A. Coujou

An in situ study of twin propagation in TiAl

Abstract An in situ study of twinning in a TiAl alloy has been performed in order to determine twin properties and to discuss the processes controlling twin propagation. This study shows that twinning is due to a/6⟨112] partial dislocations gliding in octahedral planes and that twinning is an intensive mode of deformation for some specimen orientations. Twins move slowly at an approximately constant velocity by a collective movement of partial dislocations which are separated by a constant distance during the motion. Twins in the TiAl alloy are perfect. A polar twin source is analysed. The occurrence of twinning with respect to other possible deformation modes is discussed and interpreted using the Schmid law. It is shown that the twin propagation is controlled by thermally activated frictional forces acting on partial dislocations and by the dragging of a stacking fault.

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.

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%.

Local order in industrial and model γ phases of superalloys

Abstract Different γ phase model alloys of nickel base superalloys are investigated as a function of temperature using elastic diffuse neutron scattering, macroscopic tensile tests, TEM post mortem observations and in situ straining experiments. Short Range Order (SRO) of (1 1/2 0) type resulting from nano-domains with correlation lengths of 0.5–1 nm is observed in all alloys. The dislocation distribution confirms the presence of short-range ordered nano-domains with a size of about 0.7 nm. In all the alloys, the influence of SRO on the deformation mechanisms is clear at low and medium temperature and thus explains the similarities of the deformation behaviour observed in this temperature range. From 750°C to 1000°C, the deformation microstructure becomes progressively homogeneous which suggests the decrease of SRO as illustrated by a decrease of diffuse scattering intensities at the (1 1/2 0) positions on heating. The various techniques applied prove to be complementary in the investigation of complex solid solutions.

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.

Determination of precipitate strength in aluminium alloy 6056-T6 from transmission electron microscopy in situ straining data

Abstract The transmission electron microscopy in-situ straining technique is employed to measure the breaking angles of strengthening precipitates in aluminium alloy 6056-T6 as they are sheared by dislocations. The experimental determination of the character of bowed dislocation segments when dislocations are pinned on precipitates allows us to calculate the corresponding line tensions. From this, the maximum forces F m that precipitates can sustain before being sheared by dislocations are deduced. It is suggested that F m may be regarded as a quantitative parameter which includes the effects of the various strengthening mechanisms operative in precipitation-hardened alloys. An attempt is made to relate the maximum force calculated from in-situ straining data to the macroscopic yield strength of the material.

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.