Characterization and Application of Atomic Structure Information of Liquid Hypoeutectic Aluminum-Silicon Melts

Abstract

The liquid structure of Al-Si hypoeutectic binary alloys with and without the addition of 0.04wt% Sr was characterized from diffraction experiments using a high energy X-Ray (Synchrotron) beam source. The high temperature liquid diffraction experiments were carried out for pure aluminum and Al alloys with 3, 7, 10 and 12.5 (eutectic) weight percent Si concentration, respectively. Further, two levels of Sr addition to these alloys at 0 and 0.04 weight percent were carried out. The diffraction data for all the alloys were obtained at various melt temperatures. The salient structure information such as structure factor (SF), pair distribution function (PDF), radial distribution function (RDF), coordination number (CN) and atomic packing densities (PD) have been quantified as a function of Si concentration and melt temperatures. Reverse Monte Carlo (RMC) analysis was carried out using the diffraction experimental data to quantify the partial pair correlation functions such as partial structure factor, partial pair distribution function (PPDF) and partial radial distribution function. Further, the partial pair distribution function and the liquid atomic structure information were used in a semi empirical model to evaluate the viscosity of these liquid alloys at various temperatures. The results show that Sr has a profound effect in changing the atomic structure of the liquid Al-Si hypoeutectic alloys at various Si concentrations and melt temperatures. Further, the effect of Sr is more pronounced at lower melt temperatures closer to the liquidus temperature. The study has shown conclusive evidence that the addition of trace levels of Sr changes the atomic arrangement of the liquid Al-Si alloys which could be validated by the significant change in melt viscosity values. The mechanism of modification of the morphology of the eutectic phases (both eutectic Al and Si) due to Sr addition in these alloys during solidification, from a coarse plate-like eutectic Si phase to a fine fibrous phase, and a significant refinement of the eutectic Al grains can be attributed to the changes in the nucleation mechanism of these alloys effected by the addition of trace levels of Sr. Sr causes more disorder in the atomic structure and delays the nucleation event causing significant undercooling before the evolution of the eutectic phases which in turn refines the morphology of these phases. The Al-Si hypoeutectic alloys in this study have a significant commercial value, especially in shape casting application for automotive, aerospace, domestic and defense sectors. The addition of Sr has been carried out for nearly eight decades but the mechanism for the morphological changes effected by this addition has been eluding researchers globally during all these years. This study presents conclusive evidence to propose a valid mechanism for the effect of Sr addition on the solidified microstructure of these alloys. Another significant contribution of this study would be the development of a valid methodology to evaluate partial pair distribution functions of liquid alloy structures and the evaluation of the melt viscosity using a semi-empirical model along with the atomic structure information. This methodology could be extended to evaluate other fundamental physical properties of binary metallic alloys.