approach in the open-source Tusas framework, which is a
general, flexible tool developed in C++ for solving coupled systems of
nonlinear partial differential equations. The performance of our approach is
analyzed in terms of algorithmic scalability and efficiency, while the
computational performance of Tusas is presented in terms of parallel
scalability and efficiency on emerging heterogeneous architectures. We
demonstrate that modern algorithms, discretizations, and computational science,
and heterogeneous hardware provide a robust route for predictive phase-field
simulation of microstructure evolution during additive manufacturing.
