Origins of Non-random Particle Distributions and Implications to Abnormal Grain Growth in an Al-3.5 Wt Pct Cu Alloy

The mechanisms of abnormal grain growth (AGG) in particle-containing systems have long been a mystery. Recently, we reported that a non-random particle distribution can induce a grain size advantage and trigger AGG. However, the processing conditions leading to a non-random particle distribution are far from being understood. Here, we investigate the particle distribution and concomitant grain growth behavior at different annealing temperatures and times in an Al-3.5 wt pct Cu alloy by scanning electron microscopy (SEM). At high temperatures and long times, the particle distribution evolves from random to non-random, with an accompanying transition from normal grain growth (NGG) to AGG. Analytical calculations suggest that a non-random particle distribution is introduced by residual Cu segregation even after homogenization. In short, the corresponding fluctuation of θ-Al2Cu phase distribution is amplified at elevated temperatures via particle dissolution. We quantify the spatial inhomogeneity of particles through the Gini coefficient and link this important parameter to the critical grain size necessary for AGG. The trends are conveyed succinctly in a temperature–time–(structural) transformation (TTT) diagram, which identifies the onset of AGG in an Al-3.5 wt pct Cu alloy.