Abstract
► A constant stress exponent does not signify that creep is driven by applied stress. ► Microstructure has provided insight into the controlling deformation mechanism. ► A threshold stress given by
σ
o
/
G
=
B
o
exp(
Q
o
/
RT) explains the high apparent creep activation energy.
Creep characteristics of nano-particle strengthened AA6082 (Al–Mg–Si) alloy via precipitation hardening were investigated over five orders of magnitude of strain rates. The experimental data showed that the apparent creep behavior of AA6082 differed from that of pure Al with regard to two primary features: the apparent stress exponent,
n
a
, is about 7 and the apparent activation energy for creep (206
kJ/mol) is higher than that for self-diffusion (142
kJ/mol). An examination of the data revealed a threshold stress whose temperature dependence is much stronger than that attributable to the shear modulus. Evidence based on transmission electron microscopy (TEM) investigation indicated that the origin of the threshold stresses was related to the interaction of dislocations with second phase particles, which were identified as Mg
2Si. By incorporating the threshold stress into creep analysis, it was demonstrated that the true creep behavior of AA6068 was consistent with that ascribable to high-temperature climb and that the alloy was more creep resistant than Al.