Dynamic Mechanical Response and Microstructural Evolution of High Strength AluminumScandium (Al-Sc) Alloy

The dynamic mechanical properties of high strength aluminum-scandium (Al-Sc) alloy are investigated using a compression split Hopkinson bar. Dynamic impact testing is carried out at nominal strain rates (abbreviated to strain rate hereafter) ranging from 1.2× 10^3 to 5.8× 10^3 s^-1 at room temperature. The effects of strain rate on the mechanical properties, microstructural evolution and fracture characteristics are investigated and the relationship between the mechanical properties of the alloy and its microstructure is explored. The measured strain-stress curves reveal that the dynamic mechanical behaviour of Al-Sc alloy is highly dependent on the strain rate. The flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but the fracture strain and activation volume decrease. The Zerilli-Armstrong fcc constitutive law is used to model the shear flow response of the Al-Sc alloy. A good agreement is found between the predicted and measured shear flow responses. The Al-Sc alloy specimens fracture as a result of shear band formation and crack propagation within the shear band. SEM observations indicate that the fracture features are dominated by a transgranular dimple-like structure. The density and depth of the dimples decrease with increasing strain rate. TEM microstructural observations reveal that the presence of Al3Sc precipitated particles in the matrix and at the grain boundaries prevents dislocation motion and leads to a significant strengthening effect. An analysis of the dislocation substructure indicates that a higher strain rate increases the dislocation density, thereby reducing the size of the dislocation cells. The variations of the dislocation cell structure reflect different degrees of strain rate sensitivity and activation volume, and correlate well with the impact flow stress--strain response.