What are the alloy strengthening mechanisms?

　　Alloy Strengthening

　　Alloy strengthening is a broad concept, generally understood as adding alloying elements to a metal to increase its strength; specifically, it is an effective and common method to improve a metal's resistance to plastic deformation.

　　Alloying elements can exist in the matrix in various forms: as solute atoms randomly distributed in a solid solution, forming an ordered structure with solvent atoms, forming dispersed particles with different structures and compositions from the matrix, and forming a mixture of phases with comparable sizes; their varying degrees of hindrance to dislocation movement are the direct reasons for the alloy's high strength. Alloying elements can also indirectly increase the alloy's strength by changing the matrix lattice type, refining the matrix grains, and increasing the matrix hardenability.

　　Different types of alloy strengthening processes have different strengthening mechanisms.

 

　　Solid Solution Strengthening

　　Strengthening caused by alloying elements dissolved in a solid solution is called solid solution strengthening.

　　In the most basic case, alloy atoms are randomly distributed in the matrix, forming a uniform single-phase solid solution. Due to the mechanical, chemical, and electrical interactions between alloy atoms and dislocations, and because the interaction energy is a function of the relative position of the dislocation and solute atoms, the dislocations on the slip plane are like being among randomly distributed energy peaks and valleys of various sizes, forming obstacles to dislocation movement. The overall statistical effect of these interactions determines the stress required to drive dislocation movement. The role of carbon in the martensite of steel can serve as an example of this mechanism.

　　If temperature and time conditions permit, solute atoms tend to diffuse to the most energetically favorable positions, resulting in the formation of solute atom clusters around dislocations, forming a solid solution with compositional segregation. In this case, whether the dislocation first escapes from the cluster and then moves, or moves together with the solute atom cluster, requires more work from the external force. These processes are closely related to the apparent yield point, strain aging, and blue brittleness phenomena in steel.

 

　　Second-Phase Particle Strengthening

　　Dispersed second-phase particles are often used in alloys to improve strength. The highest strength corresponds to the state where the second-phase particles are not large and are highly dispersed. These second phases are often metal compounds or oxides, much harder than the matrix.

　　If the second-phase particles are produced by precipitation from a solid solution, it 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 that produces the precipitate particles (see decomposition of solid solutions). A typical development process can be described as follows: The initial strength of the alloy is equivalent to that of a supersaturated solid solution. In the initial stage of precipitation, the new phase is coherent with the matrix, and its size is very small and dispersed. The yield strength is determined by the resistance that needs to be overcome for the dislocation to cut through the precipitate phase, including the contributions of 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 for dislocations to cut through the precipitate particles. According to the Orowan mechanism, when the radius of curvature that the dislocation line can reach is comparable to the interparticle spacing on the slip plane, the dislocation will bypass the obstacle particles in a manner similar to a Frank-Read source, leaving a dislocation loop on the second-phase particle. At this time, the interparticle spacing becomes the main factor controlling the yield strength, so in the later stage of aging, there is a phenomenon where the yield strength decreases with the prolongation of aging time.

　　Second-phase particles in alloys can also be introduced by internal oxidation, powder sintering, etc., which is technically called dispersion strengthening. High-hardness oxides are commonly used as dispersion-hardening particles.

　　Second-phase particles generally increase the work-hardening rate of the alloy.

 

　　Strengthening of Ordered Alloys

　　Due to the different bonding energies between similar and dissimilar atoms, the distribution of atoms in a solid solution is not completely random. There may be a structure superstructure, i.e., short-range or long-range order (see order-disorder phase transition), where dissimilar atoms are arranged regularly locally or globally in the lattice. Strengthening caused by this ordered structure is called ordered alloy strengthening.

　　For alloys composed of mixtures of two or more phases with comparable sizes, in addition to the above-mentioned strengthening mechanisms in each phase, the contribution of the phase interface to the strength must also be considered.

　　In special cases, alloying elements cause a decrease in the yield strength of the solid solution, such as in some silicon alloys and some iron alloys at low temperatures, which is called the solid solution softening effect.

 

　　What are the strengthening mechanisms of aluminum alloys?

　　1. Solid solution strengthening

　　Adding alloying elements to pure aluminum forms an aluminum-based solid solution, causing lattice distortion and hindering dislocation movement, resulting in solid solution strengthening and increasing its strength. According to the general rules of alloying, when forming infinite solid solutions or high-concentration solid solution alloys, not only high strength can be obtained, but also excellent plasticity and good pressure processing performance. Al-Cu, Al-Mg, Al-Si, Al-Zn, and Al-Mn binary alloys generally form limited solid solutions and have large limiting solubilities, thus exhibiting significant solid solution strengthening effects.

 

　　2. Age hardening

　　Another strengthening effect of alloying elements on aluminum is achieved through heat treatment. However, since aluminum does not undergo allotropic transformation, its heat treatment phase transformation is different from that of steel. The heat treatment strengthening of aluminum alloys is mainly due to the fact that alloying elements have high solubility in aluminum alloys and decrease sharply with decreasing temperature. Therefore, after aluminum alloys are heated to a certain temperature and quenched, a supersaturated aluminum-based solid solution can be obtained. When this supersaturated aluminum-based solid solution is placed at room temperature or heated to a certain temperature, its strength and hardness increase with time, but its plasticity and toughness decrease. This process is called aging. Aging at room temperature is called natural aging, and aging under heating conditions is called artificial aging. The phenomenon that the strength and hardness of aluminum alloys increase during aging is called age hardening or age strengthening. Its strengthening effect is achieved by relying on the age hardening phenomenon produced during aging.

 

　　3. Excess phase strengthening

　　If the amount of alloying elements added to aluminum exceeds the limit of solid solubility, then during solution heat treatment, a portion of the second phase that cannot be dissolved into the solid solution will appear, which is called the excess phase. In aluminum alloys, these excess phases are usually hard and brittle intermetallic compounds. They hinder dislocation movement in the alloy, strengthening the alloy, which is called excess phase strengthening. This method is often used in production to strengthen cast aluminum alloys and heat-resistant aluminum alloys. The greater the number of excess phases and the more dispersed their distribution, the greater the strengthening effect. However, if there are too many excess phases, both strength and plasticity will decrease. The more complex the composition and structure of the excess phase, and the higher the melting point, the better the high-temperature thermal stability.

 

　　4. Grain Refinement Strengthening

　　Many aluminum alloy structures are composed of α solid solution and excess phases. If the structure of the aluminum alloy can be refined, including refining the α solid solution or refining the excess phase, the alloy can be strengthened.

　　Since the structure of cast aluminum alloys is relatively coarse, the method of modification treatment is often used in actual production to refine the alloy structure. Modification treatment is to add a modifier (commonly used sodium salt mixture: 2/3NaF+1/3NaCl), which accounts for 2～3% of the alloy weight, to the molten aluminum alloy before casting, to increase the number of crystal nuclei and refine the structure. The aluminum alloy after modification treatment can obtain a fine and uniform eutectic plus primary α solid solution structure, thus significantly improving the strength and plasticity of the aluminum alloy.