Mechanical properties such as the strength and coarse-grained plasticity of ultra-fine grained grains in titanium metal

(a) EBSD images, recrystallized grain layers with a grain size of 1-5 μm are distributed in the nanocrystalline matrix (black contrast); (b) "soft-hard" layer microstructures of titanium (HL) with ultra-fine grain (UFG) strength and coarse-grained (CG) plasticity; (c) "soft-hard" lamellar microstructure Titanium yield strength and plastic synergistic characteristics, comprehensive mechanical properties than the typical titanium alloy even better.

Metals have two important mechanical properties, namely strength and uniform tensile elongation (referred to as plastic). High strength can make metal bear a large load before plastic deformation occurs, while large plasticity can make it undergo a large plastic deformation before destruction, to avoid sudden damage. The strength and plasticity of the material depend on the size of the internal grain size. The traditional coarse grain has a large plasticity. When the grain size is reduced to the nanometer size, the strength is significantly improved, but almost all the plasticity is lost. One of the greatest challenges people face is how to achieve both extreme performances at the same time, namely the high strength and coarse crystallinity of nanocrystalline metals, which is particularly difficult to achieve at high strength ends. Recently, the Nanomechanical Behavior Task Force of the Institute of Mechanics at the Chinese Academy of Sciences in cooperation with researchers from North Carolina State University in the United States has developed a new microstructure in metallic titanium that not only has the high strength of an ultra-fine crystalline structure, but also has traditional Coarse crystal large plasticity. They used asynchronous rolling technology and annealing to transform conventional titanium into a “soft-hard” composite lamellar microstructure in which high-strength, ultra-fine-grained “hard” lamellar substrates were used to disperse volume fractions. A "soft" layer of approximately 25%, large plastic recrystallized grains.

A notable feature of the "soft-hard" ply microstructure is its large work hardening capacity, even exceeding the coarse grain structure, which has never been observed before. Through tensile unloading/reloading experiments, they found that the "soft-hard" ply exhibited a significant Bauschinger effect. They pointed out that this is due to the back-skin hardening effect, and proposed the mechanism of back stress formation, that is, the "soft-hard" laminations are inconsistent in plastic deformation during tensile deformation, and a large number of plastic deformations come from "soft" laminations. bear. This strain redistribution forms a geometrically necessary dislocation and a dislocation packing at the interface of the "soft-hard" layer, resulting in back stress hardening. In contrast, in the homogeneous microstructure there is only hardened Liner dislocation and no back stress hardening is observed.

More specifically, although the volume fraction of "soft" laminae reaches 25%, the total strength can still reach the strength of ultra-fine grain. This contrary to the common sense of textbooks results from back stress strengthening. When the external stress reaches the "soft" ply yield strength, they try to start plastic deformation. However, they are completely surrounded by "hard" laminations and cannot be deformed, resulting in geometrically necessary dislocations plugging up at the "soft-hard" lamellar interface, forming a large back stress reinforcement until the "hard" laminae begin to yield. In other words, the "soft" ply becomes almost as strong as the "hard" ply under back stress.

Their work proposes a new idea that can simultaneously obtain high-strength and coarse-grain plasticity of ultra-fine grain. Such a perfect combination of mechanical properties has never been achieved before. At the same time, this idea has a great deal of possibilities to apply to other metal materials. Moreover, since the "soft-hard" lamination microstructures are obtained through the most common cold-rolling forming technology in the industry, it is easy to obtain large-scale production and application in the industry.

Their research work has been published online in the Proceedings of the National Academy of Sciences of the United States of America (Heterogeneous Lamella Structure Unites Ultrafine-Grain Strength with Coarse-Grain Ductility. Wu Xiaolei, Yang Muxin, Yuan Fuping , Wu Guilin, Wei Yujie, Huang Xiaoxu, Zhu Yuntian). The research work was funded by the National Natural Science Foundation of China and the “973” Nano Special Funding.