FactSage for Alloy Processing

Thermodynamic databases are available for nearly all inorganic material systems. Alloy processing can be modelled for steels, light metal alloys, copper, lead, solders or noble metals, but also for hard metals, oxide and non-oxide ceramics. As an example it is shown here, how heat treatments can be designed for a commercial Al alloy AA6016.

The Al alloy AA6016 has as major alloying elements Silicon and Magnesium. An isothermal section of the Al-rich corner of the Al-Si-Mg phase diagram can be seen in the following figure:

It can be seen that the Mg and Si solubility in the FCC phase is very dependent on the Si and Mg contents. Processing of an alloy consisting purely of Al-Si-Mg could be done by considering such ternary phase diagram sections. However, there are additional other elements, e.g. small amounts of Cu and Mn, that affect the solubilities of Si and Mg. These are taken into account in the following Equilib calculation that shows how the minimum solution annealing temperature can be calculated:

In this calculation, small amounts of Cu and Mn are added and the variable alpha, or A, is used to vary the Mg and Si contents reversely. When defining the calculation conditions, a kinetic limitation is implemented by suppressing the formation of pure Silicon. This is simply done by removing pure Si compounds and the diamond solution phase during the product selection (left part).

For the FCC solution phase, the precipitation option is set, meaning that a reverse modelling approach is used here: Instead of searching for the equilibrium phases at a given temperature, the temperature is calculated where precipitates form from the FCC matrix. The Alpha <A> parameter varies from 0 to 1.3 with a stepsize of 0.02, which results in 66 calculations. The results window will look as follows:

For every calculation (meaning every value of Alpha <A>), one tab page is written containing the results. Using the built-in postprocessor, the minimum solution annealing temperature can be plotted as function of Alpha <A>. Large differences in solution annealing temperatures of 30°C can be seen where a small change of Si and Mg content by 0.6 wt.% (Alpha from 0.3 to 0.9). For small values of Alpha, i.e. small content of Mg, a ternary Al9Mn2Si precipitate forms while for larger Si contents Mg2Si precipitates.

A metallurgist relying only on the ternary phase diagram Al-Mg-Si would easily overlook the effect of the quaternary impurity Mn with a content of 0.1 wt.% on the solution annealing temperature. This evidences the benefit of an Integrated Thermodynamic Databank System compared to classical experimental guidance using published phase diagrams since even small impurities can be decisive for phase stability!

This example showed how FactSage calculations can be used for prediction of alloy processing, e.g. solution annealing temperatures. Kinetic constraints were already taken into account, check out the next application example for further information how to model non-equilibrium processes! Read further information about the Phase Diagram and Equilib module that were used for this application example!