An investigation of phase separation in the Ir–Pt binary system

In the past two different phase diagrams of the Ir–Pt system have been proposed, where the miscibility gap in the fcc solid solution exhibits a different peak temperature and composition. In the present work, to examine the miscibility gap in the Ir–Pt system, the metallurgical microstructure was observed using scanning electron microscopy (SEM) equipped electron probe X-ray microanalyzer (EPMA). The phase identification and lattice parameter change were obtained from X-ray diffractometry (XRD) for the Ir–25, 35, 50, and 75 at.%Pt alloys at temperatures from 1123 K to 1573 K. The results show that the heat treatment of this alloy should be better than 1973 K/72 h, in order to homogenize the specimens that exhibit the usual dendrite structure, as found in as-cast specimens. Although ab initio calculations predict the existence of the miscibility gap, the estimated peak temperature of the two-phase boundary, 1000 K, is too low to obtain the two-phase structure from experiments. This is the reason there is no indication of the two-phase structure in the present work and the two-phase boundaries reported previously may be due to insufficient homogenization.