François Louchet

Directional coarsening of Ni-based superalloys: Computer simulation at the mesoscopic level

Directional coarsening of nickel based superalloys is modeled taking into account the role of anisotropic misfit relaxation by dislocations generated during creep. The directional coarsening is the response to the gradient in elastic energy induced by this anisotropic relaxation. A 3-dimensional computer simulation has been developed to describe both the morphologies and the kinetics of the phenomenon. Results of the simulation are presented and compared to experimental observations on an AM1 superalloy. Morphology maps describing the expected rafting geometry for other superalloys, as a function of misfit and applied stress, are established and discussed.

Strain induced directional coarsening in Ni based superalloys

Directional coarsening (or rafting) in Ni-based single crystal superalloys occurs after short times under stress at high temperature. This phenomenon results in a strongly anisotropic evolution of the microstructure that needs to be understood since it can occur in superalloy turbine blades during service. As the strain induced during creep seems to be responsible for the rafting phenomenon, it is worth studying the effect of a strain gradient on coarsened structures. A simple way to do this is to investigate the coarsening morphologies developed around an indentation.

Thermal ageing of an Fe-Cu alloy : microstructural evolution and precipitation hardening

Abstract The microstructural evolution of an Fe-Cu alloy during ageing has been investigated by small-angle X-ray scattering and transmission electron microscopy (TEM). Its mechanical properties have been characterized by Vickers microhardness measurements and in situ straining TEM. Cu precipitates evolve during annealing from a small coherent bcc to a larger incoherent type. In both cases, in situ straining shows that long screw dislocations move between localised cusps owing to dislocation-precipitate interactions. In the case of incoherent precipitates, a particular three-dimensional bypassing mechanism is observed, which does not form any loop around the precipitate. On the basis of these observations, we propose a model of hardening combining the viscous motion of screws and the dislocation-precipitate interaction, which yields a monotonic decrease in stress with annealing time in both cases of particle shearing and bypassing. As a consequence, the hardness peak shown by microhardness experiments is ...

Microstructure and mechanical properties of A 2091 AlLi alloy. II: Mechanical properties : yield stress and work hardening

Abstract Mechanical properties of a 2091 AlLiCuMg (Zr) alloy are investigated together with those of simpler AlLi (Zr) and AlCuMg (Zr) model alloys. The different contributions to the yield stress are estimated from the microstructure investigated in companion Paper I. Work hardening is described using Ashbys model, but taking into account the strain localization.

A model for low stress cross-diffusional creep and directional coarsening of superalloys

First stages of high temperature creep along in superalloys containing a large volume fraction of the ordered {gamma}{prime} phase are strongly related with directional coarsening (DC), i.e., rafting of {gamma}{prime} precipitates in planes parallel or perpendicular to the stress axis, depending on the signs of lattice misfit and of external loading (tension or compression). The present paper shows that, in the low stresses case, creep cannot directly involve dislocations, and that the major part of the plastic strain during the coarsening process results from a cross diffusion mechanism driven by the gradients of isostatic pressure between phases (Cross-Diffusional Creep). Here, the authors shall consider for the sake of simplicity the case of a superalloy loaded in tension along a direction, with a negative misfit ({delta} = 2({alpha}{sub {gamma}{prime}} {minus} {alpha}{sub {gamma}})/({alpha}{sub {gamma}{prime}} + {alpha}{sub {gamma}}) < 0), which is by far the most frequent situation, and in which the elastic moduli of both phases are comparable.

Elevated temperature creep of polycrystalline uranium dioxide: from microscopic mechanisms to macroscopic behaviour

Abstract Creep of polycrystalline UO 2 at intermediate temperatures is studied by compression experiments and microstructural observations at different dimensional scales. Creep data exhibit two different stress regimes in the classical power law representation, but can be described on a more physical basis by a unique hyperbolic sine creep law on the whole range of stresses explored here, which is shown to account for both bulk grain deformation and accommodated grain boundary sliding.

Ordinary dislocations in γ-TiAl: cusp unzipping, jog dragging and stress anomaly

Abstract Two models recently proposed for strength anomalies in TiAl were based on different behaviours of jogs: in the first, jogs are supposed to move conservatively along the screw direction, whereas in the other, screw dislocation motion is supposed to be controlled by jog or dipole dragging. In the present paper, we compute the flow stresses expected in both cases. The former model is shown to operate up to the stress peak temperature, and to be replaced by the latter as temperature is raised above the peak.

Analysis of in situ shearing mechanisms of γ′ precipitates in a nickel-based superalloy at 1120 K

Abstract L12 precipitate shearing processes by (a/3)〈112〉 dislocations during in situ straining of a superalloy in a transmission electron microscope are reported. Information about the core structure of such dislocations is obtained from the way they split when they get out of a precipitate. We conclude that a further dissociation into two identical Shockley partials occurs.

Microstructure and mechanical properties of a 2091 AlLi alloy—I. Microstructure investigated by SAXS and TEM

The microstructure of the 2091 alloy (AlLiCuMgZr) alloy has been studied and compared to that of two simpler AlLi and AlCuMg alloys. The SAXS, in situ SAXS and TEM techniques have been used for heat treatments at room temperature and at 150°C. The time evolution of the size and of the volume fraction of the δ′ precipitates and of the Cu-rich GPB zones have been determined. The δ′ precipitates are shown to follow a classical Lifschitz-Slyosov-Wagner law. Their interfacial energy has been estimated. S and S′ precipitates have also been characterized, respectively on grain boundaries and on helical dislocations, by TEM.

Microstructure and mechanical properties of a 2091 AlLi alloy—III. Quantitative analysis of portevin le chatelier instabilities and relation to toughness in AlLi, AlCuMg and AlLiCuMg (2091) alloys

Abstract The microstructure and mechanical properties of a 2091 alloy are studied and compared to simpler AlLi and CuMg alloys. For ageing times between 6 and 24 h at 150°C, the 2091 alloy exhibits a toughness drop and a simultaneous change in PLC characteristics (as evidenced by a combination of local and total strain measurements), but no significant change in microstructure, except for the size of δ′ precipitation. SEM in situ tests show that plastic instabilities are always related to extra damage. A quantitative model accounts for the toughness drop, based on plastic dissipation by PLC active bands.