1. Interface bonding mechanism of titanium/steel composite plate
The interface bonding between heterogeneous metal materials can be divided into three types: physical bonding, chemical bonding, and mechanical bonding. Van der Waals forces, hydrogen bonds, and other physical bonds are commonly found in polymer based composite materials. Chemical bonding refers to the interaction and chemical reaction between two materials under specific process conditions, resulting in the formation of a chemical bond, commonly found in metal based composite materials. The mechanical bonding force mainly includes frictional force, which is determined by the surface roughness of the material. The higher the surface roughness of the metal in composite materials, the greater the mechanical bonding force. Almost all composite materials have mechanical bonding force. Based on the above three types of combinations, researchers have proposed composite material composite theory from multiple perspectives.
(1) Diffusion theory
The diffusion theory holds that two heterogeneous materials will release enormous deformation heat due to close contact during large plastic deformation processing. As the deformation heat gradually accumulates, these deformation heats will stimulate diffusion between different elements, thereby forming a metallurgical bonding layer.
(2) Recrystallization theory
According to the recrystallization theory, the heterogeneous atoms in the interface contact area of the metal plate undergo rearrangement, gradually approaching the lattice constant, and the metal atoms are rearranged, resulting in the formation of common grains on the two metal plates. The recrystallization theory is only effective for the composite process of hot metal sheets and cannot explain the bonding phenomenon of metal slabs at low temperatures.
(3) Metal bond theory
When metals of different materials are pressurized, they gradually approach each other, and as the distance between atoms decreases, from repulsion to attraction, metal bonds are formed. The subsequent energy theory proposed that interface atoms must meet certain energy conditions to form metal bonds, which is a supplement to the metal bond theory.
(4) Thin film theory
When metal undergoes plastic deformation, the surface hardening layer or oxide layer is damaged, and fresh metal is squeezed out under the action of rolling force, achieving interfacial bonding.
(5) Mechanical combination theory
The so-called mechanical bonding refers to the mutual connection between the matrix and the reinforcing body solely based on mechanical bonding force. It is a composite material formed by the matrix and the reinforcing body under the action of friction, but this form of composite plate can only withstand a single longitudinal load.
The mechanism of interface bonding is complex and diverse, and the interface bonding mechanism between different metal materials is also different. It is difficult to comprehensively describe it with a single composite mechanism. In practical applications, the interface of composite materials often has multiple different bonding mechanisms simultaneously.
Fick's first law can describe steady-state diffusion well, but in most cases, diffusion belongs to non steady state diffusion. In the case of unsteady diffusion, the concentration of a substance varies depending on the diffusion distance and time, and Fick's first law does not apply. To address this issue, Fick's Second Law was proposed, which can effectively solve the problem of unsteady diffusion
Atomic theory
Atomic theory explains the mechanism of diffusion. According to atomic theory, three diffusion mechanisms can be discovered: gap, exchange, and vacancy. The schematic diagrams of the three diffusion mechanisms are shown in Figure 1-5.
Gap diffusion mechanism: When there are small-sized atoms in the crystal gaps, these atoms can diffuse in the lattice gaps. In the process of diffusion, diffusing atoms move from one gap through adjacent atoms to another lattice gap, causing lattice distortion, commonly seen in atomic diffusion in interstitial solid solutions.
Exchange diffusion mechanism: Solute atoms and solvent atoms have similar sizes and achieve diffusion through position exchange. This type of positional exchange can cause significant lattice distortion, and due to the different diffusion coefficients of dissimilar atoms, it is difficult to achieve diffusion through the exchange mechanism, which is only applicable to atoms of the same type. Therefore, its role in alloys is very limited.
Vacancy diffusion mechanism: Vacancy refers to a defect in the crystal structure that represents the missing position of an atom or ion in the crystal structure. The vacancy diffusion mechanism refers to the transition and diffusion of particles or vacancies within solid materials from one crystal lattice point to another.
2. Factors affecting diffusion
Previous studies have shown that factors such as temperature, pressure/pressure, crystal structure, internal defects of crystals, and chemical composition have a significant impact on diffusion rate
(1) Temperature
When the temperature rises, the collision frequency and energy between particles inside the substance increase, and the particle velocity also increases, which makes it easier for particles to diffuse from high concentration areas to low concentration areas.
(2) Crystal structure
The atomic diffusion mechanism described in atomic theory mainly includes vacancy diffusion, gap diffusion, or exchange diffusion. Regardless of which diffusion mechanism atoms use for diffusion, their diffusion trajectories need to pass through lattice nodes or lattice gaps. The original complete crystal structure is greatly affected by the crystal structure and crystal type due to lattice distortion caused by heterogeneous atoms.
(3) Crystal defect
Crystal structure defects can be divided into point, line, and surface defects. The diffusion rate of point defect materials has a promoting effect, while the influence of line defects and surface defects on diffusion is more complex. Different types and quantities of defects can have varying impacts on the diffusion process.
(4) Pressure/Pressure
As the pressure increases, the average spacing between particles decreases and their interactions become stronger. It makes it easier for particles to move from high concentration areas to low concentration areas, thereby increasing the speed of diffusion.
