Enhanced mechanical properties of Al-4.5 wt.% Cu single crystals with seaweed morphology

While the seaweed pattern is one the most important classes of interfacial morphologies, their mechanical properties are not well understood. Here we produce axial seaweed (AS), tilted seaweed (TS) and degenerate seaweed (DS) by controlling the orientation-dependent interfacial energy anisotropy during the directional solidification of an Al-4.5 wt.% Cu alloy. Characterization of the disordered morphology by electron backscattered diffraction (EBSD) showed that the seaweed growth was oriented in the <110> direction, which was significantly different from that of a <100> regular dendrite. The mechanical properties of seaweed morphology specimens were evaluated using a tensile-testing machine at room temperature. The ultimate tensile strength (σ_b), elongation (δ_f) and toughness of the seaweed morphology specimens reach 186.1 MPa, 45.5% and 72.9 MJ m⁻³, respectively, for AS; 151.6 MPa, 50.9% and 67.9 MJ m⁻³, respectively, for TS; and 164.3 MPa, 62.2% and 89.4 MJ m⁻³, respectively, for DS. With respect to specimens with regular dendrite, the DS specimen exhibited a significant improvement in the elongation and toughness of 109% and 96%, respectively. Additionally, the seaweed morphology specimens show a stronger work hardening effect (n = 0.33, 0.30 and 0.38 for AS, TS and DS specimens, respectively) than the dendrite specimen (n = 0.22). Continuous tip-splitting of the seaweed morphology specimen produced refined and uniformly distributed second-phase particles that contributed to the aforementioned enhancement. Our results provide the underlying insight of the mechanical properties of seaweed morphology specimens, a topic crucial to the design of aluminum alloys.