Measurement and Modeling of Second Phase Precipitation Kinetics in Zirconium Niobium Alloys

Abstract

Commercial zirconium alloys contain second phase particles (SPPs) that are precipitated during processing. These particles not only influence mechanical properties but more crucially also have a profound influence on the corrosion performance. To understand how to control evolution and size distribution of SPPs, it is necessary to know how alloy composition and process variables influence the precipitation kinetics. In this work, a detailed study has been performed of the precipitation kinetics in binary Zr-1 wt % Nb, Zr-2.6 wt % Nb, and ternary variants with added iron and tin. A numerical model has also been developed to predict the precipitation kinetics of β-Nb in niobium containing zirconium alloys. Precipitation has been tracked by synchrotron X-ray diffraction measurement of lattice parameter change in the zirconium matrix as solute is removed into SPPs. The X-ray data has been complemented by thermoelectric power measurements. The combination of these two approaches is shown to be effective in quantifying the overall precipitation kinetics of SPPs. The results confirm previous observations that without prior deformation, precipitation kinetics is very sluggish in the binary Zr–Nb system. Deformation accelerates precipitation, and this effect is much stronger for the 1 wt % Nb alloy than for 2.6 wt % Nb, because the supersaturation is least. Ternary additions also have a profound effect on the overall precipitation kinetics. Iron accelerates the rate of niobium loss from solution, whereas tin additions appear to increase the incubation time for the onset of precipitation of niobium.

The characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fe alloys: A critical review

“…In a fundamental study on the precipitation kinetics of the Zr-Nb system, parallel X-ray diffraction and thermoelectric power measurements were used to demonstrate the slow nucleation of secondary phases in such alloys, made even more sluggish by the addition of substitutional element Sn and faster by adding Fe or deformation prior to annealing [11]. This precipitation of the β-Nb phase and the ternary ZrNbFe phases in bulk α-Zr is primarily due to the low terminal solid solubility (TSS) of Nb in the α phase.…”

Coble, Orowan Strengthening, and Dislocation Climb Mechanisms in a Nb-Modified Zircaloy Cladding

“…In the case of precipitate shearing, the shear stress required to break away from the precipitates can be calculated from the particle strength which may originate from various hardening mechanisms such as misfit strengthening, chemical strengthening, modulus hardening, and order hardening. [25] Since the b-Nb precipitates in HANA-4 have no ordered structure as noticed from the diffraction pattern, only the strengthening owing to the other mechanisms can be considered [65] a p = 3.28 [62] Shear Modulus at 723 K (450°C) (GPa) l = 25 [31] l p = 30 [66] for calculating the shearing stress of the precipitates. The strengthening ðr m Þ contributed by the misfit (e) between matrix and precipitate can be determined using the relations, [60] …”

“…The interfacial energy for the Zr matrix and b-Nb precipitates was obtained from the literature to be 0:19 J/m 2 , [62,63] which is assumed for the newly formed interface due to particle sharing in HANA-4 as well. Thus, the stress required to shear the b-Nb precipitates in HANA-4 was calculated to be 308 MPa.…”

From solid solution to cluster formation of Fe and Cr in α -Zr

“…In previous papers, the authors considered the formation of SPPs [6,47] and their interaction with H [26,48]. Here, the authors are concerned with the conditions relevant to irradiated Zr alloys, in which the SPPs are partially dissolved.…”