Fluage de molybdène sous irradiation par les fragments de fission a 20 K

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Abstract
CREEP OF MOLYBDENUM IRRADIATED WITH FISSON FRAGMENTS AT LOW TEMPERATURE Length and resistivity changes of uniaxially stressed molybdenum samples have been recorded during irradiations by fission fragments, in the temperature range 20–30 K. Eleven deformation versus dose curves are shown. They were obtained with cold-rolled and annealed molybdenum ribbons irradiated under fixed or variable stress up to fluences of about 1.5 d.p.a. At low temperature there is no steady-state creep prior to 1.2 d.p.a. Because of a long primary creep period, one can follow the build up of the dislocation net- work from the first displacement cascades to its saturation. The primary creep strain is neither linear with stress nor with fluence. Stress oriented nucleation of the dislocation loops is the predominant primary creep mechanism. The analysis of the creep rate data corresponding to different fluences and different stresses suggests the following picture : every fission fragment produces approximatively 30 000 displacements; 20 000 of these Frenkel pairs escape to point defect recombinations. The nucleation of the dislocation loops initiates the build up of the dislocation network. It begins in the first displacement cascades and ends at fluences of a few d.p.a., when the dislocation network is dense enough to capture practically all defects produced in the displacement cascades. Up to a few 10−2 d.p.a., this network is highly anisotropic, because it essentially consists in isolated loops which were nucleated under stress. Then, it can produce high creep rates of about (d.p.a.)-I at 300 MPa. Dislocation recombinations slowly destroy this anisotropy; at 1.5 d.p.a. loops have com- pletely disappeared and collapsed into a dense isotropic network. This results in a much lower creep rate, whose order of magnitude corresponds to those predicted by the S.I.P.A.model (5 × 10−2 (d.p.a.)−I at 300 MPa). As to the experiments under variable stress, the creep rates after a sudden change in applied stress can be understood only if glide plays a predominant role in the reorganization of the dislocation network. At low temperature this glide can take place only in the spikes produced in every displacement cascade. Samples shows that primary creep is very sensitive to internal stresses produced by cold-working prior to irradiation.

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