Precipitates formed in the as-forged Mg–Zn–RE alloy during ageing process at 250 °C

The precipitate behavior of the Mg–10Gd–2Y–0.5Zn–0.3Zr alloy during forging process and subsequent ageing process at 250 °C is studied in the article. Different from the precipitation sequence in extruded Mg–Zn–RE alloy, only long period ordered structure and β′ phase precipitate on the Mg matrix during forging process. The slight kinking is mainly ascribed to the insufficient and uneven plastic deformation in the forged alloy, which suggests that the deformation mode of the alloy during forging process is different from that of extruded Mg–Zn–RE alloy. Amounts of long period ordered structure and β′ phases precipitate on the matrix with ageing time increasing until 10 h. The peak hardness, 100 HV, of the forged alloy during ageing process is obtained when the alloy is aged at 250 °C for 10 h. The long period ordered structure and β′ phase are the main strengthening phases in the peak-aged alloy. Therefore, the strengthening mechanism of the forged alloy is controlled by the comprehensive effects of grain refinement and dispersed precipitation of long period ordered structure and β′ phase on the matrix. The strength improvement of the peak-aged alloy results from dense precipitation of long period ordered structure and β′ phase on the matrix.

► The strengthening mechanisms of the alloy are studied.
► The optimal ageing parameters at 250 °C are obtained.
► The phase transformation of the different secondary phases is discussed.