Chinese scientists proposed a countertrend strategy of work hardening to solve the long challenge of low ductility in the nanostructure (NS). The record-leveled balance was then obtained in a multi-principal element VCoNi alloy, achieving high ductility of 20% and ultrahigh yield strength of 2 GPa simultaneously at both room and cryogenic temperatures. This work, published in Nature Materials on April 11, was conducted by Prof. WU Xiaolei from the Institute of Mechanics of the Chinese Academy of Sciences (CAS).

Yield strength and tensile ductility are two important mechanical properties in metallic materials for structural applications. Unfortunately, these two properties are conflicting mutually. Ductility almost disappears completely in a NS. The problem is the difficulty in dislocation production and accumulation. This is because the conventional dislocation sources, either inside grains or at grain boundaries, fail to work. As a result, an irreversible necking will happen prematurely, showing little ductility. The challenge, then, is how to produce and reserve dislocations.

"The history of Lüders band (LB) spans more than a century and a half. Conventionally, the LB is seen as an initial tensile response in mild steels of low strength. Generally, it has been taken for granted that deformation physics of LB propagation is similar, regardless of the yield strength of the material. However, it is not known whether there is a difference between the Lüders banding in the microstructures with UHYS and low strength, let alone the influence of LB propagation on work hardening." said Prof. WU Xiaolei.

In this study, the researchers revisited the LB and found new deformation physics when the LB appears in a NS with ultrahigh yield strength. To this end, in situ experiments of two kinds have been conducted, along with transmission electron microscopy observations and finite-element-method simulations. These tools and analyses successfully revealed an unexpected role of Lüders banding in how to rapidly produce the dislocations.

There are two findings. The first is the premature necking that happens already at the LB front once the LB begins to propagate. The second is the rapid dislocation multiplication and accumulation at the LB front. These two phenomena are unexpected and subvert conventional textbook wisdom on the LB. 

By means of the finite element simulations, triaxial stress was observed at the LB front due to geometrical inhomogeneities. Triaxial stress induces the dislocation production, triggering work hardening of two kinds. One is the forest dislocation hardening, while the other is the hetero-deformation-induced hardening. Particularly, the latter, as an extra but indispensable part, is ascribed to the interaction of moving dislocations with the local chemical orders (LCOs)— built-in microstructural heterogeneities in the VCoNi alloy. Once the LB starts to migrate in a NS, a premature necking will be induced, immediately producing work hardening which in turn restrains and stabilizes the necking. This is actually an "instability-restrained" strategy induced by an ongoing instability, rather than existing conventional strategies to "postpone the instability ". The approach can be easily transferrable to other kinds of ultrastrong alloys in which the plastically unstable LB appears. This work further shows the convincing role of LCOs in work hardening.

This work was supported by the National Key Research and Development Program of Ministry of Science and Technology of China, the Strategic Priority Research Program of CAS, and Nature Science Foundation of China.