Isothermal section of the Ho-Cо-Sn system at 770 K

Abstract

The phase equilibrium diagram of the Ho - Co - Sn ternary system was constructed over the whole concentration range at 770 K using methods of powder X-ray diffractometry, metallography and electron microprobe analysis. The alloys for investigation were prepared by direct arc melting the stoichiometric amounts of the constituent elements under high purity Ti-gettered argon atmosphere on a water-cooled copper bottom. The arc-melted ingots were then annealed at 770 K in evacuated quartz glass tubes for two months and subsequently cold water quenched. The synthesized and annealed samples are stable in atmospheric conditions. For the characterization of the annealed samples X-ray powder diffraction on DRON-2.0m diffractometer with Fe K α-radiation was performed. The chemical and phase compositions of the obtained samples were examined by Scanning Electron Microscopy REMMA-102-02 electron microscope. Interaction of holmium with nickel and tin at 770 K resulted in the formation of eight ternary intermetallic compounds: Ho 6 Co 2 Sn (Ho 6 Co 2 Ga-type), Ho 3 Co 8 Sn 4 (Lu 3 Co 7 . 77 Sn 4 -type), HoCoSn (TiNiSi-type), Ho 3 Co 6 Sn 5 (Dy 3 Co 6 Sn 5 -type), Ho 117 Co 57 . 3 Sn 111 . 5 (Dy 117 Co 57 Sn 112 -type), HoCo 6 Sn 6 (YCo 6 Ge 6 -type), Ho 5 Co 6 Sn 18 (Er 4 Rh 6 Sn 19 -type), and Ho 7 Co 6 Sn 23 (Ho 7 Co 6 Sn 23 -type). The Ho 3 Co 8 Sn 4 compound is characterized by small homogeneity range along isoconcentrate of Ho up to 4 at. % Sn. According to EPMA data the homogeneity range of the Ho 3 Co 8 Sn 4 compound is limited by Ho 20 .58 Co 52 . 3 1 Sn 27 .1 1 and Ho 21 .0 6 Co 5 6.7 8 Sn 2 2.1 6 compositions. Ho 7 Co 6 Sn 23 compound was checked using differential thermal analysis (LINSEIS STA PT 1600 device). The DTA curve measured in the heating regime shows the thermal peak at about 980 K which can be associated with the decomposition of the Ho 7 Co 6 Sn 23 phase. In the Ho–Sn, Co-Sn and Ho-Co systems a formation of the all binaries reported in the literature was confirmed. Additionally under used conditions the HoCo 5 binary with CaCu 5 -type was identified in the Ho-Co system. The formation of the interstitial-type solid solution HoCo x Sn 2 based on HoSn 2 (ZrSi 2 -type) binary compound was found up to 10 at. % Co ( а = 0.4382(2), b = 1.6193(4), с = 0.4291(2) nm for HoSn 2 ; а = 0.4393(2), b = 1.6273(7), с = 0.4334(4) nm for Ho 30 Co 10 Sn 60 sample. Solubility of the third component in other binary compounds is less than 1-2 at. %. With regard on the stoichiometry and crystal structure of the formed intermediate phases the investigated Ho-Co-Sn ternary system is similar to the Y-Co-Sn, Dy-Co-Sn and Er-Co-Sn systems. Keywords: intermetallics, ternary system, phase equilibria.