Dislocation mechanisms in a zirconium alloy in the high-temperature regime: An in situ TEM investigation

Abstract

Dislocation mechanisms responsible for the high-temperature mechanical properties of a Zr alloy have been investigated using in situ straining experiments between 250°C and 450°C. At 250°C and 300°C, the results show a steady and homogeneous dislocation motion in prismatic planes, with little cross-slip in the pyramidal and/or basal planes. At 350°C, the kinetics of mobile dislocations becomes very jerky and inhomogeneous, in agreement with a dynamic strain aging mechanism. Above this temperature, the motion is again steady and homogeneous. Extensive crossslip forms super-jogs which are efficient pinning points against the glide motion. These super-jogs move by glide along the Burgers vector direction, never by climb. The glide velocity between super-jogs is linear as a function of the total driving stress (applied stress minus line-tension stress due to dislocation curvature), in agreement with the solute dragging mechanism. The origin of the stress-strain rate dependence with an exponent larger than unity is then discussed.

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

“…The disappearance of the jogs at high temperature has also been reported elsewhere (Ecob and Donaldson, 1985). Moreover, the jogged-screw theory is challenged by a recent in-situ TEM study (Caillard et al, 2015) done at similar temperature (250 o C-450 o C) as Morrow et al (2013), where the climb of jogs is not observed.…”

“…It has been demonstrated that it increases almost linearly with the dislocation velocity, then reaches a maximum value after a short transient process, and starts to decrease afterward. The stresses acting on a moving dislocation include the applied resolved shear stress, lattice friction, solute dragging stress ( dislocations move at low velocity (of order 10 nm/s according to Caillard, 2015), which leads to a higher travel time mentioned in Eq. 7 and thus lower creep rate.…”

“…Pyramidal <c+a> slip can also be activated but with critical stress 10 to 20 times higher than the prismatic slip. Besides, pyramidal <a> is also considered in the simulations since cross-slip of screw segment from prismatic plane to pyramidal <a> and basal planes has been reported for Zr alloys (Caillard et al, 2015;Onimus et al, 2013).   1012 tensile twinning and   1122 compressive twinning are excluded from this work as their activations are suppressed at elevated temperatures (Beyerlein and Tomé , 2008;Kanitpanyacharoen et al, 2012).…”

“…In the Zr alloy used for comparison, the initial dislocation density is reported to be low (of order 10 12 m -2 ), and most of the dislocations are on the prismatic slip plane (Caillard et al, 2015;Murty, 2015a, 2015b;Moon et al, 2006;Morrow et al, 2016Morrow et al, , 2013. To comply with this observation, the initial dislocation density is set to be 2.4• 10 11 m -2 for each prismatic slip system and 0.14• 10 11 m -2 for each basal, pyramidal <a> and pyramidal <c+a> system.…”

“…This dragging stress is a function of solute atoms bulk diffusivity and depends linearly on the dislocation speed. According to Caillard et al (2015), the velocity of free traveling dislocations is of order 10nm/s, while the dragging resistance is under 50MPa. However, it has been reported that high stresses will build up at a dislocation pinned for a long period at obstacles.…”