On the strengthening response of aluminum alloys containing shear-resistant plate-shaped precipitates

The precipitate strengthening increment arising from a reasonably monotonic distribution of plate-shaped shear-resistant precipitates in an Al matrix is considered theoretically in terms of the average stress required to push dislocations between the particles. The radius of the Solute diffusion field surrounding the tips of the plates is shown to be a key microstructural parameter in controlling the distance of closest approach between precipitates and therefore the maximum strengthening increment. Three new strategies for enhancing the strengthening increment are proposed based on minimizing the size of the solute diffusion fields surrounding plates. These approaches are contrasted with those previously advocated. A model Al-Cu alloy containing plate-shaped, shear-resistant theta' (Al2Cu) precipitates is used to experimentally test the predictions of the proposed strengthening description. Quantitative agreement between experiment and theory is observed under isothermal annealing conditions at 473 K. Furthermore, one of the strategies advocated for increasing the strengthening response is the use of a decreasing transformation temperature. This is tested experimentally at three temperatures and the peak strengthening increment is shown to be in excellent agreement with the theoretical predictions. This work Suggests a reinterpretation of the origins of the strengthening observed in current high-strength Al alloys containing plate-shaped precipitates is required. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.