Viscoplastic behavior of zirconium alloys in the temperatures range 20°C-400°C : characterization and modeling of strain ageing phenomena

摘要

The anomalous strain rate sensitivity of zirconium alloys over the temperatures range 20C–600° has been widely reported in the literature. This unconventional behavior is relatedto the existence of strain ageing phenomenon which results from the combined action ofthermally activated diffusion of foreign atoms to and along dislocation cores and the longrange of dislocations interactions. The important role of interstitial and substitutional atomsin zirconium alloys, responsible for strain ageing and the lack of information about the domainwhere strain ageing is active have not been yet adequately characterized because of themultiplicity of alloying elements and chemical impurities.The aim of this work is to characterize experimentally the range of temperatures and strainrates where strain ageing is active on the macroscopic and mesoscopic scales. We proposealso a predictive approach of the strain ageing effects, using the macroscopic strain ageingmodel suggested by McCormick (McCormick, 1988; Zhang et al., 2000).Specific zirconium alloys were elaborated starting from a crystal bar of zirconium with 2.2 wt%hafnium and very low oxygen content (80 wt ppm), called ZrHf. Another substitutional atomwas added to the solid solution under the form of 1 wt% niobium. Some zirconium alloys weredoped with oxygen, others were not. All of them were characterized by various mechanicaltests (standard tensile tests, tensile tests with strain rate changes, relaxation tests withunloading). The experimental results were compared with those for the standard oxygendoped zirconium alloy (1300 wt ppm) studied by Pujol (Pujol, 1994) and called Zr702. Thefollowing experimental evidences of the age–hardening phenomena were collected and thenmodeled:• low and/or negative strain rate sensitivity around 200°C–300°C,• creep arrest at 200°C,• relaxation arrest at 200°C and 300°C,• plastic strain heterogeneities observed in laser extensometry on the millimeter scale.Relaxation experiments give information about deformation mechanisms. At lower plasticstrain rates, the macroscopic response is associated with the dragging mode (highertemperatures) and at higher plastic strain rates, the macroscopic response is associated withthe friction mode (lower temperatures). Between these two limiting modes, the behavioris unstable. For Zr702, the change in the deformation mechanism was observed between200°C and 400°C. The apparent activation volumes associated with friction and draggingmodes are almost the same for Zr702, close to 0.7 nm3.atom−1. By reconstruction of theentire relaxation curve at the temperature peak of 300C for strain ageing, an estimated”drag stress” of about 250 MPa was determined for Zr702 (1300 wt ppm oxygen). For ZrHf,the dragging mechanism was observed for lower temperatures close to 300°C. The apparentactivation volumes are close to 2 nm3.atom−1 for the friction mode and 1 nm3.atom−1 forthe dragging mode. For this alloy which contains only about 80 wt ppm of oxygen, the ”dragstress” was estimated at about 130 MPa. These relaxation tests provided also evidence thatstrong internal stresses develop in the tested specimens for both alloys.The macroscopic strain ageing model was implemented in a finite element code. An internalvariable, characterizing a global ageing time of the material and associated with a non–linearity of the constitutive equations allows to simulate plastic strain (rate) localizationunder the form of bands extending across the width of the sample. The material parameterswere identified for Zr702. A reliable prediction of the strain ageing phenomena observedexperimentally can be ensured with this model. The development of strain heterogeneousfields observed by laser scanning extensometry may be also predicted by the model.