The weakest size of precipitated alloys in the micro-regime: The case of duralumin

In the microsize regime, all crystalline metals studied to-date exhibit a smaller-is-stronger size effect. Here, we report an unusual weakest-size phenomenon in the precipitated alloy duralumin 2025, i.e., below a critical size of ∼7 μm, the strength increases as the size decreases, while above this size, the strength increases toward the bulk value with increasing size. At the critical size, strain-hardening is also slowest and the room-temperature creep is fastest. Interestingly, the reduction of strength at the weakest size is more significant for the peak-aged state of duralumin 2025 than its naturally aged state. Theoretical modeling shows that at the weakest size, both strengthening mechanisms of precipitation hardening and dislocation starvation are ineffective. The present results indicate that the conventional wisdom of precipitation hardening is not applicable in the micro-regime, and the common smaller-is-stronger understanding is incorrect when material microstructures impose internal length scales that can affect strength.