Sequential phase evolution of copper mold cast Al-Cu-Ni-(Zr) alloys: Zirconium Micro-alloying results in ultrahigh hardness (∼5 GPa)

The present study reports the evolution of different phases with bimodal microstructure in Al-Cu-Ni-(Zr) alloys during arc melting followed by copper mold casting. Various characterization techniques, including X-ray diffractometry, scanning electron and transmission electron microscopy were employed to study the microstructural features. The study provides an insight into the fundamental of phase transformation by correlating the observed phases with the corresponding ternary and binary phase diagrams. The results demonstrate that the bimodal microstructure and the formation of rod-shaped Al3Zr significantly impact the hardness of the alloy. Noticeably, the copper mold cast Zr micro-alloyed Al80Cu15Ni5 and Al75Cu15Ni10 alloys exhibit unusual high hardness (similar to 5 GPa). The findings suggest that the bimodal microstructure can be tailored to achieve superior mechanical properties in Al-Cu-Ni-(Zr) alloys, providing valuable insights into the microstructural design of such alloys for advanced engineering applications.