Metastable phases in Zr-Excel alloy and their stability under heavy ion (Kr²⁺) irradiation

Abstract

Zr-Excel alloy (Zr-3.5Sn-0.8Nb-0.8Mo, wt.%) has been proposed as a candidate material of pressure tubes in the CANDU-SCWR design. It is a dual-phase alloy containing primary hcp α-Zr and metastable bcc β-Zr. Metastable hexagonal ω-Zr phase could form in β-Zr as a result of aging during the processing of the tube. A synchrotron X-ray study was employed to study the lattice properties of the metastable phases in as-received Zr-Excel pressure tube material. In situ heavy ion (1 MeV Kr²⁺) irradiations were carried out at 200 °C and 450 °C to emulate the stability of the metastable phase under a reactor environment. Quantitative Chemi-STEM EDS analysis was conducted on both un-irradiated and irradiated samples to investigate alloying element redistribution induced by heavy ion irradiation. It was found that no decomposition of β-Zr was observed under irradiation at both 200 °C and 450 °C. However, ω-Zr particles experienced shape changes and shrinkage associated with enrichment of Fe at the β/ω interface during 200 °C irradiation but not at 450 °C. There is a noticeable increase in the level of Fe in the α matrix after irradiation at both 200 °C and 450 °C. The concentrations of Nb, Mo and Fe are increased in the ω phase but decreased in the β phase at 200 °C. The stability of metastable phases under heavy ion irradiation associated with elemental redistribution is discussed.

Introduction

Since Zr-Excel alloy (Zr-3.5Sn-0.8Nb-0.8Mo, wt.%) was proposed as a candidate material for pressure tubes for the CANDU Supercritical Water Reactor (CANDU-SWR), due to its high strength and good creep resistance, research work on the alloy which was originally developed by AECL in 1970s has been revived. Significant studies have been carried out on the thermodynamics [1], [2], [3], mechanical properties [4], irradiation induced damage [5], [6] and delayed hydride cracking (DHC) [7] of Zr-Excel alloy.

Like Zr-2.5 wt.% Nb alloy, of which the pressure tubes of current CANDU reactors are made, Zr-Excel alloy is a dual-phase material consisting of a majority of hexagonal close packed (hcp) α-Zr phase and a fraction of thermodynamically metastable body center cubic (bcc) β-Zr phase. In the Zr-2.5wt.%Nb pressure tube material, the metastable β-Zr is enriched with β stabilizing elements Fe and Nb [8], [9]. The concentration of Nb in β-Zr phase is about 20%, which corresponds to the composition at the monotectoid point of the equilibrium Zr–Nb binary phase diagram [10]. Decomposition of β-Zr to hexagonal ω phase with space group of P6/mmm or Nb enriched equilibrium cubic β-Nb phase could take place under irradiation or thermal treatment [8], [11], [12], [13], [14], [15], [16]. However the redistribution of minority alloying elements such as Fe and Nb between the α and β phases was only observed under irradiation. It was found that Fe is depleted in the β phase after both neutron irradiation [8], [14] and heavy ion (Ar⁺) irradiation [17]. An increased concentration of Fe was observed at the α-α boundary and in the α matrix [8], [14]. The concentration of Nb is increased in both the α matrix, caused by irradiation driven alloying element redistribution, and in the β-Zr matrix as a result of decomposition of β-Zr to ω phase [14], but is decreased in β-Nb if it is present before irradiation [8].

The volume fraction of β-Zr phase in Zr-Excel as-received pressure tube is reported as 13% [3]. The concentration of Nb in the β-Zr phase is just 4%, as evaluated from energy dispersive X-ray spectroscopy (EDS) [3], which is much less than the monotectoid point of the equilibrium Zr–Nb binary phase. However, the addition of Mo which is a strong β phase stabilizer and is enriched in the β-Zr phase contributes to the retention of the β-Zr phase in Zr-Excel. No ω phase particles were reported in the β-Zr phase [3], whereas, in our present study, cuboidal ω phase particles were observed in the β-Zr phase in the Zr-Excel as-received pressure tube. An early study on the stability of ω phase particles in a Zr–Nb alloy under HVEM demonstrated dissolution of coarse ω particles and re-precipitation of small fine ω particles at 350 °C [16]. However, an in-situ heavy ion irradiation at 400 °C did not show a noticeable change of the morphology of ω particles in Zr–20Nb alloy [18]. A recent ex-situ statistical study was carried out on a heat treated Zr-Excel alloy in which as-quenched ω particles were present in the β phase, showing that irradiation assists the precipitation and growth of ω phase in the β-Zr phase [6].

To date, three open questions remain for as-received Zr-Excel pressure tube alloy. 1) What are the lattice parameters of the β-Zr phase and the ω phase? 2) Are the minor phases in Zr-Excel alloy stable under irradiation? 3) How do the alloying elements redistribute between the β-Zr phase and the ω phase after irradiation? In this paper, a high resolution synchrotron X-ray diffraction experiment was carried out on the as received Zr-Excel pressure tube material to measure the lattice parameters of the two minor phases. In-situ heavy ion irradiation was carried out on a TEM disc cut from the material to directly observe the stability of the β-Zr and ω phases during irradiation. Before and after the irradiation, STEM-EDX elemental mapping was performed to characterize elemental distribution over the two metastable phases.