Kinetics of Lamellar Decomposition Reactions in U-Nb Alloys

Abstract

Discontinuous precipitation (DP) and discontinuous coarsening (DC) reactions have been observed in numerous alloy systems [1]. DP has been observed in the U-Nb system [2, 3, 4, 5]. The U-Nb phase diagram (Fig. 1) exhibits a continuous γ-BCC solid solution at high temperatures and a two-phase mixture of a-orthorhombic and γ-BCC below the 647°C monotectoid isotherm. The DP reaction occurs during continuous cooling and isothermal aging over 300-647°C. No metallographic evidence of a DC reaction in U-Nb has been published, although this is suggested from x-ray observations of distinct changes in the Nb content of the γ phase upon prolonged holding after the DP reaction [2, 3, 6]. This study will provide direct evidence for a DC reaction. Discontinuous and other aging reactions [7] are undesirable in U-Nb alloys, since they degrade corrosion resistance [5], ductility [8], and the shape-memory effect [9]. Hence, an improved understanding of the kinetics of these discontinuous phase transformations in U-Nb alloys is of practical interest.

Recent Developments on Discontinuous Precipitation

“…It is not also clear which alloy was taken for the comparison U-13at.% Nb or U-17at.% Nb. One should also note that Hackenberg [23] observed for the first time DC reaction for the U-13at.% Nb alloy for ageing at 73, 723 and 773 K. For 623 K this reaction is expected to begin after 2 years. …”

Multiscale Modeling of Discontinuous Precipitation in U‐Nb

“…the gradual increase of γ composition at increasing temperature. While the hypothesis is thermodynamically eligible, Djuric's observation is, interestingly enough, found contradicting with the recent experiments from Hackenberg et al [7]. In their study, Hackenberg et al [7] examine DP and DC of U-13Nb and U-17Nb (at.%).…”

“…To estimate thermodynamic driving force (in form of energy density [J/cm 3 ]), the previously assessed CALPHAD energetic data are used [12]; for simplicity, the used molar volumes for this estimation are assumed to be constant and take the average value of those from the initial orth and bcc phases. The interfacial energies are taken as the averages of those evaluated in [7].…”

“…• The sluggish evolution time (of orders 10 14 (s) for 450 • C and 10 11 (s) for 550 • C) results from the estimated CALPHAD's slow bulk diffusivity (in orders of 10 −23 [cm 2 /s] for 450 • C and 10 −21 [cm 2 /s] for 550 • C [12], consistent with the experimental values from Peterson and Ogilvie [32,33]). In reality, the reaction happens much faster due to the fast boundarydiffusion condition at the reaction front of DP [6] as evidenced by the measured interphase boundary diffusivity triple products in [7].…”

“…The material, however, exhibits various metastable phase transformations whose resulting microstructures strongly affect the fuel's performances (see, for instance, [1][2][3][4]). In the current work, the phase transformation of interest is discontinuous precipitation (DP) [5,6] whose lamellar precipitate is known to degrade U-Nb's corrosion resistance and ductility [7]. DP is the result of a decomposition from a supersaturated solid solution into a solute-depleted matrix and a precipitate across moving boundary [5,6].…”