Effect of microstructure on the quench sensitivity of AlZnMgCu alloys

The effect of microstructure on the quench sensitivity of 7055 type aluminum alloys was investigated by hardness testing, optical microscopy and transmission electron microscopy. The results showed that the homogenized and hot-rolled alloys without Zr and the homogenized alloy with Zr were not quench sensitive, while the hot-rolled alloy with Zr was quite quench sensitive. The reason was analyzed according to the difference in the microstructure in these alloys, i.e. type of nucleation sites including dispersoids, (sub)grain boundaries and constituent particles for heterogeneous precipitation during slow quenching. Quench-induced η phase was observed mainly on large dispersoids and (sub)grain boundaries. In the homogenized and hot-rolled alloys without Zr, grain boundaries, which were the main heterogeneous precipitation nucleation sites, resulted in low quench sensitivity. In the homogenized alloy with Zr, grain boundaries and fine and coherent Al3Zr dispersoids led to low quench sensitivity. In the hot-rolled alloy with Zr, the large number of coarse Al3Zr dispersoids and subgrain boundaries were mainly responsible for high quench sensitivity. Constituent particles seemed to have little effect on quench sensitivity.

Research highlights▶ The authors have carried out detailed investigations on the effect of microstructure on quench sensitivity of AlZnMgCu alloys with and without Zr. ▶ The mechanism for higher quench sensitivity due to deformation of Zr-containing alloy was proposed according to microstructure examination.