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.