The equilibrium diagram of the ThNi system

Abstract

The phase diagram of the Th-Ni system was investigated in the range 15– 85 at.% Ni using differential thermal analysis, micrographic analysis, X-ray diffraction and electron microscopy. Three known intermediate phases were confirmed: Th7Ni3, ThNi2 and ThNi5. ThNi was recognized as a stacking variant of the CrB-FeB type, isotypic with SrAg, and Th2Ni17 was shifted to the composition Th2Ni19. Two new compounds were determined: Th4Ni7 which crystallizes with a new monoclinic type of structure and Th2Ni7 which can exist in two structural modifications depending on temperature. The general proposed shape of the equilibrium diagram is accepted, but with some significant corrections concerning the characteristic temperatures and the melting behaviour of some compounds and eutectic compositions. Solid solubilities were found for ThNi5 towards nickel-rich compositions, whereas for ThNi2 only a very limited solubility range was detected. The 0–15 and 85–100 at,% Ni regions of the diagram were not investigated due to contamination of the alloys with the container material. They were drawn on the basis of earlier investigations.