M. Magini

Mechanical alloying of the Ti-Al system

Ti-AI alloys have been prepared by mechanical alloying (MA) of pure titanium and aluminium powders for four compositions: Ti80AI20, Ti75AI25, Ti50AI50, and Ti40AI60. The 50 < Ti(at%) < 80 compositions could be fully amorphized with a milling time strongly dependent on the starting chemical composition. For the Ti40AI60 composition only partial amorphization was observed. The investigation on the early stage of MA shows that the different systems amorphized through two different paths. On the titanium rich side (Ti=75,80 at%) the MA first leads to the formation of an h.c.p. αTi(AI) solid solution with an aluminium content of 14–16% which subsequently collapses into the amorphous state. For the aluminium rich side (Al=50,60 at%), MA sooner promotes the nucleations of disordered forms of Ti3AI and TiAl intermetallic compounds respectively. The inhibition of the ordering transition of the observed intermetallic phases is ascribed to the low temperature at which the SSR takes place. The complete (Ti50AI50) or partial (Ti40AI60) amorphization of the powder is then attained through a destabilization of the disordered Ti3AI and TiAl phases. The present results confirm the existence of a metastable f.c.c. phase as final alloying stage for the Ti75AI25 and Ti40AI60 compositions.

Synthesis of amorphous and metastable Ti40Al60 alloys by mechanical alloying of elemental powders

Ti40Al60 amorphous and metastable alloys have been prepared by mechanical alloying (MA), under controlled milling conditions in a planetary mill. Three different quantities of kinetic energy at the collision instant have been achieved by using balls of different size, φb = 5, 8 and 12 mm, keeping constant all other device parameters. Assuming the collision between the balls and the vial walls to be inelastic, during the early stage of alloying, the amount of energy transferred to the trapped powder could be estimated. The experimental results show that the milling with balls of diameter φb = 5 or 8 mm leads to a solid-state amorphization of the Ti40Al60 mixture, through the attainment of a supersaturated solid solution of aluminium into α-titanium. Otherwise, the milling causes the nucleation of the A1-fcc disordered form of the TiAl intermetallic compound. The end products of MA-induced solid-state reaction (SSR) have been ascribed to the different temperature reached by the powder during each collision and to the reaction time scale for the formation of the amorphous phase, δta, and for the nucleation of the non-equilibrium intermetallic compound, δtd. Differential scanning calorimetry has indicated that the crystallization of amorphous samples follows a two-step reaction. At a temperature Tc≈400 °C, the amorphous phase crystallizes into the A1 -fcc. TiAl phase having a measured heat of crystallization of 6.2 kJ(g at)−1. Upon further heating, the system undergoes A1 → L1o reordering transition with an enthalpy release of about 3.2 kJ (g at)−1.

X‐ray diffraction study of Na+ ions coordination in sodium borate glasses

X‐ray diffraction measurements were performed on glasses of the type Na2O–B2O3 and Li2O–B2O3, in which the maximum content of Me2O is 0.3 mol per 1 mol of B2O3. By subtracting the radial distribution functions of the Li2O–B2O3 samples from those of Na2O–B2O3 glasses having the same molar ratios, difference curves were obtained in which peaks coming from the distribution of nearest neighbor atoms around Na+ ions were observed. The quantitative analysis showed that the environment of Na+ ions is not a regular polyhedron. About 5.0–5.5 O atoms surround Na+ ions; on the average 3.4–3.6 O atoms are 2.40–2.44 A distant from Na+ ions while 1.5–2.0 other O atoms are at a distance of 2.75–2.80 A.

Fe-Co-B amorphous alloy powder by chemical reduction

Fine boride powders, prepared by reactions between sodium or potassium borohydride and aqueous (or alcoholic) solutions of transition metal salts have long been known to be good catalysts for hydrogenation reactions. New interest around them has recently grown, because they were found to exhibit amorphous structure. Classical chemical reactions may thus offer a simple route to prepare amorphous metal powders, paralleling more sophisticated methods so far used to reach the same goal. To explore the potential of these reactions, we considered the system Fe-Co-B with the aim of examining the characteristics of the product obtained in the easiest way and without any particular precautions. The structure of the amorphous alloy appears to be different from that of materials with a similar Me/B ratio prepared via melt spinning.

