On the serrated yielding in Cu-14.1 at%AI polycrystals

Abstract

Cu-14.1 at %Al polycrystals with three different grain sizes (76,113 and 157 μm) have been tested for the Portevin-LeChatlier effect under various conditions of temperature (−196 to 200°C) and strain rate (2.78 × 10⁻5 to 5.56 × 10³ sec⁻¹). In the above range of strain rate, serrated yielding was observed in the temperature range 60 to 160° C. The strain rate dependence of the onset of serrations is most probably due to the diffusion of vacancy-solute atom pairs, as indicated by the low value of the activation energy for migration (0.77 eV). The correlation governing the test variables at the onset of serrations appears to be:$\dot \varepsilon$ = (const)ε₀ ^{2.2 ± 0.2} μ^{−0.87 + 0.03}, where$\dot \varepsilon$, ε₀ and μ are strain rate, critical strain for the onset of serration and grain size, respectively. The onset of serrations is most probably due to dynamic strain ageing, although the possibility of short range ordering is not ruled out.