Debashis Mukherji

Deformation-induced dislocation networks at the γ-γ′ interfaces in the single-crystal superalloy SC16: A mechanism-based analysis

Abstract The formation of dislocation networks at the γ-γ′ interfaces in a single-crystal nickel-based superalloy SC16 has been studied after deformation at 1223 K. The dislocation segments in the networks were characterized. As a result the dislocation substructure was found to be quite different on interfaces parallel and perpendicular to the applied stress direction. Reactions between glide dislocations were essentially observed on the latter type of interface. A model of network formation based on cross-slip leading to dislocation interactions has been proposed to explain the observed networks.

Lattice misfit measurement in Inconel 706 containing coherent γ′ and γ″ precipitates

Abstract Lattice misfit is an important parameter in Ni–Fe based alloy IN706 as coherency strain influences the strength of the alloy and the stability of the microstructure. X-ray and synchrotron measurements were performed on bulk samples with precipitates in constrained lattice. Lattice parameters of different phases and the lattice misfit could be determined even though the peaks were completely overlapped.

Evaluation of anisotropic small-angle neutron scattering data; a faster approach

An improvement of the anisotropic small-angle neutron scattering (SANS) data evaluation program is presented. The program is particularly suited for treatment of data from dense precipitate systems like those appearing in single crystal Ni-base superalloys. A new model of the precipitate microstructure was implemented allowing for a significant shortening of the evaluation time which is necessary mainly for in-situ studies. The change concerns the mode in which the size distribution is calculated. The local random smearing of the size as well as the distance of particles was introduced, too, which leads to a more realistic look of the microstructure model. The form of individual cuboidal particle was changed as well. The characteristics of the new microstructure model with respect to the SANS data modeling are tested on several measured as well as simulated data.

Deformation behaviour of freestanding single-crystalline Ni3Al-based nanoparticles

Abstract Deformation on a single freestanding metallic object of submicron size is usually not performed. That is primarily because it is not easy to handle isolated single objects in this size scale. Here we demonstrate a method of performing compression testing on a freestanding cubic shaped single crystalline Ni3Al-type nanoparticle (∼300 nm). The particles were deformed with the help of a nano-manipulation system inside a scanning electron microscope. The nanoscale test specimens were obtained from a nickel-based superalloy by electrochemically extracting the Ni3Al-γ′ precipitates. Stress-strain curves are generated when a cubic particle is deformed under compression between a tungsten “micro-hammer” and a silicon “micro-anvil”. Deformations conducted on particles in two different states, undeformed and predeformed, show distinctly different deformation behaviour. Sudden strain “bursts” are observed during tests on undeformed specimens indicating that deformation is possibly governed by dislocation nu...

High temperature stability of Cr-carbides in an experimental Co–Re-based alloy

Abstract The stability of the microstructure at high temperatures was studied in an experimental Co–Re-based alloy. The experimental alloy is mainly strengthened by Cr-carbides, particularly by those in the form of thin lamellar plates. Electron microscopic investigation on samples exposed for up to 1 000 h to temperatures of 1 000 and 1 200 °C showed that Cr23C6 type carbides present in the alloy in different morphologies are unstable at these temperatures. It was also observed that the alloy hardness dropped after exposing the samples to elevated temperatures and much of this loss occurred within the first 100 h. In-situ diffraction measurements with synchrotron radiation showed that carbide dissolution started as early as 3 h of holding at 1 000 °C. Moreover, in-situ small angle neutron scattering results indicated that the carbides at the grain boundaries and the blocky carbides dissolve first and then the thin lamellar carbides. Further, the enrichment of Cr in the Co-matrix phase, which took place due to the dissolution of Cr-carbides, stabilized a Cr–Re-rich σ phase. Although the dissolution of lamellar carbides results in a significant loss of strength, the formation of σ phase with extremely high hardness partly compensated the for loss. The σ phase is stable even at 1 200 °C.

