Aging behavior and strengthening mechanisms of coarsening resistant metastable ȡ precipitates in an Al-Cu alloy

An Al-Cu alloy micro-alloyed with Mn and Zr (ACMZ) was examined to understand the thermal stability and strengthening mechanism of metastable theta'-Al2Cu precipitates with interfacial segregation after prolonged thermal exposure. The microstructure was characterized at multiple scales with techniques including synchrotron x-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, and atom probe tomography. The theta' precipitates did not exhibit measurable coarsening after thermal exposure at 300 degrees C for 5000 h. Kinetic effects of Mn and Zr interfacial segregation, which dominate over thermodynamic effects under these conditions, were necessary to understand the complete inhibition of precipitate coarsening. The theta' phase fraction was stable during the 5000 h exposure. This stable phase fraction was regarded as the metastable equilibrium value and was smaller than that predicted by the theta' solvus line of the ACMZ alloy. As expected from the observed phase stability, the alloy hardness also remained stable during the 5000 h exposure. An Orowan mechanism alone was inadequate to explain theta' precipitate strengthening. Additional strengthening mechanisms by theta' precipitates specifically related to the transformation strain may explain the observed hardness values. (C) 2020 The Authors. Published by Elsevier Ltd.