Stabilization of coherent precipitates in nanoscale thin films

Coherent precipitates, on growth beyond a critical size ($r_{bulk}^{*}$), become semi-coherent by the presence of interfacial misfit edge dislocations. In the case of precipitation in ‘small’ domains (matrix), the strain energy of the precipitate is altered with respect to its value in bulk domains, due to a lesser volume of strained material and domain deformations, resulting from the proximity of free surfaces. This results in a change in the value of critical size for the coherent to semi-coherent transition of the interface. In the current work, the Cu-2wt.þ system is used as model system to study the coherent to semi-coherent transition of precipitates. In this system, spherical Fe-2wt.%Cu precipitates form, which become cuboidal on growth beyond the critical size. Transmission electron microscopy is used to show experimentally for the first time that a coherent precipitate can be stable beyond $r_{bulk}^{*}$, in nanoscale free-standing thin films (i.e. critical size for coherent to semi-coherent transition for thin films ($r_{film}^{*}$) > $r_{bulk}^{*}$). Electron energy loss spectroscopy has been used to determine the thickness of the sample. In parallel, finite element simulations are used to compute the critical size in representative domains and to comprehend the energetic basis for the observed phenomenon. Eigenstrains are used to simulate the precipitation process and the stress state due to an interfacial misfit dislocation in the finite element model.