Atomistic study of the coherency loss during the b.c.c.-9R transformation of small copper precipitates in ferritic steels

Using 3D atomistic static simulations, we first investigate the atomic coherency between a small twinned 9R embryo (less than 6 nm in diameter) and the b.c.c. Fe matrix. We show that the (11(4) over bar)(9R) planes of the precipitate are aligned with the ( 1(1) over bar 0)(b.c.c.) planes of the matrix, and that the coherency loss of the precipitate/matrix interface is due to the formation of dislocation loops inside the precipitate. Then, we use 3D atomistic simulations to study the structure changes of an initially b.c.c. copper precipitate and show that the structure evolves towards a 9R structure. Finally, we use the above results to propose a simple geometric mechanism which may be relevant for the b.c.c./9R transformation at low temperature and when the influence of vacancies can be neglected. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.