Influence of zirconium content on microstructure and creep properties of Mo-9Si-8B alloys

X-ray and metallographic studies of the possibility of replacing niobium in NbT 2 (T  Cr, Mn, Fe, Co, Ni) by silicon or aluminium were carried out. Alloys with a Friauf-Laves structure were observed up to the compositions Nb 0.4 Si 0.6 Cr 2 , Nb 0.4 Al 0.6 Cr 2 , Nb 0.6 Si 0.4 Mn 2 , Nb 0.8 Al 0.2 Mn 2 , Nb 0.2 Si 0.8 Fe 2 , Nb 0.5 Al 0.5 Fe 2 , Nb 0.7 Si 0.3 Co 2 and Nb 0.5 Al 0.5 Co 2 . In the systems containing nickel (binary NbNi 2 does not evist) the only single-phase samples span the composition region Nb 0.9 Si 0.1 Ni 2 -Nb 0.8 Si 0.2 Ni 2 . Depending on the kind and amount of the substituent, the heat treatment and the atomic number of the transition element, the crystal structure is found to be cubic (space group, Fd 3 m ; MgCu 2 type), or hevagonal of the Mgvn 2 type (space group, P 6 3 / mmc ). The atomic arrangement within the unit cell for both structure types is determined for all alloys investigated. The variation in the unit cell parameters and interatomic distances, as well as the influence of the substituent on the stability of these phases, is briefly discussed.

Structural investigations of the Nb1−xSixT2 and Nb1−xAlxT2 (T Cr, Mn, Fe, Co, Ni) systems

A large number of samples of the general composition Zr1−xAlxT2, Zr1−xSixT2 and Zr1−xTixT2 (T ≡ Mn, Fe, Co) were prepared and investigated using X-ray diffractometry and metallography. It was found that in binary Friauf-Laves phases, ZrT2, zirconium can be replaced by aluminium or silicon up to the compositions Zr0.7A10.3Mn2, Zr0.7Si0.3Mn2, Zr0.6Al0.4Fe2, Zr0.5Si0.5Fe2, Zr0.4Al0.6Co2 and Zr0.6Si0.4Co2. In all these systems the crystal structure of the prototype compounds (the cubic MgCu2 or the hexagonal MgZn2 type) is preserved. In the Zr1−xTixT2 systems, continuous solid solutions were formed throughout the investigated region and their crystal structures depend on the composition and/or heat treatment. An ordering phenomenon was observed only in the Zr1−xALxCo2 system, where a superstructure of the MgSnCu4 type was formed at the composition Zr0.5A10.5Co2. The stability of these phases is briefly discussed in terms of the relative atomic sizes.

On Friauf-Laves phases in the Zr1−xAlxT2, Zr1−xSixT2 and Zr1−xTixT2 (T ≡ Mn, Fe, Co) systems

Based on the experimental phase equilibria and thermodynamic data available in the literature and enthalpies of formation computed from first-principles calculations, the thermodynamic reassessment of the Mo–Hf and Mo–Zr systems was carried out by means of the CALPHAD (CALculation of PHAse Diagram) method. The enthalpies of formation for stable and metastable Laves (C15, C36, C14) phases and enthalpy of mixing for the β(bcc) solid solution phase in dilute solution were predicted via first-principles calculations to supply the necessary thermodynamic data for the modeling in order to obtain the thermodynamic parameters with physical sound meaning. The relative stability of Laves C14, C15 and C36 in the systems was discussed. The solution phases, i.e. liquid, β(bcc) and α(hcp) were described by the substitutional solution model, and all the Laves phases in the systems were described using two sublattice model. A set of self-consistent thermodynamic parameters were obtained for these binary systems, which agrees well with the experimental data in the literature. Based on these results, the trend of the site occupancy fraction of Laves phase changing with temperature was predicted. The isothermal section and the liquidus projection of Mo-Hf-Zr system were also predicted by combing with the Zr–Hf system reported in the literature.

Thermodynamic assessment of the Ga-X (X=B, Ca, Sr, Ba) systems supported by first-principles calculations

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

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, P6 3 mmc ). 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.

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

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.

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

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.

Ž Blažina

Papers

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

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

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