Alloy strengthening

　　Alloy strengthening is a relatively large and extensive concept, which can be roughly understood as adding alloying elements to the metal and increasing the strength of the alloy; specifically, it is an effective and commonly used method to improve the ability of the metal to resist deformation.

　　Alloying elements can exist in the matrix in many forms: as the solute atoms are randomly distributed in the solid solution, form an ordered structure with the solvent atoms, form dispersed particles different in structure and composition from the matrix, and form a complex phase of the same size Mixtures, etc.; their different degrees of hindrance to the movement of dislocations are the direct reason for the alloy to obtain high strength. Alloying elements can also refine the matrix grains by changing the matrix lattice type, 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 situation is that the atoms of the alloying elements are randomly distributed in the matrix to form a uniform single-phase solid solution. Because there are mechanical, chemical, and electrical interactions between alloy atoms and dislocations, and the interaction energy is a function of the relative positions of the dislocations and solute atoms, the dislocations on the slip surface are like chaos. The large and small energy peaks and valleys of the distribution constitute an obstacle to dislocation slip. The statistical effect of all interactions determines the stress necessary to drive the movement of the dislocation. The role of carbon in the martensite of steel can be used as an example of this mechanism.

　　If the temperature and time conditions permit, solute atoms tend to diffuse to the most favorable energy position. As a result, solute atom clusters will be formed around the dislocations and become solid solutions with segregated components. In this case, no matter whether the dislocation breaks free from the air mass and then moves, or drags the solute atom air mass to move together, external forces are required 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 alloys, dispersed particles of the second phase are often used to increase the strength. The highest strength corresponds to the small size of the particles of the second phase and a highly dispersed state. These second phases are often metal compounds or oxides, which are harder than the matrix. Much.

　　If the second phase particles are produced by solid solution precipitation precipitation, it is called precipitation strengthening. This strengthening method is widely used in high-strength aluminum alloys, steel, and nickel-based superalloys.

　　The precipitation mechanism is related to the aging treatment of the precipitated particles (see the desolvation and 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 dispersed, and the yield strength is determined by the resistance that dislocations cut through the precipitation phase, including the contribution of factors such as coherent stress, internal structure of the precipitation phase, and phase interface effects. With the growth of the new phase and the changes in the interface and internal structure, it becomes increasingly difficult for dislocations to cut the particles of the precipitated phase. According to the Orowan mechanism, when the radius of curvature that the dislocation line can reach is equivalent to the particle spacing on the slip surface, the dislocation will bypass the obstacle particle in a form similar to the Frank-Reed source, and on the second phase particle Leave a dislocation circle. At this time, the distance between the particles becomes the main factor controlling the yield strength. Therefore, in the later stage of aging, the yield strength may decrease with the extension of the aging time.

　　The second phase particles in the 　　 alloy can also be introduced by means of internal oxidation, powder sintering, etc., which is technically called dispersion strengthening. High hardness oxides are commonly used for dispersion hardened particles.

　　The second phase particles generally increase the work hardening rate of the alloy.

　　Strengthening of ordered alloys

　　Due to the different bonding energies of similar atoms and heterogeneous atoms, the distribution of atoms in the solid solution is not completely messy. There may be structural superstructures in which heterogeneous atoms are arranged regularly or locally in the lattice, that is, short or long programs (see Order-disorder phase transition), the strengthening caused by this ordered structure is called ordered alloy strengthening.

　　For alloys composed of two-phase or multi-phase mixtures of the same size, in addition to the above-mentioned strengthening mechanism still working in each phase, the contribution of the phase interface to the strength should also be taken into account.

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

　　What are the strengthening mechanisms of aluminum alloy?

　　1. Solid solution strengthening

　　Pure aluminum is added with alloying elements to form an aluminum-based solid solution, which causes lattice distortion, hinders the movement of dislocations, plays a role of solid solution strengthening, and improves its strength. According to the general law of alloying, when infinite solid solution or high concentration solid solution alloy is formed, not only high strength can be obtained, but also excellent plasticity and good pressure processing performance can be obtained. Al-Cu, Al-Mg, Al-Si, Al-Zn, Al-Mn and other binary alloys can generally form a limited solid solution, and all have a large ultimate solubility, so they have a large solid solution strengthening effect.

　　2. Time strengthening

　　Another strengthening effect of alloying elements on aluminum is achieved by heat treatment. However, since aluminum has no allotropic transformation, its heat treatment phase transformation is different from that of steel. The heat treatment strengthening of aluminum alloy is mainly due to the large solid solubility of alloying elements in aluminum alloy, and it decreases sharply with the decrease of temperature. Therefore, after the aluminum alloy is 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. The aging performed at room temperature is called natural aging, and the aging performed under heating is called artificial aging. The phenomenon of increasing the strength and hardness of the aluminum alloy during the aging process is called aging strengthening or aging hardening. The strengthening effect is achieved by the aging hardening phenomenon produced in the aging process.

　　3. Reinforcement of excess phase

　　If the amount of alloying elements added to aluminum exceeds the limit solubility, a part of the second phase that cannot be dissolved into the solid solution appears during the heating of the solution treatment, which is called the excess phase. In aluminum alloys, these excess phases are usually hard and brittle intermetallic compounds. They hinder the movement of dislocations in the alloy and strengthen 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 diffuse the distribution, the greater the strengthening effect. But too much excess phase will reduce the strength and plasticity. The more complex the structure of the excess phase component and the higher the melting point, the better the high-temperature thermal stability.

　　4. Refine the organization and strengthen

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

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