Isothermal Transformations of Uranium--13 Atomic Percent Niobium.

Low temperature age hardening in U–13at.% Nb: An assessment of chemical redistribution mechanisms

“…Jackson identified two principal kinetic modes of aging that correspond to two separate C-curves on the T-T-T diagram for U-13% Nb [16]. Similar decomposition reactions take place at a given temperature regardless of whether the thermal path is direct isothermal or cooling transformation from the stable c phase region, or decomposition of the a 00 phase upon quenching and subsequent aging.…”

“…Similar decomposition reactions take place at a given temperature regardless of whether the thermal path is direct isothermal or cooling transformation from the stable c phase region, or decomposition of the a 00 phase upon quenching and subsequent aging. The higher temperature C-curve, spanning 300-647°C with a 550°C nose, corresponds to the cellular (discontinuous) precipitation of a lamellar a + c 1-2 product [2,[16][17][18][19][20][21][22]. The c 1-2 phase is bcc with an intermediate composition defined by a metastable extension of the equilibrium monotectoid solvus (14-35% Nb); it evolves toward the equilibrium c 2 composition ($75% Nb) with concomitant formation of additional a [2,16,17,20].…”

“…The reaction(s) corresponding to the lower C-curve [16], with a nose at 400°C, results in considerable strengthening and ductility loss [2,18,19,[21][22][23][24][25], with only marginal decrements in corrosion resistance [2]. The specific aging mechanism(s) for this low-temperature decomposition mode are unclear, though many U-Nb aging studies have proposed some variant of diffusional niobium redistribution.…”

“…The specific aging mechanism(s) for this low-temperature decomposition mode are unclear, though many U-Nb aging studies have proposed some variant of diffusional niobium redistribution. Mechanisms that have been considered include classical nucleation-and-growth (N&G) [16], spinodal decomposition [2,16,26,27], chemical ordering [26,27], and precursors to phase separation 2 involving niobium diffusion, such as clustering and segregation to defects, particularly a 00 twin boundaries [16,21,28]. Niobium (or impurity) segregation to existing twin boundaries is a potentially potent strengthening mechanism in view of the high twin mobility in as-quenched U-14% Nb.…”

“…Diffraction and dilatometry experiments provide indirect evidence that niobium redistribution occurs during low temperature (6300°C) U-Nb martensite aging [16,24,25,29,30]. Kinetic analyses based upon mechanical property changes resulted in activation energies (Q) of 121-138 kJ/mol in U-13% Nb [24] and 117 kJ/mol in U-11% Nb [21]; these values are close to the Q diff = 138 kJ/mol measured for niobium volume diffusion in c-U 3 [31].…”