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

Overview & Synopsis

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 related to the existence of strain ageing phenomenon which results from the combined action of thermally activated diffusion of foreign atoms to and along dislocation cores and the long range of dislocations interactions. The important role of interstitial and substitutional atoms in zirconium alloys, responsible for strain ageing and the lack of information about the domain where strain ageing is active have not been yet adequately characterized because of the multiplicity of alloying elements and chemical impurities. The aim of this work is to characterize experimentally the range of temperatures and strain rates where strain ageing is active on the macroscopic and mesoscopic scales. We propose also a predictive approach of the strain ageing effects, using the macroscopic strain ageing model 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 atom was added to the solid solution under the form of 1 wt% niobium. Some zirconium alloys were doped with oxygen, others were not. All of them were characterized by various mechanical tests (standard tensile tests, tensile tests with strain rate changes, relaxation tests with unloading). The experimental results were compared with those for the standard oxygen doped zirconium alloy (1300 wt ppm) studied by Pujol (Pujol, 1994) and called Zr702. The following experimental evidences of the age–hardening phenomena were collected and then modeled: • 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 plastic strain rates, the macroscopic response is associated with the dragging mode (higher temperatures) and at higher plastic strain rates, the macroscopic response is associated with the friction mode (lower temperatures). Between these two limiting modes, the behavior is unstable. For Zr702, the change in the deformation mechanism was observed between 200°C and 400°C. The apparent activation volumes associated with friction and dragging modes are almost the same for Zr702, close to 0.7 nm3.atom−1. By reconstruction of the entire 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 apparent activation volumes are close to 2 nm3.atom−1 for the friction mode and 1 nm3.atom−1 for the dragging mode. For this alloy which contains only about 80 wt ppm of oxygen, the ”drag stress” was estimated at about 130 MPa. These relaxation tests provided also evidence that strong internal stresses develop in the tested specimens for both alloys. The macroscopic strain ageing model was implemented in a finite element code. An internal variable, characterizing a global ageing time of the material and associated with a non– linearity of the constitutive equations allows to simulate plastic strain (rate) localization under the form of bands extending across the width of the sample. The material parameters were identified for Zr702. A reliable prediction of the strain ageing phenomena observed experimentally can be ensured with this model. The development of strain heterogeneous fields observed by laser scanning extensometry may be also predicted by the model.