Researchers of Xi'an Jiaotong University (XJTU) have proposed a new mechanism for strain-hardening of ultra-strong nano metals and designed appropriate novel high-performance alloys.

The results were published on April 13 in Nature under the title of Uniting tensile ductility with ultrahigh strength via composition undulation. (Link to the paper: https://www.nature.com/articles/s41586-022-04459-w)

The co-first authors of the article are Dr. Li Heng from Jilin University, and Professor Zong Hongxiang and Professor Li Suzhi from Xi'an Jiaotong University.

The co-corresponding authors are Associate Professor Han Shuang from Jilin University, Professor Ding Xiangdong and Professor Ma En from the XJTU State Key Laboratory for Mechanical Behavior of Materials, Professor Sha Gang from Nanjing University of Science and Technology, and Professor Liao Xiaozhou from the University of Sydney.

The research team used a nickel-cobalt (NiCo) alloy as the model material and the pulse electrodeposition process to construct composite nanostructures composed of nanograins (grain size 26nm) and their internal multiscale compositional fluctuations (1-10nm) in a face-centered cubic single-phase dual-principal solid solution alloy.

The deliberately exacerbated compositional fluctuations in the preparation contributed to obvious fluctuations in the stacking fault energy and lattice strain field, which occurred at a spatial scale just large enough to effectively interact with dislocations, thereby changing their dynamic behavior and making the dislocation motions appear.

The process has the characteristics of hysteresis and intermittent entanglement, which promotes effective proliferation and storage inside the nanocrystalline grains and improves the strain-hardening ability of the material.

In addition, because the dislocation lines no longer run straight and evenly, the nano-segments are "uncaptured" piece by piece. This activation process increases the strain rate sensitivity of the dislocation motion and enhances the strain rate-hardening capability.

Under the combined effect of strain hardening and strain-rate hardening, the nanoalloy exhibits a unique optimized configuration of strength and plasticity at ultra-high flow stress levels, and achieves single-phase face-centered cubic metals (including traditional solvent-solute solid solutions) at unprecedented new heights: the yield strength of the material reaches 1.6GPa, the highest tensile strength is close to 2.3GPa, and the tensile fracture strain can reach 16%.

Composite nanostructures composed of nanoscale grains (a, b, c) and intragranular multiscale compositional fluctuations (d, e, f) in Ni50Co50 alloys. 

Component fluctuations are distributed as a three-dimensional network (g), and regions of different components are separated by "component boundaries" (h). 

(i) Tensile engineering stress-strain curve of Ni50Co50 alloy. 

The image also shows the tensile curves of nanocrystalline Ni, nanocrystalline Co and multilayer nanostructured NiCo alloys at the same strain rate for reference and comparison. 

(j) The yield strength-strength-plastic product relationship diagram of nanocrystalline metal materials. 

The measured properties of different batches of nanocrystalline Ni50Co50 alloys at different strain rates are indicated by red five-pointed stars.

This study demonstrates a strengthening mechanism based on the interaction between apparent compositional fluctuations at the nanoscale (1-10 nanometers) and moving dislocations, which is different from traditional solid solution strengthening based on atomic radius differences -- that is, single solute atoms and dislocations interaction between stress fields.

When the grain size inside the metal material is reduced to the nanometer scale, the strength of the material will be greatly improved according to the Hall-Petch relationship. 

However, when the nanocrystalline metal is plastically deformed, it becomes extremely difficult for dislocations to remain inside such small grains, resulting in the material losing its strain-hardening ability, and it is easy to localize plastic deformation and cause instability.

By choosing the appropriate alloy system or preparation process, this structure-composition composite control concept is expected to open up new ideas for the design and development of new alloy materials.