Steady-state creep of α-zirconium at temperatures up to 850 °C

Abstract

Cumulative zirconium creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300 ЊC to 850 ЊC) were analyzed based on an extensive literature review. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional "five-power-law" regime. Thus, dislocation climb, rather than the often assumed glide mechanism, may be rate controlling. Power-law breakdown occurs at values of ss /D greater than approximately 10 9 cm Ϫ2 , consistent with most traditional five-power-law materials. The creep rate of zirconium at low values of /G varies proportionally to the applied stress. The rate-controlling mechanism(s) for creep within this regime is unclear. A grain-size dependency may exist, particularly at small (Ͻ90 m) sizes, suggesting a diffusional mechanism. A grain-size independence at larger grain sizes supports a Harper-Dorn mechanism, but the low observed activation energy (ϳ90 kJ/mol) is not consistent with those observed at similar temperatures at higher stresses in the five-power-law regime (270 kJ/mol) where creep is also believed to be lattice self-diffusion controlled. The stress dependence in this regime is not consistent with traditional grain-boundary sliding mechanisms.

Microstructural characterization of creep anisotropy at 673 K in the M5® alloy

“…This experimental approach could also be useful to investigate generalized dislocation climb, as several studies [4,31,32] mention that this mechanism could control dislocation mobility [4]. Although dislocation climb was not observed in the present study, there is still a probability for it to be masked by dislocation rearrangement during the cooling of crept samples.…”

“…As no dislocation line is partially or totally lying out of a slip plane, we can conclude that no evidence of dislocation climb is observed. However, this mechanism has been mentioned in recent studies of the mechanical behavior of Zr alloys investigating mechanical tests results [4,31] or mesoscopic observations [32]. That is why we will see in Section 5 if the hypothesis of dislocation climb can explain our experimental results in terms of geometrical considerations.…”

“…However, the consequences for http://dx.doi.org/10.1016/j.actamat.2012.04.001 creep behavior remain unclear. Moreover, developments of analytical descriptions of creep data, through computation using stress exponents [4] or Transmission electron microscopy (TEM) observations [5], suggest that dislocation climb could replace dislocation glide as a rate-controlling process at intermediate temperatures (673-1123 K). Thus, characterization of creep anisotropy needs new experimental data obtained at different length scales.…”

Identification of the steady-state creep behavior of Zircaloy-4 claddings under simulated Loss-Of-Coolant Accident conditions based on a coupled experimental/numerical approach

“…The literature associated to these temperature and stress ranges reports diffusion creep mechanisms in these conditions. Most of the constitutive laws for diffusion creep in the literature [33,34] …”

Coble, Orowan Strengthening, and Dislocation Climb Mechanisms in a Nb-Modified Zircaloy Cladding

“…Based on these observations, it can be argued that Zr-Nb alloys behave like a class-A alloy during creep at intermediate stresses, whereas they act as a class-M alloy at high stresses similar to a-zirconium and its alloys with tin as major alloying element such as Zircaloy-2 and Zircaloy-4. [21,22] Though there exists a consistency in the mechanistic creep parameters like stress exponent and activation energy at a range of stresses in several Zr-Nb alloys, more detailed analyses of the deformation microstructure are warranted to unambiguously detect the ratecontrolling mechanisms. With this in mind, the creep behavior of HANA-4 (Zr-1.5 pctNb alloy) cladding, which is a potential candidate for nuclear fuel claddings, has been investigated with a focus on identifying the transitions in creep mechanisms along with emphasis on deformation microstructures.…”