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[Display omitted] • Laser welded nuclear-grade FeCrAl alloys were firstly investigated using experiments and crystal plasticity modeling. • The maximum ultimate tensile strength and elongation of the welds were 652 MPa (strength coefficient of ∼70.7%) and 9.7%, respectively. • The weld softening was caused by bipolar size distribution of the Fe 2 (Nb,Mo) Laves-phase particles. • The in situ mechanical response confirmed that pore-induced stress concentration resulted in crack initiation and subsequent weld failure. The Fukushima Daiichi nuclear accident in Japan in 2011 created a strong push for the exploration of advanced accident-tolerant fuel materials. For this purpose, FeCrAl-based alloys are a promising class of materials. However, no study has been conducted on the Laves phase evolution in the weld, which is proved to be directly related to weld softening based on the observations in this study. Moreover, the failure mechanism after laser welding remains unknown. In this study, laser welded nuclear-grade FeCrAl alloys were investigated through experiments and crystal plasticity modeling. The maximum ultimate tensile strength and elongation of the welds were 652 MPa (strength coefficient of ∼70.7%) and 9.7%, respectively. Obvious softening was observed in the laser-welded FeCrAl alloy joints. The re-precipitated nanoscale Fe 2 (Nb,Mo) Laves-phase particles in the fusion zone (FZ) were smaller than 200 nm, whereas the particles in the heat-affected zone (HAZ) were coarsened over 1.6 μm owing to Ostwald ripening. The bipolar size distribution of the Laves phase was the intrinsic reason for the weld softening. In addition, porosity defects were detected in the FZ and fusion line. Most of them were less than 4 × 10−5 mm3 in volume and were distributed left-of-center or right-of-center instead of in the exact center of the FZ. The in situ mechanical response confirmed that pore-induced stress concentration triggered off crack initiation and propagation, leading to the weld failure. Therefore, enhancing the precipitation of Laves phase with concurrent elimination of the porosity defects can serve as a promising strategy to achieve high-strength FeCrAl alloy joints. The fundamental discoveries in this study can facilitate the application of laser welding in joining the nuclear-grade FeCrAl-based alloys. [ABSTRACT FROM AUTHOR]