The crystallographic and morphological evolution of the strengthening precipitates in Cu–Ni–Si alloys

High-resolution transmission electron microscopy and first-principles energy calculations reveal that, upon formation, the hardening precipitates in Cu–Ni–Si alloys are unchanged δ-Ni2Si nanocrystals. However, their crystallographic and morphological features evolve during the precipitation process. It is shown that, in terms of crystallographic orientation relationships, there are basically two types of δ-Ni2Si precipitates in the alloys, referred to as δ1-Ni2Si and δ2-Ni2Si respectively. In the early stages of aging (including peak aging), the precipitates are small and belong to the δ1 type, with the following orientation relationship with the Cu matrix: [0 1 0]δ||[1 1 0]Cu and (0 0 1)δ||(0 0 1)Cu. In the late stages, the precipitates are clearly larger and become the δ2 type, with the orientation relationship: [0 1 0]δ||[1 1 0]Cu and approximately (301)δ‖(11¯1)Cu. Governed by the minimization of its overall energy, a developing δ precipitate has to evolve from an almond-like δ1 particle with a low-index coherent habit plane to a French baguette bread-slice-shaped δ2 particle that has a high-index broad interface. This evolution is found to be in excellent agreement with predictions provided by the invariant line theory. Intermediate stages exist for a particle to accomplish such an evolution, leading to many different crystallographic and morphological appearances of these δ-Ni2Si particles being observed in the alloys.