Optimizing mechanical properties in Al-Li alloys through synergistic regulation of dislocations and precipitates during creep aging

Abstract

Creep aging is a coupled process that integrates both processing and precipitation strengthening in age-hardening alloys. However, inhomogeneous precipitation resulting from dislocation accumulation can significantly degrade the mechanical properties. In this study, an attempt was made to establish a correlation between creep stress, dislocation density and precipitation strengthening to address the degradation in plasticity. The results show that the application of creep stress does not significantly change the low-angle grain boundaries (LAGBs). The distinct microstructure of the T₁ phase and the disappearance of the precipitation-free zones (PFZs) at the grain boundaries (GBs) are the main factors responsible for the improved mechanical properties. The micro-elastic lattice strain induced by creep stress can effectively mitigate the depletion of dislocations by aging precipitation, and the nucleation kinetics of precipitates can be significantly enhanced. During the creep aging process under 150 MPa stress, the T₁ phase forms a sandwich structure via interstitial nucleation, with its number density increasing from 2.25 × 10¹⁴/m² to 4.04 × 10¹⁴/m². The dislocation density induced by creep stress also contributes to the narrowing or disappearance of the PFZs at the GBs, further improving the capacity for localized plastic deformation. Finally, the Al-Li alloy with excellent ultimate tensile strength (∼754.0 MPa), yield strength (∼718.1 MPa) and elongation (∼8.8%) was obtained. This study provides strategies for optimizing the microstructure and improving the mechanical properties of Al-Li alloys.