Discontinuous Precipitation in U-10 wt.%Mo Alloy: Reaction Kinetics, Effect of Prior γ-UMo Microstructure, the Role of Grain-Boundary Misorientation, and the Effect of Ternary Alloying Addition

Abstract

Discontinuous precipitation (DP) plays a major role during eutectoid transformation in U-10 wt.%Mo (U10Mo) alloy. DP results in the formation of a lamellar microstructure consisting of an α-U and Mo-enriched bcc γ-UMo phase along the prior γ-UMo grain boundaries. DP is the main mode of transformation at 500°C. However, continuous precipitation of γ′-U2Mo phase together with grain-boundary DP is observed at 400°C. The reaction kinetics are noted to be much slower at 400°C compared with 500°C. The degree of eutectoid transformation is related to the prior γ-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, or Co) leads to substantial alterations in the nature of the DP mechanism in U10Mo alloy. In addition, high-angle grain boundaries with misorientation of 30°–45° are observed to be the major DP initiation sites. The microstructural evolution of U10Mo alloy regarding the DP-based reaction is discussed.