Effect of rare earth Nd on the microstructural transformation and mechanical properties of 7xxx series aluminum alloys

Abstract

Al–Zn–Mg–Cu–Zr aluminum alloys have shown promise as materials for drill pipes; however, their application temperature is limited to below 120°C. This study investigates the influence of incorporating the rare earth element Nd on the microstructure and mechanical properties of Al–Zn–Mg–Cu–Zr alloys. The microstructural evolution during casting, homogenization, hot deformation, and heat treatment processes is characterized using optical microscopy and scanning electron microscopy. The composition of the rare earth phase is determined through transmission electron microscopy (TEM). Furthermore, first-principles calculations are employed to determine the formation enthalpy, cohesive energy, shear modulus, bulk modulus, Young’s modulus, and Poisson’s ratio of bulk Al8Cu4Nd. The effect of Nd addition on the mechanical properties of the alloy is investigated through hardness and tensile testing. The results indicate that the addition of Nd significantly refines the grain and dendrite sizes of the alloy and effectively suppresses recrystallization behavior during hot extrusion and solution treatment. TEM observations reveal the presence of micrometer-sized blocky Al8Cu4Nd phases and nanometer-sized Al3Nd phases. The Al3Nd phases are located near dislocations, hindering dislocation movement and thus enhancing the alloy’s mechanical properties. First-principles calculations demonstrate that the bulk Al8Cu4Nd phase exhibits superior structural stability, deformation resistance, and brittle characteristics, which negatively impact the ductility of the alloy. The alloy with Nd addition can maintain a high hardness value for an extended period at high temperature, and the tensile strength of the alloy with 0.26 wt% Nd addition reaches 396.2 MPa at 120°C. These results indicate that the rare earth element Nd can improve the high-temperature mechanical properties of the alloy.

Keywords

• al–zn–mg–cu–zr aluminum alloy
• rare earth phase
• first-principles
• recrystallization
• mechanical property