High‐Temperature Phase Diagram for the System Zr.

The melting, eutectic, peritectic, solidus, and liquidus temperatures in the system Zr‐O have been measured directly by a simple optical pyrometric technique requiring only a few hundred milligrams of sample. The saturation solubility of oxygen in α‐Zr( s ) between 1270° and 1980°C and the lower phase boundary of the ZrO 2 – α phase between 1900° and 2400°C have been measured by an isopiestic equilibration method. The oxygen solubility limit in α‐Zr( s ) agrees well with previous low‐temperature studies and reaches a maximum solubility of 35°1 at.% O at the eutectic temperature, 2065°°5°C. The maximum melting temperature of α‐Zr( ss ) is 2130°°10°C and corresponds to a composition of 25°1 at.% O. Both of these temperatures are approximately 150° higher than previously reported. Liquidus compositions above the eutectic temperature were obtained via mass spectrometry from the kinetic behavior of the liquid solution‐ZrO 2–x ( s ) mixture as it approached equilibrium at 2125°°5°C. The lower phase boundary or solidus of the ZrO 2–x phase departs appreciably from ideal stoichiometry above 1900°C and smoothly reaches its most reduced composition, 61 at.% (ZrO 1.56 ), near 2300°C. The solidus is retrograde at higher temperatures. The melting temperature of the stoichiometric dioxide is 2710°°15°C.