Inconel 718 Alloy

Alloy name: Inconel 718
Diagram No.: 872
Type of diagram: TTT
Chemical composition in weight %: 0.03% C, 18.04% Fe, 18.5% Cr, 53.21% Ni, 5.3% Mo, 1.00% Ti, 0.52% Al, 0.06% Si, 0.13% Co
Alloy group: Nickel-based alloys
Note: TTT diagram for Inconel 718 [1].
Study of published TTT diagrams of for Inconel 718 shows, that no significant microstructural changes should take place at exposure to 700 C for such a short time (7.5h). Based on this fact, observed changes must be caused by superposition of other factors, as only a temperature exposure. To understand these changes it is necessary to understand mechanisms of plastic deformation and the interaction of precipitates with the dislocations in the precipitation hardenable materials, like the nickel base superalloy Inconel 718. During cyclic plastic deformation in Inconel 718, in the first stage increase the dislocation density, which causes a slight increase of resonant frequency of the tested specimen. Dislocations are pinning on the precipitates, where the increase of dislocation density causes the increase of the stress required for the further formation and movement of the dislocations. When dislocation interacts with the precipitate, there are known two basic phenomena, depending on the size of the precipitates. The first case, when precipitates are small enough, the dislocation can shear them and the second case, typical for larger precipitates, a so called Orowan looping takes place (HULL B. 2005, PINEAU A. 2009, SIMS S. H. 1987). In Inconel 718, the particles of Gamma'' has a disc shape, with the diameter of 30-60 nm and thickness 5-9 nm, so shearing is the preferred interaction of dislocation with the precipitates Gamma''. Gamma double prime is an ordered precipitate with the D022 crystal structure and for its shearing are necessary super dislocations with the double Burger's vector, than a regular dislocation in FCC matrix (REED C. 2006). Superdislocations are high energy configuration state, so their formation begins in the second stage of cyclic plastic deformation. At the beginning of the second stage, movement of the free dislocations is exhausted, so any other plastic deformation can be accommodated only by the shearing of the precipitates by super dislocations, whose formation begins as a consequence of increasing critical stress for dislocation movement. This process will show up in the decrease of resonant frequency of the tested specimen. During shearing precipitates by super dislocations, some phenomena can occur (mechanism of shearing of the ordered precipitates is complicated process, due to decrease of energetic demands, which typically splits into to a super partial dislocations etc. This mechanism is described in the work in detail (HULL B. 2005). At first, shearing of Gamma''. can result in creation so small particles of Gamma'', so they are not thermodynamically stable, and they dissolve in the matrix, which result in an increase of local concentration of Nb in the matrix. The second very important phenomenon is the creation of stacking fault, between leading and trailing super partial dislocations. In the area of stacking fault, a small area with the similar atomic configuration as in the Delta phase in certain atomic planes is formed, from which, due to relatively high temperature and the fact, that a part of Nb content is dissolved in the matrix as a residue from dissolved Gamma''. particles, particles of Delta phases can nucleate and grow. These claims are supported by observation of a manner, how the Delta phase precipitates, where from the Fig. it can be concluded that the Delta phase precipitates at some certain crystallographic planes, and these planes are considered to be {111} planes, which are the primary slip planes in the FCC metals.
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