Heterostructure control enabling outstanding strength-crack tolerance synergy in a dilute Mg-Al-Mn-Zn-Ce-Nd alloy

Abstract

There exists a severe strength-crack tolerance trade-off in dilute magnesium (Mg) alloys. Herein, a heterogeneous Mg-0.6Al-0.6Mn-0.5Zn-0.2Ce-0.2Nd (A200-10) alloy with a high density of dislocations was obtained through low-temperature extrusion and short-term annealing. The microstructure consists of recrystallized (RXed) and unrecrystallized (unRXed) regions, with a precisely controlled volume fraction ratio of 3:1. The heterogeneous A200-10 alloy exhibits a high tensile yield strength (TYS) of ∼306 MPa and a superior tensile elongation (TEL) of ∼18.4%. Based on quasi-in-situ electron backscattered diffraction (EBSD) and scanning electron microscope (SEM)-digital image correlation (DIC) analysis, we find that plastic deformation occurs preferentially in the RXed regions, mediated by the mobile <a> dislocations. As strain increases, strain gradient gradually accumulates at the interface between RXed and unRXed regions, generating hetero-deformation induced (HDI) strengthening and hardening. Besides, there is significant intergranular slip transfer in RXed regions, which can coordinate partial strain incompatibility. Furthermore, heterogeneous interfaces play a crucial role in enhancing crack tolerance. The heterogeneous interface functions as a bridging ligament to withstand stresses, and activates non-basal slips in the unRXed grains near the crack tip. Such activation of extra dislocations not only alleviates stress concentration but also dissipates the energy essential for microcrack propagation, thus effectively blunting the crack tip. Accordingly, the heterogeneous A200-10 alloy obtains an excellent strength and elongation combination. This work is anticipated to provide a valuable avenue for the development of Mg alloys with outstanding performance by regulating the appropriate heterostructure.