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

The effect of holmium addition to the Al-Si system has been experimentally studied, as the isothermal section at 500 ̊C. The constitution of the alloys has been determined by means of scanning electron microscopy (SEM), electron microprobe analysis (EDXS) and X-ray powder diffraction. The knowledge of the phase relationships in the R-Al-Si ternary systems (R: rare earths element) is essential to deeply understand the technological properties of the Al-Si based alloys, that are useful in different industrial fields. In the system investigated have been identify nineteen ternary fields and twelve two phase fields. Three ternary compounds have been found in this ternary section: τ2-Ho2Al3Si2 (mS14-Y2Al3Si2), τ4-Ho2AlSi2 (oI10-W2CoB2) and τ5-Ho6Al3Si (tI80-Tb6Al3Si). The results obtained can be useful compared with the other known R-Al-Si systems also for predictive purposes.

. 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.

Experimental Techniques
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.

Ternary Isothermal Section
In the present work the system is characterized by nineteen three-phase fields and twelve two-phase fields. According with the previous study of the Ho-Al-Si system [16], the HoAl 2 Si 2 (τ 1 ) compound was not found in the isothermal section at 500˚C, this is an unusual behavior when compared with all the other known R-Al-Si systems all showing the tie triangle (Al), (Si) and τ 1 . Furthermore, in the Er-Al-Si isothermal section at 600˚C [17] the ErAl 2 Si 2 phase was not     Table 1).
found and the tie triangle in the Al and Si rich part of the system shows the same vertices as the Ho-Al-Si system. The micrographs of four representative samples are reported in Figure 2. Figure 2(a)) shows the appearance of the sample n.8 that describes the triangle constituted by the two phases HoAl (2−x) Si x and HoAl (3−x) Si x (respectively grey and black and both at their maximum silicon solubility), plus light Ho 2 Al x Si (2−x) (τ 4 ) at its maximum aluminum solubility value.
In Figure 2

Ternary Compounds
Five ternary intermetallic compounds, are reported in literature: τ 1 -HoAl 2 Si 2 (hP5-CaAl 2 Si 2 ), τ 2 -Ho 2 Al 3 Si 2 (mS14-Y 2 Al 3 Si 2 ), τ 3 -HoAlSi (oS12-YAlGe), τ 4 -Ho 2 AlSi 2 A. M. Cardinale, N. Parodi (oI10-W2CoB 2 ) and τ 5 -Ho 6 Al 3 Si (tI80-Tb 6 Al 3 Si). In this work only three compounds were identified at 500˚C; the τ 3 structure seems not to form in the system at 500˚C and the τ 1 phase was not found in the system as discussed in the previous section. The τ 2 compound shows a homogeneity range with constant Ho content, dissolving up to 45 at % Al (stoichiometric composition is located at 42.8 at % Al). As the τ 4 compound has been proved the substitution with Al for Si in a range from 20.0 at % Al (stoichiometric composition) to 25 at % Al.

Binary Compounds
As expected different boundary binary compounds entry the ternary system, the homogeneity ranges minor of 2 at % are not taking into account. For each compound involved the solubility limit of the third element has been assumed as the me-

Conclusions
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 τ 2 -Ho 2 Al 3 Si 2 (mS14-Y 2 Al 3 Si 2 ), τ 4 -Ho 2 AlSi 2 (oI10-W2CoB 2 ) and τ 5 -Ho 6 Al 3 Si (tI80-Tb 6 Al 3 Si) have been found in the system at 500˚C, according with [16]. The τ 2 and τ 4 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 RAl 2 Si 2 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)/RAl 2 Si 2 occurs. Many R-Si and R-Al compounds extend in the ternary system forming solid solutions at a constant R-content.

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