ANALYSIS OF ANISOTROPIC SANS PATTERNS OBTAINED FROM Ni-Al-Mo ALLOYS WITH BIMODAL PARTICLE SIZE DISTRIBUTIONS. H.A. Calderon, J.G. Caba-As-Moreno, J.J. Cruz, Dept. Ing. Met., E.S.I.Q.I.E.-I.P.N., Apdo. Postal 75-874, Mexico 07738, And J.S. Pedersen Dept. Sol. State Physics, Riso National Lab., Dk-4000 Roskilde, Denmark.

The coarsening kinetics of bimodal distributions of coherent *' particles in the alloys Ni-14 at.%Al, Ni-10at%Al-5 at.%Mo and Ni-6.5 at.%Al-10 at.%Mo have been investigated by means of small angle neutron scattering (SANS) and by transmission electron microscopy (TEM). Mo has been added to the binary Ni-Al alloys in order to tune the elastic interaction between particles during ripening. In solids, the elastic strain energy can affect the coarsening process of particles, as theoretically predicted by several investigators. The bimodal distributions are created by aging at two different temperatures. Aging at high temperature produces a class of large precipitates in short times. Their coarsening behavior has been investigated by microscopical techniques since they cannot be detected by SANS. Aging at lower temperatures gives rise to the formation of a second set of smaller particles. SANS has been used to determine size, shape and arrangement of particles as a function of the aging time for this set of smaller particles. Single crystalline specimens oriented with the plane (110) parallel to their surface have been used for the SANS measurements. Some of the patterns are markedly anisotropic showing interference peaks along directions [001] and non-circular shapes at large scattering vectors (q). This indicates that particles have preferential arrangements in space and that their shape is not spherical. Sequeira et al. [1] analyzed such anysotropic patterns using a technique developed for isotropic patterns [2]. They found large inconsistencies with TEM measurements. In the present case, an evaluation technique for anisotropic patterns has been developed and the results are compared to TEM measurements.

[1] A.D. Sequeira, H.A. Calderon, G. Kostorz, J.S. Pedersen, Acta Metall. Mater. 43, 3427 (1995).

[2] J.S. Pedersen, J. Appl. Crystallogr. 21, 729 (1991).