The Al-Si alloys, both in the as cast and after thermal treatment conditions, have been recognized as interesting materials for the industries (e.g. automotive, heat exchanger, etc.). The addition of a rare earth element, at a very low concentration, can modify the property of the alloys, improving some useful characters as low density and thermal expansion coefficients, good casting performance and weld ability, high wear resistance and temperature strength, good corrosion resistance [1] [2] [3] [4]. All of the above leads to an increasing interest in the study of R-Al-Si based alloys (R being a trivalent rare earth element). The knowledge of the phase equilibria and the transformations that take place during the solidification pathway of foundry aluminum based alloys are crucial, especially in planning and develop new materials. The industrially relevant R-Al-Si alloys have usually a concentration lying near the binary Al-Si eutectic composition and mischmetal (alloy of rare earth metals, whose typical composition includes approximately 50% Ce, 25% La and smaller small amounts of Nd and Pr) is often added.

Moreover, owing to the definition of pseudo-lanthanide [5] it is possible to predict the behavior of an intermetallic phase not prepared yet, when experimental data are available for the adjacent members of this series. Taking into account the aforementioned considerations, investigations of a number of R-Al-Si systems have been carried out by our research group.

A set of eighteen Ho-Al-Si alloys have been prepared starting from pure elements (aluminum, silicon and holmium at 99.999, 99.99 and 99.9 mass % purity respectively all supplied by Newmet Koch, Waltham Abbey, England). The starting metals, prepared in little pieces, have been weighed in a proper amount and then arc melted on a water cooled copper plate under vacuum, the samples were re-weighed after melting to check for any mass losses that were always less than 0.5 mass %. The samples have been annealed (in an alumina crucibles sealed in quartz tube under vacuum) at 500˚C for 720 hours and then quenched in water.

To characterize the samples after annealing the techniques used have been scanning electron microscopy (SEM), electron probe microanalysis based on energy dispersive X-ray spectroscopy (EDXS) and X-ray diffraction analysis (XRPD). As the metallographic analysis, the samples incorporated in a conductive resin support were prepared according to the standard method by SiC paper and diamond paste polishing. After preparation the samples have been analyzed applying an acceleration voltage of 20 kV for 50 s, and a cobalt standard was used for calibration. For the quantitative analysis the software packaging Inca Energy (Oxford Instruments, Analytical Ltd., Bucks, UK) was employed to process X-ray spectra.

To determine the crystal structures, and calculate lattice parameters of the different phases, the samples were prepared crushing them into powder in an agate mortar; then XRPD analysis was performed by the vertical diffractometer X’Pert MPD (Philips, Almelo, and The Netherlands). The indexing of the obtained diffraction data was achieved by comparison with literature or calculated data (the program Powder Cell [20]), the lattice parameters of the phases were calculated using the program LATCON [21].

Table 1 reports data obtained from the eighteen samples synthesized and characterized by using SEM/EDXS and X-ray powder diffraction, while Figure 1 reports the Ho-Al-Si isothermal section at 500˚C. It was drawn basing on the experimental data in Table 1 and contains the samples composition, the field described by means of dotted lines are hypothesized on the basis of the experimental data and the knowledge of the different R-Al-Si systems. In Figure 2 are shown the micrographic appearance of some selected samples after annealing and quenching, while Figure 3 reports the PXRD pattern of a three phase sample.

The Ho-Al-Si-isothermal section at 500˚C has been studied in the whole composition range, as the phase relationships the main conclusions are the following:

• The studied system consists of nineteen three-phase fields and twelve two-phase fields.

• Of the five known ternary compounds only τ₂-Ho₂Al₃Si₂ (mS14-Y₂Al₃Si₂), τ₄-Ho₂AlSi₂ (oI10-W2CoB₂) and τ₅-Ho₆Al₃Si (tI80-Tb₆Al₃Si) have been found in the system at 500˚C, according with [16]. The τ₂ and τ₄ phases dissolve aluminum up to 45 at % and 25 at % respectively.

• Different binary compounds dissolve the third element extending into the ternary system.

• By comparing the different known ternary isothermal sections, some points can be highlighted. All the sections are characterized by the presence of intermediate phases with R content up to 60 at % rare earth. The number of phases decreases on going from the light (Pr, Nd, Sm) to the heavy rare earths (Gd, Tb, Dy, Er). Only the RAl₂Si₂ compounds form along the whole lanthanides series as point compounds; nevertheless in the previously studied Ho-Al-Si [16] and Er-Al-Si [17] isothermal sections this compound has not been founded. From La to Dyat low R content the three-phase equilibrium: (Al)/(Si)/RAl₂Si₂ occurs. Many R-Si and R-Al compounds extend in the ternary system forming solid solutions at a constant R-content.

The authors declare no conflicts of interest regarding the publication of this paper.

Cardinale, A.M. and Parodi, N. (2019) Experimental Study of the Phase Equilibria in the R-Al-Si Ternary Systems (R: Rare Earth Element) the Ho-Al-Si Isothermal Section at 500˚C. Journal of Materials Science and Chemical Engineering, 7, 9-17. https://doi.org/10.4236/msce.2019.78002