Dependence of hardness on interlamellar spacing of discontinuous precipitates in cast AZ91 magnesium alloy

The relationship between the hardness and interlamellar spacing of discontinuous precipitates was investigated for a cast AZ91 (Mg-9%Al-1%Zn-0.3%Mn) alloy. After the specimens were solution-treated at 693 K for 24 h, the specimens were isothermally aged for 48 h at different temperatures ranging from 403 K to 473 K, in order to obtain discontinuous precipitates with various interlamellar spacings. It was found that aging below 423 K yielded nodular discontinuous precipitates as the only observed structure at the grain boundaries, while aging above 423 K yielded both discontinuous and continuous precipitates. The hardness of the discontinuous precipitates decreased with an increase in the aging temperature, while the interlamellar spacing of the discontinuous precipitates increased with increasing aging temperature. The hardness of the discontinuous precipitates in the AZ91 alloy was correlated with the interlamellar spacing and can follow a Hall-Petch type relation as in the case of lamellar-structured pearlite in steel.