AA7050 Alloy

Alloy name: AA7050
Diagram No.: 858
Type of diagram: TTT
Chemical composition in weight %: 6.2% Zn, 2.3% Cu, 2.35% Mg, 0.13% Zr, balance is Al
Alloy group: Aluminium-based alloys
Note: Simulated Continuous Cooling Curves for AA7050 showing the high temperature, medium temperature and low temperature precipitation regions. The solid lines showing the start temperatures and dashed lines showing the end temperatures of precipitation.
Figure shows the cooling curves for the Al-6.2Zn-2.3Cu-2.35Mg-0.13Zr (AA7050), as predicted by the numerical model for cooling rates varying over a range of 4 orders of magnitude. The curves have the start (solid lines) and end temperatures (dashed lines) of the precipitation of different phases through high, medium and low temperature precipitation reactions. The start temperature is when number density for that particular phase has its first non 0 value and the end temperature, when it becomes constant. The nucleation is not evenly distributed over the range of temperatures shown. The nucleation rate is more initially and decreases with time.
The precipitation regions of the S phase start at lower temperatures at higher cooling rates due to the fact that higher supersaturation is needed to overcome slower diffusion for nucleation at lower temperatures. As nucleation and growth is limited, the supersaturation grows as the temperatures drop, leading nucleation continuing at lower temperatures at higher cooling rates, leading to a large S phase precipitation region at higher cooling rates. The number density of S phase is higher at higher cooling rates but they are finer in size due to limited growth. These smaller platelets more easily dissolve during preheating before extrusion. There is an overlap of the high and medium temperature precipitation reaction stages at higher cooling rates. The medium and the low temperature precipitation reactions begin early at higher cooling rates due to availability of solute in the alloy. No precipitation at temperatures below 50C is observed.
The S (490C), Eta(470C), T(480C) and Omega(540C) phases are low melting temperature phases which, if they do not dissolve during preheat (> 0.6 µm) , may melt during extrusion6. S phase is brittle and reduces the toughness of the alloy. If larger than a critical size (> 1 µm) , they may also cause particle stimulated nucleation of recrystallization. To have a favorable microstructure with precipitates < 0.6 µm, the cooling rates need to be > 500C/hr.
In case of cooling after solution heat treatment before aging, any S phase is undesirable as it might remain in the final microstructure. Elimination of S phase is possible if we quench the alloy at very high cooling rates.
Reference: Pikee Priya, Microstructural evolution during the homogenization heat treatment of 6XXX and 7XXX aluminum alloys, PhD Thesis, Purdue University, 2016, pp. 144-145.

Transformation Diagram

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