Variations of Internal Stresses within a Rafted Superalloy during High Temperature Mechanical Testing: An in situ XRD St
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Variations of Internal Stresses within a Rafted Superalloy during High Temperature Mechanical Testing: An in situ XRD Study Alain Jacques1, Laura Dirand1, Thomas Schenk1, Jean Philippe Chateau-Cornu1, Olivier Ferry1, and Pierre Bastie2 1 Institut Jean Lamour – SI2M (UMR CNRS – UdL 7198, labex DAMAS) parc de Saurupt, 54000 Nancy, France 2 LIPhy (UMR CNRS –UJF N° 5588) 140 Avenue de la Physique - BP 87, 38402 Saint Martin d’Hères, France ABSTRACT Modeling of the mechanical behavior of a two-phased material, even with a simple microstructure such as a single crystal superalloy remains a difficult task, for lack of phase specific experimental data. The combination of Three Crystal Diffractometry with high energy synchrotron radiation and in situ experiments can give access to such data in real time. A few examples are given on load transfer between phases, dislocation densities, and the stress – strain behavior of a phase. INTRODUCTION Single-crystal nickel-based superalloys are currently widely used in aircraft engines for their creep resistance under tension at high temperature. These alloys are two-phased materials, with a disordered fcc γ matrix containing a high relative concentration of ordered L12 γ’ precipitates. During high temperature creep, the microstructure of specimens with a [001] tensile axis changes to a rafted one [1]: the microstructure becomes a semi-coherent stack of so called γ corridors and γ’ rafts perpendicular to [001]. The fcc γ corridors deform by glide of perfect Shockley dislocations which glide along {111} planes and leave trapped dislocations at the γ/ γ’ interfaces. As these trapped dislocations have the same edge component with their half plane on the raft side, they may partly or fully relieve the coherence stresses due to a negative lattice parameter mismatch between both phases δ = 2 ⋅ (a '− a ) (a '+ a ) ( a and a' are the stress-free lattice parameters of the γ and γ’ phases). Recent papers [2-4] suggested that the high temperature plastic strain of the γ’ phase involved correlated climb of dislocations. Physical modeling of the mechanical behavior of such a composite requires data on the internal stresses between both phases, especially the σxx = σyy stress perpendicular to the tensile axis. The sign and the magnitude of these components is determined by the difference between the effective interface mismatch in the interface plane δ ⊥ = 2 ⋅ (a '200 − a200 ) (a'200 + a200 ) = ρ ⋅ be and the “natural” mismatch δ. (ρ and be are the linear density and edge component of interface dislocations) [5]. As the interface dislocation density may (and does) vary during a test, very precise and short measurements of the lattice mismatch δ ⊥ are needed. It has been shown [6-9] that this may be achieved in situ diffraction methods using neutrons or high energy synchrotron radiation, especially Three Crystal Diffractometry (TCD). Thanks to the high energy, the high resolution of TCD and the intensity of the high energy beam of beamlines such as ID 15 (ESRF) and BW5 and P07 at DESY, it is possible
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