Efficient Atomic-Scale Kinetics through a Complex Heterophase Interface

Atomic-scale imaging and first-principles modeling are applied to the heterophase interface between the Al-Cu solid solution (${\alpha }_{\mathrm{Cu}}$) and ${\theta }^{\prime }$ (${\mathrm{Al}}_2\mathrm{Cu}$) phases. Contrary to recent studies, our observations reveal a diffuse interface of complex but well-defined structure that enables the progression from ${\alpha }_{\mathrm{Cu}}$ to ${\theta }^{\prime }$ over a distance of $\approx 1\mathrm{nm}$. We demonstrate that, surprisingly, the observed interfacial structure is not preferred on energetic grounds. Rather, the excess in interfacial energy is compensated by efficient atomic-scale kinetics of the ${\alpha }_{\mathrm{Cu}}\to {\theta }^{\prime }$ phase transformation.

Figure 1

(a) HAADF-STEM image showing the three orientations of ${\theta }^{\prime }$ precipitates and their faceting along the $⟨100⟩$ and $⟨110⟩$ directions. C and SC refer to the coherent and semicoherent ${\theta }^{\prime }$-matrix interfaces. The SC (b) $\{100\}$ and (c) $\{110\}$ interfaces, respectively, exhibit a 0.5-nm-wide ${\theta }^{\prime \prime }$ region and a transitional region ${\theta }_t^{\prime }$. The crystal structures for $\alpha$ (matrix phase) and ${\theta }^{\prime }$ are overlaid on enlarged insets in (b) and (c). Small blue disks with or without crosses represent Al atoms at $z=0.5/0$ and large orange disks with or without circles Cu atoms at $z=0.5/0$, where $z$ is the unit cell fraction normal to the page.

Figure 2

Structural atom model for the $\alpha -{\theta }^{\prime }$ interfaces imaged in Fig. 1. The interfacial regions of ${\theta }^{\prime \prime }$ and ${\theta }_t^{\prime }$ identified in Fig. 1 are framed. The brown Cu disks correspond to the ${\theta }^{\prime }$ lattice positions in the bulk of the precipitate and the yellow Cu disks to the interstitial sites inside that same ${\theta }^{\prime }$ structure [12]. The arrow at the ${\theta }^{\prime \prime }\mathrm{\text{−}}{\theta }_t^{\prime }$ interface indicates the Burgers vector of a misfit dislocation.

Figure 3

Variation of formation energy per plane $\Delta E^f/n$ vs $1/n$ used to deduce the energy of the sharp and diffuse interfaces.

Figure 4

Step-by-step structural atom model for the growth (lengthening) of a $1.5c_{{\theta }^{\prime }}$-thick unit of ${\theta }^{\prime }$ via motion of the $\alpha -{\theta }^{\prime }$ diffuse interface to the left, i.e., in the $[0\overline{1}0]$ direction, as viewed along [100]. Circles and arrows indicate atomic diffusional shifts. The interface progresses from (a) to (d) via single atomic movements, which include (b) the glide of the dislocation, (c) further glide of the dislocation and the shift of Cu atoms, and (d) additional Cu atom shifts.