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The effect of heat treatment on the plane-strain fracture toughness in forged AZ80 magnesium alloy was studied. Two different kinds of heat treatment technologies (T5 and T6) were performed on the forged samples. The values of plane-strain fracture toughness ( K) were obtained from the stretched zone (SZ for short) analysis. The results showed that the as-forged specimen would produce a large quantity of β-MgAl precipitates both in the interior of the grains and at grain boundaries after T5 and T6 heat treatments. The β-MgAl displayed a precipitation strengthening. Compared with the as-forged specimen, the ultimate tensile strengths of T5 and T6 specimens were improved by 42.8 and 30 MPa, respectively. And the K of T5 and T6 specimens also increased to 23.8 and 21.0 MPa m while that of the as-forged sample was 17.9 MPa m. The average grain size of T5 sample was similar to that of the as-forged one, but was finer than that of T6 specimen. The best mechanical properties and fracture toughness of forged AZ80 magnesium alloy were achieved after T5 treatment. [ABSTRACT FROM AUTHOR]