Study of the iodine induced stress corrosion cracking mechanisms of zircaloy-4. Strain hardening effects

The iodine induced stress corrosion cracks of zirconium alloys initiate in an intergranular way, then the propagation becomes transgranular and leads to the rupture of the metal by plastic instability. In another way, the microstructural defects induce by the irradiation of the metal by neutrons (dislocation loop), lead to an important modification of the mechanical behaviour of the zircaloy-4, which has also bad consequences on the stress corrosion cracking resistance of this alloy, in favoring particularly the transgranular propagation of the cracks. In order to progress in the identification of the parameters leading to the transgranular propagation of the SCC cracks, the effect of a strain hardening by deformation on the propagation modes of the cracks is studied. The fractographic analysis of pre-strain hardened or not traction test specimens, under imposed charge in presence of iodine methanol until rupture, shows that the strain hardening favors the transgranular cracking. This last one is then accompanied by a significant acoustic emission. A parallel is established with the behaviour in SCC of the irradiated material. (O.M.)