Optimizing the mechanical and corrosion properties of an ultrafine-grained Al-Cu-Mg alloy through cyclic deformation: Clusters and lattice defects

Abstract

To optimize its corrosion and mechanical properties, an Al-Cu-Mg alloy is cyclically processed by high-pressure torsion (HPT), involving a 1/2 turn clockwise and a 1/2 turn counter-clockwise per cycle. The dislocation density increases to 3.04×1014 m−2 on 1 cycle HPT and decreases gradually to 2.23×1014 m−2 for higher cyclic rotations (i.e. 5 cycle HPT). Due to the rapid elimination of the geometrically necessary dislocations, the refinement of grain size in cyclic HPT is more efficient than the monotonic HPT (i.e. it decreases from 2.48 μm to 112 nm on 1 cycle HPT). The HPT samples are electrochemically corroded in a simulated seawater environment. 1-cycle HPT process causes a strong increase in hardness (to 232 HV) and an increase in corrosion potential (to −0.517 V). The trade-off between hardness/strength and corrosion resistance is broken on cyclic HPT processing. Aided by excess vacancies induced by HPT, Cu-Mg clusters form on and near grain boundaries through a ‘vacancy-pump’ mechanism, causing solute depleted zones (SDZ) adjacent to GBs. The combined effects of ultrafine grains, solute cluster segregation and SDZs enhance the strength and corrosion resistance of the Al-Cu-Mg alloys. This study provides a potential strategy to design the microstructures of an Al alloy, and to achieve a higher hardness and a better corrosion performance of that material.