Mechanisms For Stabilizing Theta '(Al2cu) Precipitates At Elevated Temperatures Investigated With Phase Field Modeling

While most Al-Cu and Al-Si-Cu alloys strengthened by the metastable theta' phase exhibit extensive microstructural degradation above 200 degrees C, recent experimental work has demonstrated that theta' precipitates can be stabilized to 350 degrees C by microalloying additions of Mn and Zr, resulting in improved mechanical properties at elevated temperatures. The present work utilizes phase field modeling to study the relationship between microalloying solute elements and the coarsening resistance of theta'. Simulations are designed to parse out the relative influence of various stabilization mechanisms on microstructural evolution of theta' precipitates at elevated temperatures. Specifically, a ternary alloying element is added to a virtual microstructure to study the operation and effectiveness of stabilization mechanisms including solute drag, diffusion barriers, interfacial energy reduction, and lattice strain modification. Simulation results are compared with atom probe tomography observations. The simulations rationalize experimental observations of microstructural evolution and solute segregation in Al-Cu-Mn-Zr alloys, and reveal the interlinked thermodynamic and kinetic mechanisms that determine the elevated temperature stability of theta' precipitates.