Ultra-strong nanotwinned Al-Ni solid solution alloys with significant plasticity

Phase Engineering of Nanostructural Metallic Materials: Classification, Structures, and Applications

Metallic materials are usually composed of single phase or multiple phases, which refers to homogeneous regions with distinct types of the atom arrangement. The recent studies on nanostructured metallic materials provide a variety of promising approaches to engineer the phases at the nanoscale. Tailoring phase size, phase distribution, and introducing new structures via phase transformation contribute to the precise modification in deformation behaviors and electronic structures of nanostructural metallic materials. Therefore, phase engineering of nanostructured metallic materials is expected to pave an innovative way to develop materials with advanced mechanical and functional properties. In this review, we present a comprehensive overview of the engineering of heterogeneous nanophases and the fundamental understanding of nanophase formation for nanostructured metallic materials, including supra-nano-dual-phase materials, nanoprecipitation- and nanotwin-strengthened materials. We first review the thermodynamics and kinetics principles for the formation of the supra-nano-dual-phase structure, followed by a discussion on the deformation mechanism for structural metallic materials as well as the optimization in the electronic structure for electrocatalysis. Then, we demonstrate the origin, classification, and mechanical and functional properties of the metallic materials with the structural characteristics of dense nanoprecipitations or nanotwins. Finally, we summarize some potential research challenges in this field and provide a short perspective on the scientific implications of phase engineering for the design of next-generation advanced metallic materials.

First-principles calculations for understanding microstructures and mechanical properties of co-sputtered Al alloys

Recent experimental studies show that co-sputtering solutes with Al, together, can refine columnar grain size around few tens of nanometers and promote the formation and enhance the stability of planar defects such as stacking faults (SFs) and grain boundaries (GBs) in Al alloys. These crystal defects and fine columnar grains result in high strength, enhanced strain hardening and thermal stability of Al alloys. Using first-principles density-functional theory (DFT) calculations, we studied the role of eleven solutes in tailoring kinetics and energetics of adatoms and clusters on Al {111} surface, stable and unstable stacking fault energies, and kinetic energy barriers for the migration of defects. The calculations show that most solutes can effectively refine columnar grain size by decreasing the diffusivity of adatoms and surface clusters. These solutes do not necessarily decrease the stacking fault energy of Al alloys, but reduce the formation energy of faulted surface clusters and increase the energy barriers for the recovery of faulted surface clusters. Correspondingly, the formation of SFs is kinetically promoted during sputtering. Furthermore, solutes are segregated into the core of Shockley partial dislocations and play a pinning effect on SFs, SF arrays and twin boundaries, enhancing the thermal stability of these crystal defects. These findings provide insights into the design of high-strength Al alloys for high-temperature applications.

Remarkable toughness of a nanostructured medium-entropy nitride compound

A novel medium-entropy nitride (MEN) - CrCoNiN doped with Al and Ti was prepared using magnetron sputtering. The new MEN possesses a single-phase face-centered cubic (FCC) structure, offering a superior combination of hardness (∼21.2 GPa) and fracture toughness (∼4.53 MPa m^1/2) that surpasses those of most of the conventional and high-entropy ceramics. The ultrahigh hardness value is attributed to a combined effect of lattice friction, solid solution, nanograin structure and compressive residual stress. The exceptional damage tolerance of the new nitride is underlain by the formation and operation of multiple steady shear bands and amorphization mediated by dislocation accumulations. The discovery of the deformation-induced amorphization and extensive shear banding in the MEN, in conjunction with the mechanistic understanding of the critical roles of high dislocation density and large lattice resistance in dislocation-mediated solid-state amorphization, opens up a new frontier for the development of damage-tolerant MPENs for application under extreme loading conditions.

Epitaxial nanotwinned metals and alloys: synthesis-twin structure–property relations

Recent works of epitaxial nanotwinned metals and alloys with different stacking fault energies are reviewed to elaborate the relationship among synthesis conditions, intrinsic factors, twin structure and various properties.

Design of super-strong and thermally stable nanotwinned Al alloys via solute synergy

Al alloys have widespread industrial applications. However, their mechanical strength is often much lower than steels. Here, we investigate the influence of solutes on achieving ultrahigh strength and thermal stability of nanotwinned Al alloys. In situ micropillar compression tests show the addition of a small amount of Ti can significantly increase the mechanical strength of Al-Ni alloys to 2 GPa. Deformation induced detwinning, Ni segregation and grain coarsening as discovered in binary Al-Ni alloys are mostly absent in the ternary Al-Ni-Ti alloys. Moreover, the ternary Al-Ni-Ti alloys have outstanding thermal stability. Density function theory calculations reveal the synergetic pinning effect of Ni-Ti solute pairs on incoherent twin boundaries. This study demonstrates that the proper selection of synergistic solute pairs is critical to improve the thermal stability and mechanical properties of nanotwinned Al alloys.