This study uses a torsional split-Hopkinson bar to investigate the shear response and fracture characteristics of unweldable Al-Sc alloy during mechanical testing at shear strain rates of 800 s À1 , 1500 s À1 , 2200 s À1 and 2800 s À1 and temperatures of À150 C, 25 C and 300 C. The experimental results show that both the shear strain rate and the temperature have a significant effect on the shear properties of the Al-Sc alloy. At a constant temperature, the shear stress, fracture shear strain, work hardening rate, yielding shear strength, work hardening coefficient, strain rate sensitivity and temperature sensitivity all increase with increasing strain rate. However, inverse tendencies are observed with increasing temperature at a constant strain rate. It is found that the Kobayashi and Dodd constitutive equation provides accurate predictions of the high strain rate shear plastic behaviour of unweldable Al-Sc alloy. SEM fractographic observations reveal that the fracture surfaces are characterized by a dimple-like structure. The density of the dimples increases with increasing strain rate at a constant temperature or with increasing temperature at a constant strain rate. SEM observations indicate that specimen fracture initiates at the interface of the matrix and the Al 3 Sc precipitates. Finally, twisted shear bands are observed on the equatorial plane of the gauge length section of the deformed specimens. The microhardness of these shear bands increases with increasing strain rate, but decreases with increasing temperature.