DISLOCATION CLIMB, PRECIPITATION HARDENING AND NEWTONIAN VISCOUS DEFORMATION MECHANISMS OF HIGH TEMPERATURE CREEP IN A NIOBIUM-MODIFIED ZIRCALOY

摘要

Zirconium based alloys are commonly used as thin walled tubing to clad radioactive fuel as well as intermediate and support grids in light water reactors and a thorough knowledge of the creep mechanism(s) of zirconium alloys along with their constitutive equations to define the creep rates under varying stress and temperature conditions becomes essential not only for predicting the life of the claddings but also for developing more creep-resistant microstructures. Creep behavior of a newly-developed HANA4 cladding (Zr-l.5Nb-0.4Sn-0.2Fe-0.1Cr) was investigated through internal pressurization tests by applying a range of hoop stresses from 8.4×10~(-5)E to 2.4×10~(-3)E, at 0.3-0.4 T_m (E and T_m are the elastic modulus and the melting point in Kelvin respectively) with an emphasis on the transitions in creep mechanisms. The underlying deformation mechanisms have been identified based on the creep mechanistic parameters such as stress exponent (n) and creep activation energy (Qc), and detailed TEM analyses of deformation microstructures. Similar to the previous studies on Zirlol (Zircaloy-4 with around l%Nb) we noted transitional creep mechanisms with stress exponents of 1, 3 and 5 as higher stresses are approached. While n=3 in the intermediate stress regime was earlier interpreted to be due to solute atom locking (microcreep) such as is observed in class-A alloys, presence of Nb in HANA4 was observed to form β -Nb precipitates in zirconium matrix resulting in dislocations bypassing the precipitates through climb as the rate-controlling mechanism. Further, Coble creep and general dislocation climb were attested as the dominant mechanisms at low and high stresses as described earlier.