R. Ferro

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.

The Al–R–Mg (R=Gd, Dy, Ho) systems. Part II: Thermodynamic modelling of the binary and ternary systems

Abstract The thermodynamic modelling and optimisation of the Al–R, R–Mg and Al–R–Mg (R=Gd, Dy, Ho) systems has been carried out, the Al–Mg system being already optimised in literature. The Compound Energy Formalism (CEF) has been used to describe the thermodynamic functions of either solutions and stoichiometric phases present in the systems. In particular, the order/disorder relation between cI2-W (A2) and cP2-CsCl (B2) phases present in the systems has been taken into account and thermodynamically modelled. Moreover γ-(Mg,Al) and R5(Mg1-xAlx)24 (R=Dy, Ho) phases, related to the cI58-αMn (A12) type structure, have been modelled as different sublattice occupations of the same A12 phase. The thermodynamic modelling of the ternary systems is mainly based on the experimental investigation carried out in our laboratory and reported in a separate paper in this issue [1]. As a result, a complete description of the solid-solid and solid-liquid phase equilibria in the six binary and three ternary systems has been obtained.

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 Yue5f8Al and Smue5f8Al 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 Smue5f8Al 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.

The Al-Er-Mg ternary system Part II: Thermodynamic modeling

A thermodynamic modeling and optimization of the Al-Er, Er-Mg, and Al-Er-Mg systems has been carried out, the Al-Mg system having already been optimized in the literature. The sublattice model has been used to describe the thermodynamic functions of both the solution phases and the stoichiometric phases present in the systems. In particular, the order/disorder relationship between the couples of phases A2/B2 and A1/L12 present in the system as bcc-(Er,Mg)/Er(Mg1−xn Aln xn ) and fcc-(Al)/(Al1−xn Mgn xn )3Er, respectively, has been taken into account and thermodynamically modeled. Moreover γ-(Al,Mg) and Er5(Mg1−xn Aln xn )24 phases, both related to the cI58-αMn (A12) type structure, have been modeled as different sublattice occupations of the same A12 phase. The thermodynamic modeling and the experimental investigation of the phase equilibria (reported in part I of this work) have been carried out by a recursive procedure. As a result, complete descriptions of the solid-solid and solid-liquid phase equilibria have been obtained.

The isothermal section at 500 °C of the Y-La-Mg ternary system

Ternary Y-La-Mg alloys have been studied by X-ray diffraction, optical microscopy, scanning electron microscopy, and electron probe microanalysis. Phase equilibria have been established in the isothermal section at 500 °C in the 50 to 100 at. Pct Mg composition range. The sections (YxLa1-x) Mg (continuous solid solution), (YxLa1-xMg2 (cF24-Cu2Mg type for 0 ≤x ≤ 0.72 and hP12-MgZn2 type for 0.82 ≤x ≤ 1 solid solutions), and YxLa1-xMg3 (0 ≤x ≤ 0.47) have been especially studied. The ≈YxLa1-xMg5 (0.35 ≤x ≤ 0.77) ternary compound (cF440-GdMg5 type) has been found. A reinvestigation of selected regions of the binary La-Mg and Y-Mg systems proved necessary. The La5Mg41 phase, not previously reported, has been found by the annealing of samples at 600 °C. The YMg2 phase shows a wide homogeneity range up to 73 at. Pct Mg. The ternary section has been compared with a simulated one that was created by placing sections of binary systems of Mg with different rare earth elements side by side. The results are related to the pseudolanthanide concept.

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.

Computer coupling of thermodynamics and phase diagrams: the gadolinium-magnesium system as an example

Abstract A thermodynamic analysis of the binary Gd-Mg system is presented, and its description is optimized using the experimental phase diagram and thermodynamic values. The excess Gibbs energies of the liquid and solid (α-Gd, β-Gd and Mg) solutions were described according to the Redlich-Kister polynomial expansion. The intermediate compounds (GdMg, GdMg2, GdMg3 and GdMg5) were assumed to be stoichiometric phases. A good agreement between the experimental and the computed phase diagram is shown. The results are briefly discussed and compared with those for other binary rare earth-Mg alloys.

ON THE THERMOCHEMISTRY OF THE RARE EARTH ANTIMONIDES. THE Dy-Sb SYSTEM

Abstract The data in the literature relevant to the rare earth antimonide phase diagrams and the crystallochemical and thermodynamic properties of the rare earth antimonides are summarized. The data obtained in an investigation of Dy-Sb compounds are reported. This investigation was performed using X-ray and metallographic analyses and calorimetric measurements of the heats of formation. The values of Δ H form (kJ (g atom) −1 ±2.0) for the following compounds were obtained for the reaction in the solid state at 300 K (the crystal structure data have also been confirmed): Dy 5 Sb 3 (hP16-Mn 5 Si 3 -type),−105.5; Dy 4 Sb 3 (cI28-anti-Th 3 P 4 -type), −111.5; DySb (cF8-NaCl-type), −114. These data, together with those relevant to the other rare earth antimonides, are discussed and their trends are in good agreement with the relationships proposed by Gschneidner. The experimental data are in agreement with those computed according to Miedemas model and, in the case of the rare-earth-rich alloys, also agree with those calculated according to Kubaschewskis suggestion based on the effective coordination numbers.

The system Ce–Ag–Sn: phase equilibria and magnetic properties

Abstract Phase relations in the ternary system Ce–Ag–Sn have been established for an isothermal section at 750°C. Experimental techniques used were optical microscopy, EPMA and X-ray powder analysis of arc-melted samples which had been annealed at 750°C for 10 days and quenched to room temperature. Phases equilibria are characterized by the formation of three ternary compounds: CeAgSn (CaIn 2 -type), Ce 5 AgSn 3 (Hf 5 CuSn 3 -type), and Ce 3 Ag 4 Sn 4 (Gd 3 Cu 4 Ge 4 -type). CeAg 2 Sn 2 , earlier reported to form and crystallize in the CaBe 2 Ge 2 -type, was not observed in the present study. All these ternary phases order magnetically at low temperature; our measurements reveal that Ce 5 AgSn 3 is ferromagnetic below T C =5xa0K, and Ce 3 Ag 4 Sn 4 is antiferromagnetic below T N =9xa0K.

Thermodynamic modelling of the Co–Ti system

Abstract In the Co–Ti system many solid solutions are present: A1, L12, A2, B2, A3, and Laves C15 and C36 phases. Two of them exhibit order/disorder transitions: A1 and L12, A2 and B2. In this optimisation the sublattice model has been used to describe the thermodynamic functions of the above-mentioned phases. The order/disorder transformation A1↔L12 has been described by modelling the L12 phase with either two or four sublattices both descriptions being mathematically equivalent. The A2↔B2 relation was also modelled in a similar way. The cited solid solutions, together with the liquid (Redlich–Kister excess model) and the CoTi2, considered as stoichiometric, phases have been optimised by means of the Thermo-Calc software, on the basis of the literature data concerning both phase equilibria and thermodynamics.