Paraskevas Kontis

In-situ high-temperature tensile testing of a polycrystalline nickel-based superalloy

An in-situ testing method for high-temperature time-resolved surface observations inside the scanning electron microscope is described and used to improve the mechanistic understanding of a new polycrystalline nickel-based superalloy. The in-situ observations are able to elucidate surface mechanisms of damage in different deformation regimes. Tensile tests from ambient conditions to 750 °C determine the effect of temperature on the prevalent modes of deformation. At low and intermediate temperatures, the formation of discrete slip bands and the cracking of the harder precipitates (carbides) are observed directly. Under this regime, both inter- and trans-granular failure are observed. At the highest temperatures tested, the mechanism of damage is different; the grain boundaries, their orientation and their precipitates play a crucial role. Void growth is directly observed as a consequence of small hard-particle decohesion from the matrix, within those grain boundaries of relative orientation which promotes the shear deformation. As deformation increases, voids grow and coalesce, forming grain boundary cracks thus leading to catastrophic intergranular failure: this manifests itself as low-ductility particularly at the high-temperature conditions.

The Role of Oxidized Carbides on Thermal-Mechanical Performance of Polycrystalline Superalloys

Oxidized MC carbides which act as main crack initiation sites in a polycrystalline superalloy under thermal-mechanical fatigue (TMF) conditions at 850xa0°C were studied. Microstructural observations in the TMF tested specimens were compared to findings from bulk samples exposed isothermally in air at 850xa0°C for 30 hours in the absence of any external applied load. Carbides were found to oxidize rapidly after exposure at 850xa0°C for 30 hours resulting in surface eruptions corresponding to oxidation products, from where micro-cracks initiated. Plastic deformation due to volume expansion of the often porous oxidized carbides led to high dislocation densities in the adjacent matrix as revealed by controlled electron channeling contrast imaging. The high dislocation density facilitated the dissolution kinetics of γ′ precipitates by segregation and diffusion of chromium and cobalt along the dislocations via pipe diffusion, resulting in the formation of soft recrystallized grains. Atom probe tomography revealed substantial compositional differences between the recrystallized grains and the adjacent undeformed γ matrix. Similar observations were made for the TMF tested alloy. Our observations provide new insights into the true detrimental role of oxidized MC carbides on the crack initiation performance of polycrystalline superalloys under TMF.

Consequences of a Room-Temperature Plastic Deformation During Processing on Creep Durability of a Ni-Based SX Superalloy

Ni-based single crystalline superalloys are used for high-pressure parts of aero-engines due to their superior mechanical properties and very good oxidation resistance at high temperature. However, shocks or unexpected mismatch in thermal contraction between molds and castings can occur during casting process and subsequent heat treatments, inducing plastic deformation of the alloy at low temperature. To mimic such events, a tensile plastic deformation is applied at room temperature on solutioned AM1 specimens and followed by standard aging heat treatments. Faster growth of the γ′ precipitates inside plastically deformed bands is obtained after full heat treatment with no lattice rotation or recrystallization. It has however been evidenced that the applied deformation has a detrimental impact on the creep properties, especially at high temperature (above 950xa0°C). It partly results from a highly localized failure process along former slip bands in which recrystallization is observed. The evolution of the microstructure during creep tests of prior deformed and nondeformed specimens has been thoroughly investigated to better identify under which conditions recrystallization occurs inside the bands during a creep test and by which mechanism.

Thin-Wall Debit in Creep of DS200 + Hf Alloy

The thin-wall debit in creep life of the directionally solidified DS200xa0+xa0Hf alloy at 900xa0°C has been investigated. A range of different applied loads and various directions with respect to the solidification direction was investigated. A direct comparison of creep properties in air between thin and massive specimens of DS200xa0+xa0Hf was studied in detail. Creep results have shown that a substantial thin-wall debit in creep life and creep ductility is obtained along transverse directions compared with the longitudinal direction. The above creep performance was compared with the thin-wall loss in creep of the 〈001〉 single-crystal DS200xa0+xa0Hf, for which almost no thickness debit in creep life was observed. The thin-wall debit in creep was mainly ascribed to the preferential oxidation of the grain boundaries. Besides, oxidized carbides were found to be cracked, and recrystallization was found in their vicinity. Finally, based on the produced experimental outcome, a coupled creep-oxidation modeling approach has been proposed to account for the thin-wall debit in creep life. This model takes into account creep anisotropy through the normalization by the ultimate tensile stress in both the Norton and Kachanov laws used in this modeling framework.

On the segregation of Re at dislocations in the γ' phase of Ni-based single crystal superalloys

Abstract We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of γ precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750u202f°C under an applied stress of 800u202fMPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the γ/γ interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design.

On the Microstructural Optimisation of a New Polycrystalline Superalloy for Industrial Gas Turbines

Polycrystalline superalloys are still utilized effectively as components on hot sections of industrial gas turbines, such as guide vanes, owing to their complex geometries, which cannot be produced by single crystal castings. A heat treatment plan for the optimization of carbides precipitation at grain boundaries, for a new polycrystalline nickel-based superalloy, was developed. Prior to experiments, the heat treatment window was defined by using THERMO-CALC software. Moreover, the relationship between grain boundary character and mechanical behaviour of the optimum designed microstructure was investigated through a number of creep tests, which were performed under different stresses. A strong effect of silicon and boron on creep resistance was observed, which deteriorate creep ductility. In addition, it was found that M23C6 carbides engulfed in γ’ layers are beneficial for creep resistance by obstructing crack propagation.