Alloy strengthening is a relatively large and extensive concept, which can be roughly understood as the addition of alloying elements to metals and the strength of alloys; in particular, effective and common methods for improving the resistance of metals to normal deformation.
The alloying elements may exist in the matrix in various forms: as the solute atoms are disorderly distributed in the solid solution, and the solvent atoms constitute an ordered structure, forming a dispersed particle which is different in structure and composition from the matrix, and forming a complex phase of equivalent size. Mixtures, etc.; their different degrees of hindrance to dislocation motion are the direct cause of high strength of the alloy. By changing the matrix type of the matrix, the alloying elements can also refine the grain of the matrix, increase the hardenability of the matrix, and indirectly increase the strength of the alloy.
Different types of alloy strengthening processes have different forms of strengthening mechanisms.
Solid solution strengthening
The strengthening caused by the alloying elements dissolved in the solid solution is called solid solution strengthening.
The most basic case is that the atoms of the alloying elements are randomly distributed in the matrix to form a uniform single-phase solid solution. Since there are mechanical, chemical, and electrical interactions between alloy atoms and dislocations, and interaction energy is a function of the relative position of dislocations and solute atoms, the dislocations on the slip surface are as chaotic. The distribution of large and small energy peaks and energy valleys constitutes an obstacle to dislocation slip, and the statistical effect of all interactions determines the stress necessary to drive dislocation motion. The role of carbon in the martensite of steel can be used as an example of this mechanism.
If temperature and time conditions permit, the solute atoms tend to diffuse to the most favorable position of the energy, and as a result, a solute atomic gas mass is formed around the dislocations, becoming a solid solution having a component segregation. In this case, no matter whether the dislocation first breaks out from the air mass and then moves, or drags the solute atomic air mass to move together, it needs external force to do more work. These processes are closely related to the obvious yield point, strain aging and blue brittleness in steel.
Second phase particle enhancement
In the alloy, the dispersed second phase particles are often used to increase the strength. The highest intensity corresponds to the second phase particle size and is highly dispersed. These second phases are often metal compounds or oxides, which are harder than the matrix. Much more.
For example, the second phase particle is produced by solid solution desolvation precipitation, which is called precipitation strengthening. This strengthening method is widely used in high-strength aluminum alloys, steels, and nickel-based superalloys.
The precipitation mechanism is related to the aging treatment of the precipitated particles （see desolvation decomposition of solid solution）, and the typical development process can be described as follows. The initial strength of the alloy is equivalent to a supersaturated solid solution. In the initial stage of precipitation, the new phase is coherent with the matrix, the size is small and diffuse, and the yield strength is determined by the resistance to be overcome by the dislocation cut through the precipitate phase, including the coherent stress, the internal structure of the precipitate phase and the phase interface effect. As the new phase grows and the interface and internal structure change, it becomes increasingly difficult to dislodge the precipitated phase particles. According to the Orovan mechanism, when the radius of curvature that can be achieved by the dislocation line is equal to the particle spacing on the slip surface, the dislocations bypass the obstacle particles in the form of a Frank-Reed source, and on the second phase particles. Leave a circle of dislocations. At this time, the particle spacing becomes the main factor controlling the yield strength, and therefore, the yield strength is prolonged and decreased in the late aging period.
The second phase point in the alloy can also be introduced by means of internal oxidation, powder sintering, etc., which is technically known as dispersion strengthening. Diffuse hardened particles are commonly used in high hardness oxides.
The second phase point generally increases the work hardening rate of the alloy.
Strengthening of ordered alloys
Due to the different bonding energies of the atoms and heterogeneous atoms, the atomic distribution in the solid solution is not completely disordered. There may be structural superstructures that are regularly or collectively arranged in the lattice in a matrix, ie, short programs or long programs （see Ordered-disordered phase transitions, the reinforcement caused by this ordered structure is called ordered alloy strengthening.
For alloys consisting of two-phase or multi-phase mixtures of comparable size, in addition to the above-described strengthening mechanism in each phase, the contribution of the phase interface to the strength is also taken into account.
In special cases, alloying elements cause a decrease in the yield strength of solid solutions, such as certain silicon alloys and certain iron alloys at low temperatures, known as solid solution softening effects.
What are the strengthening mechanisms of aluminum alloys?
Solid solution strengthening
The addition of alloying elements to pure aluminum forms an aluminum-based solid solution, which causes lattice distortion, hinders the movement of dislocations, and acts as a solid solution strengthening agent to increase the strength. According to the general rule of alloying, when an infinite solid solution or a high concentration solid solution type alloy is formed, not only high strength but also excellent plasticity and good press workability can be obtained. Binary alloys such as Al-Cu, Al-Mg, Al-Si, Al-Zn, and Al-Mn generally form a limited solid solution and have a large limit solubility, and thus have a large solid solution strengthening effect.
2. Time strengthening
Another strengthening effect of the alloying elements on aluminum is achieved by heat treatment. However, since aluminum has no isomeric transformation, its heat treatment phase change is different from steel. The heat treatment strengthening of the aluminum alloy is mainly due to the large solid solubility of the alloying elements in the aluminum alloy and the sharp decrease with the decrease of the temperature. Therefore, after the aluminum alloy is heated to a certain temperature for quenching, a supersaturated aluminum-based solid solution can be obtained. When the supersaturated aluminum-based solid solution is placed at room temperature or heated to a certain temperature, its strength and hardness increase with time, but the plasticity and toughness decrease. This process is called aging. The aging at room temperature is called natural aging, and the aging under heating is called artificial aging. The phenomenon that the strength and hardness of the aluminum alloy is increased during the aging process is called age strengthening or age hardening. The strengthening effect is achieved by the age hardening phenomenon generated in the aging process.
3. Excessive phase strengthening
If the amount of the alloying element added to the aluminum exceeds the limit solubility, when the solution treatment is heated, a part of the second phase which cannot be dissolved in the solid solution appears, which is called a surplus phase. In aluminum alloys, these excess phases are typically hard and brittle intermetallic compounds. They hinder dislocation motion in the alloy and strengthen the alloy, which is called excess phase strengthening. This method is often used in production to reinforce cast aluminum alloys and heat resistant aluminum alloys. The more the number of excess phases, the more dispersed the distribution, the greater the strengthening effect. However, if there are too many excess phases, both strength and plasticity will be reduced. The more complex the structure of the excess phase component, the higher the melting point, the better the high temperature thermal stability.
4. Refine organizational strengthening
Many aluminum alloy structures are composed of alpha solid solution and excess phase. If the microstructure of the aluminum alloy can be refined, including refining the alpha solid solution or refining the excess phase, the alloy can be strengthened.
Since the microstructure of the cast aluminum alloy is relatively large, the metamorphic treatment method is often used in actual production to refine the alloy structure. The modification treatment is to add a modifier （usually a sodium salt mixture: 2/3 NaF + 1/3 NaCl） in an amount of 2 to 3% by weight of the alloy to the molten aluminum alloy before casting to increase the crystal core and refine the structure. The modified aluminum alloy can obtain a fine uniform eutectic plus a primary α solid solution structure, thereby significantly improving the strength and plasticity of the aluminum alloy.