AbstractThermomechanical processing has been employed to control the grain and subgrain structure of a powder produced SiC particle reinforced (3μm; 17 vol.-%) Al–Cu–Mg alloy. The present research investigates the effect of grain structure on fracture toughness. Variation in grain size and structure was accompanied by significant changes (up to 50%) in plane stress fracture toughness; toughness was observed to decrease with decreasing grain size. No pronounced modification in fracture path/mode was observed to explain this behaviour. However, a dependency of fracture toughness on work hardening rate was observed. A detailed analysis was performed to interpret the flow properties of the materials studied, particularly the effect of grain structure on work hardening–relaxation mechanisms. The lower work hardening rate and hence toughness exhibited by the fine grained composite is postulated to emanate from stress relaxation by grain boundary sliding in the highly stressed SiC particle/matrix interface region.MST/3002