In Situ Study of Microstructure Evolution in Solidification of Hypereutectic Al-Si Alloys with Application of Thermal Analysis and Neutron Diffraction

Abstract

Understanding of the kinetics of solid-phase evolution in solidification of hypereutectic aluminum alloys is a key to control their as-cast microstructure and resultant mechanical properties, and in turn, to enhance the service characteristics of actual components. This study was performed to evaluate the solidification kinetics for three P-modified hypereutectic Al-19 pct Si alloys: namely, Al-Si binary alloy and with the subsequent addition of 2.8 pct Cu and 2.8 pct Cu + 0.7 pct Mg. Metallurgical evaluation included thermodynamic calculations of the solidification process using the FactSageä 6.2 software package, as well as experimental thermal analysis, and in situ neutron diffraction. The study revealed kinetics of solid a-Al, solid Si, Al 2 Cu, and Mg 2 Si evolution, as well as the individual effects of Cu and Mg alloying additions on the solidification path of the Al-Si system. Various techniques applied in this study resulted in some discrepancies in the results. For example, the FactSage computations, in general, resulted in 281 K to 286 K (8°C to 13°C) higher Al-Si eutectic temperatures than the ones recorded in the thermal analysis, which are also~278 K (~5°C) higher than those observed in the in situ neutron diffraction. None of the techniques can provide a definite value for the solidus temperature, as this is affected by the chosen calculation path [283 K to 303 K (10°C to 30°C) higher for equilibrium solidification vs non-equilibrium] for the FactSage analysis; and further complicated by evolution of secondary Al-Cu and Mg-Si phases that commenced at the end of solidification. An explanation of the discrepancies observed and complications associated with every technique applied is offered in the paper.

A New Approach in Numerical Modeling of Inoculation of Primary Silicon in a Hypereutectic Al-Si Alloy

“…Higher melt temperature (785°C and 850°C/1562°F) in Al-20 pct Si, to some extent, is found to have a positive effect on the size and distribution of the primary Si crystals particles which was attributed to presence of heterogeneous clusters of short range Si atoms existed above the liquidus temperature [11]. This was later shown using in-situ Neutron diffraction analysis of Al-(672°C) due to the agglomeration of Si clusters in the melt [12][13].…”

Electrochemical Degradability of Al-20% Mg and Al-22% Si Alloys in an Acidic Environment in Relation with their Microstructure

“…In order to reveal the eutectic silicon morphology, cast Al-22% Si alloy etching was performed with a picral solution, for one minute. Many studies [20,[31][32][33][34] reported that the silicon's primary particles morphology depends on the casting process, cooling rates during melt solidification and on the silicon content. The primary silicon particles size was found to decrease with a rise in the cooling rate during solidification.…”

Neutron Diffraction Analysis of Light Alloys: A Review

“…A relatively new application of neutron diffraction (pioneered by Sediako and Kasprzak [11]) involved the in-situ study of alloy solidification, where the growth of primary and secondary phases was tracked as a function of temperature. The fraction of solid for the solidifying alloy was determined by calculating the area under each diffraction peak for each temperature (increasing peak size and volume fraction of solid with decreased temperature, as shown in Figure 6) and relating this to the peak area at the temperature where the phase has completely solidified [11][12][13][14]. The ability to quantify the solidification kinetics of individual planes for each phase is a major advantage of neutron diffraction compared to traditional thermal analysis techniques.…”