Precipitate-Induced Dynamic Strain Aging and Its Effect on the Strain Rate Sensitivity of Precipitation Hardened Aluminum Alloys

Abstract

We examine precipitate-induced dynamic strain aging in precipitation-hardened Al-Cu alloys by combining atomistic simulations, kinetic Monte Carlo, and analytical rate theory. Atomistic simulations were used to characterize (1) the energetics of nearest neighbour Cu<->Al exchanges at dislocation - precipitate junctions and (2) the subsequent change in obstacle strength. For robustness, the simulations were performed with two distinct interatomic potentials. The resulting catalog of local Cu-Al exchange events was used as input for a kinetic Monte Carlo model of the time-dependent evolution of obstacle strength during dislocation pinning at the precipitate. The predicted strengthening kinetics were then embedded in an analytical dynamic strain aging model to predict the strain-rate sensitivity parameter. On the whole, the modeling predicts a low strain-rate sensitivity across a broad range of intermediate quasi-static strain rates, consistent with experimental observations for precipitate-strengthened alloys. The results therefore identify a mechanistic origin of the low strain-rate sensitivity in precipitation hardened aluminum alloys, emerging directly from the kinetics of dislocation-precipitate interactions when nearest neighbour Cu<->Al exchanges are considered.

Figures (8)

• Figure 1: Cu$\leftrightarrow$Al exchanges when the dislocation is far from the GP zone in the Al--Cu alloy. (a) Exchange between a Cu atom from the GP zone and Al atoms at the dislocation core. Six candidate Al sites (A--F) are selected based on their positions relative to the compressive and tensile sides of the dislocation core. The Cu atom chosen for the exchange corresponds to the site with the highest potential energy within the GP zone, resulting in the Cu atom occupying the GP-zone boundary, and the configuration is selected to minimize the energy difference $\Delta W_{ij}$. (b) Single-hop (nearest-neighbour) Cu$\leftrightarrow$Al exchange within the GP zone and its surroundings. The Al atom G is selected to minimize $\Delta W_{ij}$. The corresponding $\Delta W_{ij}$ values for all labeled sites (A--G) are summarized in Table \ref{['tab:deltaWvalues']}.
• Figure 2: Probability distributions of energy changes $\Delta W_{ij}$ for all Cu$\leftrightarrow$Al nearest-neighbour exchanges when the dislocation is pinned at the GP zone. Blue indicates unfavourable exchanges ($\Delta W_{ij}\ge0$), while red highlights favourable exchanges ($\Delta W<0$). The applied shear stress $\sigma_{\mathrm{ap}}$ for each configuration is indicated in the panel titles.
• Figure 3: Energetically favourable Cu$\leftrightarrow$Al nearest-neighbour exchanges for the ADP potential. Panel (i) shows Cu atoms coloured according to if they have any energetically favorable single-hop exchanges, $W_{ij,\mathrm{min}}<0$. Panel (ii) shows Al atoms from two perspectives that are associated with favorable exchanges and colored according to their most favorable exchange energy.
• Figure 4: Correlation between energy change ($\Delta W_{ij}$) and strength change ($\Delta\tau_{ij}$) for Cu$\leftrightarrow$Al exchanges in the GP zone. Top row: ADP potential; bottom row: NNP potential. Blue: strengthening ($\Delta\tau_{ij}\ge 4$ MPa); red: weakening ($\Delta\tau_{ij}\le -4$ MPa); black: neutral.
• Figure 5: Strengthening-type favourable exchanges ($\Delta W_{ij}<0$ and $\Delta\tau_{ij}\ge4$ MPa) for the 60$^\circ$ orientation under tensile (a) and compressive (b) loading. Brown spheres: Cu in GP zone; blue spheres: Cu atoms undergoing exchanges; red spheres: corresponding Al atoms. Black lines indicate the dislocation cores. Panels (i) and (ii) correspond to views taken along the $+y$ and $-y$ directions, respectively, providing complementary perspectives of the same exchange event on opposite sides of the GP zone. The green arrows show the direction of dislocation glide.