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Xijiaotong University small: discovery of a new mechanism of dislocation regulating the growth of zirconium hydride

    Zirconium alloy has excellent comprehensive properties and is widely used as cladding and pressure tube of nuclear fuel. Zirconium alloy is easy to absorb hydrogen to form brittle zirconium hydride distributed along the base plane, which accelerates the embrittlement and failure of zirconium alloy cladding. Clarifying the nucleation and growth mechanism of hydride is of great significance to inhibit the precipitation of hydride and prolong the service life of zirconium alloy cladding. This paper reveals a new mechanism of precipitation and growth of zirconium hydride under dislocation control. In situ study shows that the hydride growth process presents an alternating sequence of dislocation emission hydride growth dislocation re emission hydride re growth.

    According to the migration characteristics of atoms, the solid-state transformation of metals can be divided into diffusion type transformation and non diffusion type transformation (such as martensite transformation). There is another kind of transformation that not only produces shear, but also needs the assistance of solute atom diffusion, that is, mixed type transformation, such as bainite transformation. The precipitation of zirconium hydride has similar characteristics. It is generally believed that the slip of shawclay dislocation and the diffusion of solid solution hydrogen atoms coordinate the precipitation of hydride. However, the detailed mechanism of precipitation and growth is lack of in-depth and intuitive in-situ research. In addition, the hydride in zirconium alloys is always wrapped by complex dislocation structures with bird's nest or butterfly shape. Even if the hydride decomposes during heating, the dislocation structures around it can still be preserved. Therefore, clarifying how to form such a complex dislocation structure in the precipitation process of zirconium hydride and clarifying the relationship between the complex dislocation structure and the phase transition of zirconium hydride are of great significance to understand the phase transition of zirconium hydride, regulate the precipitation of hydride, and inhibit the hydrogen embrittlement of zirconium hydride.

    Figure 1 Dislocation emission regulates hydride growth. (A-D) hydride growth accompanied by front dislocation emission at room temperature; (E, f) dislocation reaction at the front of hydride; (g) The curve of hydride length with time, and the letters in the figure correspond to figure (A-F).

    The precipitation of zirconium hydride usually occurs in the cooling process from high temperature to room temperature. With the decrease of temperature, the solid solubility of hydrogen in zirconium gradually decreases, which is conducive to the nucleation and growth of hydride. This unique growth process also poses a challenge for the in-situ study of zirconium hydride. Recently, the research group of Professor hanweizhong from the school of materials, Xi'an Jiaotong University found that electron beam irradiation can induce the nucleation and growth of hydride in zirconium at room temperature, which provides a simple and easy new method for in-situ study of the phase transition process of zirconium hydride. Based on this method, the whole process of hydride nucleation and precipitation was systematically studied by the research group using transmission electron microscope. Hydride precipitation includes two steps: dislocation emission and hydride growth. In the study, the phase transition process of zirconium hydride is slowed down by reducing the hydrogen concentration (zirconium absorbs a small amount of hydrogen in the double jet process). Using a high-resolution camera, it is captured that dislocation emission and hydride growth are an alternating process, such as dislocation emission hydride growth dislocation re emission hydride re growth (Fig. 1), rather than a synchronous process. High resolution electron microscope observation found that there were uneven steps at the interface between the hydride front and the matrix, which provided favorable conditions for dislocation emission at the hydride front (Fig. 2).

    Figure 2 Atomic structure and stress field of hydride front. (a-c) γ- Hydride and parent phase α- Atomic image of the interface between Zr. (d) Shape model of acicular hydride. (e) Schematic diagram of hydride and prism slip system in Zr. (f) Von Mises stress around the hydride front. (G-I) shear stress component τ The projection of YZ on three cylindrical slip systems.

    Density functional theory calculations show that the hydride phase transition produces tensile stress in the zirconium matrix and compressive stress in the hydride. The tensile stress produced by the volume expansion of zirconium hydride phase transition is enough to stimulate the dislocation slip of the cylinder, cone and base plane in zirconium, which makes it possible to form a complex dislocation structure around the hydride (Fig. 2). Through the calculation and analysis of free energy, it is found that the volume expansion caused by hydride phase transition forms compressive stress, which is not conducive to the continuous precipitation and growth of hydride. When the hydride phase changes, the tensile stress inside the matrix increases the solid solubility of hydrogen at the front of the hydride interface, and the driving force of phase change decreases. When the dislocation is emitted from the hydride front, the tensile stress of the surrounding matrix decreases, and the solid solubility of hydrogen at the interface front decreases, reaching the hydrogen supersaturation state, which promotes the further precipitation of hydride. In addition, dislocation emission can increase the concentration gradient of hydrogen between the hydride front and the zirconium matrix, accelerate the precipitation rate of zirconium hydride, and promote the growth of hydride (Fig. 3).

    Figure 3 Gibbs free energy curve of hydride precipitation. (a) The Gibbs free energy of each Zr atom in HCP and FCT structures varies with hydrogen concentration and stress. (b) Effect of dislocation emission on Zr Gibbs free energy curves of hydride and parent phase. The red curve is α- Gibbs free energy curve of Zr, when hydride grows, α- Tensile stress is accumulated in Zr, Gibbs free energy is reduced, and hydrogen is α- The solubility in Zr increases; When the dislocation is emitted, the stress is released and hydrogen is released in α- The solubility in Zr decreases and promotes the migration of hydride interface. (c) Variation of hydride length with time and its growth rate curve. (d) Hydrogen concentration and hydrogen concentration gradient at the interface before and after dislocation emission.

    In conclusion, the precipitation of zirconium hydride is a mixed phase transition process controlled by dislocation emission and hydrogen diffusion. The precipitation of zirconium hydride causes tensile stress in the surrounding zirconium matrix, which promotes the enrichment of hydrogen to the hydride tip, but the tensile stress increases the solid solubility of hydrogen, which is not conducive to further phase transformation; Dislocation emission releases the tensile stress in the zirconium matrix, reduces the solid solubility of hydrogen, and promotes the growth of zirconium hydride; The growth rate of zirconium hydride after dislocation emission shows the characteristics of first fast and then slow, which is due to the gradual increase of tensile stress in zirconium matrix and internal compressive stress of hydride caused by further precipitation of zirconium hydride, until the phase transition is completely suppressed. The volume expansion stress of zirconium hydride precipitates excites Multiple slip systems in the zirconium matrix, and the dislocation interaction on Multiple slip systems forms a complex bird's nest or butterfly dislocation structure around the hydride.

    The research results were published in the academic journal small under the title of "location mediated hydrid precision in zirconium", with an impact factor of 13.281. Doctoral student liusimian is the first author of the paper, and Professor hanweizhong is the first corresponding author of the paper. The collaborators include Dr. Akio Ishii, Professor Mi Shaobo, Professor Shigenobu Ogata and Professor Li Ju.