PNNL

Abstract

Eutectoid transformation in U-10 wt.% Mo (U10Mo) alloy initiates through discontinuous precipitation (DP) resulting in the formation of a lamellar microstructure that consists of an a-U and Mo-enriched bcc ?-UMo phase along the prior ?-UMo grain boundaries. DP is the main mode of transformation at 500°C. However, continuous precipitation (CP) of ?’ U2Mo phase together with grain-boundary DP is observed at 400°C. The reaction kinetics are noted to be very sluggish at 400°C compared to 500°C. The degree of eutectoid transformation is related to the prior ?-UMo grain size, and a smaller average grain size leads to a larger amount of transformation at a given reaction temperature. Minor addition (0.2 wt.%) of a third alloying element (Cr, Ni, Co) leads to significant changes in the nature of the DP mechanism in U10Mo alloy. In addition, high-angle grain boundaries with a misorientation of 30–45° are observed to be the major DP initiation sites. The microstructural evolution of U10Mo alloy with regard to DP-based reaction is discussed.