Measurement of γ′ precipitate morphology by small angle neutron scattering

Modern Ni-base superalloys contain large volume fraction of ordered Ni{sub 3} (Al, Ti) precipitates ({gamma}{prime} phase) which are coherently embedded in a Ni-Al solid solution ({gamma} phase). Superalloys are strengthened by the {gamma}{prime} precipitates and it is known that the precipitate morphology, their distribution in the matrix and their volume fraction strongly influence the strength of the alloys. In this paper the authors results from the measurements by small angle neutron scattering (SANS) and use special data analysis procedures to quantify various {gamma}{prime} morphological parameters. The authors use complementary measurements by scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM) to verify and augment the results of SANS. They find SANS is a very useful tool for these kind of measurements as it measures over a large volume of specimen. Further, the 3-Dimensional microstructural parameters could be obtained quantitatively from measurements on a single specimen with the help of a cradle used to in-situ align the specimen with respect to the neutron beam.

Oxidation behaviour of experimental Co–Re-base alloys in laboratory air at 1000°C

Abstract The oxidation behaviour of experimental Co – Re-based alloy at 1000 °C was studied. A set of binary, ternary and quaternary alloys from the Co – Re – Cr – C system was used as model alloys to understand the role each alloying element plays on oxidation. The morphology and composition of the oxide scale that formed was analysed by X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. It was found that the present Co – Re alloys with 23 at.% and 30 at.% Cr additions behaved very similarly to Co – Cr binary alloys with equivalent Cr content. The oxide scale was multilayered, consisting of a dense CoO outer layer, a porous mixed oxide layer containing Co-oxide and Co – Cr spinel, and a discontinuous and non-protective Cr3O2 layer. The binary Co – Re alloy behaved differently in oxidation, and it formed only a monolithic CoO scale. However, Re in combination with Cr promotes Cr – Re-rich phase formation, which oxidises preferentially compared to the Co matrix. Carbon ties up part of the Cr to form Cr23C6 type carbides. However, these carbides are not stable at 1000 °C and dissolved with time, therefore C had only a minor role in the oxidation behaviour. In general, increasing Cr content in the alloy improved oxidation resistance.

Microstructural study of boron-doped Co–Re–Cr alloys by means of transmission electron microscopy and electron energy-loss spectroscopy

Abstract The development of corrosion-resistant alloys with sustained thermal and mechanical stabilities is the focus of current research on high-temperature alloys. It has been shown that the creep behavior and oxidation resistance of Co–Re alloys can be improved by small additions of, e.g., B and Zr. To understand the effect of these elements on mechanical properties, we have characterized the phases and microstructures in the Co–Re–Cr system using transmission electron microscopy. Zr forms isolated large crystals in the matrix, while B is accommodated in small intergranular (Cr,Re)2B borides. The incorporation of up to 23 at.% Re into the Cr2B structure results in increased unit cell constants. The elongated borides occur pervasively along grain boundaries, suggesting that they improve the grain boundary cohesion and thereby the mechanical properties of Co–Re–Cr alloys.

Determination of γ/γ interface types in a γ-TiAl alloy using convergent beam electron diffraction

Abstract The domain orientations and lamellar interfaces play a critical role in the mechanical properties of TiAl based alloys. An umambiguous experimental determination of all six domain orientation in lamellar structure and the interfaces between neighboring γ laths in a (α 2 +γ) two-phase TiAl alloy containing Mn and Nb were analyzed by means of convergent-beam electron diffraction. The frequency of different interface types are reported.

Character of dislocations at the γ/γ′ interfaces and internal stresses in nickel-base superalloys

Abstract It is experimentally observed in Ni-based superalloys that the dislocation substructure after deformation at elevated temperatures is different on horizontal and vertical γ/γ′ interfaces. Edge dislocation networks are predominantly observed at horizontal interfaces while screw dislocation segments are seen mostly on the vertical interfaces. A crystallographic model of the critical shear stress for dipole expansion for screw and 60° dislocation segments in the γ/γ′ interfaces is used to explain the observations in high γ′ volume fraction (∼70%) alloys. However, the same model is found to be inadequate in explaining the substructure observations in those alloys with lower γ′ volume fraction (∼40%) which have wider matrix (γ) channels ( ≈180 nm ). It is shown by superimposition of the dislocations stress field, the internal coherency stresses and the externally applied stress that between 60° and screw dislocation segments the former increase the internal stresses in vertical interfaces more than the latter.