Phase field approaches for damage and fracture have gotten increasing attention in the last 15 years, see e.g. [1]. They offer two main advantages over local damage models and methods for discrete crack modelling. The first one is the definition of a material dependent internal length scale parameter which allows for convergence of the predicted crack pattern with respect to the numerical resolution [2]. The second advantage is related to avoiding complex interface tracking by using a continuous phase field variable to describe a crack’s topology [3]. This provides great flexibility regarding the location of crack initiation and monitoring the path of an individual crack. In the presented work these advantages are exploited by combining phase field damage with a crystal elastic-plastic constitutive law applied to a volume element representing a brittle-ductile dual-phase microstructure. The resulting microcrack pattern is used to calculate an effective energy release rate which is evaluated at the point of maximum macroscopic stress. The influence of key microstructural parameters is studied under different loading conditions. The microcrack pattern and the effective energy release rate depend strongly on the microstructure and the loading conditions. Virtual tensile tests with an increasing volume fraction of the ductile phase as well as an increasing mechanical contrast between the phases are performed. Both parameter studies show a sharp transition from brittle failure with little energy release controlled by a single microcrack to a quasi-brittle failure with large energy release due to several microcracks. The results of virtual compression tests highlight the importance of modelling the intrinsic friction at defect interfaces. Overall, the phase field based approach applied on the microscale gives valuable insights into the role of microcrack initiation and propagation in the complex failure of dual-phase materials. For quantitative predictions involving compressive loading the friction at defect interfaces has to be described more detailed. Therein, a numerically robust method for the determination of the interface orientation poses a major challenge in complex 3D crack networks [4]. REFERENCES [1] P.K. Kristensen, C.F. Niordson and E. Martínez-Pañeda. An assessment of phase field fracture: crack initiation and growth. Philos Trans A Math Phys Eng Sci, Vol. 379, 2021. [2] T.K. Mandal, V.P. Nguyen and J.-Y. Wu. Length scale and mesh bias sensitivity