Microstructure and calorimetric analysis of the UMn binary system

The microstructure, growth kinetics, and the high temperature phase equilibria of the UMn binary system are investigated using differential scanning calorimetry. Alloys with various compositions are prepared in a positive-pressure arc melting furnace in order to examine all possible phase changes within the system. Phase composition and morphology were analyzed using XRD, SEM and EDX. Transformation temperatures and enthalpies have been assessed pertaining to: (i) allotropic phase changes αMn → βMn → γMn →δMn, (ii) two eutectic isotherms UMn2 + βMn → L and U6Mn + UMn2 → L and (iii) melting transitions as a function of composition. The development of the two intermetallic phases U6Mn and UMn2 are observed. A faceted morphology accompanies the formation of UMn2 in a matrix of U6Mn, while grain boundary decomposition is found of UMn2 in αMn. The transformation temperatures are compared to existing phase diagrams and are slightly higher than reported in literature. Experimental observations of the allotropic transformations of αMn to βMn and βMn to γMn are in agreement with a calculated phase diagram. The eutectic composition for UMn2 and αMn is found to be 55 wt%Mn, in agreement with previously reported values, while the enthalpy of fusion associated with this eutectic point is found to be 54 ± 6.5 kJ mol−1. The enthalpies of formation for the two intermetallic phases U6Mn and UMn2 are 71 ± 8.5 kJ mol−1 and 42 ± 5 kJ mol−1 respectively.