The study on the coarsening process and precipitation strengthening of Al3Er precipitate in Al-Er binary alloy

The aging strengthening behavior and coarsening process of Al3Er precipitates were investigated in Al-0.045Er (in at.%) alloys aged isothermally at 300-400 °C, using Vickers micro-hardness measurement and transmission electron microscopy (TEM) observation. The isothermal hardness curves showed that the peak hardness decreased slightly with the increase of temperature. From the TEM observations, the radius of Al3Er precipitates at peak hardness was measured to be 2.4 ± 0.4 nm. The coherency of Al3Er precipitates started to get lost at the radius of 8.0-9.1 nm, which were much larger than the predicted value. The difference was speculated to be due the reduction of misfit between Al3Er precipitates and matrix when particles were very small. From the analysis of the coarsening of coherent Al3Er particles using LSW theory, the diffusion activation energy and the diffusivity pre-exponential constant of Er in Al were deduced to be 77.2 ± 5.3 kJ/mol and (4.3 ± 2.2) × 10−12 m2 s−1, respectively. Finally, the strength increment caused by Al3Er particles was evaluated using precipitation strengthening theory. It was found that when the particle radius was larger than 2.4 nm, the Orowan bypass mechanism dominated. In radius of 0-1.4 nm, a less coherency strengthening contribution indicated a reduction of lattice misfit between particles and matrix. From the strength plateaus in radius of 1.4-2.4 nm, the anti-phase boundary (APB) energy of Al3Er precipitates was deduced to be 0.60 ± 0.03 J/m2.