Diagram of constituent crystalline phases in a Nd–Fe–B–Cu sintered magnet by in-situ high-temperature synchrotron X-ray diffraction and its thermodynamic interpretation

A crystalline phase diagram of a Nd–Fe–B–Cu sintered magnet up to 1100 °C was determined through in-situ synchrotron X-ray diffraction using a new sample mounting method that prevents high-temperature contact reactions between the sample and the quartz capillary tube container. Using this newly developed method, we successfully observed almost identical phase diagrams during heating and cooling, indicating thermodynamic equilibrium. In addition, we obtained changes in the constituent phase fraction after quenching. Temperature-reversible changes in the dhcp-Nd fraction were observed from 475 °C to 650 °C, corresponding to a eutectic reaction in the Nd–Cu phase. The fcc-NdO_x fraction decreased with the increase in the Nd₂O₃ fraction above 1000 °C, which behavior was attributed to a phase change from fcc-NdO_x to high-temperature liquid and hcp-Nd₂O₃ phases. The measurements verify the thermodynamic database recently constructed for CALPHAD calculations of the Nd–Fe–B–Cu–O system by assuming the local thermodynamic equilibrium of the Nd oxides within the microstructure of the magnet.