Intrinsic mechanism of grain size effect and grain boundary misorientation angle effect on crack propagation in martensitic steels

•A polycrystalline phase field mode is proposed to investigate the effects of grain size and grain boundary misorientation angle on the crack propagation process.•This model constructs a grain boundary energy function and uses a specific order parameter to track crack propagation in the grain boundary area.•The secondary crack typically initiates at grain boundaries and propagates along them.•The crack mainly bifurcates near the trifurcated grain boundary.•The crack tip tends to induce transgranular fracture in low-angle grain boundaries and intergranular fracture in high-angle grain boundaries. Grain size and grain boundary misorientation angle as two important parameters in the polycrystalline system have a significant influence on the mechanical properties of advanced martensitic steels. A phase field model is used to investigate the intrinsic mechanism of the effects of grain size and grain boundary misorientation angle on the crack propagation process which considers the coupling damage behavior between grains and grain boundaries in martensitic steels by means of the finite element finite method. The transgranular (through grain boundaries) and intergranular (along grain boundaries) failure can be simulated by this model through constructing a grain boundary energy function and using a specific order parameter to track crack propagation in the grain boundary area. This model can directly display the crack propagation path inside the grain (martensite and austenite phases) and grain boundary area with the stress distribution. The preferential cleavage planes in austenite and martensite phases are 111γ and 100α in grains with different orientations, respectively. Moreover, the simulation results show that the crack mainly bifurcates near the trifurcated grain boundary. The crack tip tends to induce transgranular fracture in low-angle grain boundaries and intergranular fracture in high-angle grain boundaries when it reaches the grain boundary area. Consequently, this model will be a powerful tool to investigate the influence of grain size and grain boundary misorientation angle on the mechanical properties of polycrystalline martensitic steels.