Selective Growth of Low Stored Energy Grains During δ Sub-solvus Annealing in the Inconel 718 Nickel-Based Superalloy

Abstract

The microstructure stability during d sub-solvus annealing in Inconel 718 was investigated, focusing on the conditions that may lead to the development of very large grains (about 100 lm) in a recrystallized fine grained matrix (4 to 5 lm) despite the presence of second-phase particles. Microstructure evolution was analyzed by EBSD (grain size, intragranular misorientation) and SEM (d phase particles). Results confirm that, in the absence of stored energy, the grain structure is controlled by the d phase particles, as predicted by the Smith-Zener equation. If the initial microstructure is strained (e < 0.1) before annealing, then low stored energy grains grow to a large extent, despite the Zener pinning forces exerted by the second-phase particles on the grain boundaries. Those selectively growing grains could be those of the initial microstructure that were the least deformed, or they could result from a nucleation process. The balance of three forces acting on boundary migration controls the growth process: if the sum of capillarity and stored energy driving forces exceeds the Zener pinning force, then selective grain growth occurs. Such phenomenon could be simulated, using a level set approach in a finite element context, by taking into account the three forces acting on boundary migration and by considering a realistic strain energy distribution (estimated from EBSD measurements).

Grain boundary precipitation in Inconel 718 and ATI 718Plus

“…This is done by a mechanism known as Zener pinning [68,69]. However, it is established within the field that low quantities of δ and η precipitation are sufficient for grain boundary control and that excessive amounts are actually detrimental to the mechanical properties of alloy.…”

An Investigation on the Softening Mechanism of 5754 Aluminum Alloy during Multistage Hot Deformation

“…The flow behaviors are very complex during hot deformation, and the parameters of hot deformation, such as temperature and strain rate, affect the flow stress significantly. Therefore, the control of deformation parameters is critical in optimizing the final mechanical properties [16][17][18][19][20]. Figure 4 indicates the true representative stress-stain curves of the studied alloy under various deformation conditions.…”

Misorientation-Dependent Twinning Induced Hardening and Texture Evolution of TWIP Steel Sheet in Plastic Deformation Process

“…It is also well-known that the stored energy depends on crystallographic orientation and plays a significant rule in the formation of texture [39]. Thus, the intragranular misorientation corresponds to the stored energy indirectly.…”

“…The detailed discuss is given. The intragranular misorientation reflects strain generated during deformation with dislocation glide, particularly related to the density of geometrically necessary dislocations [38,39]. With the increasing strain from 0.1 to 0.4, the dislocation accumulation would occur at the grain boundaries via dislocation gliding, which generates local stress and leads to inhomogeneous deformation at the grain boundaries.…”