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The effect of particle deformation zone (PDZ) on the microstructure and mechanical properties of SiCp/Mg-5Zn composites was studied. Meanwhile, the work hardening and softening behavior of SiCp/Mg-5Zn composites influenced by PDZ size were analyzed and discussed using neutron diffraction under in-situ tensile deformation. The evolution of FWHM (full width at half maximum) extracted from the diffraction pattern of SiCp/Mg-5Zn composites was used to interpret the modification of dislocation den-sity during in-situ tension, which discovered the effect of dislocation on the work hardening behavior of SiCp/Mg-5Zn composites. In addition, the tensile stress reduction (Δ Pi ) values during in-situ tension test were calculated to analyze the effect of PDZ size on the softening behavior of SiCp/Mg-5Zn com-posites. The results show that the work hardening rate of SiCp/Mg-5Zn composites increased with the enlargement of PDZ size, which was attributed to the grain size of SiCp/Mg-5Zn composites increased with the enlargement of PDZ size. Moreover, the stress reduction (ΔPi ) values increased continuously during in-situ tensile for SiCp/Mg-5Zn composites due to the increased stored energy produced during plastic deformation, which provided a driving force for the softening effect. However, the effect of grain size on the softening behavior is greater than that of the stored energy, which led to the tensile stress reduction (ΔPi ) values of P30 (dPDZ=30μm)-SiCp/Mg-5Zn composite were higher than that of P60 (dPDZ=60μm)-SiCp/Mg-5Zn composite when theε ri were 0.25, 0.5, 0.75 and 1, respectively.