Aluminum is a only non ferrous material of structural importance that can be effectively heat-treated to enhance its mechanical properties. The heat treatment of aluminum involves all the distinct steps as with ferrous materials. The mechanism of this heat treatment that has a commercial importance is known as precipitation hardening.
PRECIPITATION HARDENING : AGE HARDENING
Precipitation is the decomposition of a solid solution into two phases of different composition - the precipitate, and the solid solution. And precipitation hardening is the process whereby hardening of an alloy is caused by precipitation of constituent from a supersaturated solid solution by heating to some elevated temperature. Precipitation hardening is sometimes referred to as age hardening which is defined as the spontaneous increase of hardness at room temperature with lapse of time. Both terms are used interchangeably owing to their identical mechanism of hardening.
The phenomenon of precipitation can occur only in those alloys in which there is a decrease in solid solubility with decreasing temperature resulting in a supersaturated solution.
In essence, the situation is depicted which is characteristic of all alloys which age -hardened and the aluminum copper alloys have been used as typical example. Aluminum forms a solid solution with copper in which the latter has a maximum solubility of 5.7 percent at the eutectic temperature (548*C). For an example duralumin contains 4 % copper and small quantities of other alloying elements at temperature 500*C. At this point, all the compound is dissolved to form a homogenous solid solution. When the alloy is slowly cooled from 500*C, the solubility is reduced and the compound is precipitated as relatively coarse particles, visible under the microscope, until at room temperature about 0.5 per cent of copper remains in solution.
After slow cooling, alloys containing up to 0.5 per cent copper will have a single - phase structure ( Alpha solution of copper in aluminum). If the copper content ranges from 0.5 to 5.0 per cent , a two -phase structure ( Alpha + CuAl2) will be obtained.
On rapid cooling ( quenching ), however, there is not enough time for diffusion to bring about the nucleation and growth of the precipitates and a super -saturated solid solution is retained at room temperature. This condition is unstable and as a function of time, at room temperature and at elevated temperatures up to 200*C the alloy ( Alpha + CuAl2) ages and tends to reject the excess crystals of CuAl2 and the alloy acquires a single-phase structure , Alpha - Solid Solution., and the precipitate. This results in an increase in strength and hardness. The maximum hardness is attained before any precipitate is visible under the microscope, although some atomic changes must have occurred.
Other aluminum alloys require precipitation hardening at about 165*C. To prevent the onset of normal age-hardening storage in a refrigerator at a temperature down to -20*C is used. This is especially done where riveting or pressing operations are involved on solution treated alloys.
The above phenomenon shows that the relative amount of the precipitate is determined by the rate of decrease in solubility of one constituent of the solid solution, the rate of temperature of change, and the time. To obtain optimum properties , therefore, control is must for both time and temperature with the proper combination depending on the alloy.
It is generally believed that the first stage in precipitating reaction is the formation of nuclei of the precipitating phase , followed by the growth of the nuclei in larger particles. This results in the lattice distortion which is responsible for the hardening of alloys. The actual cause of age - hardening , how ever , is the obstruction to the motion of dislocation set up by the fine transition precipitate particles and the strains produced by the mismatch of the transition products with the matrix. Controlling the progress of the transformation so as to obtain the maximum obstruction to dislocation motion is the aim of the heat treatment of this alloy.
Similar precipitation phenomena occurs in several other systems such as Al-Ag, Al-Mg, and Cu-Be. In fact, there are some 200 precipitation hardening alloys in commercial use. But it must be remembered that non one pattern appears to be followed by all alloys capable of precipitation hardening., but , rather a number of patterns are possible.
If an alloy is held for a long period of time at the treatment temperature, or if the temperature is raised to 250*C and higher, coagulation of the particles, formed at the centers of decomposition, is observed and the intermediate phase becomes stable. This is called over aging.
In general, precipitation is accompanied by changes in the physical , chemical and mechanical properties of alloys. Electrical conductivity generally increases (except in Al-Cu Alloys ) , and hardness increases by passing through a maximum and then decreases. Strength also increases with hardness but ductility decreases.
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