Z. Ban

X-ray studies in the system ThNi5−xAlx

Abstract A set of twelve alloys in the system ThNi5−xAlx was studied by means of the X-ray powder diffraction method. The upper limit of the Ni Al substitution appears to be 2 3 . The determination of the relevant structural parameters indicates that the aluminium atoms enter the Kagome net consisting originally of nickel atoms. On basis of the microhardness measurements, which indicated that the maximum hardness occurs at the composition ThNi4Al, a special kind of ordering of small atoms in the structure was postulated. A brief discussion of the possible valenceelectron concentration influence is also included.

Study of substitution in the systems UNi5−xMx and ZrNi5−xMx(M = In, Sn, Zn)

Abstract X-ray powder diffraction studies and metallographic examinations in the systems UNi 5−x M x and ZrNi 5−x M x ( M = In, Sn, Zn ) revealed the existence of single phase regions up to the composition 1 : 4 : 1, i.e. up to UNi 4 M and ZrNi 4 M, respectively. Comparison between calculated and observed intensity values showed that nickel atoms, in the crystallographic positions 4(c) of the isostructural binary compounds UNi 5 or ZrNi 5 (UNi 5 -type, S.G. F43m), are substituted by M atoms.Vegards rule is obeyed in the single phase regions of all investigated systems. The crystal structures of UNi 4 M and ZrNi 4 M are superlattice structures of the MgSnCu 4 -type (S.G. F43m). Structural correlations of these phases with corresponding binary prototypes are made in terms of stacking sequences.

Selective oxidation of zirconium in Zr3Al

Abstract The oxidation of Zr3Al in oxygen at temperatures ranging from 740 to 1050 K results in the formation of the following sequence of layers: cubic ZrO2, monoclinic ZrO2, Zr3Al and Zr2Al. During the oxidation, aluminium diffuses from the oxide layer into the Zr3Al bulk, thus forming a Zr2Al phase at some distance from the alloy-oxide interface. The excess zirconium is therefore selectively oxidized, yielding monoclinic and/or cubic ZrO2 containing no alumina.

Metal-metalloid exchange in the Zr1−xMxMo2 (M ≡; Ge, Si, Al) system

An extensive study of the possibility of substituting zirconium in ZrMo2 by germanium, silicon or aluminium was carried out using X-ray diffractometry and metallography. In the Zr1−xGexMo2 system the crystal structure of the prototype compound ZrMo2 (cubic Friauf-Laves phase; space group, Fd3m) was retained up to the composition Zr0.8Ge0.2Mo2. Substitution in the Zr1−xSixMo2 and Zr1−xAlxMo2 systems was possible up to the compositions Zr0.75Si0.25Mo2 and Zr0.3A1{0.7}Mo2, respectively, but the cubic crystal structure transformed into a hexagonal MgZn2-type structure (space group, P63mmc). This structure was stabilized if the substituent contents were in excess of 5 at.% Si or 6.7 at.% Al. A high temperature b.c.c. ternary solid solution was also observed in all the systems investigated. The unit cell parameters, cell volumes and interatomic distances were determined for the observed phases of all three systems. Some semiquantitative correlations of these phases with those found in the related Hf1−xMxMo2 (M ≡; Ti, Ge, Si, Al), Zr1−xTixMo2, Zr1−xMxW2 and Hf1−xMxW2 (M ≡; Ti, Si, Al) systems were also made.

High temperature equilibria between B.C.C. and MgCu2-type structures in the Zr1−xMxW2 AND Hf1−xMxW2 (M ≡ Al, Si) Systems

Abstract X-ray and metallography investigations of the Zr 1− x Al x W 2 , Zr 1− x Si x W 2 , Hf 1− x Al x W 2 and Hf 1− x Si x W 2 systems were carried out. All the ternary systems investigated showed marked similarities with respect to the limiting concentrations of substituents which corresponded to the formulae Zr 0.5 M 0.5 W 2 and Hf 0.5 M 0.5 W 2 (M ≡ Al, Si). Below a critical temperature all the systems consist of two coexisting phases, a b.c.c. ternary solid solution and a ternary Friauf-Laves structure of the MgCu 2 type. The precipitation and/or dissolution temperatures of the MgCu 2 phase in the b.c.c. matrix were determined for samples of all the systems investigated. Intensity calculations showed that the substitution in the MgCu 2 -type structure was unequivocally associated with the 8a positions in the Fd 3 m space group. No ordering phenomena were observed in the high temperature b.c.c. solid solutions. The surprisingly small variations in the lattice constants of both phases and the corresponding short W-W distances are discussed.

