Understanding the influence of high-strength submicron precipitate on the fracture performance of additively-manufactured aluminum alloy

Abstract

• The formation process of submicron precipitate and its influence mechanism on the mechanical performance are first revealed in the Al-Cu-Ni alloy during additive manufacturing-heat treatment. • The microstructure evolution from manufacturing to fracture is analyzed systematically, and the coordinated strength-ductility performance is achieved by microstructure regulation. • The in-situ dislocation motion and crack propagation are newly studied in Al-Cu-Ni alloy to provide direct evidences for its fracture failure mechanism. • Novel findings are grained that the submicron grain-boundary Al 7 Cu 4 Ni precipitate and dislocation entanglement present the obstruction effect on the intergranular and intragranular cracks, respectively. • Combining ex-situ TEM and DFT calculation reveals the intact and stable structure of the Al 7 Cu 4 Ni precipitate with submicron dimension and high bonding strength. The formation of intermetallic compound has been widely considered as an effective strengthening approach in Al alloy. Its precipitate dimension is a key factor influencing the mechanical performance. Except for the pinning effect of nanosized precipitate, the contribution of submicron precipitate is also nonnegligible. Therefore, establishing the mechanism framework for the relationship of manufacturing process-precipitate structure-fracture performance is of great significance, which is essential and foundational for optimizing the practical service performance of alloys parts. Herein, by taking the Al-Cu-Ni series alloy (e.g. RR350) as background, the study reveals the microstructure evolution of high-strength submicron Al 7 Cu 4 Ni precipitate from fabrication (additive manufacturing-heat treatment) to failure, and its influence mechanism on the fracture behavior. Through the microstructure regulation, a high elongation rate of ∼28.5% and slightly-deteriorated ultimate tensile strength of ∼305.2 MPa are achieved. The in-situ and ex-situ characterizations are employed to analyze the synergy mechanism of strength-ductility performance. Some novel findings are obtained that the submicron grain-boundary precipitates can interrupt the intergranular crack by influencing the stress status, thus decreasing the crack propagation rate and altering its propagation pathways. The entangled dislocation also presents an obstruction impact on the intragranular crack extension by its hardening effect. Moreover, the submicron Al 7 Cu 4 Ni precipitates with high bonding strength can withstand the concentrated stress to maintain a stable structure during alloy fracture, meanwhile present a strengthening effect on α-Al matrix to ameliorate the deterioration of tensile strength. The characterization of dislocation and microcrack evolution, provides direct evidence to the mechanism framework above, and could also provide insights into the strength-ductility coordination for other Al alloys.