High-resolution electron microscopy studies on coherent plate precipitates and nanocrystals formed by low-temperature heat treatments of amorphous Ti-rich Ti-Ni thin films

Abstract

Further high-resolution electron microscopy studies have been performed on coherent thin non-equilibrium precipitates and unique nanocrystals with an identical orientation, which are produced from sputter-deposited amorphous Ti-48.2 at.° Ni thin films by heat treatments near or below the crystallization temperature T_c . Abundant evidence is provided for the previously proposed mechanisms for formation of the subnanometric precipitates near T_c and the nanocrystals below T_c ; it is as follows. For heat treatment below T_c , firstly a thin plate precipitate with the {100} habit plane, which has a bct lattice with its tetragonal axis perpendicular to the habit plane, is frequently observed at the crystal-amorphous interface, secondly the bct precipitates constituting the boundaries between nanocrystals are arranged in such a way that they can truncate an approximately spherical nanocrystal, while keeping the coherency with the B2 matrix at the {100} habit plane, thirdly enrichment of Ti atoms is detected at the bct precipitates wherever they may lie, fourthly Ti enrichment is found in the amorphous regions near the advancing crystal-amorphous interface and fifthly the sizes of the nanocrystals are nearly the same. For heat treatment near T_c , firstly extremely thin bct precipitates formed on the {100} plane are found to have a thickness of only about 0.3nm which corresponds to three {200} lattice planes of the B2 matrix or to one unit cell of the bct lattice of the precipitate, secondly the perfect matching of the (100) atom rows of the bct precipitate with the (100) atom rows of the B2 matrix is confirmed by inverse Fourier transform images, thirdly a large matrix region around the precipitate is elastically strained and fourthly Ti enrichment is also detected at this subnanometric plate precipitate. Besides these experimental findings, a plate precipitate with the C11_b (Si₂Mo) structure is found at the boundary between nanocrystals, which seems to be energetically more stable than a very high tetragonal phase. The possibility of the production of similar nanometric substructures in other intermetallic compounds is suggested.