Abstract: An Al₂O₃ dispersion strengthened (ADS) alloy with an ultra-high softening temperature of ∼1200 K was fabricated by the in-situ internal oxidation and reduction methods. The evolution of the nanometer Al₂O₃ particles, grain size, and consequently the softening behavior of this ADS alloy, were investigated by conducting the annealing treatments in the range from 673 K to 1273 K for 60 min. These refined nanometer Al₂O₃ particles were found to be highly stable at elevated temperatures, leading to the high dislocation density and grain boundary stability of the matrix. The average grain size was found to increase extremely slowly from ∼0.60 μm to ∼0.74 μm with increasing annealing temperatures from 773 K to 1273 K. A criterion for grain boundaries migration and softening was established based on the competition between grain growth and pinning effect of Al₂O₃ particles. The strong pinning effect of Al₂O₃ particles was found when the grain size was between the lower limit (about 0.4-0.5 μm) and upper limit (2.18 μm). The occurrence of softening behavior was attributed to the rapid increase of the proportion of grains larger than the upper limit. A modified Hall-Petch relationship was established by introducing the integration of the grain size distribution, which can describe this correlation between softening behavior and the pinning effect of Al₂O₃ particles. The current study not only sheds light on the further understanding of the softening mechanism of ADS copper alloy but also provides a useful route for designing copper alloy with high softening resistance.

Key words: ADS copper alloy, Nanometer-scaled Al₂O₃ particles, Grain growth, Softening mechanism, Softening model