Al-4Mg (wt. pct) was processed by upset forging in the temperature range 300K-800K and strain rate range 0. 02 to 200s** minus **1. Hardness and tensile properties have been studied. On the basis of microstructure, ductility, and equations in the literature, the temperature-strain rate regimes of damage due to cavity formation at hard matrix particules and wedge cracking at grain boundaries have been isolated. The tensile fracture features are in confirmity with these damage mechanisms.