V.K Portnoy

Solid state reactions in Ni–Al–Ti–C system by mechanical alloying

Abstract Several Ni–Al–Ti–C compositions from different areas of the Ni–Al phase diagram containing various amounts of Ti and C were prepared by mechanical alloying. Synthesized samples were investigated by X-ray diffraction and differential scanning calorimetry methods. Solid state reactions in Ni–Al–Ti–C powder mixtures with an equiatomic Ni/Al ratio lead to the formation of NiAl and TiC equilibrium phases. Metastable solid solution of Ni(Al,Ti,C) and non-stoichiometric TiC x are formed in the mixtures with C Ni /C Al ratio of 1.7–3. The grain size of metastable intermetallic phases is less than 5 nm, if Ni and Al content in the mixtures corresponds to the dual-phase and Ni 3 Al-areas of the Ni–Al phase diagram.

Crystallographic data of new ternary Sm5Ge4-type R2Ti3Ge4 compounds (R=Gd-Er)

Abstract Investigations made by powder X-ray diffraction on five new ternary R2Ti3Ge4 compounds (R=Gd-Er) are reported. The (Gd0.97)2Ti3Ge4 (a=0.7042(1) nm, b=1.3494(2) nm, c=0.7186(1) nm), (Tb0.99)2Ti3Ge4 (a=0.7019(1) nm, b=1.3457(2) nm, c=0.7156(1) nm), (Dy0.97)2Ti3Ge4 (a=0.6987(1) nm, b=1.3409(2) nm, c=0.7122(1) nm), (Ho0.96)2Ti3Ge4 (a=0.6981(1) nm, b=1.3399(2) nm, c=0.7117(1) nm) and (Er0.91)2Ti3Ge4 (a=0.6962(1) nm, b=1.3367(2) nm, c=0.7099(1) nm) crystallize in the orthorhombic Sm5Ge4-type structure (space group Pnma).

New Ternary Compounds R117Fe52Ge112 (R = Gd, Dy, Ho, Er, Tm) and Sm117Cr52Ge112 of the Tb117Fe52Ge112-Type Structure

Abstract The new compounds Gd117Fe52Ge112 [a = 2.8711(4) nm], Dy117Fe52Ge112 [a = 2.8398(5) nm], Ho117Fe52Ge112 [a = 2.8293)(5) nm], Er117Fe52Ge112 [a = 2.8108(7) nm], Tm117Fe52Ge112 [a = 2.802(2) nm], and Sm117Cr52Ge112 [a = 2.9098(6) nm] belonging to the Tb117Fe52Ge112-type structure (space group Fm3m) were prepared and characterized using powder X-ray diffraction.

GdTiGe (CeScSi-type structure) and GdTiGe (CeFeSi-type structure) as the coherent phases with different magnetic and hydrogenization properties

Abstract The results of investigations of new ternary coherent GdTiGe compounds and corresponding hydrides by X-ray powder diffraction and magnetic measurements are reported. Coherent phases GdTiGe with CeScSi-type structure and GdTiGe with CeFeSi-type structure were found to coexist in the GdTiGe alloy when the sample mass was 5 g (melting in electric arc furnace in argon atmosphere). Annealing at 1070 K for 200 h in argon atmosphere led to an increase in GdTiGe (CeScSi-type structure) phase in the GdTiGe alloy. The GdTiGe with CeScSi-type structure ( a = 0.4065(1), c = 1.5450(1) nm) was characterized by Curie point T c = 376 K, while GdTiGe with CeFeSi-type structure ( a = 0.4065(1), c = 0.7716(1) nm) demonstrated Neel point T N = 412 K. The mechanical hydrogenization of the GdTiGe alloy led to formation of the GdTiGeH ∼4 hydride with CeScSi-type structure and to a sharp decrease in magnetization; GdTiGe with CeFeSi-type structure remained passive. The X-ray single crystal diffraction study confirmed that the Gd(Ti 0.8 Zr 0.2 )Ge compound has the CeScSi-type structure (space group I 4/ mmm , a = 0.4060(1), c = 1.5400(3) nm).

Mechanical alloying of NiNb in different atmospheres

Abstract The formation of NiNb alloys prepared by mechanical alloying (MA) in different atmospheric conditions was examined by X-ray diffraction. Ball milling of elemental Ni and Nb powders under an argon atmosphere for alloys with Nb contents up to 15 at.% forms a supersaturated solid solution on the base of Ni. Amorphous alloys are formed if the Nb content is more than 20 at.%. The interaction of metallic powders Ni and Nb was absent and a b.c.c. phase was formed for all compositions up to pure Nb, when MA of NiNb powders was performed in the presence of water, alcohol or acetone in Ar and air atmospheres. This phase, identified as the interstitial phase of oxygen in niobium, has the lattice parameter a = 3.390 A and an O content of about 34 at.%.

Hydrogen sorption in homologous lanthanum and cerium nickel silicides

Abstract The interaction of hydrogen with the La 15 Ni 8 Si 9 H 31 , Ce 15 Ni 9 Si 8 H 35 (Pr 15 Ni 7 Si 10 type structure) and CeNi 0.6 Si 1.4 (AlB 2 -type structure) has been studied. These compounds belong to the homologous R n 2 +3 n +2 T 2 n 2 +2 (R=La, Ce; T=Ni, Si, n =2, 3, 4, ∞) series with the Ce 6 Ni 2 Si 3 , Ce 2 NiSi, Pr 15 Ni 7 Si 10 and AlB 2 structure types. The new hydride phases La 15 Ni 8 Si 9 H 31 and Ce 15 Ni 9 Si 8 H 35 have been synthesised and characterised by X-ray diffraction and thermodesorption. CeNi 0.6 Si 1.4 ( n →∞, AlB 2 -type structure) did not react with hydrogen at a pressure up to 200 MPa at room temperature. The hydrides preserved the structure type of the starting compounds with anisotropic unit cell distortion: the lattice parameter a and the unit cell volume v increased (d a / a ≈0.07, d V / V ≈0.09), while the parameter c decreased (d c / c ≈−0.04). The homologous R n 2 +3 n +2 T 2 n 2 +2 (R=La, Ce; T=Ni, Si) compounds form hydrides with general formula R n 2 +3 n +2 T 2 n 2 +2 H ∼(2 n 2 +3 n +4) ( n =2, 3, 4) with anisotropic unit cell distortion: the lattice parameter a and the unit cell volume v increased, while the parameter c decreased. Some suggestions on hydrogen localisation in the crystal lattice and their anisotropic distortion have been proposed.

Hydrogen in Ce2Ni1-xSi1+x and Ce6Ni2Si3 compounds

Abstract Hydrogen interaction with Ce2Ni0.8Si1.2,Ce2NiSi, Ce2Ni1.2Si0.8 and Ce6Ni2Si3 compounds were studied. The new hydride phases containing 3.7, 4.4, 4.9,and 10.9 hydrogen atoms per formula unit correspondingly, have been synthesized andcharacterized by X-ray diffraction and thermodesorption methods. All hydrides preservedstructure type of the starting compounds with pronounced anisotropic distortion of crystal lattice.Some suggestions on hydrogen localization in interstitial sites have been proposed.