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    . 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  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.
2. Literature Data
2.1. Boundary Binary Systems
As far as the binary systems are concerned, the Al-Si phase diagram is a simple eutectic, with the eutectic reaction at 12.2 at % Si and 577˚C and with no intermetallic compounds found in this system . Phase relationships in the Ho-Al system were assessed by . There are five intermetallic compounds: Ho2Al (oP12-Co2Si type), Ho3Al2 (tP20-Al2Zr3 type), HoAl (oP16-ErAltype), HoAl2 (cF24-MgCu2 type) and HoAl3 (hR60-Al3Ho). The Ho-Si system, was recently assessed by ; at 500˚C temperature there are six intermetallic compounds in it as well: Ho5Si3 (hP16-Mn5Si3 type), Ho5Si4 (oP36-Sm5Ge4type), HoSi (oP8-FeB type), Ho4Si5, HoSi2−b (hP3-AlB2 type, ~38 at % Ho) and HoSi2−a (oI12-GdSi2 type, ~35 at % Ho). The Ho4Si5 compound was investigated  by means of single crystal X-ray diffraction and its structure was assumed to be Ho3Si4oS24-Ho2Si2.67 type.
2.2. R-Al-Si Ternary Systems
To our best effort literature data on R-Al-Si systems isothermal sections (in the whole range of concentrations) and liquid us projections mainly deal with the following: La-Al-Si (0 - 33 at % La) , Ce-Al-Si , Pr-Al-Si  , Nd-Al-Si , Sm-Al-Si ( and refs therein ), Eu-Al-Si , Gd-Al-Si, Al-Si-Tb and Al-Si-Dy ( and refs therein), Ho-Al-Si (0 - 33 at % Ho) , Er-Al-Si  and Y-Al-Si (0 - 33 at % Y) . In the Ho-Al-Si ternary system, in literature , are reported the following compounds: HoAl2Si2 hP5-CaAl2O2 (τ1), Ho2Al3Si2 mS14-Y2Al3Si2 (τ2), HoAlSi oS12-YAlGe (τ3), Ho2Al1+xSi2−x oI10-W2CoB2 (τ4) and Ho6Al3Si tI80-Tb6Al3Si (τ5).
3. Experimental Techniques
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 ), the lattice parameters of the phases were calculated using the program LATCON .
4. Results and Discussion
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.
4.1. 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 , the HoAl2Si2 (τ1) 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 τ1. Furthermore, in the Er-Al-Si isothermal section at 600˚C  the ErAl2Si2 phase was not
Figure 1. The Ho-Al-Si ternary isothermal section at 500˚C, position and code of the samples are superimposed.
(a) (b) (c) (d)
Figure 2. Back scattered electrons (BSE) images of some selected Ho-Al-Si samples after annealing and quenching, the composition is reported in Table 1: (a) Sample n.8, primary light Ho2AlxSi(2−x) (τ4), surrounded by small quantity of grey HoAl(2−x)Six phase plus black HoAl(3−x)Six compound; (b) Sample n.5, two phase sample constituted by primary dark HoAl(2−x)Six phase plus light HoAl(1−x)Six compound; (c) Sample n.14, primary gray HoAl(1−x)Six crystals surrounded by small quantity of light peritectic Ho5AlxSi(4−x) compound plus the black HoAl(2−x)Six phase; (d) Sample n.16 primary light polygonal crystal of Ho5Si4 compound surrounded by peritectic formed Ho2AlSi2 (τ4) and HoAlSi(2−b) containing 6 at % Al.
Table 1. SEM-EDX and XRPD data on the Ho-Al-Si samples annealed at 500˚C and quenched.
Figure 3. X-ray powder pattern of sample n.16 (see 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)Six and HoAl(3−x)Six (respectively grey and black and both at their maximum silicon solubility), plus light Ho2AlxSi(2−x) (τ4) at its maximum aluminum solubility value. In Figure 2(b), the sample n.5, pertaining to the two HoAl(2−x)Six and HoAl(1−x)Six phase field, shows dark primary HoAl(2−x)Six crystals and the binary eutectic between the primary phase and HoAl(1−x)Six compound, the medium eutectic composition is Ho 42 at % and Al 46 at % very close to the composition of the sample as confirmed by the micrographic feature. Ongoing throughout the system to the Ho richest part, in Figure 2(c) the sample n.14 is a three phase sample with gray primary HoAl(1−x)Six compound (at its maximum silicon solubility) surrounded by a small quantity of light Ho5AlxSi(4−x) of peritectic formation, plus black HoAl(2−x)Six phase. The sample n.16, in Figure 2(d) belongs to the Ho2AlxSi(2−x) (τ4), Ho5Si4 and HoSi2−btietriangle; the PXRD pattern of sample n.16 is reported in Figure 3, the main reflections of the three phases identify are highlighted.
4.2. Ternary Compounds
Five ternary intermetallic compounds, are reported in literature: τ1-HoAl2Si2 (hP5-CaAl2Si2), τ2-Ho2Al3Si2 (mS14-Y2Al3Si2), τ3-HoAlSi (oS12-YAlGe), τ4-Ho2AlSi2 (oI10-W2CoB2) and τ5-Ho6Al3Si (tI80-Tb6Al3Si). In this work only three compounds were identified at 500˚C; the τ3 structure seems not to form in the system at 500˚C and the τ1 phase was not found in the system as discussed in the previous section. The τ2 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 τ4 compound has been proved the substitution with Al for Si in a range from 20.0 at % Al (stoichiometric composition) to 25 at % Al.
4.3. 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 medium value of the different experimental measurements in the different samples investigated. From the Ho-Al system, the HoAl3, HoAl2 and HoAl compounds dissolve respectively 7 at %, 3 at % and 15 at % silicon. Ongoing from the Si to the Ho richest part of the binary Ho-Si phase diagram some compounds dissolve aluminium in the following percentage: HoSi2−b 4 at %, Ho5Si4 5 at % and Ho5Si3 10 at %.
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 . 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  and Er-Al-Si  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.