Determination of Coherency Strain Fields Around Coherent Particles in Ni-Al Alloys by HREM and CBED
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their excellent mechanical properties at high temperature is the ordered Ni 3A1 or y' phase (L12 structure). This phase appears in the form of coherent ordered particles homogeneously distributed in a Ni basis solid solution (fcc structure). Use at high temperature of Ni-based superalloys implies a coarsening mechanism of the particle distribution that promotes a degradation of the mechanical properties. Coarsening of particles is controlled by the decrease of the total energy of the system. In 'he case of solids, two important contributions can be found i.e., the reduction of the interfacial and the elastic strain energies. There is an important reduction of the interfacial energy when larger particles grow at the expense of smaller ones. However the lattice mismatch between particles and matrix induces a strain field that gives rise to elastic interaction between particles. The elastic energy influence depends on the particle volume and thus larger particles imply an increase of the elastic contribution, especially in the late stages of the coarsening mechanism. There is clear evidence of the elastic effects e.g. particle shape changes as a function of size, particle alignment along elastically soft directions, formation of particle arrangements, particle splitting, etc. [1, 2]. There is also a great deal of investigations on the subject based on numerical simulation [see e.g. 3, 4]. According to Johnson et al. [3], it is possible to have inverse coarsening i.e., smaller particles growing at the expense of larger ones, for some elastic conditions. In the same way it has been suggested that growing particles can achieve a critical size beyond which they will split into smaller particles [4]. In this investigation microscopical techniques are employed to evaluate the strain fields around coherent particles in the late stages of the coarsening mechanism i.e., when the influence of the elastic interaction is strong. 99 Mat. Res. Soc. Symp. Proc. Vol. 589 @2001 Materials Research Society
EXPERIMENTAL PROCEDURE Monocrystalline samples of the alloy Ni -12 at.% Al were aged at two different temperatures to form a bimodal particle size distribution. The first aging was made at 1133 K and the second one at 923 K. This microstructure was selected in an attempt to create an especially strong elastic interaction between particles. High resolution electron microscopy (HREM) and convergent beam electron diffraction (CBED) are used to evaluate elastic deformations around particles with a high spatial resolution. The quantitative analysis of the HREM images has been carried out by means of the software Darip. CBED patterns have been evaluated by means of the software ems. RESULTS AND DISCUSSION Fig. 1. Dark field image of the alloy Ni12 at.% Al after aging 1700 h at 923 K The evaluation of strain fields around y' partiand 5h at 1133 K. cles is addressed to the late stages of the coarsening process. The samples are initially constituted by two different particle size distributions. A first aging at a temperature very close to th
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