High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system

Abstract

High-entropy alloys (HEAs) with high melting points and low thermal neutron cross-section are promising new cladding materials for generation III+ and IV power reactors. In this study a recently developed high throughput computational screening tool Alloy Search and Predict (ASAP) has been used to identify the most likely candidate single-phase HEAs with low thermal neutron cross-section, from over a million four-element equimolar combinations. The selected NbTiVZr HEA was further studied by density functional theory (DFT) for moduli and lattice parameter, and by CALPHAD to predict phase formation with temperature. HEAs of NbTiVZrx (x = 0.5, 1, 2) were produced experimentally, with Zr varied as the dominant cross-section modifier. Contrary to previous experimental work, these HEAs were demonstrated to constitute a single-phase HEA system; a result obtained using a faster cooling rate following annealing at 1200°C. However, the beta (BCC) matrix decomposed following aging at 700°C, into a combination of nano-scale beta, alpha (HCP) and C15 Laves phases.

Data on the annealing of NbTiVZr at 1200 °C with slow cooling rate

“…Additional peaks were suggestive of hexagonal closed packed (hcp) alpha as well as secondary bcc phases, these were studied in further detail following alternative heat treatment in the associated work by King et al. [1] .…”

“…X-ray diffraction (XRD), and scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) were used to characterize the HEA composition NbTiVZr (chosen for investigation due to its relatively low thermal neutron absorption cross-section and potential use as a nuclear fuel cladding [1] ). The bulk sample was annealed at 1200 °C for 24 hours followed by cooling at a rate of ∼2 °C/min.…”

“…This data provides the scientific community with the knowledge that a single-phase is not achievable at ∼2 °C/min for the NbTiVZr HEA. In combination with similar work on this system [1] , [2] , this data can be used to determined what the threshold cooling rate is required to achieve a single-phase. This data is also qualitatively useful for diffusivity research as it shows that the sluggish diffusion, often claimed a HEA property, does not overpower the thermodynamically-driven chemical segregation of NbTiVZr at low temperatures.…”

“…In combination with similar work on this system [1] , [2] , this data can be used to determined what the threshold cooling rate is required to achieve a single-phase.…”

Short communication: ‘Low activation, refractory, high entropy alloys for nuclear applications’

“…Although the majority of irradiation damage studies have been targeted at FCC HEAs based on transition metals (CoCrFeMnNi), refractory HEAs that offer a low thermal neutron absorption cross-section i.e. TiVZrNb [28] and TiVCrNbMo [29], have been proposed for nuclear applications. However, as pointed out in [19], the suitability of these alloys for use in the next generation of advanced nuclear reactors is somewhat restricted by the high activation elements Nb and Mo.…”