On the coordination of iron ions in sodium borosilicate glasses. I: A wide angle X-ray diffraction investigation

Abstract Three sodium borosilicate glasses, containing approximately 4%, 8% and 12% by weight of Fe2O3, have been investigated by X-ray diffraction technique in order to determine the structural arrangement of iron ions in these glasses. The radial distribution functions of the iron glasses have been compared to suitable distribution functions obtained from an appropriate iron-free sodium borosilicate matrix. The analysis of the resulting distribution difference curves leads to the unambiguous conclusion that in 4% and 8% glasses iron ions are surrounded by four oxygen atoms at the distance expected for a tetrahedral coordination. The structural arrangement of iron ions in the 12% glass is very likely to be the same, although the analysis of the data is complicated by an initial crystalline phase separation within the amorphous network. In addition to the structural parameters obtained for the first iron shell, a tentative model to explain the second order interactions around iron ions is discussed.

Chloro‐complexes formation in a ZnCl2–CdCl2 aqueous solution: An x‐ray diffraction study

A solution containing 1 mol/l of ZnCl2 and CdCl2 was examined by x‐ray diffraction in order to study the mean environments of the two cations. The results were compared with those obtained from two solutions in which the same salts were separately dissolved. Zn2+ ions turned out to be hexa‐coordinated both in the single and the mixed solution, the coordination shell being composed of about five water molecules and one Cl− ion. On the other hand, Cd2+ ions, which in the CdCl2 solution are nearly hexa‐coordinated, become tetra‐coordinated, in the mixed solution. This transition is accompanied by a significant increase of the Cd2+–Cl− complexation degree.

X‐ray diffraction investigation of Co(II) ions in borosilicate glasses

The radial distribution functions of cobalt glasses with 6%, 8% and 14% CoO are compared with those of suitable cobalt‐free borosilicate matrices leading to ‘‘difference distribution curves’’ representative of the cobalt structural arrangement. Analysis of the curves indicates that cobalt ions are surrounded by approximately four oxygen neighbors at the distance expected for fourfold coordination.

X‐ray diffraction study of CoCl2–LiCl aqueous solutions

Three 1 M solutions of CoCl2 at Cl−/Co2+ ratios of 4, 6, and 7 were examined by x‐ray diffraction. The Fourier transformed scattering data indicated inner complex formation between Co2+ and Cl− ions for the two solutions more enriched in Cl− ions. Coordination parameters of Co2+ ions were obtained by direct fitting of the structure functions.

Early and Late Mechanical Alloying Stages of the Pd-Si System

Four Pd-Si compositions (Pd/Si=2.5/1; 3/1; 4/1 and 5/1) were mechanically alloyed by a ball-milling technique. X-ray diffraction and fluorescence analysis were used to monitor the mechanical alloying (MA) process which was carried out in planetary and vibratory ball mills. The first step of the milling process consists in a very fine fragmentation of the silicon particles into the palladium matrix. After this early stage of milling, formation of the intermetallic line compound Pd3Si can occur for the 2.5/1, 3/1 and 4/1 composition depending on the milling conditions and/or milling apparatus adopted: i.e. on the conditions of energy transfer experimentally realized. Subsequent milling indicates that amorphization probably occurs starting from the previously formed Pd3Si compound. Long milling times, up to 56 h, promote a demixing process towards the parent elements for the 2.5/1 and 4/1 compositions. Thermal treatments of the long-term milled samples confirm the final products obtained from the near room-temperature solid state reaction induced by MA. For the Pd5Si composition the conditions for Pd3Si formation and subsequent amorphization were never reached.

X-ray diffraction investigation of the structure of lead metavanadate PbV2O6 glass

Abstract Lead metavanadate glass PbV 2 O 6 has been investigated via X-ray diffraction. The analysis of the radial distribution function leads to the conclusion that vanadium atoms are surrounded by about five oxygen atoms at an average distance of 1.73 A. Comparison with models derived from the three known PbV 2 O 6 crystalline phases has been attempted.