Microstructure Characteristics and Elevated-Temperature Tensile Properties of Al-7Si-0.3Mg Alloys with Zr and Hf Addition

The microstructure characteristics and the elevated-temperature tensile behavior of Al-7Si-0.3Mg alloys with Zr/Hf additions were studied. The results showed that the individual addition of Zr promoted the precipitation of β" precipitates and the best effect was achieved by their combined addition. The individual addition of Zr or Hf and the combined addition of Zr and Hf in alloy could cause a reduction in the elevated-temperature tensile strength. Al-7Si-0.3Mg-0.14Zr-0.44Hf alloy displayed a noticeable increase in the ductility coupled with a remarkable decrease in the strength at elevated-temperature tensile test. The degradation in the strength of all the alloys was attributed to the phase transformation and coarsening behavior. The pre-β and β precipitates played the main strengthening effect because of the fact that the additional Zr and/or Hf containing dispersoids (i.e., Si-Zr/Hf) had relatively large size, nanobelt/rectangle-like morphology, and thereby low number density and volume fraction.