F. Pettinari-Sturmel

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.

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.

Local disordering and reordering phenomena induced by mobile dislocations in short-range-ordered solid solutions

Abstract Dislocation motion in short-range-ordered solid solutions under stress are investigated in situ by transmission electron microscopy. For the first time, local reordering created by the back slip of mobile dislocations is proven to exist. Dislocation movement within pile-ups produces local disordering by forward slip during loading, and local reordering by back slip during unloading. Within a pile-up, the driving force for this mechanism is the diffuse antiphase-boundary energy created by the movement of the leading dislocation.

Characterisation of short-range order using dislocations

Abstract The mobile dislocation pile-ups observed during in situ straining experiments have been used as probes to describe the short-range order (SRO) present in the phase of nickel-base superalloys. An experimental approach, based on the measurement of the dislocation positions, to characterise the SRO has been developed. This paper focuses on the main results we have obtained on this topic. A quantitative analysis allows to get the chemical forces associated with SRO, and also the SRO degree. For the first time, the energies associated with SRO are evaluated along a dislocation pile-up as a function of temperature. More generally, a description of the SRO is proposed: SRO is seen as a heterogeneous distribution at the micrometer scale of nanometric areas. Another characteristic has been evidenced: the reversible character of the SRO creation.

A new TEM method for the characterization of the tertiary γ' nano-precipitates in a PM disk superalloy : influence of ageing

An original method for characterizing the γ′-phase tertiary precipitates in a Ni-based superalloy manufactured by powder metallurgy is described. This investigation is made using post mortem transmission electron microscopy (TEM). It is based on the analysis of sheared areas within crept specimens, which allows the precipitates revealed by the dislocations in their glide plane to be observed. The characteristics of these nano-precipitates, i.e. their size, their volume fraction and the channel width between them, are determined for two different heat treatments (HTs). The results show a wide distribution of the microstructural parameters for a given HT, but only slight differences between the microstructures produced by the two different HTs. This microstructural information allows a better understanding of the wide variety of the deformation micromechanisms observed during creep at high temperature.

Weak-beam study of dislocations in D022-Al3Ti deformed at 400 C

Polycrystalline samples of Al3Ti with the D022-type structure have been plastically deformed at 400°C. Observations using weak-beam transmission electron microscopy show activation of dislocations with <100] or <110] Burgers vectors. The fine structure of [100] and [010] dislocations reveals strong stabilization effect on the pure edge character. [110] superdislocations, resulting from the interactions between [100] and [010] perfect dislocations, are dissociated in two identical superpartials separated by an antiphase boundary (APB). Their segmented shape together with the separation distances between the [110] superpartials point to a large anisotropy of APB energies whose estimated values on (001) and (111) are 13 ± 3 mJ/m2 and 64 ± 5 mJ/m2, respectively. The low mobility of <100] and <110] dislocations is highlighted and related to the low ductility of this material.

Decorrelated Dislocation Movement in the γ-Matrix Channels of a Ni-Based Superalloy: Experiment and Dislocation Dynamics Simulation

The mechanical behavior of the polycrystalline NR3 Ni-based superalloy has been investigated at the microscopic scale. The elementary deformation mechanisms have been analyzed using transmission electron microscope observations as well as in situ straining experiments. Under low stress and relatively low strain rate conditions, a large variety of shearing micromechanisms has been observed depending on the local microstructure and the local effective stress. The influence of the smallest precipitates on the creep behavior has been enlightened: they induce narrow channels which act as obstacle for the movement of the dislocations. In the case of the narrowest channel, the deformation can operate by the propagation of Shockley dislocations or else, by the only propagation of the leading partial resulting from the partial dislocation decorrelation. The occurrence of the observed micromechanisms has been quantitatively analyzed using a nodal dislocation dynamics simulation.

Short range order heterogeneity on plastic mechanisms in γ-phase nickel-based superalloys

The fine structure of slip lines in the γ-phase of Ni-based superalloy single crystals has been investigated by in situ atomic force microscopy observations of the specimen surface during deformation. For the first time, the dislocation structure is discussed in terms of short range order heterogeneity. A stress-driven unlocking mechanism is observed of dislocation pile-ups scanning the short range order in the bulk material. This is strongly enhanced by the propagation of other pile-ups a few hundreds of nanometers apart. A modelling based on elasticity is proposed and discussed.