The effect of thermal aging on phase transformation and impact toughness of an as-cast duplex stainless steel was investigated at room temperature.After long-term thermal aging,the impact toughness decreases significantly and the cracks initiate and propagate more easily.The plastic deformation ability of the ferrite phase decreases after thermal aging,which leads to the degradation of impact toughness.High stress concentration occurs on the grain boundaries of the austenite phase in the aged materials.Meanwhile,high-stress concentration areas are also observed in the austenite phase near the grain boundaries.After long-term thermal aging,pinned dislocations in ferrite and along phase boundaries lead to the high stress concentration.Micro-cracks preferentially initiate in the ferrite phase and propagate via separation of phase boundaries.The blocking influences of spinodal decomposition precipitates and G-phase precipitates are stronger than the effect of grain boundaries and phase boundaries on the dislocation movement.