Elsevier

Journal of Nuclear Materials

Volume 481, 1 December 2016, Pages 164-175
Journal of Nuclear Materials

The effect of low-temperature aging on the microstructure and deformation of uranium- 6 wt% niobium: An in-situ neutron diffraction study

https://doi.org/10.1016/j.jnucmat.2016.09.004Get rights and content

Abstract

The mechanical properties of uranium-niobium alloys evolve with aging at relatively low temperatures due to subtle microstructural changes. In-situ neutron diffraction measurements during aging of a monoclinic U-6Nb alloy at temperatures to 573 K were performed to monitor these changes. Further, in-situ neutron diffraction studies during deformation of U-6Nb in the as-quenched state and after aging for two and eight hours at 473 K were completed to assess the influence of microstructural evolution on mechanical properties. With heating, large anisotropic changes in lattice parameter were observed followed by relaxation with time at the aging temperature. The lattice parameters return to nearly their initial values with cooling. The active plastic deformation mechanisms including, in order of occurrence, shape-memory de-twinning, mechanical twinning, and slip-mediated deformation do not change with prior aging. However, the resistance to motion of the as-quenched martensitic twin boundaries increases following aging, resulting in the observed increase in initial yield strength.

Introduction

The crystal structure of solid pure uranium at high temperature (1049 K < T < 1405 K) is body-centered cubic (γ phase) [1]. On cooling, uranium experiences a solid-solid phase transformation to the tetragonal β phase followed by a second transformation to an orthorhombic α phase, which is stable at room temperature. To improve the room temperature mechanical properties and corrosion resistance, attempts have been made to alloy uranium with other elemental metals such as Mo [2], [3], Nb [4], Zr [5] and Ti [6], [7], each of which is soluble in the high temperature γ phase. Although these elements have minimal solubility in the β or α phases of uranium, metastable martensitic phases can be formed by moderate to rapid quenching from the high-temperature γ phase. The addition of niobium to uranium is of particular interest because it significantly increases the oxidation resistance and increases the ductility [8].

When quenched from high temperatures to prevent diffusional decomposition, the uranium-niobium alloy system has a rich metastable phase diagram [4], [9], [10], [11], [12], [13]. At low niobium ([Nb] < 3.5 wt%) content the metastable alloy has an α′ orthorhombic structure, similar to pure uranium, with an expanded b and contracted a lattice parameter. As niobium content is increased to intermediate values (3.5 wt% < [Nb] < 6.5 wt%), the alloy assumes an α′′ monoclinic structure that is characterized by an increase of the γ angle of the orthorhombic structure. Further niobium enrichment (6.5 wt% < [Nb] < 9 wt%) yields a tetragonal distortion of the body-centered cubic structure of γ-U, termed γo. Finally, above roughly 9 wt% niobium the bcc γ phase is retained to room temperature with quenching. The transformation temperatures between the phases during quenching and heating of alloys with up to 10 wt% niobium have recently been reported [14], but early studies indicated that both the austenitic and martensitic transformations were dependent on heating and/or cooling rate [9]. The lattice parameters of each phase have been determined with x-ray diffraction and reported [4], [10], [11].

The deformation of U-Nb alloys in the α′′ phase field (near 6 wt%) have been studied extensively [15], [16], [17], [18] because the superior corrosion resistance and ductility of this particular composition leads to use in several applications. In-situ neutron diffraction during deformation to monitor the evolution of crystallographic texture with tensile deformation [18] coupled with ex-situ electron microscopy studies [19], [20] have shown that U-6wt%Nb (U-6Nb) exhibits distinct dominant deformation mechanisms, de-twinning of martensitic variants formed during transformation, formation of mechanical twins, and finally slip mediated deformation, with increasing strain.

