G. Borzone

Thermodynamic measurements and assessment of the Al–Sc system

Abstract A study of the binary Al-Sc phase diagram has been performed by means of thermodynamic calculations and experimental measurements. The enthalpy of formation of all intermetallic compounds has been determined and a cursory examination of the phase equilibria carried out, for compositions greater than 40 at% Sc. Two new invariant reactions have been identified in the Sc-rich part of the diagram: L ↔ (βSc)+Sc2Al at 1185°C and (βSc) ↔ Sc2Al+(αSc) at 970°C. A coherent set of Gibbs energy expressions for all the phases in the system has been generated by a least square optimisation procedure using all the experimental data available. The overall agreement is satisfactory but some uncertainties still persist, especially concerning the ScAl phase, owing to experimental difficulties.

Contribution to the study of the alloys and intermetallic compounds of aluminium with the rare-earth metals

General properties of aluminium alloys of the rare earth metals are briefly summarised. Their phase equilibria and the crystal structures of the different intermediate phases are presented and discussed. The results obtained in the experimental investigation of the YAl and SmAl systems are reported. The formation enthalpies of YAl2 and SmAl2 have been re-measured, resulting in −50.5 kJ/(mol at) and −55.0 kJ/(mol at) at room temperature, in a good agreement with literature data. Phase equilibria investigation in the SmAl system has been carried out and the results obtained via thermal analysis, micrographic examination, microprobe and X-ray diffraction analyses described and discussed in the framework of the general behaviour of the rare earth alloys. The experimental Sm-Al phase diagram, collated with previous literature data, is compared with the results of a thermodynamic optimisation. The following intermetallic compounds exist: Sm2Al (peritectic decomposition, at 860 °C) SmAl (per.dec.960 °C), SmAl2 (melting point 1480 °C), SmAl3 (peritectoidal decomposition at 1130 °C), Sm3Al11 (melting point ~1380 °C). The following eutectic equilibria have also been determined or confirmed: 20 at% Al and 760 °C, 75 at% Al and 1340 °C, 97.0 at% Al and 635 °C. A eutectoidal equilibrium occurs at 10 at% Al and 700 °C.

Hydrogen evolution reaction on NiRE (RE=rare earth) crystalline alloys ☆

NiRE (RE=Y, Ce, Pr and Sm) crystalline alloys have been characterized by means of microstructural and electrochemical techniques in view of their possible application as electrocatalytic materials for the hydrogen evolution reaction (HER). The microstructure of the electrodes has been investigated by optical and electron microscopy. The electrocatalytic efficiency has been evaluated on the basis of electrochemical data obtained from Tafel curves and a.c. impedance measurements carried out in 1 M NaOH solution at 298 K. The microstructural features play a fundamental role in determining the electrocatalytic activity of the investigated alloys. The overall experimental data indicate that interesting electrocatalytic performances are obtained with Ni94Pr6 and Ni95Ce5 electrodes, which show the highest activities towards the HER.

On the alloying behaviour of cerium with tin

Abstract A cursory re-examination of the Ce-Sn system was carried out using metallographic and X-ray analyses. The existence of the following phases was observed or confirmed: Ce3Sn, α-Ce5Sn3, β-Ce5Sn3, Ce5Sn4, Ce11Sn10 (approximate formula), Ce3Sn5 (approximate formula), Ce3Sn7, Ce2Sn5 and CeSn3. The lattice parameters of selected compounds were measured. The heats of formation of a number of Ce-Sn alloys were determined using a direct isoperibol calorimeter and the following values (kcal (g atom) −1) were found: Ce3Sn, −11.8 ± 0.5; Ce5Sn3, −17.5 ± 1.0; Ce5Sn4, −20.0 ± 1.0; Ce3Sn5, −18.0 ± 1.0; CeSn3, −14.0 ± 0.5. The results obtained are discussed briefly and are compared with those already known for other rare earth alloys with p-block elements.

