Influence of the solute nature on the tensile behaviour and on the microstructure of γ-phases of nickel based superalloys

F. Pettinari, M. Jouiad, P. Caron, H. Calderon, A. Coujou, N. Clément

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Abstract

Ni-based single crystal superalloys are currently being developed for gas turbine blades and vanes applications. Their face centred cubic austenitic γ matrix, considered a priori to be disordered, is strengthened by a high volume fraction (up to 70 %) of finely dispersed ordered γ' Ll2 precipitates. These materials contain a number of alloying elements (between 8 and 10), each of them having a specific role in their mechanical and environmental properties. This work is aimed at determining the influence of refractory elements such as tungsten (W) rhenium (Re) or ruthenium (Ru) on the mechanical behaviour of different γ phases. In order to identify their specific role, three different simplified γ phase single crystals model alloys were specially elaborated at ONERA to be compared with a single γMC2 industrial phase. Both tensile tests performed in a wide range of temperature (-196°C to 1050°C) and post mortem T.E.M. observations of macroscopically deformed samples were conducted in order to determine the controlling mechanisms in such complex alloys. As a result, all alloys do not exhibit identical macroscopic tensile properties nor deformation microstructures. By T.E.M., in all alloys, two different domains are clearly determined : at low and intermediate temperatures a strong localization of the deformation exists while at high temperature (1050°C) an homogeneous deformation is observed : - at low temperature (T ≤ 500°C), in the four alloys investigated, the temperature dependence of the yield stress is the same and appears to be thermally activated ; - in this temperature domain, post mortem T.E.M. investigations indicate that deformation occurs through the propagation of pile ups of dislocations. One or more pairs of dislocations are observed at the head of the pile-ups ; - at high temperature (T ∼ 1000deg;C), the macroscopic flow stresses are practically the same. At 1050°C, the microstructure appears to be homogeneous in all alloys with dislocations moving independently. Several cross slip events are detectable. Nevertheless, some differences can be pointed out at intermediate temperatures (350°C ≤ T≤ 1050°C) : - compared with γMCW and γMC2, dynamic ageing appears later in γMCRe and γMCRu. ; - in the industrial γMC2 alloy, the deformation is controlled by the presence of long range ordered γ' microprecipitates whereas in the model alloys, the deformation is controlled by short range order; - then, in the industrial alloy, the yield stress peak observed, between 500 and 800°C, can be attributed to the precipitation of the LRO γ' small precipitates around 760°C. As in such an alloy, pairs of dislocations are necessary to shear the γ' particles, the number of dislocation pairs allows to determine the size of the precipitates. These precipitates are observed to grow when temperature increases with d ≈ 0,7 nm at -196°C (only one pair is observed), d ≈ 2 nm at 500°C (3 pairs are observed) and d ≈ 3,5nm (5 pairs). At high temperature, because of the dissolution of the precipitates, the dislocations are able to move independently. - In the model alloys, the presence of short range order is determined, therefore deformation mainly proceeds by dislocation pile ups with only one pair of dislocations at the head of these pile ups. In fact, in this case, the deformation controlling parameter is the diffuse antiphase boundary energy γOCD-2 which corresponds to the configurational energy change caused by the glide of the two firsts or more dislocations in the slip plane. At 750°C, in the intermediate temperature domain, short pile ups remain but now, at their head, no more pairs of dislocations are visible. At 1050°C, the deformation is homogeneous because of the disordering of the solid solution which begins before 900°C for γMCW, and between 900 and 1050°C for γMCRe and γMCRu. This decrease of short range order is quantified by a decrease of the diffuse antiphase boundary energy.
Original languageAmerican English
Pages (from-to)1038-1046
Number of pages933
JournalRevue de Metallurgie. Cahiers D'Informations Techniques
StatePublished - 1 Dec 2000

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