There are many schools on the mechanism of hydrogen embrittlement: hydrogen adsorption theory, pressure expansion theory, interaction theory between hydrogen and dislocation, lattice embrittlement theory, hydride or hydrogen-rich phase precipitation theory, hydrogen assisted fracture (HAC) theory, etc. . Each school has a certain experimental basis and can explain some hydrogen embrittlement phenomena.
In 1952, NTPP (NTPetch) and P. Stabk proposed hydrogen adsorption theory. According to the theory, due to the adsorption of hydrogen on the tip of the crack, the surface energy r of the metal is lowered. According to the Griffith theory, the fracture strength σc of the metal is proportional to r1/2. With the decrease of the surface energy r, the fracture strength σc is also Reduced, causing material embrittlement. NT Pecs et al. believe that the crack surface reduces the surface energy due to the adsorption of argon atoms. When the crack tip is in the cathode state, a large amount of hydrogen atoms are generated due to the cathode reaction, and according to the fracture mechanics, the surface at the tip of the high stress crack will effectively promote the surface adsorption of the hydrogen atom.
The theory of hydrogen pressure expansion was proposed by C. ZapHe in 1947. Hydrogen pressure theory believes that hydrogen is segregated under the action of stress to the pores, holes, mosaic structures, dislocations and other defects inside the material, and combined into hydrogen molecules, causing great pressure in the micropores. 9.8l × 10 ^ 5MPa). The internal pressure and the internal or external stress of the material are superimposed, which will cause the crack to expand and cause cracking. Since the high pressure is controlled by the diffusion rate of hydrogen atoms, the crack propagation is determined by the ability of hydrogen to diffuse in the material. At lower temperatures, hydrogen embrittlement is slow and even stops. Hydrogen pressure expansion theory better explains the formation mechanism of fisheye white spots. When the material is subjected to sufficient tensile stress, microcracks will be generated at the interface between the pores and the substrate or at the interface between the inclusions and the substrate or the inclusion itself, and the hydrogen atoms will be segregated toward the cracks and combined into hydrogen molecules to generate tremendous pressure. Under the action of external tensile stress, the explosion becomes a local brittle fracture zone, and the fisheye white spot with the pores or inclusions as the core is displayed on the fracture.
The theory of interaction between hydrogen and dislocation is considered. The hydrogenogen accumulated in the three-direction stress region at the leading edge of the crack or crack interacts with the dislocation, so that the dislocation is pinned, cannot move freely, and causes local hardening. Under the external force, the matrix cannot be relaxed by plastic deformation. Energy can only be released in the form of cracks. After the crack enters the hydrogen-depleted zone, the dislocation motion recovers freely, and the stress can be relaxed by plastic deformation. The crack stops growing. When the hydrogen concentration at the front end of the crack reaches a critical value, the crack grows again until it finally breaks.
In 1960, ARTroiano proposed the theory of lattice embrittlement. He believes that the high concentration of dissolved oxygen at the grain boundary and phase boundary reduces the bonding force between atoms in the metal lattice. When the tensile stress in a local region is greater than the inter-atomic bonding force reduced by hydrogen, the bonding force between the atoms is destroyed, and brittle fracture occurs.
In 1969, DG West Laka et al. studied the embrittlement caused by hydrides such as Zr-H, Nb-H, VH, etc., and proposed the theory of hydride or hydrogen-rich phase precipitation. The metal material containing hydrogen only causes the plasticity of the material to decrease and embrittle occurs when the hydride or hydrogen-rich precipitate phase precipitates.
In 1972, CDBeachem proposed the theory of hydrogen assisted fracture. He believes that the degree of plastic deformation at the front end of the crack depends on the stress intensity factor K and the concentration of hydrogen. When K is sufficiently high that the hydrogen concentration is sufficiently high, the crack front has a large plastic deformation zone. If K is small, quasi-cleavage or intergranular fracture will occur.
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