Anomalous strain-energy-driven macroscale translation of grains during nonisothermal annealing

We report a mode of grain growth, involving the macroscopic translation of grain centers during nonisothermal annealing. Through synchrotron high-energy x-ray diffraction microscopy, we find dissolution of semicoherent precipitates generates dislocations, thereby raising the stored strain energy within grains. The subsequent evolution of grains shows unexpected grain translations over length scales of 10–100 μm. Phase-field simulations reveal that such translations are not uncommon in strain-energy-driven grain growth, wherein different regions of a grain may grow and shrink simultaneously.

Figure 1

(a) Grain maps at 650 °C ($S_1$, left) and 730 °C ($S_2$, right) colored based on the inverse pole figure relative to the specimen $\widehat{z}$ axis. Reconstructed datasets are displayed at an azimuth angle of 20 ° and an elevation of 10 ° for perspective. Grain A undergoes significant displacement and is marked for reference. Particles are excluded for clarity. (b) Dislocation density maps in a cross section of states $S_1$ (left) and $S_2$ (right) with grain A indicated. Yellow regions correspond to high dislocation density and blue the converse. Red regions represent α particles. The arrows point to a notch used for sample alignment.

Figure 2

Mean dislocation density averaged over each grain at 650 °C $vs$ the volume fraction of α precipitates within the grain at 505 °C (α solvus: 726 °C), showing a positive correlation. Each point represents one grain tracked between the two datasets. Grain A is indicated for reference. A linear fit with Pearson correlation coefficient of 0.52 is shown, along with 95% confidence interval bounds.

Figure 3

Phase-field simulations of grain growth. (a) Initial arrangements of the grains. (b) Intermediate state with 77 grains and (c) final state with 69 grains (with a stored energy term). (e) Intermediate state with 77 grains and (f) final state with 69 grains (without stored energy term). (d) Average displacement of the grains’ center of mass. The time axis of the capillary-driven case is rescaled so that two curves visually have the same initial slope. (g) Translations of grains A and B. Blue, green, and red outlines indicate the positions of GBs at time $t^{*}=0$, 250 and 500, respectively, of the two grains. Dislocation density shown in the figures is normalized with respect to the mean ($1.25\times {10}^{13}{\mathrm{m}}^{-2}$) of the dislocation densities of the grains in the initial condition. Color indicates the normalized dislocation density where the stored energy is considered (a)–(c) and indicates different grains otherwise.