Dislocation cross-slip controlled creep in Zircaloy-4 at high stresses

Abstract

a b s t r a c tUniaxial creep tests were performed on Zircaloy-4 sheet in the temperature range of 500-600 1C at high stresses ( 410 À 3 E), to uncover the rate-controlling mechanism. A stress exponent of 9.3-11 and a stressdependent activation energy in the range of 220-242 kJ/mol were obtained from the steady state creep data. TEM analyses revealed an extensive presence of hexagonal screw dislocation network on the basal planes indicating recovery of screw dislocations by cross-slip to be the dominant mechanism. The creep data was therefore analyzed in the light of Friedel's cross slip model for HCP metals according to which the stress-dependency of the activation energy determined from the creep data was written in the form,The constriction energy of screw dislocations of 150 kJ/mol is in agreement with the values reported in the literature for zirconium and other HCP metals. Further analysis of the yield strength and the activation volume data obtained from stress relaxation tests in the temperature range 500-600 1C favors cross-slip of screw dislocations as the rate controlling mechanism over the test conditions.

NEAMS Burnup Extension Accomplishments and Remaining Modeling Gaps

“…experimental and simulated minimum creep rate as a function of stress (between 5 and 150 Mpa and temperature between 500 and 600°C) [56,57], (c), irradiation growth strain in the principal direction vs. dose, and (d) simulated strain rate vs. time curves for multiple creep stress obtained by varying solid solution H content, from 60 to 180 ppm. ...........21 Figure 22.…”

Modeling irradiation creep of graphite using rate theory

“…The creep strain behavior then can be rationalized from the unit processes that closely correlate to the energy barriers. Such an approach is extensively used for describing thermal creep in materials, especially for metals [19][20][21].…”

“…At higher stresses, the stress exponent changes to 3, approximately, followed by an even higher stress exponent. A stress exponent of 3 typically indicates a deformation mechanism through dislocation glide in metals; higher values of stress exponent indicate a crossover to a dislocation climb and cross-slip mechanisms [20].…”

Mechanism-based modeling of solute strengthening: Application to thermal creep in Zr alloy

“…While the jog density is very low at higher temperatures, fully formed subgrain boundaries have been detected by Murty (2015a, 2015b), consisting of pure edge dislocations and the special 'honeycomb' wall structure. The 'honeycomb' boundary, also recognized as Frank network (Hayes et al, 2002;Poirier, 1976), is reported in Kombaiah and Murty (2015a) as a 2-D structure on the basal plane. According to Murty (2015a, 2015b), its formation starts with the interaction between the screw dislocations on different prismatic planes, followed by cross-slip.…”