High resolution TEM study of Ni4Ti3 precipitates in austenitic Ni51Ti49

Binary NiTi with a composition of 51 at.% Ni was heat treated to form lens-shaped Ni4Ti3 precipitates that are coherent or semi-coherent with the B2 matrix. High resolution transmission electron microscopy (HRTEM) was used to study the internal structure of the precipitates, precipitate-precipitate and matrix-precipitate interfaces and the deformation of the B2 matrix near a precipitate. Observations were made in the <1 1 0>B2 and <1 1 1>B2 zones and compared with computer simulated high resolution images. The <1 1 1>B2 observations made it possible to study the [0 0 1]H zone orientation of Ni4Ti3 (direction defined according to the hexagonal unit cell of Ni4Ti3) which corresponds to the normal of the central plane of the discs. In these images the superperiodicity of the 4:3 ordering is clearly visible confirming the known atomic structure. Close to the precipitate the B2 matrix is deformed, as determined by measuring the interplanar spacing from the HRTEM images. The observed deformations are compared with theoretical models for the stress field