SEDL / STP / STP1354-EB / STP14313S

In-Process Investigation of Precipitate Growth in Zirconium Alloys

Herb, B
Albany, Oregon

Ruhmann, H
Siemens AG, Power Generation Group (KWU), Erlangen,

König, A
Siemens AG, Power Generation Group (KWU), Erlangen,

Pages: 23    Published: Jan 2000

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Abstract

The nucleation and growth of second-phase precipitates was investigated in ELS (Extra Low Sn-Sn below, Fe and Cr above ASTM specification) and Zircaloy-4 alloys used for DUPLEX cladding. DUPLEX cladding is typically used for the most demanding irradiation conditions in reactors with high power densities, coolant temperatures, and discharge burnups. Extensive in-process sampling was performed after the β-quenching, hot working, and α-annealing steps during manufacturing. SEM and TEM evaluations were performed to characterize the precipitate distribution and composition.

The β-quench step produces a significant number of fine precipitates along the grain boundaries during the β → α+β phase transformation. Intergrain nucleation of irregular-shaped precipitates was also observed. The subsequent hot-working steps in the α-phase tend to increase the uniformity and size of the precipitates. Zircaloy-4 materials processed with a forge-Quench-Forge (FQF) process generally have a log-normal precipitate distribution during the early processing steps. However, the log-normal distribution transforms into a bimodal distribution after α-annealing. The low-Sn ELS alloy shows broader distributions very early in the process with two or even three Gaussian peaks needed to describe the distribution curve. TEM evaluations suggest that the small particles in the bimodal distribution are a Zr-Si-Fe phase with very slow growth kinetics. The larger particles are the typical Laves phase Zr(Fe,Cr)² with significantly faster growth rates. Separation of these two precipitate classes allows for a more accurate correlation to the Particle Growth Parameter (PGP) annealing parameter concept.

The precipitate distributions from FQF processing were also compared to a similar Zircaloy-4 alloy fabricated with a Forge-Forge-Quench (FFQ) process. This material was β-quenched and then extruded directly without α-phase forging.