The poor stability of nanograins in metals, characterized as a strong tendency to coarsen at much lower temperatures compared with their coarse grain counterpart, has become a major bottleneck restricting the preparation and application of nanograined metals.

The traditional methods for stabilizing nanograins are mainly to reduce grain boundary energy or pin the grain boundary through alloying. However, the grain size of pure metals prepared by severe plastic deformation (such as equal-channel angular pressing, etc.) is usually in submicron scale, and it is difficult to activate the grain boundary relaxation mechanism during processing.

In a study published in Science Advances, Prof. LI Xiuyan's group from Institute of Metal Research (IMR) of the Chinese Academy of Sciences found that annealing twins can be introduced by a new approach of rapid heating in nanograined copper, resulting in thermally-induced grain boundary relaxation and improving the thermal stability of the nanograins. Generally, the temperature for grain coarsening (~393 K) in unstable nanograins is usually lower than that for annealing twinning (473-523 K).

Based on the Kissinger effect, the researchers proposed that the grain coarsening temperature can be promoted obviously by increasing heating rate, while the formation of annealing twins is little dependent on heating rate, which provides a potential to initiate annealing twinning before grain coarsening.

In the experiment, the pure Cu with a grain size of about 80 nm was rapidly heated to 523 K at a rate of 160 K / min and then cooled. The grain size did not change significantly after heating, but the number of twins increased remarkably, leading to grain boundary relaxation. After rapid heating, the apparent grain coarsening temperature of grains increased from 393 K to above 773 K.

The approach of thermally-induced grain boundary relaxation can be used to improve the stability of submicron grains and nanograins produced from severe plastic deformation, which is of great significance for the development of highly stable nanomaterials and the promotion of nanomaterials application.

Systematic works on stability of nanograined metals have been conducted at Shenyang National Laboratory for Materials Science of IMR. In 2018, the researchers discovered the abnormal grain size dependence of thermal stability in nanograined pure copper and pure nickel produced from plastic deformation (Science). As grain size is below a critical size, the dominated deformation mechanism of material changes from full dislocations activities to partial dislocations activities, leading to grain boundary relaxation, which promotes the thermal stability of nanograins. Subsequently, the grain size dependence of mechanical stability in nanograined metals were also revealed (Phys. Rev. Lett.), which is highlighted by Science entitled "A size limit for softening" (Science).