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The processability of laser powder bed fusion (LPBF) fabricated Al alloys can be significantly improved by the introduction of grain refinement inoculates. However, tensile testing results showed that the fabricated alloys exhibited unusual deformation characteristics, including discontinuous yielding, Lüders elongation and low work hardening behaviors. In this study, the underlying tensile deformation mechanisms of the inoculates modified Al alloys after LPBF processing were elucidated through various characterization techniques, using the recently developed Al–Mn-Sc alloy as an example. It was found that the "extra hardening" effect caused by the fine grain structures promoted the discontinuous yielding behavior, while lack of mobile dislocations within the fine grain regions triggered the Lüders bands propagation. On the other hand, the high initial strain hardening rate contributed to the strong work hardening behaviors of the fabricated alloy with lower fine grain fractions, whereas all alloys with different heterogeneous grain structures proved similar dynamic recovery behaviors at higher strains. • The tensile deformation behaviors of LPBF fabricated Al–Mn-Sc alloys with various heterogeneous grain structures were studied. • The fine grain structures promoted the discontinuous yielding behavior, while lack of mobile dislocations within fine grains triggered the Lüders bands propagation. • The high initial strain hardening rate contributed to the strong work hardening of the fabricated alloy with lower fine grain fractions. [ABSTRACT FROM AUTHOR]