It is well known that U-Nb alloys near 6 wt% are age hardenable by heat treatments well below the 920 K monotectoid. Aging at temperature greater than roughly 575 K results in significant hardening with corresponding loss of ductility and loss of corrosion resistance [8]. The hardening is due to non-lamellar precipitation at early time followed by cellular decomposition of the α′′ toward the equilibrium phases of niobium-poor α and niobium-rich γ phase [8], [9], [21], [22], [23], [24], [25] at long times. However, significant hardening is also observed after aging at temperatures as low as 475 K [23], [26], but experimental determination of the mechanism of this low temperature hardening has remained elusive [23], [26]. Using transmission electron microscopy (TEM), Hsiung interprets the microstructural evolution (at nominally 6 wt%Nb) as being consistent with spinodal decomposition after aging at 473 K for as little as 16 h. In contrast, Clarke et al. [23] did not observe statistically relevant chemical heterogeneity (beyond their quoted uncertainty of 1%) with atom probe tomography at the nm scale after as much as 70 days at 473 K, which is inconsistent with spinodal decomposition. It is worth noting that Clarke et al. did observe significant chemical variations after aging at a higher temperature, 573 K, which is consistent with prior work at this temperature [8], [9], [21], [22], [27], [28]. Moreover, a thermodynamics-based argument against spinodal decomposition as a low temperature aging mechanism was presented by Clarke et al. [23].

In order to better understand the microscopic origins of the aging process and the concomitant change in mechanical response, we have carried out in-situ neutron diffraction measurements: (1) during low temperature aging of a U-6Nb alloy at 373 K, 473 K, 523 K, and 573 K for up to 40 h, and (2) during deformation of U-6Nb in the as-quenched condition as well as after aging at 473 K. As probe particles, neutrons have an advantage over x-rays and electrons in that they penetrate distances on the order of centimeters into most materials, including uranium. This allows the bulk microstructure (as opposed to a surface layer) to be monitored while under non-ambient conditions. Changes in lattice parameters during heating which are large compared to the expected thermal expansion of α-U are observed, followed by time dependent relaxation of the lattice parameter during subsequent isothermal holds. Moreover, based on the evolution of the texture, we find that, while the initial yield strength of the material increases after low temperature aging (473 K), the active deformation mechanisms do not change. The motion of transformation twin boundaries, which is responsible for the initial yield phenomenon [17], [18] becomes more difficult, but remains the predominant active deformation mode at small strains. However, the current data cannot unambiguously identify the microscopic mechanism of the increased resistance to motion of the transformation twin boundaries with low temperature aging.

Section snippets

Sample preparation

Two different sources of nominally U-6Nb material were used for the experiments [14]. Niobium concentration-dependence and aging experiments were performed on samples made in small batches specifically intended to yield (compositionally) homogenized samples [14], [29]. Small-scale (∼50 g) U-Nb alloy buttons were synthesized by arc melting high-purity, unalloyed depleted uranium and elemental niobium under an argon partial pressure. The challenges of mixing elements with dissimilar melting

Dependence on [Nb]

Neutron diffraction patterns from homogenized U-Nb samples with several different niobium concentrations are shown in Fig. 1. Each structure in the uranium-rich side of the phase diagram is represented. At the lowest niobium concentration studied, 0.25 wt%, the alloy is α′ (orthorhombic). At a niobium concentration of 2.0 wt%, the crystallographic structure is still orthorhombic, but the peaks have shifted significantly and are considerably broader. The alloy is well into the α′′ (monoclinic)

Discussion

To summarize the observations and ease the discussion, the stresses and strains at which various macroscopic phenomenon (e.g. primary yield) and diffraction effects (e.g. increase in (130) pole density) were observed are listed in Table 4. Clearly, low temperature aging (i.e. at 473 K) has a pronounced effect on the mechanical response of U-6Nb, as well as the underlying micro-mechanics of the deformation. In as-quenched U-6Nb, the immediate increase of (130) pole density with strain (Fig. 10)

Conclusions

In-situ neutron diffraction measurements during and aging heat treatment and deformation of 473 K aged nominally U-6Nb materials were completed in an attempt to understand the microstructural origin of the reported increased strength of material following aging heat treatments. At the initial low-stress yield point of U-6Nb, strain is accommodated by preferential selection of specific martensitic variants through motion of transformation twins formed during quenching through the martensitic

Acknowledgments

This work has benefited from the use of the Lujan Neutron Scattering Center at LANSCE, which was funded by the Office of Basic Energy Sciences (DOE). Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.

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