Local Order and Associated Deformation Mechanisms in the γ Phase of Nickel Base Superalloys

Abstract

Abstract. 2014 Using X-ray diffraction, atom probe nanoanalysis, TE.M. associated with in situ deformation, the presence of local order in the 03B3 matrix of a monocrystalline MC2 superalloy is demonstrated. In Ni2CoCr the model alloy of this phase, as well as in the 03B3 MC2 matrix, the existence at room temperature of a strong short range order of {11/2 0} special point type is emphasized. It induces high friction stresses opposing the movement of dislocations which have to propagate in planar arrays in the corresponding materials. In addition, long range ordered L12 hyperfine precipitates appear in the 03B3 MC1 samples annealed at 750 °C. They induce several pairs of dislocations at the head of the moving pile-ups in order to cut them.

Quantitative measurements in in situ straining experiments in transmission electron microscopy

“…11) on γ‐phases confirm the presence of dislocation pile‐ups which are heterogeneously distributed in the foil. The pile‐up spearhead consists of paired dislocations which appear as a signature of the presence of local order (Clément et al ., 1996). The two first dislocations are always observed to move together as though they were strongly attracted, so that the distance between them remains approximately equal to 30 nm.…”

Pile-ups in thin foils: application to transmission electron microscopy analysis of short-range-order

“…In the g phase of concentrated Ni-based alloys, it is now well established that this behaviour is a consequence of the existence of short-range order (SRO) of ð1 1 2 0Þ type (Cle´ment et al 1996, Pettinari et al 2001, Prem et al 2002. Indeed the glide of a dislocation D 0 on a given plane G of a short-range-ordered alloy destroys, at least partially, the SRO, which needs an energy 0 per unit surface swept by the dislocation and corresponds to a frictional force 0 per unit length opposing the Philosophical Magazine…”

“…Introduction In-situ and post mortem transmission electron microscopy (TEM) observations have demonstrated that deformation may proceed by the motion of planar arrays of dislocations, resulting in slip bands or pile-ups of dislocations (Cle´ment 1984, Olfe and Neuha¨user 1988, Gerold and Karnthaler 1989. This is the case when the movement of dislocations in a given plane is enhanced by the movement in the same plane of previous dislocations.In the g phase of concentrated Ni-based alloys, it is now well established that this behaviour is a consequence of the existence of short-range order (SRO) of ð1 1 2 0Þ type (Cle´ment et al 1996, Pettinari et al 2001, Prem et al 2002. Indeed the glide of a dislocation D 0 on a given plane G of a short-range-ordered alloy destroys, at least partially, the SRO, which needs an energy 0 per unit surface swept by the dislocation and corresponds to a frictional force 0 per unit length opposing the Philosophical Magazine…”

Local Order and Mechanical Properties of the γ Matrix of Nickel-Base Superalloys

“…Here we have to deal with the question of order vs. disorder complicated by the fact that in the matrix of complex industrial alloys, not only short range order, but also long range order (for example, DO22 as well as L12 type) may be present. For Cr (20 -30 at %) rich Ni-Cr alloys, neutron scattering, deformation experiments and in situ electron microscopy indicate the presence of short range order, of the type Ni2Cr [ 4,6 ] or NisCr [3,5,7,8, 91. Short range order of the Ni-Cr pairs has also been found by X-ray scattering in NizCoCr, and the same was confirmed to exist in the MC2 matrix single crystals used in the present study [3, lo].…”