Elsevier

Journal of Nuclear Materials

Volume 497, 15 December 2017, Pages 10-15
Journal of Nuclear Materials

Texture evolution during annealing of hot extruded U-10wt%Zr alloy by in situ neutron diffraction

https://doi.org/10.1016/j.jnucmat.2017.09.008Get rights and content

Abstract

Texture evolution during annealing of a U-10wt%Zr alloy hot extruded in the (α+δ) region was studied by in situ neutron diffraction. The extruded alloy had a lamellar (α+δ) microstructure and the initial texture consisted of (100)α, (110)α, and the (0001)δ poles oriented along the extrusion direction. The β phase, after the αβ transformation, showed no preferred orientation while the γ phase, after the βγ transformation, had (110)γ and (111)γ poles oriented along the extrusion direction. After a temperature cycle through the γ phase, there was a complete loss of texture in the α phase while the δ phase retained the initial texture indicating a memory effect.

Introduction

High thermal conductivity and the ability to incorporate minor actinides makes metallic fuels an attractive choice for fast reactors [1]. Additionally, irradiation data from one of the alloy compositions, i.e., uranium-10 wt% zirconium (U-10Zr) alloys available from the Experimental Breeder Reactor-II (EBR)1 indicate their viability as a potential fuel form [2]. U-Zr alloy fuel rods can be produced by different processes like injection casting, machining, and extrusion. Injection casting requires very high temperatures and expensive crucibles while machining results in large material losses. Extrusion may be a favourable alternative to other methods since it is more cost effective. It is a common fabrication process that is performed on an industrial scale around the world to cost-effectively manufacture many simple shapes from many materials such as aluminum, copper, and steel [3]. Extrusion of uranium or its alloys is not a common practice; however, it has been previously investigated to some detail where extruded fuel pins were tested in EBR-I [1]. The present study is aimed towards the evaluation of extrusion as a potential manufacturing process for U-10Zr fuel pins by understanding the microstructural changes of a hot extruded billet during annealing.

Uranium and its alloys undergo several phase transformations with temperature: pure uranium occurs in the orthorhombic α-phase at room temperature, followed by a transition at 669 °C to the tetragonal β-phase, and at 776 °C to the body centered cubic (bcc) γ-phase [4]. In the U-Zr system studied here, α, β, γ along with δ are relevant within the context of this work. Here, the δ-phase is hexagonal and exists along with α at room temperature (U-Zr phase diagram is shown in Fig. 1). Because of the strong thermoelastic and plastic anisotropy exhibited by α-U, any thermo-mechanical processing in the α phase results in the texture development [5], [6], [7].

Since the present study deals with hot extrusion of U-10Zr alloy in the (α+δ) region, relevant literature on the textures developed in α-U during various thermomechanical processing conditions is briefly reviewed. It has been observed that the texture developed in α-U not only depends on the nature of the deformation (compression, tension, rolling, extrusion), but also on the temperature at which the specimen is deformed [8], [9], [10], [11], [12], [13], [14]. This is due to the strong dependence of the relative importance of the deformation modes on temperature (i.e., slip and twinning) in uranium [7], [12], [15]. Daniel et al. [7], by deforming the specimens under tension and compression, have shown that at all temperatures, slip occurs most easily in the [100] direction, and depending on the temperature, it would occur on the (010) plane below 500C, or on (001) plane at higher temperatures. Slip on the (110) plane and [110] direction was observed at temperatures above 150C. Ivanov et al. [12] have observed that at rolling temperatures above 300C, twinning becomes less pronounced while the dominant slip system is {110}110. It has been pointed out that, with increasing temperature, the critical stress for activating slip decreases much more rapidly than for twinning [15].

