PhD Thesis Colloquium: Mr. Avik Mondal (23/03/26)
Thesis title:
Synergistic effect of Si, Mg and Fe on solidification, viscosity, corrosion and diffusional growth kinetics of Al-Si-Mg-Fe alloy system
Faculty advisor(s):
Prof. Aloke Paul
When?
23th March, 2026 (Monday), 11:00 AM (India Standard Time)
Where
KPA Auditorium, Department of Materials Engineering
Abstract:
Al-based alloys exhibit an excellent strength-to-weight ratio, superior corrosion resistance, and good resistance to liquid metal embrittlement. Owing to these advantages, they are widely used in aeronautical and automotive applications. Alloying elements such as Si, Mg (added intentionally), and Fe (from bath hardware and steel) play a crucial role in modifying the microstructure, diffusional phase transformations, and physicochemical properties of Al-based alloys, including viscosity and corrosion resistance. The primary objective of this thesis is to systematically investigate the synergistic effects of Si, Mg, and Fe on solidification behavior, microstructure evolution, viscosity, corrosion response, and diffusional phase transformation, as well as the growth kinetics of intermetallic phases between steel and Al–Si–Mg alloys using a combined CALPHAD and experimental approach.
The synergistic effects of Si, Mg, and Fe on the solidification behavior and microstructure development of Al–Si–Mg, Al–Si–Fe, and Al–Si–Mg–Fe alloys were investigated using CALPHAD-based thermodynamic calculations followed by experimental validation. Various Si-, Mg-, and Fe-rich intermetallic phases were observed in the alloys. The alloy compositions strongly influenced the solidification path and resulting microstructure. Binary, ternary, and quaternary eutectic reactions were identified in the alloy systems. In particular, the concentrations of Si and Mg and the Si/Mg ratio significantly affected the phase fractions of the intermetallic compounds. Additionally, Si, Mg, and Fe contents strongly influenced the liquidus temperature and the solubility limits of elements in the alloys.
Viscosity is a critical parameter governing the flow of liquid metal and strongly influences industrial processes such as casting and hot-dip coating. However, accurate viscosity measurements in liquid metals are challenging due to high temperatures, oxidation, and vaporization of volatile elements. Therefore, predictive models for multicomponent alloy systems are highly desirable. In this thesis, viscosity models for multicomponent alloys were developed by integrating the Seetharaman–Du Sichen (SDS), Kaptay, and Associate models with the CALPHAD methodology. Viscosity predictions were performed for the Al–Si, Al–Mg, Al–Fe, and Al–Si–Mg systems and were validated against available experimental data. The SDS model combined with CALPHAD provided reliable predictions for Al–Si, Al–Mg, and Al–Si–Mg alloys, whereas the Associate model showed the highest accuracy for the Al–Fe system.
Although Al–Si–Mg alloys are known for their excellent corrosion resistance, the roles of alloy composition and microstructure in the corrosion mechanism remain insufficiently understood. In this thesis, the synergistic effects of Si, Mg, and Mn on the galvanic and long-term corrosion behavior of Al–Si–Mg–Fe alloys were systematically investigated. The corrosion response was correlated with the microstructure of the alloys. The localized corrosion occurred due to the presence of multiple phases with different Volta potentials, leading to the formation of anodic and cathodic regions. The volume fraction and morphology of these phases significantly influenced the corrosion mechanism. Mg addition promoted the formation of a compact and uniform oxide layer, resulting in high polarization resistance. Mg-rich alloys exhibited thick double-layered corrosion products consisting of Al–O, Al–Si–O, and Si–O.
Furthermore, diffusional phase transformations and intermetallic growth kinetics were investigated using solid–liquid diffusion couples between Al-based alloys and steel, relevant to industrial processes such as hot-dip coating. The effects of Si and Mg in the alloy and Mn in the steel substrate on phase formation and growth behavior were systematically examined. The alloy composition was found to play a critical role in determining the growth of intermetallic phases in the diffusion couples. In solid–liquid diffusion couples, a thin layer and a thick phase developed in the interdiffusion zone. The phase exhibited parabolic growth kinetics in Mg-free alloys, whereas Mg addition resulted in a pronounced thinning of the layer. Mn in the steel substrate did not show a significant influence on phase formation and growth kinetics in the solid–liquid diffusion couples.
Overall, this thesis provides a comprehensive understanding of the effects of Si, Mg, and Fe on solidification behavior, microstructure evolution, viscosity, corrosion mechanisms, and diffusional phase transformations in Al–Si–Mg–Fe alloys. The integration of CALPHAD modeling with experimental investigations offers valuable insights for optimizing alloy design and industrial processes involving Al-based alloys.