In search of θ-(Pu,Zr) in binary Pu–Zr: Thermal and microstructural analyses of Pu − 30Zr alloy

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Abstract

The existing Pu–Zr binary phase diagrams report the stability of the compound θ-(Pu,Zr) in the low temperature region between 300 and 0 °C. Furthermore, the current understanding is that θ-(Pu–Zr) is thermodynamically favored over the metastable δ-(Pu,Zr) phase. In an effort to shed light on the phases formed in Pu–Zr binary alloys and reduce uncertainties in the poorly defined boundary between the θ-(Pu,Zr), (θ+δ), δ-(Pu,Zr), and (θ+α-Zr) regions, Pu − 30Zr (in wt.%, equivalent 53 at.%) alloys were subjected to microstructural characterization, annealing, and differential scanning calorimetry (DSC). The results indicate that the alloy is composed of δ-(Pu,Zr) matrix, with a number of smaller, randomly distributed α-Zr precipitates. The phase transition temperatures (determined based on the DSC data) and phases identified in Pu − 30Zr alloys (based on crystallographic data) compare well to those predicted by the phase diagrams with the exception of the θ-(Pu–Zr) phase. Our data indicates that θ-(Pu–Zr) is metastable and can be observed only within a small temperature window (100–300 °C). The consecutive heating cycles remove θ-(Pu–Zr) from the system, and no traces of θ-(Pu–Zr) remain at room temperature, as evidenced by microstructural characterization. This calls for reevaluation of the binary Pu–Zr phase diagram, with particular attention paid to the existence of θ-(Pu–Zr) and its stability.

Introduction

The complex and time-varying microstructure of metallic fuels contributes to and evolves with a number of coupled irradiation behaviors, which negatively influence the performance of metallic fuel elements. Because of this complexity, the metallic U-Pu-Zr fuels are lacking a sound scientific foundation for the development of computational models that can mechanistically predict their behavior. In this contribution, we scrutinize the phases in binary Pu–Zr alloys as a first step to understanding the thermodynamic behavior of ternary U-Pu-Zr fuels.

The key features of the Pu–Zr phase diagram (provided in Fig. 1 for the reader’s convenience) are continuous solubility between β-Zr and ε-Pu, extensive solubility of Zr in δ-Pu, limited solubility of Zr in α-Pu, β-Pu, γ-Pu, and δ′-Pu, and existence of the θ-(Pu–Zr) intermediate phase, which is stable at room temperature [1]. The Pu-rich (80–100 at.% Pu) portion of the phase diagram is debated by a number of research groups [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. Our previous work has indicated that heat treatment will anneal out metastable κ-PuZr2 from Pu − 10Zr (in wt.%, equivalent to 12 at.%) and result in formation of δ′-Pu and β-Pu phases within δ-(Pu,Zr) matrix [7].

To avoid the complexities of Pu-rich region, this manuscript focuses on the less frequently debated part of the phase diagram (50–60 at.% Zr/50–40 at.% Pu), denoted with the red box in Fig. 1. The alloy exists in liquid form at high temperatures (>1350 °C) and transitions to a mixture of liquid and (ε-Pu,β-Zr) solid solution upon cooling. Further cooling leads to decomposition of (ε-Pu,β-Zr) into the mixture of (ε-Pu,β-Zr) and δ-(Pu,Zr), followed by transition into δ′-Pu and then δ-(Pu,Zr) around 580 °C. Around 250 °C, phase diagrams indicate that δ-(Pu,Zr) will start transforming into intermetallic θ-(Pu–Zr), and then mixture of θ-(Pu–Zr) and α-Zr, which is predicted to be stable between 300 °C and room temperature (RT).

Of particular interest to this contribution was the region between 550 °C and RT, which should include primarily δ-(Pu,Zr) and θ-(Pu–Zr) matrix with α-Zr precipitates. These precipitates are commonly referred to as “oxygen-stabilized α-Zr,” but little is known about their formation mechanisms [[10], [11], [12]]. The compound θ-(Pu,Zr) (also known as Pu4Zr or Pu6Zr) has been said to occur at about 20 at.% Zr but has been notoriously difficult to observe experimentally [3]. If the phase diagram shown in Fig. 1 is accurate, it can be assumed that even if the matrix phase does not predominantly consist of θ-(Pu,Zr), it should contain at least traces of θ-(Pu,Zr). However, little to no agreement over the (θ+α-Zr) region exists, and further work is needed to determine the transition points for these phases.

The goal of this manuscript is to compare the experimental data to the existing phase diagrams and determine if the current understanding of the Pu–Zr system is sufficient for development of accurate computational models of its microstructure. To this end, we conducted microstructural characterization of Pu − 30Zr alloys and investigated the phases formed in this binary system using microscopy-based techniques. The phase transition temperatures were determined and correlated to the observed microstructural features, and gaps in the community’s scientific understanding of these alloys were identified.

Section snippets

Materials and methods

Alloys with nominal compositions of Pu − 30Zr (in wt.%, equivalent 53 at.%) were fabricated in a similar fashion to the metallic (U-Pu-Zr) fuel alloys. The alloys were arc cast in an inert atmosphere glovebox containing <50 parts per million (ppm) oxygen and homogenized between consecutive melts. The cast slugs were sectioned into pieces for microscopy and thermodynamic property measurements. The described arc casting procedure has been routinely used to produce metallic U and Pu fuel alloys

Results and discussion

Both as-cast and annealed alloys were examined in an SEM, and the backscattered electron micrographs from both specimens are shown in Fig. 2. The microstructure of the as-cast alloy contained a large number of darker Zr-rich inclusions, which were uniformly distributed across the lighter contrast Pu–Zr matrix (Fig. 2(a)). Annealing refined the microstructure of the alloy and brought the system closer to the equilibrium, as can be seen from Fig. 2(b), in which Zr-rich inclusions increase in size

Conclusions

In this manuscript, we scrutinized the microstructure and phase transition temperatures of the Pu − 30Zr alloys to identify the phases that can be observed in a binary Pu–Zr system. The microstructural characterization revealed that the matrix is consistent with the δ-(Pu,Zr) phase and contains a large number of α-Zr precipitates. Based on microstructural characterization alone, casting and annealing do not induce the expected δ-(Pu,Zr)→θ-(Pu,Zr)+α-Zr phase transformation. This observation

Declaration of competing interest

None.

Acknowledgements

This work was supported by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07-05ID14517, as part of Fuel Cycle Research and Development (FCRD) program of the US DOE and INL Laboratory Directed Research and Development (LDRD) program. Authors would like to acknowledge the staff of Fuel Manufacturing Facility (FMF), Electron Microscopy Laboratory (EML), and Analytical Laboratory at the Materials and Fuels Complex (MFC) at Idaho National

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