NuFuel & MMSNF 2015

First Workshop on Research into Nuclear Fuel in Europe
and Materials Modeling and Simulation for Nuclear Fuels Workshop
Karlsruhe, Germany, November 16th to 18th, 2015

Updated: Tue 08 Dec 2015, 14:27

Poster 3.6: Kinematic hardening in creep of Zircaloy 2

Radan Sedlacek and Dietmar Deuble
  • AREVA GmbH, Erlangen, Germany

Abstract

Kinematic hardening is an increase in flow stress in the course of plastic deformation. In contrast to isotropic hardening, the flow stress decreases upon load reversal. The latter phenomenon is also known as the Bauschinger effect. It is well known that kinematic hardening occurs during plastic deformation of Zircaloy. There is a limited number of slip systems in the hexagonal crystallographic structure of alpha-Zirconium which results in intergranular deformation incompatibilities and buildup of back stress. The back stress opposes the deformation during prestrain, and favours it upon load reversal.

The situation is less clear for creep: here, the kinematic hardening would manifest itself by strain rate decrease followed by its increase (in magnitude) upon stress reversal. In theory, it could be possible that due to long times at high temperatures, the back stress would relax to a large extent, e.g. by some diffusional process, and the kinematic hardening in creep would be less pronounced than in plastic deformation. On the other hand, creep experiments with stress change reported in the literature indicate that isotropic hardening alone cannot explain the observed transient creep behavior. However, a systematic investigation of kinematic hardening in creep of Zircaloy is not available in the literature so far.

The present work decribes a dedicated creep experiment with stress reversal on Zircaloy 2 to investigate possible kinematic hardening. Gas filled cladding tube samples were deformed in furnace (internal pressure, resulting hoop stress of 75 MPa at 380°C) and in autoclave (outer overpressure, resulting hoop stress of –75 MPa at 380°C). The samples were periodically removed after each 7 days in furnace/autoclave to measure the outer diameter. Two of the samples were removed from the furnace and put into the autoclave after 35 days, two were moved from the autoclave to the furnace. For comparison, two other samples were continuously deformed in the furnace, and two others in the autoclave.

The creep rate measured after the stress reversal is in both cases (furnace to autoclave as well as autoclave to furnace) even higher than the primary creep rate of the fresh material, indicating a strong kinematic hardening. The experimental results will be presented in detail and an interpretation in terms of a mixed isotropic and kinematic hardening model will be proposed.