The changes of microhardness, microstructures and precipitation phase morphologies of as--cast and aged AZ91 alloys with different La were characterized by TEM, OM, SEM and Vickers. The influences of discontinuous β phase and Al11La3 on AZ91 magnesium alloy were discussed and the diffusion coefficient of Al atom in the aging process was calculated. The results showed that the alloy with 0.16%La aged at 170 ℃ for 24 h has the largest hardness (138 HV). By extending aging time and adding La element, the discontinuous precipitation phases was transformed into continuous lath--sharped phases and the diffusion coefficient of Al atom was decreased, the volume and distribution of β phase were changed. During the process of heat treatment, the topography of Al11La3 in grain boundary was not changed, but the anchoring effect of Al11La3 restricted the growth of β phase. By controlling the content of La and parameters of aging treatment, the volume and size of β phase were changed, in the mean time the strengthening and toughness properties of alloys can been improved effectively.
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