Comparison of the oxidation behaviour of α-ZrAl and Zr3Al

Abstract A comparative study of the oxidation behaviour of Zr 3 Al and α-Zr-1wt.%Al was performed in dry oxygen at temperatures of 647–1050 K. The oxidation of both phases follows a parabolic rate law. The rate of oxidation of α-ZrAl is higher than that of Zr 3 Al. The oxide layer on α-ZrAl consisted mainly of monoclinic ZrO 2 with traces of cubic ZrO 2 . An activation energy of 274.4 kJ mol −1 was estimated. Only cubic ZrO 2 was formed on Zr 3 Al up to temperatures of about 790 K. At temperatures above 790 K monoclinic ZrO 2 was formed in addition to cubic ZrO 2 . Two values of the activation energy for the oxidation of Zr 3 Al were estimated: 114.0 kJ mol −1 at temperatures below 800 K and 53.1 kJ mol −1 at temperatures above 800 K.

Oxidation of stoichiometric Zr3Al-based alloy

Abstract Stoichiometric Zr 3 Al-based alloy consisting of α-Zr(Al) and Zr 3 Al phases has been exposed isothermally to dry oxygen at temperatures of 701–828 K. Cubic ZrO 2 was formed below 750 K, above that temperature monoclinic ZrO 2 appeared beside the cubic phase. The α-Zr(Al) phase was found to be more reactive than Zr 3 Al. Oxidation obeys a parabolic rate law, with an activation energy of 161.95 kJ.

The effect of silicon, aluminium and germanium on the stabilization of the C14 polymorph of HfMo2

Abstract Depending on the heat treatment, the crystal structure of HfMo2 is hexagonal (space group, P6 3 mmc ; MgNi2 or MgZn2 type) or cubic (space group, Fd3m; MgCu2 type). X-ray and metallography investigations revealed that hafnium can be substituted by silicon, aluminium or germanium up to the composition Hf0.7M0.3Mo2 (M ≡ Si, Al or Ge). At substituent concentrations above a critical value (3.3 at.% Si, 6.7 at.% Al or 3.3 at.% Ge) only the MgZn2 polymorph of HfMo2 was stabilized. All the Friauf-Laves structures transform near their melting points to b.c.c. solid solutions. The variations in the unit cell parameters, the cell volumes and the Mo-Mo distances are briefly discussed. Some correlations between these phases and those found in related Zr1−xMxW2 and Hf1−xMxW2 (M ≡ Si or Al) systems are also made.

Homogeneity regions in the systems ThT5−xAlx (T = Mn, Fe, Co)

Abstract An X-ray powder diffraction study of the systems ThMn 5− x Al x ., ThFe 5−x Al x and ThCo 5− x Al x revealed the existence of solid solutions in the ternary isothermal section at 800 °C which are separated from the pure binary systems. Their compositions span the regions ThMn 3.5 Al 1.5 - ThMn 3 Al 2 , ThFe 3.75 Al 1.25 — ThFe 2.75 Al 2.25 and ThCo 4 Al — ThCo 3 Al 2 , respectively. Special care was taken in order to minimize the oxygen content. From the calculated X-ray intensities for ThFe 3 Al 2 and ThCo 3 Al 2 it is concluded that aluminium statistically replaces transition metal atoms in the Kagome net of the prototype structure (CaCu 5 type, space group P 6/mmm, No. 191). A correlation with the previously reported results for the system ThNi 5− x Al x and the ambivalent electron donor-acceptor role of aluminium are discussed.

On some “pseudosilicides” having the cubic AB5 structure

Abstract X-ray and metallographic investigations indicated that in the binary intermetallic compounds UNi 5 and ZrNi 5 one nickel atom may be substituted by combinations of some elements of the IIIa-Va and IIb-VIa groups, i.e. that “pseudosilicon” can play the role of a substituent in some AB 5 phases. Single phase alloys were obtained in the systems of the general formula UNi 4 Y (Y = In + As, In + Sb, Zn + Te, Cd + Te) and ZrNi 4 Y (Y = In + As, In + Bi, Zn + Te, Cd + Te). The homogeneity regions (ratio of the constituent elements of the Y component) were also determined. In all the systems investigated so far, at the equiatomic compositions of the substituents (VEC = 4), contraction of the unit cell parameters and an increase of microhardness values were observed. Intensity calculations showed that the substitution takes place in positions 4(c) in the space group F43m. The resulting structures may also be considered as superlattice of the MgSnCu 4 type. The stability of these phases is discussed in terms of relative atomic sizes and valence electrons concentrations.