Temperature–Composition Sections of the Hf–Rh–Ir System

Abstract

Alloys of the Hf-Rh-Ir system are refractory materials similar to Ni-based superalloys with higher melting temperatures, low values of thermal expansion coefficients, good mechanical properties at high temperatures and oxidation resistance. Binary alloys based on HfRh3 or HfIr3 are proposed as thermal barrier coatings, on Hf2Rh — as hydrogen adsorbents. To develop high-temperature functional materials based on the Hf-Rh-Ir alloys, an understanding of phase equilibria in this system is essential. The alloys of the Hf-Rh-Ir system of 16 compositions are prepared from iodide processed Hf, Rh wire and refined Ir powder (purity of 99.98%, 99.97% and 99.97%, respectively) by arc melting. The alloys are investigated in as-cast and annealed at subsolidus temperatures states by optical microscopy, scanning electron microscopy, electron probe microanalysis, differential thermal analysis, melting points measurements (Pirani-Alterthum technique) and X-ray diffraction. Based on the results of the alloys studies, solidus surface of the Hf-Rh-Ir system [1] as well as liquidus surface, melting diagram and Scheil diagram are represented. Also the vertical temperature-composition sections of the system phase diagram by the ray Ir : Rh = 1 : 1, along the isopleths at 15,0 at.% Ir, 10,0 and 42,5 at.% Rh as well as at 30,0; 33,0 and 62,5 at.% Hf are constructed for the first time. These sections demonstrate characteristic peculiarities of phase equilibria in the system in particular temperature ranges of alloys crystallization and character of phase transformations. So, the vertical section by the ray Ir : Rh = 1 : 1 shows the influence of Hf on the phase equilibria character. It was established that liquidus and solidus curves of the continuous series of solid solutions between high-temperature modifications of HfRh and HfIr with CsCl-type structure decrease quite steeply (vertical sections by the ray Ir : Rh = 1 : 1 and along the isopleths at 15,0 at.% Ir and 10,0 at.% Rh).