Strengthening of aluminum: 3 mechanism
Pure aluminum - soft and ductile
Clean aluminum, the content of aluminum 99,8 %, in the annealed condition has a yield strength less 20 MPa (2 kg / mm2) and elongation more than 40 %. To make such aluminum suitable for use as a structural material thereto using various methods of hardening.
Plastic deformation of aluminum
All metals - aluminum and also - have a crystalline atomic lattice. Plastic deformation of metals is due to the existence in their atomic lattice of linear defects - dislocations. Plastic deformation takes place by the movement of dislocations, So, for example, as it shown on the picture 1.
Figure 1 - Plastic deformation by dislocation movement
atomic lattice through plastic metal
Three mechanisms hardening aluminum
SUMMARY metal reinforcement is, that its lattice order or otherwise introduced obstacles to dislocation motion.
For effective aluminum are three main strengthening mechanism. It:
- strain hardening (work hardening, hardening);
- hardening due to the formation of a solid solution of the alloying element in aluminum (hardening)
- hardening as a result of the precipitation of secondary phases in aluminum (aging).
In turn, Aluminum alloys can be classified by their mechanism of hardening prevailing.
Strain hardening aluminum
Dislocations move in the most densely packed planes of the crystal lattice. These planes are called slip planes. Since the crystal lattice of aluminum is fcc, that it has four equivalent slip planes with three areas of each slip. This gives a total of 12 sliding systems. Depending on the prevailing stress state are usually several active slip systems. Therefore, when aluminum deformations constantly interact dislocations different slip planes. As a result, forming dense tangles of dislocations, which are obstacles to further dislocation motion. Around these impediments arise field intense local stresses. This mechanism works for all metal alloys, which it is subjected to plastic deformation.
Strain hardening by cold rolling, drawing or stretching is an effective way to increase the strength of aluminum alloys, which can not be thermally hardened. Curves strain hardening - cold rolling - annealed sheets of such aluminum alloys, 1100, 3003, 5050 and 5052 shown in Figure 2. Good visibility, that the increase in strength of alloys is accompanied by a decrease in ductility, which is measured as a percentage of elongation at tensile tests samples.
Figure 2 - Curves of work hardening of aluminum (1100),
aluminum-manganese alloy 3003 and
aluminum-magnesium alloys 5050 and 5052.
Hardening by forming solid solution
The alloying elements in solid solution with dislocations interact mainly by local stress fields, which provide additional frictional forces when moving dislocations. This hardening mechanism increases the efficiency of work hardening (work hardening, hardening). 3xxx series aluminum alloys and the 5xxx alloys are typical examples of, receiving hardening by forming solid solution of magnesium and manganese, respectively in the atomic lattice of aluminum.
The figure 3 shows the effect of Mg content in solid solution of aluminum on the yield strength and elongation for the most popular aluminum-magnesium alloys industrial.
Figure 3 - The correlation between the yield stress, elongation, and content of magnesium in aluminum alloys 5xxx series
Hardening by precipitation of secondary phases
The separated particles of secondary phases in aluminum is very effective barriers to the movement of dislocations. Efficacy particles as a obstacle to the motion of dislocations depends upon the particle size, and the distance between them.
Small coherent precipitates are not a significant obstacle for dislocation - they simply cut. With the increase in the secondary phase particle size, as well as the loss of their coherence with the atomic lattice of the aluminum matrix, degree of resistance to particle motion of dislocations increases. The increase in hardness up to a maximum at the artificial aging of aluminum alloys is explained by the release of the secondary progressive phase. On the other hand, reduction of hardness during overaging aluminum alloy is due to the increased distance between the particles of second phase.
Strengthening of aluminum alloys due to aging - natural or artificial - occurs precisely by the hardening mechanism due to the separation of secondary phases from a supersaturated solid solution (figure 4). This supersaturated solid solution alloying elements in aluminum created by heating the aluminum alloy to complete dissolution of alloying elements and rapid cooling, for example, to room temperature.
Figure 4 - Curves of natural and artificial aging
extrusions of alloy 6082
In the range from room temperature to 60 ° C causes the formation of "clusters", which are coherent with the atomic lattice of aluminum. This process is called "natural aging". It leads to states of aluminum alloy T1 and T4.
In the temperature range from 60 to 220 ° C causes the formation of intermediate coherent and secondary phases semicoherent. This process is called "artificial aging". It gives the state of aluminum alloy T5 and T6.
Aging curves in Figure 4 show the effect of aging temperature on the tensile and elongation properties of the compacted alloy 6082. Note higher ductility and a lower strength after aging at room temperature.
Handbook of Aluminum: Vol. 1: Physical Metallurgy and Processes / Ed. by George E. Totten, D. Scott MacKenzie