Preferentially-orientated gradient precipitates enable unique strength-ductility synergy in Mg-Sn binary alloys

Abstract

Conventional manufacturing approaches, including casting, thermal deformation and annealing, have faced great challenges in achieving both exceptional strength and ductility for Mg alloys. Herein, we report an effective strategy for simultaneously enhancing the tensile yield strength (YS = 341 ± 9.6 MPa) and elongation (EL = 15% ± 1%) of a Mg-4Sn (at.%) binary alloy at room temperature, which has been prepared by an ultrahigh-pressure treatment followed by Joule-heat treatment (UPJT). More attractively, the aging condition (80 μs, 500 Hz, 600 s) is the most time-efficient mode for aged Mg alloys. The reason is mainly associated with the presence of a unique preferentially-orientated gradient precipitate structure, as confirmed by transmission electron microscopy observations, density functional theory calculations and molecular dynamics simulations. Both experimental and theoretical results demonstrate that twin boundary-induced precipitation followed by precipitate-assisted twin boundary migration accounts for the formation of gradient precipitate structures. The fine Mg₂Sn particles can effectively pin dislocation movement to enhance its strength. Comparatively, the coarse Mg₂Sn particles can undergo plastic deformation and shear deformation, contributing to its high ductility. The strategy of preferentially orientated gradient structure provides a budding perspective for designing new Mg alloys with superior mechanical properties.