The influence of deformation by equal-channel angular pressing on the ageing response and precipitate fracturing: case of the Al–Ag alloy

Abstract

Severe plastic deformation (SPD) processes reduce drastically the grain size of metals, leading to a substantial increase of mechanical strength. Additionally, it has been observed that the precipitation kinetics in some Al alloys is accelerated after SPD, and thus suitable heat treatments can be used as a strategy for further hardening. In this work an Al–17 % Ag alloy was processed by equal-channel angular pressing (ECAP) and aged at 400 °C for different lengths of time with the purpose of observing and quantifying the kinetics of precipitation and competitive growth of the Al₂Ag phase, as well as the precipitate fracturing and dissolution by ECAP. Microstructure observation of samples aged before ECAP deformation showed that precipitate bending rather than fracturing was the dominant event. The Al₂Ag plates precipitation kinetics were studied on samples aged in the coarse-grained condition and after ECAP; results show that in the former group of samples the kinetics is well described by the Ham, Horvay and Cahn analysis. In the ECAP processed samples, a much faster than expected Ostwald ripening type of growth was observed. Such behaviour was attributed to a dislocation substructure stabilized by the Al₂Ag precipitates.

Appendix

• (i) HHC analysis—parameter β

  $$ \varOmega = \exp \left( \beta \right)\beta^{3/2} A^{2} \int\limits_{\mu = \beta }^{\mu = \infty } {\frac{{{ \exp }( - \mu )\partial \mu }}{{\left\{ {\beta \left( {A^{2} - 1} \right) + \left. \mu \right\}\mu^{1/2} } \right.}}} $$

  The parameters Ω, β and A were defined in the text. The integral of the mathematical parameter μ must be evaluated numerically. The equation was solved using MAPLE 8.0.

• (ii) Boyd and Nicholson equation:

  $$ \bar{l}^{3} - \bar{l}_{0}^{3} = \frac{128}{9}\frac{{\gamma AC_{0} DV_{m}^{2} }}{\pi RT}(t - t_{0} ) $$

  $\overline{l }$ and $\bar{l}_{0}$ are, respectively, the precipitate average diameter after t and t ₀ ageing time, γ the precipitate/matrix interface energy (0.5 J m⁻²), and V _m the molar volume (1 × 10⁻⁵ m³ mol⁻¹). R is the universal gas constant (8.4 J mol⁻¹ K⁻¹) and T is the ageing temperature (673 K). A, C ₀ and D were defined in the text. For the calculation, $\bar{l}_{0}$ and t ₀ were equated to zero.