Abstract: This study was undertaken to understand the effects of incorporating a trace amount (0.1 at.%) of Ti and/or Mn with low diffusivities on the thermal stability of the precipitation morphologies of the T-Al₆Mg₁₁Zn₁₁ phase in a heat-resistant Al-Mg-Zn-Cu-Ni quinary alloy in terms of the locations of the added solute elements. In the Ti-added alloy, solute Ti was concentrated in the center of the dendritic α-Al phase via a peritectic reaction during solidification. The Ti distribution was retained even after solution treatment at 480 °C. In contrast, in the Mn-added alloy, Mn appeared localized around granular intermetallic phases (or formed Mn-rich phases), resulting in a slight amount of solute Mn in the α-Al matrix. Solute Mn slightly affected the precipitation morphology of the T-phase at 200 or 300 °C. Nevertheless, much finer precipitates were observed in the α-Al matrix (resulting in finer precipitation morphology) with higher solute Ti concentration, indicating suppressed growth and coarsening of the T-phase precipitates by solute Ti. Introducing both Ti and Mn enhanced local coarsening of the precipitates inside the dendritic α-Al phases with higher Ti solute concentrations. The coexistence of solute Ti and Mn promoted the formation of a significantly coarsened stable Al₁₈Mg₃(Ti, Mn)₂ phase accompanied by the decomposition of initial fine T-phase precipitates, leading to mechanical degradation of the alloy in a high-temperature environment for a long period. These results provide new insights into element selection for the design of heat-resistant Al alloys with superior high-temperature creep performances.

Key words: Al alloy, Precipitation, Coarsening, Atom probe tomography, Transmission electron microscopy