The Ni-rich part of the Al–Ge–Ni phase diagram

Abstract

The Ni-rich part of the ternary system Al–Ge–Ni (xNi > 50 at.%) was investigated by means of optical microscopy, powder X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM). The two isothermal sections at 550 °C and 700 °C were determined. Within these two sections a new ternary phase, designated as τ4, AlyGe9−yNi13±x (hP66, Ga3Ge6Ni13-type) was detected and investigated by single crystal X-ray diffraction. Another ternary low temperature phase, τ5, was found only in the isothermal section at 550 °C around the composition AlGeNi4. This compound was found to crystallise in the Co2Si type structure (oP12, Pnma). The structure was identified by Rietveld refinement of powder data. The NiAs type (B8) phase based on binary Ge3Ni5 revealed an extended solid solubility of Al and the two isotypic compounds AlNi3 and GeNi3 form a complete solid solution.Based on DTA results, six vertical sections at 55, 60, 70, 75 and 80 at.% Ni and at a constant Al:Ni ratio of 1:3 were constructed. Furthermore, the liquidus surface projection and the reaction scheme (Scheil diagram) were completed by combining our results with previous results from the Ni-poor part of the phase diagram. Six invariant ternary reactions were identified in the Ni-rich part of the system.

Thermodynamic modelling of the general NiAs-type structure: A study of first principle energies of formation for binary Ni-containing B8 compounds

“…However, the large negative values become noticeable in the ternary, where extended Al-solubilities in other NiAs compounds are possible. For example, in the Ni-Al-Ge system the binary NiAs phase Ge 3 Ni 5 dissolves up to 20 at% Al [42] and in the system Ni-Al-Si the θ-Ni 2 Si phase dissolves even up to 21 at% Al [64].…”

“…Especially Ni-Pt-Sn alloys are widely discussed as electrocatalysts for ethanol [34][35][36] and ethylene glycol [37,38] oxidation in fuel cells. Furthermore, many higher order systems that are studied in the context of joining technologies will show the formation of NiAs-type structures and solid solutions, such as, systems for transient liquid phase bonding (TLP) [39][40][41][42] and leadfree solders [43,44].…”