The strengthening methods of copper and copper alloys are deformation strengthening, fine grain strengthening, solid solution strengthening, precipitation strengthening, dispersion strengthening, composite strengthening and micronutrient addition.

1.Deformation hardening

The strength and hardness of copper alloy can be improved by plastic deformation, and it is one of the most commonly used strengthening methods. Because the crystal defects produced by cold working have little effect on the electrical conductivity of the material, this strengthening method can improve the strength and at the same time make the alloy have high electrical conductivity. The characteristic of deformation strengthening is that the plasticity decreases rapidly while the strength increases, and the electrical conductivity decreases slightly with the increase of dislocation density. In addition, when the service temperature rises, the material will be softened by recovery and recrystallization, and the strength of the alloy will be improved by single deformation strengthening, so it is often used together with other strengthening methods.

2. Fine Grain Strengthening

Fine grain strengthening is to obtain fine grains by rapid solidification or heat treatment during casting. Some trace alloy elements can also be added to refine grains. Grain size decreases, the strength of the alloy increases, and the electrical conductivity of the alloy has little effect. So fine grain strengthening has become one of the main strengthening methods for copper alloys. The outstanding advantage of fine grain strengthening is that the plasticity of the material can be improved as well as the strength. After grain refinement, the stress concentration caused by dislocation stacking at grain boundaries can be effectively alleviated and the initiation of cracks can be delayed, and large deformation can be achieved before fracture. Grain refinement is also widely used because of this advantage.

3. Solid solution strengthening

The phenomenon that the strength and hardness of the metal increase by melting some solute elements to form solid solution is called solid solution strengthening. The solid solution strengthening is caused by the distortion of the crystal lattice of the solvent metal and the increase of the resistance when the dislocations move. It has been proved that proper control of solute content in solid solution can significantly improve the strength and hardness of the material, while still maintaining good plasticity and toughness. For example, by adding 19% nickel into copper, the B of the alloy can be increased from 220 MPA to 380ー400 MPA, the hardness from HB44 to HB70, and the plasticity still remains 50% . If copper is strengthened in the same way by other means, such as work hardening during cold deformation, its plasticity will be almost completely lost. Solid solution strengthening is a strengthening method to increase the flow stress by the interaction between solute atoms and moving dislocation in solid solution. In general, the strength of the alloy will be improved by adding proper amount of alloy elements to form solid solution. According to Mott-Nabbaro's theory, for dilute solid solutions, the variation of yield strength with the concentration of solute elements can be expressed as follows: In = o + KCM, the yield strength of alloy, the yield strength of pure metal, and the mass concentration of solute atom K and M are constants that depend on the properties of the Matrix and alloy elements, where M is between 0.5 and 1.

4.Aging precipitation strengthening

The basic principle of aging precipitation strengthening is that the alloy elements with extremely low solid solubility at room temperature and high solid solubility at high temperature are added to the copper. By high temperature solid solution treatment, the alloy elements can form supersaturated solid solution basically, at this point, the strength of copper and pure compared to improve. After aging, the supersaturated solid solution was decomposed, the alloy elements were precipitated and dispersed to form precipitation phase. The precipitated phase can effectively prevent the movement of grain boundary and dislocation, thus greatly improving the strength of the alloy. There are two conditions for producing precipitation strengthened alloy elements: first, the solid solubility of copper at high and low temperature is quite different, so that enough strengthening phases can be produced during aging; second, the solid solubility of copper at room temperature is very small, to ensure high conductivity of the Matrix. Precipitation strengthening is the most widely used strengthening method in high strength and high conductivity copper alloys. In copper alloys, TI, Co, P, NI, SI, MG, CR, Zr, Be, Fe and so on are added to produce precipitation strengthening effect. The greatest advantage of aging precipitation strengthening is that the strength of the material is greatly increased while the damage of electrical conductivity is minimal.

5. Dispersion strengthening

Dispersion strengthening is the preparation of powders with a certain shape and size of dispersion strengthening phase, which are thoroughly mixed with copper powder, by means of Powder metallurgy, etc. . The second phase particles (Al2O3, ThO2, ZRO2, etc.) are dispersed in the Copper Matrix, and the strength of the copper alloy is enhanced by dispersion strengthening. This method has little effect on the conductivity and thermal conductivity of copper while increasing its strength. In order to obtain the dispersed second-phase particles in the copper matrix, it can be considered that the dispersed second-phase particles can be produced in situ by adding the second-phase particles into the copper matrix or by a certain process. The specific methods are: Mechanical Mixing, co-precipitation, internal oxidation, reverse GEL precipitation, electrolytic precipitation and so on. The mechanisms of dispersion strengthening are mainly the Orowan mechanism and the Ansell-lenier mechanism. 

(1) the Orowan mechanism. In plastic deformation, the dislocation line can not cut through the second phase particles directly, but under the action of external force, the dislocation line can bend around the second phase particles, and finally a dislocation ring is left around the second phase particles to allow the passage of the dislocation. The dislocation bending will increase the lattice distortion energy in the dislocation affected zone, which increases the resistance of the dislocation line movement and the slip resistance.

 (2) Ansell-lenier mechanism. ANSELL ET AL have proposed an alternative dislocation model for the yield of dispersion-strengthened alloys. They regard the fracture of dispersed second phase particles caused by dislocation packing as the criterion of yield. When the Shear stress on the particles is equal to the fracture stress of the dispersed particles, the dispersion strengthened alloy will yield.

6. In Situ composite strengthening of fibers

This method mainly refers to the addition of excessive alloy elements (CR, FE, V, NB, etc.) into copper to obtain two-phase complex. The dendrite structure of the alloy is changed into the fiber structure after the large deformation tensile test, and the fiber increases the resistance of dislocation movement, so the material is strengthened.

7. Add Micronutrient

The addition of micronutrient to the base can not only strengthen the alloy, but also be an effective means of developing wear and corrosion resistant materials. Some of these micronutrient can strengthen the alloy by forming dispersion phase, and some can purify the matrix structure, but none of them can obviously reduce the corrosion resistance of the alloy, thus improving the comprehensive properties of the alloy. 

Source: The Web

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