Final report on in-reactor creep-fatigue deformation behaviour of a CuCrZr alloy: COFAT 1

At present, practically nothing is known about the deformation
behaviour of materials subjected simultaneously to external cyclic
force and neutron irradiation. The main objective of the present
work is to determine experimentally the mechanical response and
resulting microstructural changes in CuCrZr(HT1) alloy exposed
concurrently to flux of neutrons and creep-fatigue cyclic loading
directly in a fission reactor. Special experimental facilities were
designed and fabricated for this purpose. A number of in-reactor
creep-fatigue experiments were successfully carried out in the BR-2
reactor at Mol (Belgium). In the present report we first describe the
experimental facilities and the details of the in-reactor creep-fatigue
experiments carried out at 363 and 343K at a strain amplitude of
0.5% with holdtimes of 10 and 100s, respectively. For comparison
purposes, similar creep-fatigue tests were performed outside of the
reactor. (i.e. in the absence of neutron irradiation).

During in-reactor tests, the mechanical response was continuously
registered throughout the whole test. The results are first presented
in the form of hysteresis loops confirming that the nature of
deformation during these tests was truly cyclic. The temporal
evolution of the stress response in the specimens is presented in the
form of the average maximum stress amplitude as a function of the
number of cycles as well as a function of displacement dose
accumulated during the tests. The results illustrate the nature and
magnitude of cyclic hardening as well as softening as a function of
the number of cycles and displacement dose. Details of the
microstructure were investigated using TEM and STEM techniques.
The fracture surface morphology was investigated using SEM
technique. Both mechanical and microstructural results are briefly
discussed. The main conclusion emerging from the limited amount
of present results is that neither the irradiation nor the duration of
the holdtime have any significant effect on the lifetime (in terms of
number of cycle to failure) of the material.