On a simple high temperature direct reaction calorimeter

Several commercial and laboratory-made instruments have been described in the literature based on a drop technique and containing an isoperibolic differential detector. The instrument presented here consists of two cells set one above the other and located in ceramic and metallic tubes for thermal equalization. The temperature difference between the two cells (working and reference cells) is measured by a 20-couple thermopile. The calorimeter is enclosed in a furnace with three independently controlled zones which can be heated up to 1200 °C. A typical run consists of dropping from a room temperature thermostat a small capsule containing about 2 g of a metal mixture. The heat effect is evaluated by dropping, before and after the capsule, a number of weighed Ag specimens. Subsequently, in a second run, the same capsule containing the reacted sample is dropped again into the calorimeter. The difference between the heat effects obtained in the two runs gives the room temperature heat of formation of the alloy. A brief description of the instrument, of the calibration procedure and of the necessary auxiliary examinations is reported and the results obtained in a first set of measurements on a number of alloys are given.

The samarium-magnesium system: A phase diagram

Abstract The samarium-magnesium system has been investigated by means of differential thermal analysis, X-ray diffraction, metallography and microprobe analysis. Five intermetallic compounds have been found to exist: SmMg (m.p. 800°C), SmMg2 (m.p. 750°C), SmMg3 (decomposes at 700°C), SmMg5 (decomposes at 565 °C) and Sm5Mg41 (peritectic decomposition). Eutectic points occur at 45.0 at.% Mg (785°C), 64.5 at.% Mg (735°C) and 92.0 at.% Mg (530°C). A eutectoidal reaction is observed at 580 °C and 20.0 at.% Mg. The crystal structures of the intermediate phases have been confirmed or determined and the lattice parameters measured.

Thermochemistry of binary alloys of rare earths: A comparison between experimental and calculated heats of formation

Abstract The experimental results obtained during the course of a systematic investigation of the alloy thermochemistry of the rare earths are compared with the heats of formation calculated from a model suggested by Miedema. In general there is good agreement between the measured and calculated data.

The Al-rich region in the Y–Ni–Al system: microstructures and phase equilibria

The Al-rich region of the Y–Ni–Al ternary system has been investigated mainly by using different micrographic techniques. In the range 60<at% Al<100 the 500°C isothermal section has been partially defined. The compound Y4Ni6Al23 and its role in phase equilibria have been highlighted. The YNi3Al16 phase previously suggested has not been confirmed. Two ternary tie-triangles in this region have also been defined: Al+NiAl3+Y4Ni6Al23 and Al+YAl3+Y4Ni6Al23. Two ternary eutectic equilibria: E1, Ni-rich: at about 96.5 at% Al, 2.6 at% Ni, 0.9 at% Y and E2, Y-rich: at about 96.0 at% Al, 1.0 at% Ni, 3.0 at% Y have been identified. The crystallization behaviour of the Al-rich alloys is described and typical micrographic appearances presented.

Contribution to the evaluation of rare earth alloy systems

It is well known that within the family of the rare earth elements (especially the “trivalent” ones) several properties change according to well-recognized and systematic patterns. A general consideration of the constitutional properties (thermodynamic properties, crystal structures, and phase diagrams) of alloys formed by the various rare earths enables a number of empirical regularities to be deduced. This behavior can constitute a prediction scheme and a reliability criterion in the evaluation of the data concerning series of various rare earth alloys with the same element. Examples of the application of this behavior to phase diagram assessment and prediction are mentioned. Special attention is given to the binary rare earth alloys formed with magnesium and aluminum. The predicted versions of the Tb-Mg and Tb-Al phase diagrams are presented at the end.

Aluminium compounds of the rare earths: enthalpies of formation of YbAl and LaAl alloys

Abstract The standard molar enthalpies of formation for the different solid LaAl and YbAl alloys have been measured by means of direct calorimetrr. The composition and equilibrium state of the samples were checcked by micrographic and X-ray diffraction techniques. The following values have been obtained 1kJ (mol atoms)1): LaAl, ΔformH° = −46±2; LaAl2 ΔformH° = −50.5±2; LaAl3 ΔformH° = −44.0±2; La3Al11 ΔformH° = −41.0±2; Ybal2 ΔformH° = −39.5±2; YbAl2 Δform H° = −32.5±2. Experimental results are discussed and compared with literature data.