Effects of Long-Term Service on The Aging Behavior of A Water-Quenched U6Nb Alloy
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0986-OO07-01
Effects of Long-Term Service on The Aging Behavior of a Water-Quenched U6Nb Alloy Jikou Zhou and Luke L. Hsiung Lawrence Livermore National Laboratory, Livermore, CA, 94551 Abstract: Microhardness testing and transmission electron microscopy are used to study the effects of long-term service on the aging behavior of a water-quenched U-6wt.% Nb alloy when subjected to isothermal aging at 200°C. The original α″ phase in the WQ-U6Nb alloy is found to become partially ordered over 18 years of aging at ambient temperatures, i.e., natural aging, forming a microstructure that is featured by antiphase domain boundaries (APBs). When subsequently aged at 200 °C, an ordered phase U3Nb is precipitated through a nucleation-andgrowth mechanism, suppressing spinodal decomposition that occurs when the water-quenched alloy is artificially aged at the same temperature. The different phase transformation paths lead to different microhardness changes during artificial aging: the naturally aged alloy is more slowly hardened, but to a greater microhardness peak value.
1. Introduction U6Nb alloy contains 14 at.% niobium (U-6wt.% Nb), which is close to the content of eutectoid uranium-niobium alloy (13.3 at.% Nb [1]). The room-temperature equilibrium phases of this alloy are orthorhombic α uranium and niobium-rich body-centered cubic (BCC) γ phase. The two-phase microstructure significantly degrades the corrosion resistance of the alloy. Hence, heat treatment, particularly high temperature homogenization followed by water-quenching (WQ) to room temperature, is generally employed to minimize niobium segregation [2]. However, several variants of martensite may be formed in the rapid quenching process. These includeα′, α′′, and γo martensite [3]. They are metastable phases supersaturated with niobium, tending to decompose over time. The question is how long it takes to decompose, particularly at room temperature. On the other hand, mechanical components made from water-quenched (WQ) U6Nb have been in service at ambient temperatures for about two decades. This is equivalent to natural aging (NA) for many years. The major concern with the application is how long they can be reliably used [4]. Hence, we need to know what types of microstructural/phase change have occurred in the NA alloy so far and what could be the consequences of such changes.
In this study, we first study the possible microstructure change in the NA alloy by comparing the microstructures of the WQ and NA alloys. The effects of such changes on the subsequent microstructure instability and mechanical property degradation are then evaluated using artificial aging (AA) experiments that are carried out at the same 200 °C for the two alloys. The differences in aging behavior suggest that the long-term service at ambient temperatures has significantly changed the phase transformation pathway of the WQ-U6Nb alloy.
2. Experimental procedure The U6Nb alloy used for this investigation was wrought processed from vacuum arc refined (VAR) ingot at the BWXT/Bechtel Y-12 pl
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