Zr inclusions in actinide–Zr alloys: New data and ideas about how they form

High-Zr inclusions are common in actinide-Zr alloys (e.g., [1-4]), despite phase diagrams indicating that these alloys should not contain a high-Zr phase. The inclusions may contain enough Zr to cause significant differences between bulk compositions and those of inclusion-free areas [1,3,5], leading to possible errors in interpreting data if the inclusions are not considered. Previous researchers noted a correlation between the oxygen concentration during casting and the volume fraction of high-Zr inclusions in U-Pu-Zr alloys [4], suggesting that the inclusions might be oxygen-stabilized Zr. Other researchers suggested that the inclusions might be stabilized by other impurities such as C, N, and Si, or might represent a previously unknown high-Zr phase [1,5,6]. However, little is known about the inclusions, and previous researchers attempting to explain them have concluded that further investigation is required. This letter presents detailed microstructural and microchemical data from individual inclusions, leading to new ideas about how the inclusions form.