The hardening properties and the operating fracture mechanisms in Al-40wt% Zn have been studied through stress-strain relations obtained in the temperature range from 503 to 623K. The linear work-hardening coefficient χ, the fracture time t f , the yield stress σ y , and the fracture stress σ f of alloy samples slowly cooled or quenched from 643K to room temperature decreased with increasing deformation temperature and exhibited two minima at both 533 and 593K. Two corresponding maxima exist at the same temperatures for fracture strain f and fracture strain rate f . The energies activating the operating fracture mechanisms in the low-, the intermediate- and the high-temperature ranges were found to be 29.4, 64.9 and 71.76kJ/mol, respectively, indicating a temperature-dependent phase transformation.