The crystallography of heterogeneous nucleation in a Ni-45 wt% Cr alloy

Abstract

Abstract A detailed transmission electron microscopy study has been made of the crystallographic characteristics of heterogeneous nucleation at grain boundaries, twin boundaries and dislocations in a Ni-45 wt% Cr alloy. Nucleation at incoherent twin boundaries and grain boundaries was observed to be easier than at coherent twin boundaries or dislocations. In all cases a unique orientation relationship, Kurdjumov-Sachs (K-S) was developed between the precipitates and matrix. Nucleation at grain boundaries was favoured if the plane of the grain boundary lay close to (121)f, the low energy habit plane of the precipitate. Growth of the precipitate generally occurred into the grain with the irrational orientation relationship, but at coherent twins or grain boundaries which had a near twin orientation relationship the precipitates grew into both grains. At (121) incoherent twin steps, the nucleus formed with a (121)f habit plane parallel to the twin step and grew into the matrix with which it kept a K-S relationship. Nucleation at dislocations was favoured when the dislocation line lay nearly parallel to the invariant line of the lath-shaped precipitates and the Burgers vector of the dislocation was closet to the maximum misfit vector of the precipitate.