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Fe–MM (MM = mischmetal) 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 alloys investigated (Fe95MM5, Fe85MM15, Fe80MM20, Fe70MM30) has been characterized by phase analysis techniques (SEM-EPMA, XRD). The electrocatalytic efficiency has been evaluated on the basis of electrochemical data obtained from cathodic polarization curves and ac 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 the Fe80MM20 electrode, which exhibits the highest activity towards the HER.

The Dy–Pt and Ho–Pt alloy systems have been investigated in the 0–50 at.% Pt composition range by using differential thermal analysis, metallographic analysis, X-ray powder diffraction and electron probe microanalysis. The R3Pt, R2Pt, R5Pt3 ,R 5Pt4 and RPt intermetallic phases (R = Dy, Ho) have been found and their structures confirmed. All these phases form through peritectic reactions, with the exception of the equiatomic phases DyPt and HoPt melting congruently. The electrocatalytic behaviour of the DyPt and HoPt equiatomic phases towards the hydrogen evolution reaction (her) has been studied by means of cathodic polarization curves and electrochemical impedance measurements, both performed in oxygen-free 1 M NaOH solution at 298 K. The higher catalytic efficiency observed for RPt electrodes compared with pure Pt is explained on the basis of improved threedimensional hydridic features of composite electrocatalysts. © 2004 Elsevier B.V. All rights reserved.

The electrochemical behavior of five Au–Gd alloys in three alkaline electrolytes has been characterised by the cyclic voltammetry technique. The alloys studied were: Au96Gd4, Au69.5Gd30.5, Au53Gd47, Au25Gd75 and Au5Gd95. The electrolytes used were: (i) Na2CO3, (ii) Na2B4O7 and (iii) NaF — each with a concentration of 0.2 M. It was observed that the alloy composition, rather than the microstructure, plays an important role in determining the electrochemical behavior of these alloys. The oxidation process is more influenced by the specific adsorption of B4O72− and F− than by the CO32−.

The Smith thermal analysis has been used in the experimental investigation of several Pr–Mg alloys to achieve a better definition of some of the phase equilibria characteristic of the Pr–Mg system. This technique has several advantages with respect to conventional thermal analysis methods, even though it is more time consuming and it can be applied only to relatively narrow ranges of temperature in any one experiment. Solvus lines in the Pr-rich region and temperatures of the invariant reactions of the Mg-rich region of the Pr–Mg system were re-determined.

The intermetallic compounds of rare earths with RAg 0.5 Ga 1.5 composition (R=Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y) have been studied. Their crystal structures have been found to be related to the hP6–CaIn 2 type. The stability range of this structure, which often occurs when the valence electron concentration is near to 8/3, has been analysed for the Gd–Ag–Ga alloys. The study of the GdAg 2 –GdGa 2 section has revealed that with increasing GdGa 2 -content the following sequence of structure types occur: oI 12-CeCu 2 , hP 6-CaIn 2 , hP 3-CeCd 2 , hP 3-AlB 2 . The YbAg 2 –YbGa 2 section has also been partially analysed.

The alloying behavior of the different rare earths, which varies according to systematic patterns, can be used to predict constitutional properties of rare earth alloys with the same element. The applicability to prediction of the TmMg phase diagram is presented here. The predicted version is compared with that determined experimentally by analyzing a few key compositions using X-ray powder diffraction, optical and scanning electron microscopy, electron probe microanalysis (EPMA) and differential thermal analysis (DTA). The following phases were identified and their crystal structures confirmed or determined: β phase, ≈Tm 2 Mg (cubic, cI 2, W type), TmMg (cubic, cP 2, CsCl type), TmMg 2 (hexagonal, hP 12, MgZn 2 type) and ≈Tm 5 Mg 24 (cubic, cI 58, α-Mn type). All the phases show peritectic formation. Two other equilibria were also observed: a eutectic reaction at 575 °C and 89 at.% Mg and the eutectoid decomposition of the β phase at 670 °C and approximately 30 at.% Mg.

The NdSn phase diagram was studied in the 55–80 at.% Sn composition range by differential thermal analysis, X-ray powder diffraction, optical and scanning electron microscopy and electron probe microanalysis. Five intermediate phases were identified in this range and their melting behavior was determined: NdSn3 (cubic, cP4AuCu3, m.p. 1150°C), Nd2Sn5 (orthorhombic, oC28Ce2Sn5, peritectic decomposition 1135 °C), Nd3Sn7 (orthorhombic, oC20Ce3Sn7, per.dec.1145 °C), NdSn2 (oC12NdSn2, m.p.1180 °C) and Nd3Sn5 (per.dec.1140 °C). Two eutectic equilibria were found at 1130 °C and 60.5 at.% Sn and 1130 °C and 73 at.% Sn.