Low-Temperature Aging Kinetics of a 15-Year Old Water-Quenched U-6wt.% Nb Alloy

It is well known that U-6wt. % Nb (U-14at.% Nb) alloy has a microstructure containing martensitic phases supersaturated with Nb that can be obtained by rapid quenching the alloy from {gamma} (bcc)-field solid solution to room temperature. The high cooling rate forces the {gamma}-phase solid solution to transform to variants of the low-temperature {alpha} (orthorhombic) phase in which Nb is forced to retain in the supersaturated solid solution. However, the crystal lattice of supersaturated solution formed by rapid quenching is in unstable conditions and is severely distorted since the solubility of Nb in the {alpha} phase at room temperature is nearly zero under an equilibrium condition. Two variant phases, a monoclinic distortion of {alpha} phase that is designated as {alpha}{double_prime} martensite and a tetragonal distortion of {gamma} phase that is designated as {gamma}{sup o} phase, can form in the as-quenched alloy, as shown in Fig. 1. We have learned from our previous TEM studies on the low-temperature aging of a water-quenched U6Nb (WQ-U6Nb) alloy that there are two possible transformation pathways for phase decomposition of the alloy supersaturated with 14 at. % of Nb upon aging at temperatures below 200 C, i.e., (1) supersaturated solid solution {alpha}{double_prime} {yields} spinodal decomposition {yields} {alpha}{submore » 1} (Nb-lean) + {alpha}{sub 2} (Nb-rich) at 200 C and (2) supersaturated solid solution {alpha}{double_prime} {yields} spinodal ordering {yields}{alpha}{double_prime}{sub po} (partially ordered phase) {yields} phase decomposition and precipitation {yields} {alpha} (U) + {alpha}{sub o} (U{sub 3}Nb) at ambient temperatures [1]. The mechanisms for the spinodal transformation occurred at 200 C and the spinodal ordering occurred at ambient temperatures are quite similar; both are caused by the composition modulation of Nb except that the wavelength ({lambda} {approx} 3 nm) of modulation for spinodal decomposition is larger than that ({lambda} {approx} 0.5 nm) of modulation for the spinodal ordering, as illustrated in Fig. 2. Since the Nb modulation for the spinodal ordering can occur within the unit cell of {alpha}{double_prime} phase through the nearest jumps of atoms along the [001] direction, the degree of long-range order (S) increases from 0 to 0.16 as a result of the Nb modulation, as illustrated in Fig. 3. As we accelerated the ordering transformation by thermal heating a 15-year old alloy at 200 C, decomposition of the {alpha}{double_prime}{sub po} phase into {alpha} (U) and a fully ordered {alpha}{sub o} (U{sub 3}Nb) phase occurred, as shown in Fig. 4. Figure 5 shows the results of microhardness measurement and TEM analysis of the microstructural evolution in the 15-old alloy samples thermally heated at 200 C. Here, it can be clearly seen that the {alpha}{double_prime}{sub po} phase with a swirl-shape feature of antiphase boundaries (APBs) vanishes upon heating with the formation of U{sub 3}Nb precipitates, which gives rise to the increase of microhardness (precipitation hardening). Figure 6 shows the changes of tensile properties of the 15-old alloy thermally heated at 200 C. It can be readily seen that in addition to the increase of tensile strength (precipitation hardening), the ductility reduces from {approx}40% to {approx}14% after heating for 96 hours. In view of these adverse changes in tensile properties upon aging, we accordingly pursued a precipitation kinetics study on the 15-year old WQ-U6Nb alloy in order to develop an empirical time-temperature-transformation model for predicting the remaining lifetime of the WQ-U6Nb alloy in the stockpile. « less