Dynamic impact response of nanostructured and hybrid aluminum alloys

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Aluminum alloys are choice materials for application in aerospace structures and as protective armor in military combat vehicles due to their low density and high specific strength. They offer unique advantages of weight reduction leading to fuel efficiency in aircraft and easy maneuverability in combat vehicles. In these applications, Al alloys must have good failure resistance to impact load, whether from bird strike for aircraft or projectiles in combat vehicles. When exposed to dynamic shock loading, intense strain localization culminating in development of adiabatic shear bands (ASBs) occurs in metallic alloys. This often triggers catastrophic failure. The ability of an alloy to withstand impact failure is determined by its resistance to formation of ASBs. Several high strength aluminum alloys fail catastrophically under dynamic impact loading due to their high susceptibility to formation of ASBs. With increasing incidences of bird strikes due to quieter engines in modern aircraft, and the need for improved lightweight protective armor, development of aluminum alloys with enhanced resistance to impact damage is inevitable. In the proposed study, the microstructures of selected aluminum alloys will be engineered with a view to making them more resistant to impact failure. ******Grain size inhomogeneity is one of the major factors promoting the initiation of ASBs in metals. The bigger grains with lower yield strength deform preferentially and act as initiation sites for ASBs. These bands propagate along the paths of least resistance. Improved Al alloys with homogeneous ultrafine grained (UFG) and nanostructured hybrid structure will be produced and investigated under dynamic impact loading. Al alloys containing UFG and/or hybrid layered structures have been developed and characterized under static loading. However, their resistance to adiabatic shear failure under dynamic impact loading is not well understood. It is anticipated that the homogeneity of the ultrafine grains will reduce the tendency for ASBs formation while the layered structure in the hybrid alloys will offer discontinuity on the ASBs paths and hinder their propagation. This will be verified in this study. Development of UFG aluminum alloys with enhanced resistance to dynamic impact failure is the long term goal. The novelty lies in an optimum combination of the merits of UFG and layered structures to achieve high impact resistance. The study will provide a new body of knowledge that will help to understand the effects of grain size and layered structures on formation of ASB in Al-alloys. It will be beneficial to Canadian aluminum and transportation industries. The research findings will also benefit the Department of National Defence in the area of protective armor improvement for combat vehicles and military helicopters. Highly Qualified Personnel will be trained to contribute to the national economy.