As we all know, for the steel with high carbon content, immediately after quenching, the hardness is very high, while the plasticity is very low. However, this is not the case for aluminum alloys. After quenching, the strength and hardness of aluminum alloys do not increase immediately, and the plasticity does not decrease, but increases. However, the strength and hardness of this quenched alloy will increase significantly after a period of time (such as after 4-6 days and nights), while the plasticity will decrease significantly. The phenomenon that the strength and hardness of quenched aluminum alloy increase significantly with time is called aging. Aging can occur at room temperature, called natural aging, or in a temperature range higher than room temperature (such as 100 ~ 200 ℃), called artificial aging.
Age hardening of aluminum alloys is a rather complex process, which is not only determined by the composition of the alloy, the aging process, but also depends on the defects caused by the alloy during the production process, especially the number and distribution of vacancies and dislocations. It is generally accepted that age hardening is the result of solute atoms deviating to form hardened areas.
When the aluminum alloy is quenched and heated, vacancies are formed in the alloy, and during quenching, due to the rapid cooling, these vacancies are "fixed" in the crystal in time to move out. Most of these vacancies in the supersaturated solid solution are bound to solute atoms. Since the supersaturated solid solution is in an unstable state, it is inevitably transformed to an equilibrium state, and the presence of vacancies accelerates the diffusion rate of solute atoms, thus accelerating the bias of solute atoms.
The size and number of hardening zones depend on the quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the concentration of vacancies, the greater the number of hardened zones, and the size of the hardened zone decreases. The greater the quenching and cooling rate, the more vacancies are fixed in the solid solution, which is conducive to increasing the number of hardened zones and reducing the size of the hardened zones.
A basic feature of precipitation-hardening alloy system is the temperature-dependent equilibrium solid solution, i.e., the solid solution increases with temperature, and most heat-treatable reinforced aluminum alloys meet this condition

 

 


 

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