Analyze the strengthening methods of copper alloy materials
The commonly used strengthening methods for copper and copper alloys include: deformation strengthening, fine grain strengthening, solid solution strengthening, age precipitation (precipitation) strengthening, dispersion strengthening, composite material strengthening, and adding trace elements.
1. Deformation strengthening
Deformation strengthening is to improve the strength and hardness of copper alloy through plastic deformation. It is one of the most commonly used strengthening methods of copper alloy. Since the crystal defects produced by cold working have little effect on the conductivity of the material, this strengthening method improves the strength while still making the alloy highly conductive. The characteristic of deformation strengthening is that while the strength of the material increases, its plasticity decreases rapidly, and the electrical conductivity will also decrease slightly due to the increase in dislocation density. In addition, when the service temperature rises, the material will undergo recovery and recrystallization processes and soften, and single deformation strengthening can only increase the strength of the alloy to a limited extent, so it is often used together with other strengthening methods.
2. Fine grain strengthening
Fine grain strengthening is to use rapid solidification measures or heat treatment methods to obtain fine grains during casting. Certain trace alloy elements can also be added to refine the grains. The grain size is reduced, the strength of the alloy is increased, and it has little effect on the electrical conductivity of the alloy. Therefore, fine grain strengthening has become one of the main strengthening methods for copper alloys. The outstanding advantage of fine-grain strengthening is that it can improve the plasticity of the material while improving the strength of the material. This is because after grain refinement, the stress concentration caused by dislocation accumulation at the grain boundary when the material is deformed can be effectively alleviated, delaying the initiation of cracks, and a larger amount of deformation can be achieved before the material fractures. Grain refinement is widely used because of this advantage.
3. Solid solution strengthening
The phenomenon of increasing the strength and hardness of a metal by incorporating certain solute elements to form a solid solution is called solid solution strengthening. Solid solution strengthening occurs because the dissolution of solute atoms causes distortion in the crystal lattice of the solvent metal, thereby increasing the resistance to dislocation movement. Practice has proved that proper control of the solute content in the solid solution can significantly improve the strength and hardness of the material while still maintaining good plasticity and toughness. For example: adding 19% nickel to copper can increase the phib of the alloy from 220MPa to 380~400MPa, and the hardness from HB44 to HB70, while the plasticity still maintains ψ=50%. If copper were to achieve the same strengthening effect through other means (such as work hardening during cold deformation), its plasticity would be nearly completely lost. Solid solution strengthening is a strengthening method that uses the interaction between solute atoms and moving dislocations in the solid solution to cause an increase in flow stress. By adding an appropriate amount of alloying elements to the base to form a solid solution, the strength of the alloy will generally be improved. According to the Mott-Nabbaro theory, for thin solid solutions, the change in yield strength with the concentration of solute elements can be expressed as: б=бo+kCm. In the formula, б is the alloy yield strength; бo is the yield strength of pure metal; C is the solute atomic mass concentration; k and m are constants determined by the properties of the matrix and alloy elements, where the value of m is between 0.5 and 1.
4. Aging precipitation (precipitation) strengthening
The basic principle of age precipitation strengthening is to add alloying elements to copper that have a very small solid solubility at room temperature and a large solid solubility at high temperatures. Through high-temperature solid solution treatment, the alloying elements form a supersaturated solid solution in the base. This The strength is improved compared with pure copper. Then through aging, the supersaturated solid solution decomposes, the alloy elements precipitate in a certain form, and are dispersed and distributed in the base to form a precipitation phase. The precipitated phase can effectively prevent the movement of grain boundaries and dislocations, thereby greatly improving the strength of the alloy. The alloying elements that produce precipitation strengthening should meet the following two conditions: first, the solid solubility in copper at high and low temperatures is quite different, so that enough strengthening phases can be produced during aging; second, the solid solubility in copper at room temperature is very different. The solubility is extremely small to ensure high conductivity of the matrix. Precipitation strengthening is the most widely used strengthening method in high-strength, high-conductivity copper alloys. In copper alloys, in order to produce the aging precipitation strengthening effect, elements added include Ti, Co, P, Ni, Si, Mg, Cr, Zr, Be, Fe, etc. The biggest advantage of aging precipitation strengthening is that it greatly improves the strength of the material while minimizing damage to the electrical conductivity.
5. Diffusion enhancement
Dispersion strengthening is a material prepared by powder metallurgy and other methods after fully mixing a dispersion strengthening phase powder of a certain shape and size with copper powder. The second phase particles (Al2O3, ThO2, Zro2, etc.) are dispersed and distributed in the copper matrix, and the strength of the copper alloy is improved due to the dispersion strengthening effect. This method has little impact on the electrical and thermal conductivity of copper while improving strength. In order to obtain dispersedly distributed second phase particles in the copper matrix, it can be considered that second phase particles are added to the copper matrix or dispersedly distributed second phase particles are generated in situ in the copper matrix through a certain process. The specific methods include: mechanical mixing method, co-precipitation method, internal oxidation method, reverse gel precipitation method, electrolytic precipitation method, etc. The main mechanisms of dispersion strengthening include Olowan mechanism and Ansel-Lenier mechanism.
(1) Orowan mechanism. During plastic deformation, the dislocation line cannot directly cut through the second phase particle, but under the action of external force, the dislocation line can bend around the second phase particle, and finally a dislocation ring is left around the second phase particle and gives way. Wrong pass. The bending of dislocations will increase the lattice distortion energy in the dislocation-affected area, which increases the resistance to the movement of dislocation lines and increases the slip resistance.
(2) (2) Ansel-Lenier mechanism. G.S. Ansell et al. proposed another dislocation model for the yielding of dispersion-strengthened alloys. They used the fracture of dispersed second phase particles due to dislocation accumulation as the criterion for yield. When the shear stress on the particles is equal to the fracture stress of the dispersed particles, the dispersion-strengthened alloy yields.
6. Fiber in-situ composite reinforcement
This method mainly refers to adding excess alloying elements (Cr, Fe, V, Nb, etc.) to copper to obtain a two-phase complex. The excess elements exist in the solidified alloy in the form of a single phase and a dendritic structure. After that, the alloy is stretched with a large deformation, so that the dendritic structure of the alloy elements is transformed into a fiber structure. The presence of fibers increases the resistance to dislocation movement, thereby strengthening the material.
7. Add trace elements
Adding certain trace elements to the base to alloy it can not only strengthen the alloy, but is also an effective means for developing corrosion-resistant materials. Some of these trace elements strengthen the alloy by forming dispersed phases, and some by purifying the matrix structure, but none of them significantly reduce its corrosion resistance, thereby improving the overall performance of the alloy.
