Nano metals have poor stability due to the introduction of a large number of grain boundaries. Generally speaking, the growth temperature of nanocrystalline grains is much lower than the recrystallization temperature of coarse crystals, and some nanocrystalline pure metals grow up even at room temperature. Poor stability has become a significant bottleneck restricting the preparation and application of nanometals. The traditional method of stabilizing nanocrystals is mainly to reduce the interfacial energy or drag the grain boundary migration through alloying.
In 2018, nanometal scientists from the Chinese Academy of Sciences discovered the abnormal grain size effect of thermal stability of nanocrystals in nanocrystalline pure copper and pure aluminum prepared by plastic deformation, that is, smaller than the critical size. The deformation mechanism changes from dominated by dislocations to governed by incomplete fractures, the grain boundary relaxation mechanism is activated, and the stability of nanocrystals does not decrease but rises. Subsequently, they found that, despite the difference in the internal mechanism of grain boundary migration under heating conditions, this abnormal grain size effect also exists in the mechanical stability of nanocrystals under stress.
It is understood that in some nanometals, such as pure copper, nano-grains grow even at room temperature. This inherent instability brings difficulties to the preparation of nano-metallic materials; on the other hand, it also limits the practical application of nanometals.
The study also found that the abnormal stability of nanocrystals not only occurs in low- and mid-level fault energy metals such as pure copper but also in high-level fault-energy pure nickel. The discovery of ultra-high stability nanocrystals is not only significant for us to understand the deformation mechanism of nanocrystals and the behavior of grain boundaries at nanometer size but also shows the possibility of developing nanocrystals used at high temperatures.
However, the grain size of the pure metals prepared by the currently commonly used severe plastic deformation methods (such as equal channel extrusion, lap rolling, etc.) is usually in the submicron scale, and it is challenging to initiate the grain boundary relaxation mechanism during processing. For example, the grain size of pure copper prepared by severe plastic deformation is mostly in the range of 100-200 nm, and its stability is poor. Recently, researchers have discovered that rapid temperature increase can introduce annealing twins into nanocrystalline copper, thereby achieving "thermal relaxation" of the nanocrystalline grain boundaries and improving the thermal stability of the nanocrystals. One of the difficulties faced by the introduction of annealing twins in nanocrystalline copper is that unstable nanocrystals have a grain growth stability of only 393 K, which is far below the temperature at which annealing twins are generated (——523 K) During the heating process, twins are not produced, and the grains have grown up. Based on the Kissinger effect, the researchers proposed that increasing the heating rate can increase the grain growth temperature without affecting the twinning growth temperature. Therefore, rapid temperature rise not only avoids grain growth but also produces growth twins. The pure Cu with a grain size of about 80 nm was rapidly heated to 523 K at a rate of 160 K / min for 15 minutes and then cooled. The grain size of the material did not change significantly, but the number of twins increased significantly. Like deformed twins, these grown twins can also relax the grain boundaries and enhance the thermal stability of the nanocrystals. After heat treatment, the apparent growth temperature of the nanocrystals increased from below 393 K to above 773 K.
The thermal relaxation method of rapid temperature increase to improve the stability of nanocrystals can be used to enhance the security of submicron and nanocrystals obtained by general severe plastic deformation and is of great significance for the development of highly stable nanomaterials and the promotion of nanometal applications.
Figure 1. Nanocrystalline copper stabilized by rapid heating. (A) Typical cross-sectional scanning electron micrographs of the surface gradient nanostructures of the as-prepared samples and samples heated to 523K at different heating rates (1, 80, 160K / min), respectively. (BD) Preparation samples and samples after heat treatment at different heating rates from the surface-typical transmission photos of nanocrystals at a depth of 25 μm (corresponding depth is shown in the dotted frame in Figure A, the average grain size of the prepared samples at this depth About 80nm). (E) The range of grain size corresponding to the surface stabilized nanocrystals in the example after heating to 523K at different heating rates. The reliable and hollow dots in the figure, respectively, represent the grain size corresponding to the upper and lower boundaries of the stable nanocrystalline layer observed in the experiment. The dotted line represents the upper limit of the average grain size of the stable nanocrystals during the 523K heat treatment of the prepared sample (D * —— 60nm ), The error bar refers to the standard statistical error of the average grain size.
Figure 2. After rapid heating, a large number of twins are formed inside the nanocrystals, and the grain boundaries are planarized. (A) A typical high-resolution photo of nanocrystals in the area shown in Figure 1D. The inset in the upper left corner is an atomic image of the nano twins in the grain. (B, C) Typical grain boundary character distribution in the sample after preparation and heating to 523K at a heating rate of 160K / min. Different colors represent different types of grain boundaries, red is the twin grain boundary, gray is the small-angle grain boundary, black is the significant conventional angle grain boundary, and other colors are other individual bulky lattice interfaces (Σ <29). (D, E) High-resolution photos of typical grain boundaries in the area shown in Figure 1D. The illustration in the upper left corner is an enlarged view of the solid red frame.