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Phase Stability in Al-Si-Sr Alloys

Strontium is becoming an important modifier of Al-Si alloys, being increasingly preferred over sodium and antimony since it is easier to handle and has less detrimental environmental effects. Additionally, strontium is easy to add to aluminum based alloys as this system presents several intermetallic phases, facilitating the fabrication of master alloys. When Sr is added in small quantities to Al-Si alloys, their eutectic structure changes from thick flakes to fine interconnected fibbers, consequently improving their mechanical properties. Although Sr is added in relatively small amounts, segregation during processing may lead to the formation of several intermetallic phases that affect the overall performance of Al-Si alloys. Thus, it is of interest to understand the phase stability of the Al-Si-Sr ternary system. As a first approach, we have currently developed the thermodynamic model for the Sr-Si system.

Results
  • Within the CALPHAD approach, two different thermodynamic models for the binary system have been developed. One of them considers the liquid phase as a random solution, while the second model uses the associate formalism considering short range ordering in the liquid phase.

  • Additionally, the SrSi2 phase was considered to have a significant degree of non-stoichiometry. Evidence for this comes from experimental compositional analysis as well as XRD lattice parameter determinations. Current work in our group supports these observations by performing first-principles calculations.

Effect of Sr addition on microstructure of Al-Si eutectic alloys.

Predicted and experimental liquid mixing enthalpy


Predicted and experimental Sr-Si phase diagram.