On the Thermal Stability of Theta Prime Precipitates in Al-Cu-Mn-Zr alloys

Al-Cu-Mn-Zr (ACMZ) alloys demonstrate vastly improved mechanical properties at temperatures between 250 C and 350 C compared to conventional theta’-strengthened Al alloys. The improved high temperature mechanical properties of Al-Cu-Mn-Zr are due to the thermal stability of strengthening theta’ precipitates against transformation to the detrimental theta-phase. This is illustrated by a study of the high-temperature compression response of an Al-Cu-Mn-Zr alloy that retains theta’ precipitates at elevated temperatures and a conventional alloy that does not. It is shown that the divergence in microstructure leads to a divergence in deformation mechanisms.The thermal stability of strengthening theta’ particles is due to their coarsening resistance. This is because the onset of theta’ to theta transformation is controlled by the morphology of theta’, as determined by a combination of thermodynamic analysis, experimental observations, and phase field modeling.The coarsening resistance of theta’ particles can be achieved through a reduction in the thermodynamic driving force for coarsening, or the introduction of a kinetic rate-limiting step in the coarsening process. This is shown by a parametric phase field study.The most effective means of controlling the thermodynamics and kinetics of theta’ evolution is a combination of interfacial energy reduction and solute drag caused by the segregation of slow-diffusing alloying elements.Based on these mechanisms, phase field simulations are able to reproduce both the coarsening resistance and solute segregation profiles observed in Al-Cu-Mn-Zr-Si alloys, in which Mn and Zr are slow-diffusing elements with the potential to reduce interfacial energy.Finally, the effectiveness of Mn and Zr in stabilizing theta’ precipitates is dependent on the Si content of Al-Cu-Mn-Zr alloys. An optimized Si content yields alloys with reduced hardness after aging but greater hardness after high temperature thermal exposure.It is concluded that the elevated temperature performance of Al-Cu-Mn-Zr alloys depends upon a synergistic combination of thermodynamic and kinetic characteristics that are influential both in the aging step and the alloys’ response to extended thermal exposure.