Previously observed deformation textures in α-U correspond, in the case of warm rolling, to (010) poles aligning with the rolling direction and (001) poles aligning along the normal direction to rolling [9], [14], [16], and in extruded uranium and U-2.4 wt%Nb alloy, to (110) poles aligning in the extrusion direction in samples subjected to low extrusion ratio and near the (310)–(100) region in samples subjected to a high extrusion ratio, respectively [11], [13]. Complete loss of α texture was observed after the αβγβα transformation cycle while a texture memory effect was observed after the αβα transformation cycle, the reason for which was attributed to the retention of small nuclei of the parent α in the β phase ([17] and references there in). Deformation textures of the intermetallic UZr2-δ phase in U-Zr alloys, which is hexagonal, have not been published in the literature and the texture development for the specific case where thermo-mechanical processing is performed in the (α+δ) region, is not yet reported.

In regards to metallic uranium or any uranium based metallic alloy fuels, it was observed that the α-U phase undergoes anisotropic irradiation growth [18]. Specifically, due to the anisotropic properties of the orthorhombic crystal structure of α-U, mismatched strains develop between individual grains due to their anisotropic growth during irradiation. The stresses thus developed accumulate and can be released at grain boundaries causing “tearing or cavitation swelling” [2]. Since cavitational swelling is the consequence of anisotropic irradiation growth in α-U, preferred orientation of α-U grains resulting from the manufacturing process plays very important role in influencing the shape changes of fuel pin during irradiation. In U-Zr alloy fuel pins, cavitational swelling was observed in the region where the fuel pin had (α+δ) microstructure [2]. However, the irradiation response of monolithic δ phase is not yet known. In order to preserve the dimensional stability of the fuel pins during service, the fuel material must be texture free, which imposes challenges on the manufacturing process. Due to the complex interplay of phase transformations, alloying element redistribution, recrystallization etc during heat treatments, experimental in situ data is of great value to understand and ultimately predict and optimize the microstructure during manufacturing [2]. In the present paper, we report the results from our study on the texture evolution during annealing of a U-10Zr alloy extruded at 600C, in the (α+δ) region in situ, using neutron diffraction. The results will be used to optimize the extrusion process parameters and subsequent heat treatments to obtain a microstructure, which is texture and residual stress free.

Section snippets

Experimental

Depleted uranium (DU) pieces were pickled in 25 vol% nitric acid to remove the oxide layer before casting. DU pieces and crystal bar zirconium were then weighed such that the alloy contained 10 wt% Zr, and melt cast in yttrium oxide crucibles under argon atmosphere in a high temperature furnace with a heating rate of 20C/min. The alloy was held isothermally for 2 h at 1900C (U-10Zr composition melts at around 1400C) after which it was cooled (i.e., furnace cooled under argon atmosphere) at a

Results

Fig. 2a shows the microstructure of extruded alloy, which has a lamellar microstructure consisting of α and δ-UZr2 phases. The high density of dark contrast features correspond to the δ-UZr2 phase while the highest contrast features, which are elongated in the extrusion direction (indicated by an arrow in Fig. 2a), correspond to zirconium stringers. Volume fractions of the phases were not estimated from the image. The neutron diffraction pattern collected at the 150° detector bank for 22.5°

Discussion

In the present study, the preferred orientation developed in α-U after extrusion is in agreement with the ones reported in literature [11], [13]. While the complete loss of α texture after the αβγβα transformation cycle observed in this study in situ by capturing the texture changes in various phase fields is in accord with the published literature (note, in previous studies these were performed ex situ at room temperature, after the transformation cycle), perhaps the most interesting

Conclusions

In summary, we have carried out an in situ neutron diffraction study to monitor the texture evolution during annealing in a U-10Zr alloy hot extruded in (α+δ) region. The results show a complete loss of α texture and suggest a texture memory effect for the δ phase after the αβγβα and δγδ transformation cycles, respectively.

Acknowledgements

This work has benefited from the use of HIPPO diffractometer at the Lujan Centre at Los Alamos Neutron Science Centre. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.

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      If in U-alloys that do undergo α–β phase transition, much of the pre-existing α texture that exists following extrusion could be removed via the phase transformation. The previous study reveals that β phase exhibits no preferred orientation [15], so the pre-existing α texture would be randomized by the α–β phase transition during thermal cycling. However, as instead the β region is not thermodynamically stable for the U-10Zr alloy and there is an variant selection between (001)α and (110)γ, the textures would be retained.

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