Powder Metallurgy Combined with Cryogenic Laser Shock Peening: a Novel Surface Plastic Deformation Process to Achieve Advanced Nanotwinned Aluminum/graphene-CNT Composites

The trade-off between stacking fault energy and capability of twining has been a roadblock in aluminum (Al) composites, due to low dislocation storage and weak strain hardening ability.

The recent extra strengthening and work hardening in gradient twinned architectures had provided an alternative approach to increase balance between nucleated twins, high density of dislocation and stacking faults.

However, there is still a huge challenge to achieve large scale strengthening bulk Al which generally nucleate sporadically and under extreme conditions.

We aim to develop a practical but innovative technique with combining stress concentration and high strain rate deformation at low temperature via powder metallurgy (PM) combined with cryogenic laser shock peening process (CLSP) to fabricate advanced, large scale, high strength twinned Al/graphene-CNT composites with uniform and controlled alignment including nucleated twins and stacking faults.

The results are interpreted by both molecular dynamics simulation and experiments.

During the cryogenic process, the pinning effect of CNTs hinders the escape of dislocations from pile-ups resulting in high stresses in front of graphene-CNT and controlling plasticity via both high strain rate and high pressure.

As local stresses in front of both graphene and CNT exceed the critical stress for twin nucleation, high-density deformation twins can be formed.

PM combined with CLSP enables us to tailor specific deformation nanotwins architecture in bulk Al composite otherwise cannot be achieved by present methods.

Parameters of shock pressure, strain rate and loading temperature for optimal thermomechanical properties and even shock loading direction effect on alignment of graphene and CNTs for better strengthening effect and twinning nucleation in Al are discussed in details.

We expect to demonstrate the feasibility of tailoring nanotwinned architecture in advanced Al composites via CLSP process, which could be put into mass production