N. Clément

On the shearing mechanism of γ′ precipitates by a single ( a /6)⟨112⟩ Shockley partial in Ni-based superalloys

The weak-beam technique of transmission electron microscopy has been used to analyse a new shearing configuration of γ′ precipitates after creep at 700°C of a Ni-based superalloy for gas turbine discs. The shearing configurations are made up of superlattice extrinsic stacking faults, matrix stacking faults and individual (a/6)⟨112⟩ Shockley dislocations. This mechanism is initiated by the decorrelated movement of the two Shockley partials of a single (a/2)⟨110⟩ matrix dislocation. The propagation of the leading partial creates this shearing process. This phenomenon that occurs in small γ channels owing to the flexibility of dislocations can be used to evaluate microstructural evolutions during ageing in the alloy.

An in situ study of cube glide in the γ′-phase of a superalloy: I. The controlling mechanism

Abstract An in situ study of the plastic deformation of single crystals of the γ′-phase of a CMSX2 superalloy (L12 structure) has been performed between 120 and 1150 K, in order to analyse the role and the mechanisms of glide of superdislocations in {001} cube planes. Extensive cube glide and dislocation sources are observed from very low-temperatures (140 K) to high-temperatures (1150 K), for a tensile axis close to ⟨110⟩. Glide on cube planes is controlled by a locking—unlocking mechanism acting on screw dislocations, in the whole temperature range. This mechanism is described in detail and compared with that presented earlier for prismatic glide in beryllium. It corresponds to a series of cross-slip events, on super partial dislocations which are alternately extended in one intersecting octahedral plane, and in the cube plane. On the other hand, non-screw dislocations interact with small clusters or solute atoms. The explanation for the activation of cube glide at very low-temperatures is given in part...

Stacking fault energy in short-range ordered γ-phases of Ni-based superalloys

Several aspects on the stacking fault energy (SFE) in short-range ordered -phase model alloys of new nickel based superalloys are discussed. Using transmission electron microscope weak beam observations and computer simulated images of dissociation widths, the SFE was determined as a function of temperature. The values for both alloys are close (31 4m J m −2 for the Re-containing alloy and 28 6m J m − 2 for the Ru-containing alloy). In both cases, the dissociation widths remain quite constant up to 350 °C and a slight decrease is observed at higher temperature. The deformation micromechanisms and the constancy of the SFE up to 750 °C are analysed in connection with short-range order. The deformation at low temperature is characterised by dislocation pile ups, the influence on the dissociation widths of the internal stress due to such configuration is analysed. The calculation reveals a strong effect on the leading dislocation of the pile-up whose dissociation distance can be reduced at most by 50%.

Decorrelated movements of Shockley partial dislocations in the γ-phase channels of nickel-based superalloys at intermediate temperature

In nickel-based superalloys with high volume fraction of γ′ precipitates, dislocations have to experience high curvatures in order to enter narrow channels by glide in the {111} planes of the fcc γ matrix. Observations of in situ dynamic sequences performed in a transmission electron microscope on several industrial superalloys have shown the occurrence of decorrelated movements of Shockley partial dislocations, originating from perfect dislocation dissociation. By evaluating the effective stress acting on each one of these partial dislocations, as well as their respective flexibility, the possible occurrence of such movements for some particular dislocation characters and channel widths is accounted for. These movements can play an important role in the creep behaviour of these materials in the low deformation rate regime.

Stacking‐fault energy at room temperature of the γ matrix of the MC2 Ni‐based superalloy

Abstract The γ-phase of the MC2 Ni-based superalloy has been characterized using both in situ experiments and post-mortem observations (bright field and weak beam) in transmission electron microscopes. A mean value of the stacking-fault energy (about 31 mJ m−2) has been obtained through the measurement of dissociated nodes and of the dissociated width of dislocations. Furthermore, the plastic deformation has been shown to be controlled by the collective movement of dense pile-ups, and the existence of a local order has been deduced.

Friction stresses in the γ phase of a nickel-based superalloy

Abstract A method is proposed to evaluate the glide plane resistance τf opposing the propagation of dislocation groups in a concentrated solid solution. The method is based on the measurement of the position of dislocations as observed in a relaxed pile-up. Two types of material have been investigated: single crystals of the γ phase of the MC2 supernalloy and a model alloy of this phase especially designed for this study. At 500°C, τf (which in fact exceeds the yield stress τy) stems from the contributions of solid solution hardening and, more important, of the ordering which takes place in these materials. The contributions from short-range and long-range order can be distinguished.

TEM in situ straining of the MC2 superalloy at room temperature

Abstract By deforming the Mc2 nickel-based superalloy at room temperature in a TEM, we were able to study the succession of mechanisms that control the movement of the dislocation through the channels of they phase, at the interfaces and on shearing of the γ′ precipitates. We demonstrated the occurrence of small pile-ups that locally multiply the stress and adapt it to the various sites of resistance within the material. The results led us to reconsider the relationships from the literature giving the critical propagation stresses in the channels of the matrix and crossing the interfaces. We were thus able to account for the dynamic observations made and, in particular, to predict the number of dislocations in the small pile-ups that allow various obstacles to be crossed.

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

The chemical force acting on dislocations in a short-range-ordered concentrated Ni-based alloy is investigated by post mortem and in-situ transmission electron microscopy. For this purpose, the positions of the dislocations in a pile-up are calculated, taking into account the stress relaxation at the free surfaces. The calculation shows that the distribution of the first dislocations of the pile-up is only slightly affected by the presence of the free surfaces, while the length of the pile-up strongly depends on the thickness of the foil. Analysis of pile-ups in two short-range-ordered Ni-based alloys shows that the chemical force resulting from short-range order (SRO) is noticeable up to the sixth dislocation of the pile-up. These results indicate the presence of very small short-range-ordered clusters, rather than a homogeneously distributed SRO.

Dynamic friction stresses in the γ phase of a nickel-based superalloy

Abstract In-situ deformation experiments were performed on single crystals of the short-range-order-containing γ phase of a nickel-based superalloy. The dislocation pile-ups moving under stress are used as a probe exploring this complex solid solution and giving precise information on the local glide resistance encountered. Their analysis gives access for the first time to several contributions to this glide resistance. The threshold friction stress, which pile-ups must overcome in order to move is equal to the dynamic friction stress, which the pile-up experiences when propagating at a constant velocity. In comparison, the glide resistances deduced from the analysis of relaxed configurations are shown to be significantly underestimated. The softening effect due to the destruction of short-range order is also quantitatively determined.

In situ deformation experiments on a γ/γ′ superalloy Strengthening mechanisms

Abstract Through in situ straining experiments performed at 850 °C, on a γ / γ′ MC2 superalloy in a transmission electron microscope (TEM), the elementary mechanisms that control deformation were identified. On the recorded dynamic sequences, it is shown that deformation preferentially localizes in channels perpendicular to the [100] traction axis. Expanding from small loops, the creation and propagation of dislocations is observed. Both cross slip and climb events are observed on the moving dislocations which promote a tridimensionnal workhardening of these channels.