Influences of small addition of Sc and Zr on grain structure and quenching sensitivity of Al-Zn-Mg-Cu alloys

The Al-Zn-Mg-Cu alloys without Sc and Zr and with Sc and Zr subjected to water quenching and air quenching and subsequent aging are used to investigate the influences of small addition of Sc and Zr on the grain structure and quenching sensitivity of Al-Zn-Mg-Cu alloys. Due to a large quantity of coherent Al3(Sc, Zr) dispersoids, the recrystallized fraction and the (sub)grain size are significantly smaller for the aged alloy with Sc and Zr than for the aged alloy without Sc and Zr. The precipitates induced by air quenching are major eta phases and minor T phases for both alloys without Sc and Zr and with Sc and Zr. The alloy with Sc and Zr has higher quenching sensitivity for yield strength than the alloy without Sc and Zr, mainly because of the greater quantity of (sub) grain boundaries in the alloy with Sc and Zr. A small quantity of incoherent Al3(Sc, Zr) dispersoids caused by growth and recrystallization also contribute to the higher quenching sensitivity of the alloy with Sc and Zr.