Effect of quenching rate on strengthening behavior of an Al-Zn-Mg-Cu alloy during natural ageing

The effect of quenching rate on strengthening behavior of an Al-Zn-Mg-Cu alloy 7085 during natural ageing has been investigated by hardness, tensile testing, differential scanning calorimetry, transmission electron microscopy, high resolution transmission electron microscopy and three dimensional atom probe. Quench-induced Y phase forms in the air-cooled condition and contributes a remarkable 100 MPa to the as-quenched strength. The air-cooled condition continues to exhibit higher hardness and strength than for the rapid, water-cooled condition during natural ageing for up to about 30000 h. With the increase of natural ageing time, the hardness difference tends to decrease slowly within the first 0.5 h, then sharply until about 20 h, and finally approaches a constant, small difference. The reasons for this unusual quench sensitivity are discussed based on the different precipitation features arising during quenching and natural ageing.